Control System Toolbox For Use with MATLAB ® Using the Control System Toolbox Version 6 How to Contact The MathWorks: www.mathworks.com comp.soft-sys.matlab Web Newsgroup [email protected] Technical support Product enhancement suggestions Bug reports Documentation error reports Order status, license renewals, passcodes Sales, pricing, and general information 508-647-7000 Phone 508-647-7001 Fax The MathWorks, Inc. 3 Apple Hill Drive Natick, MA 01760-2098 Mail [email protected] [email protected] [email protected] [email protected] [email protected] For contact information about worldwide offices, see the MathWorks Web site. Using the Control System Toolbox COPYRIGHT 2000-2004 by The MathWorks, Inc. The software described in this document is furnished under a license agreement. The software may be used or copied only under the terms of the license agreement. No part of this manual may be photocopied or reproduced in any form without prior written consent from The MathWorks, Inc. FEDERAL ACQUISITION: This provision applies to all acquisitions of the Program and Documentation by, for, or through the federal government of the United States. By accepting delivery of the Program or Documentation, the government hereby agrees that this software or documentation qualifies as commercial computer software or commercial computer software documentation as such terms are used or defined in FAR 12.212, DFARS Part 227.72, and DFARS 252.227-7014. Accordingly, the terms and conditions of this Agreement and only those rights specified in this Agreement, shall pertain to and govern the use, modification, reproduction, release, performance, display, and disclosure of the Program and Documentation by the federal government (or other entity acquiring for or through the federal government) and shall supersede any conflicting contractual terms or conditions. If this License fails to meet the government's needs or is inconsistent in any respect with federal procurement law, the government agrees to return the Program and Documentation, unused, to The MathWorks, Inc. MATLAB, Simulink, Stateflow, Handle Graphics, and Real-Time Workshop are registered trademarks, and TargetBox is a trademark of The MathWorks, Inc. Other product or brand names are trademarks or registered trademarks of their respective holders. Printing History: November 2000 June 2001 July 2002 June 2004 Online only Online only Online only Online only Revised for Version 5 (Release 12) (Reorganized and a name change) Revised for Version 5.1 (Release 12.1) Revised for Version 5.2 (Release 13) Revised for Version 6.0 (Release 14) Contents LTI Models 1 Linear, Time-Invariant Models . . . . . . . . . . . . . . . . . . . . . . . . . LTI Model Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Using LTI Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LTI Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precedence Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Viewing LTI Systems As Matrices . . . . . . . . . . . . . . . . . . . . . . . LTI Command Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2 1-2 1-3 1-3 1-5 1-5 1-6 Creating LTI Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-8 Transfer Function Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-8 Zero-Pole-Gain Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-12 State-Space Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-14 Descriptor State-Space Models . . . . . . . . . . . . . . . . . . . . . . . . . 1-16 Frequency Response Data (FRD) Models . . . . . . . . . . . . . . . . . 1-17 Discrete-Time Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-19 Data Retrieval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-23 LTI Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Generic LTI Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Model-Specific Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Setting LTI Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Accessing Property Values Using get . . . . . . . . . . . . . . . . . . . . Direct Property Referencing Using Dot Notation . . . . . . . . . . Additional Insight into LTI Properties . . . . . . . . . . . . . . . . . . 1-25 1-25 1-27 1-29 1-31 1-32 1-33 Model Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Explicit Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Automatic Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Caution About Model Conversions . . . . . . . . . . . . . . . . . . . . . . 1-39 1-39 1-40 1-40 i Time Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Functions that Support Time Delays . . . . . . . . . . . . . . . . . . . . Specifying Input/Output Delays . . . . . . . . . . . . . . . . . . . . . . . . Distillation Column Example . . . . . . . . . . . . . . . . . . . . . . . . . . Specifying Delays on the Inputs or Outputs . . . . . . . . . . . . . . . Specifying Delays in Discrete-Time Models . . . . . . . . . . . . . . . Retrieving Information About Delays . . . . . . . . . . . . . . . . . . . . Padé Approximation of Time Delays . . . . . . . . . . . . . . . . . . . . . 1-42 1-43 1-43 1-44 1-47 1-49 1-51 1-51 Simulink Block for LTI Systems . . . . . . . . . . . . . . . . . . . . . . . 1-53 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-55 Operations on LTI Models 2 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2 Precedence and Property Inheritance . . . . . . . . . . . . . . . . . . 2-3 Extracting and Modifying Subsystems . . . . . . . . . . . . . . . . . . Basic Subsystem Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Referencing FRD Models Through Frequencies . . . . . . . . . . . . . Referencing Channels by Name . . . . . . . . . . . . . . . . . . . . . . . . . Resizing LTI Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5 2-5 2-7 2-8 2-9 Arithmetic Operations on LTI Models . . . . . . . . . . . . . . . . . . Addition and Subtraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Multiplication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inversion and Related Operations . . . . . . . . . . . . . . . . . . . . . . Transposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pertransposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11 2-11 2-13 2-13 2-14 2-14 Model Interconnection Functions . . . . . . . . . . . . . . . . . . . . . 2-16 Concatenation of LTI Models . . . . . . . . . . . . . . . . . . . . . . . . . . 2-17 Feedback and Other Interconnection Functions . . . . . . . . . . . 2-18 ii Contents Continuous/Discrete Conversions of LTI Models . . . . . . . . Zero-Order Hold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . First-Order Hold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tustin Approximation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tustin with Frequency Prewarping . . . . . . . . . . . . . . . . . . . . . Matched Poles and Zeros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discretization of Systems with Delays . . . . . . . . . . . . . . . . . . . 2-20 2-20 2-22 2-23 2-23 2-23 2-24 Resampling of Discrete-Time Models . . . . . . . . . . . . . . . . . . . 2-26 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-27 Model Analysis Tools 3 General Model Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 3-2 Model Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4 State-Space Realizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7 Arrays of LTI Models 4 The Concept of an LTI Array . . . . . . . . . . . . . . . . . . . . . . . . . . . When to Use an LTI Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . When to Collect a Set of Models in an LTI Array . . . . . . . . . . . Restrictions for LTI Models Collected in an Array . . . . . . . . . . Where to Find Information on LTI Arrays . . . . . . . . . . . . . . . . . Visualizing LTI Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Higher Dimensional Arrays of LTI Models . . . . . . . . . . . . . . . . 4-2 4-2 4-3 4-3 4-3 4-4 4-6 iii Dimensions, Size, and Shape of an LTI Array . . . . . . . . . . . . 4-7 I/O and Array Dimensions of LTI Arrays . . . . . . . . . . . . . . . . . . 4-7 Accessing the Dimensions of an LTI Array Using size and ndims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9 Using reshape to Rearrange an LTI Array . . . . . . . . . . . . . . . . 4-11 Building LTI Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Building LTI Arrays Using for Loops . . . . . . . . . . . . . . . . . . . . Building LTI Arrays Using the stack Function . . . . . . . . . . . . Building LTI Arrays Using tf, zpk, ss, and frd . . . . . . . . . . . . . Generating Random LTI Arrays Using rss . . . . . . . . . . . . . . . 4-12 4-12 4-15 4-17 4-19 Indexing into LTI Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Accessing Particular Models in an LTI Array . . . . . . . . . . . . . Extracting LTI Arrays of Subsystems . . . . . . . . . . . . . . . . . . . Reassigning Parts of an LTI Array . . . . . . . . . . . . . . . . . . . . . . Deleting Parts of an LTI Array . . . . . . . . . . . . . . . . . . . . . . . . . 4-21 4-22 4-23 4-23 4-24 Operations on LTI Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Example: Addition of Two LTI Arrays . . . . . . . . . . . . . . . . . . . Dimension Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Special Cases for Operations on LTI Arrays . . . . . . . . . . . . . . Other Operations on LTI Arrays . . . . . . . . . . . . . . . . . . . . . . . . 4-25 4-26 4-27 4-28 4-30 Preliminaries for Customization 5 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2 The Property and Preferences Hierarchy . . . . . . . . . . . . . . . 5-3 iv Contents Setting Toolbox Preferences 6 Toolbox Preferences Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 Opening the Toolbox Preferences Editor . . . . . . . . . . . . . . . . . . 6-2 Units Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3 Style Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4 Characteristics Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5 SISO Tool Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6 Setting Tool Preferences 7 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2 LTI Viewer Preferences Editor . . . . . . . . . . . . . . . . . . . . . . . . . Units Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Style Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characteristics Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3 7-4 7-5 7-6 7-7 SISO Design Tool Preferences Editor . . . . . . . . . . . . . . . . . . . 7-8 Units Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-9 Style Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10 Options Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-12 Line Colors Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-14 v Customizing Response Plot Properties 8 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-2 Response Plots Property Editor . . . . . . . . . . . . . . . . . . . . . . . . Labels Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Limits Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Units Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Style Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characteristics Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4 8-5 8-6 8-7 8-8 8-9 Property Editing for Subplots . . . . . . . . . . . . . . . . . . . . . . . . . 8-11 Customizing Plots Inside the SISO Design Tool . . . . . . . . . Root Locus Property Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . Labels Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Limits Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Options Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open-Loop Bode Property Editor . . . . . . . . . . . . . . . . . . . . . . . Labels Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Limits Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Options Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Open-Loop Nichols Property Editor . . . . . . . . . . . . . . . . . . . . . Labels Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Limits Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Options Pane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prefilter Bode Property Editor . . . . . . . . . . . . . . . . . . . . . . . . . 8-12 8-12 8-13 8-14 8-16 8-18 8-19 8-20 8-21 8-22 8-23 8-24 8-25 8-25 Design Case Studies 9 Yaw Damper for a 747 Jet Transport . . . . . . . . . . . . . . . . . . . . 9-2 Computing Open-Loop Eigenvalues . . . . . . . . . . . . . . . . . . . . . . 9-4 Open-Loop Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-5 Root Locus Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-9 Washout Filter Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-14 vi Contents Hard-Disk Read/Write Head Controller . . . . . . . . . . . . . . . . Deriving the Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Model Discretization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adding a Compensator Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . Adding a Lead Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Design Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-20 9-20 9-21 9-24 9-24 9-27 LQG Regulation: Rolling Mill Example . . . . . . . . . . . . . . . . . Process and Disturbance Models . . . . . . . . . . . . . . . . . . . . . . . . LQG Design for the x-Axis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LQG Design for the y-Axis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-Coupling Between Axes . . . . . . . . . . . . . . . . . . . . . . . . . . MIMO LQG Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-31 9-31 9-34 9-42 9-43 9-47 Kalman Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discrete Kalman Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Steady-State Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time-Varying Kalman Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . Time-Varying Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-50 9-50 9-51 9-57 9-58 9-61 Reliable Computations 10 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-2 Conditioning and Numerical Stability . . . . . . . . . . . . . . . . . 10-4 Conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-4 Numerical Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-6 Choice of LTI Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-8 State Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-8 Transfer Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-8 Zero-Pole-Gain Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-13 vii Scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-15 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-17 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-18 SISO Design Tool 11 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-2 Importing and Exporting Models . . . . . . . . . . . . . . . . . . . . . . Importing Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Exporting Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Exporting to Disk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Exporting Multiple Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-4 11-4 11-5 11-6 11-6 Tuning Compensators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-7 Root-Locus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-8 Open-Loop Bode Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-9 Open-Loop Nichols Plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-10 Prefilter Bode Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-11 Using the Right-Click Menu to Design Compensators . . . 11-12 Adding Dynamics to Your Compensator . . . . . . . . . . . . . . . . . 11-12 Example: Adding a Complex Pair of Poles . . . . . . . . . . . . . . . 11-13 Viewing Loop Responses and System Data . . . . . . . . . . . . 11-16 Viewing Specialized Loop Responses . . . . . . . . . . . . . . . . . . . 11-16 Viewing System Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-18 Storing and Retrieving Designs . . . . . . . . . . . . . . . . . . . . . . 11-19 viii Contents Creating and Editing Design Constraints . . . . . . . . . . . . . Design Constraints for the Root Locus . . . . . . . . . . . . . . . . . . Example: Adding Damping Ratio Constraints . . . . . . . . . . . . Design Constraints for Open-Loop and Prefilter Bode Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Example: Adding Upper Gain Limits . . . . . . . . . . . . . . . . . . . Design Constraints for Open-Loop Nichols Plots . . . . . . . . . . Example: Adding a Closed-Loop Peak Gain Constraint . . . . Editing Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Deleting Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-20 11-20 11-22 11-23 11-24 11-26 11-27 11-29 11-31 Current Compensator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-32 Customizing the SISO Design Tool . . . . . . . . . . . . . . . . . . . . 11-33 LTI Viewer 12 Basic LTI Viewer Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-2 Using the Right-Click Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-4 Setting Characteristics of Response Plots . . . . . . . . . . . . . . . . 12-4 Importing and Exporting Models . . . . . . . . . . . . . . . . . . . . . . 12-9 Exporting Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-10 Selecting Response Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-11 Right-Click Menu: Plot Type . . . . . . . . . . . . . . . . . . . . . . . . . . 12-11 Plot Configurations Window . . . . . . . . . . . . . . . . . . . . . . . . . . 12-11 Analyzing MIMO Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-14 I/O Grouping for MIMO Models . . . . . . . . . . . . . . . . . . . . . . . 12-15 Selecting I/O Pairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-16 Customizing the LTI Viewer . . . . . . . . . . . . . . . . . . . . . . . . . 12-17 ix Index x Contents 1 LTI Models Linear, Time-Invariant Models (p. 1-2) An introduction to LTI models Creating LTI Models (p. 1-8) How to create LTI models in various formats LTI Properties (p. 1-25) Converting between LTI model formats Model Conversion (p. 1-39) Converting between LTI model formats Time Delays (p. 1-42) Adding time delays to LTI models Simulink Block for LTI Systems (p. 1-53) Describes the LTI Systems block, a Simulink® block that you can use to import models into Simulink References (p. 1-55) Relevant control theory literature 1 LTI Models Linear, Time-Invariant Models The Control System Toolbox offers extensive tools to manipulate and analyze linear time-invariant (LTI) models. It supports both continuous- and discrete-time systems. Systems can be single-input/single-output (SISO) or multiple-input/multiple-output (MIMO). In addition, you can store several LTI models in an array under a single variable name. See Chapter 4, “Arrays of LTI Models,” for information on LTI arrays. This section introduces key concepts about the MATLAB® representation of LTI models, including • “LTI Model Formats” on page 1-2 • “Using LTI Models” on page 1-3 • “LTI Objects” on page 1-3 • “Precedence Rules” on page 1-5 • See “Viewing LTI Systems As Matrices” on page 5. • “LTI Command Summary” on page 1-6 LTI Model Formats You can specify LTI models as • Transfer functions (TF), for example, s+2 P ( s ) = -------------------------2 s + s + 10 • Zero-pole-gain models (ZPK), for example, H(z ) = 2 ( z – 0.5 ) ------------------------z ( z + 0.1 ) 2 (z + z + 1) --------------------------------------------( z + 0.2 ) ( z + 0.1 ) • State-space models (SS), for example, dx ------ = Ax + Bu dt y = Cx + Du where A, B, C, and D are matrices of appropriate dimensions, x is the state vector, and u and y are the input and output vectors. 1-2 Linear, Time-Invariant Models • Frequency response data (FRD) models FRD models consist of sampled measurements of a system’s frequency response. For example, you can store experimentally collected frequency response data in an FRD. Using LTI Models You can manipulate TF, SS, and ZPK models using the arithmetic and model interconnection operations described in Chapter 2, “Operations on LTI Models,” and analyze them using the model analysis functions, such as bode and step. FRD models can be manipulated and analyzed in much the same way you analyze the other model types, but analysis is restricted to frequency-domain methods. Using a variety of design techniques, you can design compensators for systems specified with TF, ZPK, SS, and FRD models. These techniques include root locus analysis, pole placement, LQG optimal control, and frequency domain loop-shaping. For FRD models, you can either • Obtain an identified TF, SS, or ZPK model using system identification techniques • Use frequency-domain analysis techniques Other Uses of FRD Models FRD models are unique model types available in the Control System Toolbox collection of LTI model types, in that they don’t have a parametric representation. In addition to the standard operations you can perform on FRD models, you can also use them to • Perform frequency-domain analysis on systems with nonlinearities using describing functions. • Validate identified models against experimental frequency response data. LTI Objects Depending on the type of model you use, the data for your model may consist of a simple numerator/denominator pair for SISO transfer functions, four matrices for state-space models, and multiple sets of zeros and poles for MIMO zero-pole-gain models or frequency and response vectors for FRD models. For convenience, the Control System Toolbox provides customized data structures 1-3 1 LTI Models (LTI objects) for each type of model. These are called the TF, ZPK, SS, and FRD objects. These four LTI objects encapsulate the model data and enable you to manipulate LTI systems as single entities rather than collections of data vectors or matrices. Creating an LTI Object: An Example An LTI object of the type TF, ZPK, SS, or FRD is created whenever you invoke the corresponding constructor function, tf, zpk, ss, or frd. For example, P = tf([1 2],[1 1 10]) creates a TF object, P, that stores the numerator and denominator coefficients of the transfer function s+2 P ( s ) = -------------------------2 s + s + 10 See “Creating LTI Models” on page 1-8 for methods for creating all of the LTI object types. LTI Properties and Methods The LTI object implementation relies on MATLAB object-oriented programming capabilities. Objects are MATLAB structures with an additional flag indicating their class (TF, ZPK, SS, or FRD for LTI objects) and have predefined fields called object properties. For LTI objects, these properties include the model data, sample time, delay times, input or output names, and input or output groups (see “LTI Properties” on page 1-25 for details). The functions that operate on a particular object are called the object methods. These can include customized versions of simple operations such as addition or multiplication. For example, P = tf([1 2],[1 1 10]) Q = 2 + P performs transfer function addition. 2 2s + 3s + 22 Q ( s ) = 2 + P ( s ) = ----------------------------------2 s + s + 10 The object-specific versions of such standard operations are called overloaded operations. For more details on objects, methods, and object-oriented 1-4 Linear, Time-Invariant Models programming, see Classes and Objects” in the MATLAB documentation. For details on operations on LTI objects, see Chapter 2, “Operations on LTI Models.” Precedence Rules Operations like addition and commands like feedback operate on more than one LTI model at a time. If these LTI models are represented as LTI objects of different types (for example, the first operand is TF and the second operand is SS), it is not obvious what type (for example, TF or SS) the resulting model should be. Such type conflicts are resolved by precedence rules. Specifically, TF, ZPK, SS, and FRD objects are ranked according to the precedence hierarchy. FRD > SS > ZPK > TF Thus ZPK takes precedence over TF, SS takes precedence over both TF and ZPK, and FRD takes precedence over all three. In other words, any operation involving two or more LTI models produces • An FRD object if at least one operand is an FRD object • An SS object if no operand is an FRD object and at least one operand is an SS object • A ZPK object if no operand is an FRD or SS object and at least one is an ZPK object • A TF object only if all operands are TF objects Operations on systems of different types work as follows: the resulting type is determined by the precedence rules, and all operands are first converted to this type before performing the operation. Viewing LTI Systems As Matrices In the frequency domain, an LTI system is represented by the linear input/output map y = Hu This map is characterized by its transfer matrix H, a function of either the Laplace or Z-transform variable. The transfer matrix H maps inputs to outputs, so there are as many columns as inputs and as many rows as outputs. 1-5 1 LTI Models If you think of LTI systems in terms of (transfer) matrices, certain basic operations on LTI systems are naturally expressed with a matrix-like syntax. For example, the parallel connection of two LTI systems sys1 and sys2 can be expressed as sys = sys1 + sys2 because parallel connection amounts to adding the transfer matrices. Similarly, subsystems of a given LTI model sys can be extracted using matrix-like subscripting. For instance, sys(3,1:2) provides the I/O relation between the first two inputs (column indices) and the third output (row index), which is consistent with y1 H ( 1, 1 ) H ( 2, 1 ) y 2 = H ( 2, 1 ) H ( 2, 2 ) u 1 u 2 H ( 3, 1 ) H ( 3, 2 ) y3 for y = Hu. LTI Command Summary The next two tables give an overview of the main commands you can apply to LTI models. Creating LTI Models 1-6 Command Description drss Generate random discrete state-space model. dss Create descriptor state-space model. filt Create discrete filter with DSP convention. frd Create an FRD model. frdata Retrieve FRD model data. get Query LTI model properties. Linear, Time-Invariant Models Creating LTI Models (Continued) Command Description set Set LTI model properties. rss Generate random continuous state-space model. ss Create a state-space model. ssdata, dssdata Retrieve state-space data (respectively, descriptor state-space data). tf Create a transfer function. tfdata Retrieve transfer function data. zpk Create a zero-pole-gain model. zpkdata Retrieve zero-pole-gain data. 1-7 1 LTI Models Creating LTI Models The functions tf, zpk, ss, and frd create transfer function models, zero-pole-gain models, state-space models, and frequency response data models, respectively. These functions take the model data as input and produce TF, ZPK, SS, or FRD objects that store this data in a single MATLAB variable. This section shows how to create continuous or discrete, SISO or MIMO LTI models with tf, zpk, ss, and frd. This section contains the following topics: • “Transfer Function Models” on page 1-8 • “Zero-Pole-Gain Models” on page 1-12 • “State-Space Models” on page 1-14 • “Descriptor State-Space Models” on page 1-16 • “Frequency Response Data (FRD) Models” on page 1-17 • “Discrete-Time Models” on page 1-19 • “Data Retrieval” on page 1-23 Transfer Function Models This section explains how to specify continuous-time SISO and MIMO transfer function models. The specification of discrete-time transfer function models is a simple extension of the continuous-time case (see “Discrete-Time Models” on page 1-19). In this section you can also read about how to specify transfer functions consisting of pure gains. SISO Transfer Function Models A continuous-time SISO transfer function n(s) h ( s ) = ----------d(s) is characterized by its numerator n ( s ) and denominator d ( s ) , both polynomials of the Laplace variable s. There are two ways to specify SISO transfer functions: • Using the tf command • As rational expressions in the Laplace variable s 1-8 Creating LTI Models To specify a SISO transfer function model h ( s ) = n ( s ) ⁄ d ( s ) using the tf command, type h = tf(num,den) where num and den are row vectors listing the coefficients of the polynomials n ( s ) and d ( s ), respectively, when these polynomials are ordered in descending powers of s. The resulting variable h is a TF object containing the numerator and denominator data. For example, you can create the transfer function h ( s ) = s ⁄ ( s 2 + 2s + 10 ) by typing h = tf([1 0],[1 2 10]) MATLAB responds with Transfer function: s -------------s^2 + 2 s + 10 Note the customized display used for TF objects. You can also specify transfer functions as rational expressions in the Laplace variable s by 1 Defining the variable s as a special TF model s = tf('s'); 2 Entering your transfer function as a rational expression in s For example, once s is defined with tf as in 1, H = s/(s^2 + 2*s +10); produces the same transfer function as h = tf([1 0],[1 2 10]); 1-9 1 LTI Models Note You need only define the variable s as a TF model once. All of the subsequent models you create using rational expressions of s are specified as TF objects, unless you convert the variable s to ZPK. See “Model Conversion” on page 1-39 for more information. MIMO Transfer Function Models MIMO transfer functions are two-dimensional arrays of elementary SISO transfer functions. There are several ways to specify MIMO transfer function models, including • Concatenation of SISO transfer function models • Using tf with cell array arguments Consider the rational transfer matrix s–1 -----------s+1 . H(s ) = s+2 ---------------------------s 2 + 4s + 5 You can specify H ( s ) by concatenation of its SISO entries. For instance, h11 = tf([1 1],[1 1]); h21 = tf([1 2],[1 4 5]); or, equivalently, s = tf('s') h11 = (s 1)/(s+1); h21 = (s+2)/(s^2+4*s+5); can be concatenated to form H ( s ). H = [h11; h21] This syntax mimics standard matrix concatenation and tends to be easier and more readable for MIMO systems with many inputs and/or outputs. See “Model Interconnection Functions” on page 2-16 for more details on concatenation operations for LTI systems. 1-10 Creating LTI Models Alternatively, to define MIMO transfer functions using tf, you need two cell arrays (say, N and D) to represent the sets of numerator and denominator polynomials, respectively. See Structures and Cell Arrays in the MATLAB documentation for more details on cell arrays. For example, for the rational transfer matrix H ( s ) , the two cell arrays N and D should contain the row-vector representations of the polynomial entries of N(s) = s – 1 s+2 D( s) = s+1 s 2 + 4s + 5 You can specify this MIMO transfer matrix H ( s ) by typing N = {[1 1];[1 2]}; % cell array for N(s) D = {[1 1];[1 4 5]}; % cell array for D(s) H = tf(N,D) MATLAB responds with Transfer function from input to output... s 1 #1: ----s + 1 #2: s + 2 ------------s^2 + 4 s + 5 Notice that both N and D have the same dimensions as H. For a general MIMO transfer matrix H ( s ), the cell array entries N{i,j} and D{i,j} should be row-vector representations of the numerator and denominator of H ij ( s ), the ijth entry of the transfer matrix H ( s ) . Pure Gains You can use tf with only one argument to specify simple gains or gain matrices as TF objects. For example, G = tf([1 0;2 1]) produces the gain matrix 1-11 1 LTI Models G = 1 0 2 1 while E = tf creates an empty transfer function. Zero-Pole-Gain Models This section explains how to specify continuous-time SISO and MIMO zero-pole-gain models. The specification for discrete-time zero-pole-gain models is a simple extension of the continuous-time case. See “Discrete-Time Models” on page 1-19. SISO Zero-Pole-Gain Models Continuous-time SISO zero-pole-gain models are of the form ( s – z 1 ) ... ( s – z m ) h ( s ) = k ------------------------------------------------( s – p 1 ) ... ( s – p n ) where k is a real- or complex-valued scalar (the gain), and z 1 ,..., z m and p 1 ,..., p n are the real or complex conjugate pairs of zeros and poles of the transfer function h ( s ) . This model is closely related to the transfer function representation: the zeros are simply the numerator roots, and the poles, the denominator roots. There are two ways to specify SISO zero-pole-gain models: • Using the zpk command • As rational expressions in the Laplace variable s The syntax to specify ZPK models directly using zpk is h = zpk(z,p,k) where z and p are the vectors of zeros and poles, and k is the gain. This produces a ZPK object h that encapsulates the z, p, and k data. For example, typing h = zpk(0, [1 i 1+i 2], 1-12 2) Creating LTI Models produces Zero/pole/gain: 2 s -------------------(s 2) (s^2 2s + 2) You can also specify zero-pole-gain models as rational expressions in the Laplace variable s by 1 Defining the variable s as a ZPK model s = zpk('s') 2 Entering the transfer function as a rational expression in s. For example, once s is defined with zpk, H = 2s/((s 2)*(s^2 + 2*s + 2)); returns the same ZPK model as h = zpk([0], [2 1 i 1+i ], 2); Note You need only define the ZPK variable s once. All subsequent rational expressions of s will be ZPK models, unless you convert the variable s to TF. See “Model Conversion” on page 1-39 for more information on conversion to other model types. MIMO Zero-Pole-Gain Models Just as with TF models, you can also specify a MIMO ZPK model by concatenation of its SISO entries (see “Model Interconnection Functions” on page 2-16). You can also use the command zpk to specify MIMO ZPK models. The syntax to create a p-by-m MIMO zero-pole-gain model using zpk is H = zpk(Z,P,K) where 1-13 1 LTI Models • Z is the p-by-m cell array of zeros (Z{i,j} = zeros of H ij ( s ) ) • P is the p-by-m cell array of poles (P{i,j} = poles of H ij ( s ) ) • K is the p-by-m matrix of gains (K(i,j) = gain of H ij ( s ) ) For example, typing Z = {[], 5;[1 i 1+i] []}; P = {0,[ 1 K = [ 1 1];[1 2 3],[]}; 3;2 0]; H = zpk(Z,P,K) creates the two-input/two-output zero-pole-gain model H(s ) = –1 -----s 3(s + 5) -------------------2 (s + 1) 2 ( s 2 – 2s + 2 ) --------------------------------------------------( s – 1 ) ( s – 2 )( s – 3 ) 0 Notice that you use [] as a place-holder in Z (or P) when the corresponding entry of H ( s ) has no zeros (or poles). State-Space Models State-space models rely on linear differential or difference equations to describe the system dynamics. Continuous-time models are of the form dx ------ = Ax + Bu dt y = Cx + Du where x is the state vector and u and y are the input and output vectors. Such models may arise from the equations of physics, from state-space identification, or by state-space realization of the system transfer function. Use the command ss to create state-space models sys = ss(A,B,C,D) For a model with Nx states, Ny outputs, and Nu inputs 1-14 Creating LTI Models • A is an Nx-by-Nx real- or complex-valued matrix. • B is an Nx-by-Nu real- or complex-valued matrix. • C is an Ny-by-Nx real- or complex-valued matrix. • D is an Ny-by-Nu real- or complex-valued matrix. This produces an SS object sys that stores the state-space matrices A, B, C, and D. For models with a zero D matrix, you can use D = 0 (zero) as a shorthand for a zero matrix of the appropriate dimensions. As an illustration, consider the following simple model of an electric motor. θ d 2 θ- + 2 d ------ + 5θ = 3I --------2 d t dt where θ is the angular displacement of the rotor and I the driving current. The relation between the input current u = I and the angular velocity y = dθ ⁄ dt is described by the state-space equations dx ------ = Ax + Bu dt y = Cx where θ x = dθ -----dt A = 0 1 – 5 –2 B = 0 3 C = 01 This model is specified by typing sys = ss([0 1; 5 2],[0;3],[0 1],0) to which MATLAB responds a = x1 x2 x1 0 5.00000 x1 u1 0 x2 1.00000 2.00000 b = 1-15 1 LTI Models x2 3.00000 y1 x1 0 y1 u1 0 c = x2 1.00000 d = In addition to the A, B, C, and D matrices, the display of state-space models includes state names, input names, and output names. Default names (here, x1, x2, u1, and y1) are displayed whenever you leave these unspecified. See “LTI Properties” on page 1-25 for more information on how to specify state, input, or output names. Descriptor State-Space Models Descriptor state-space (DSS) models are a generalization of the standard state-space models discussed above. They are of the form dx E ------ = Ax + Bu dt y = Cx + Du The Control System Toolbox supports only descriptor systems with a nonsingular E matrix. While such models have an equivalent explicit form dx ------ = ( E – 1 A )x + ( E –1 B )u dt y = Cx + Du it is often desirable to work with the descriptor form when the E matrix is poorly conditioned with respect to inversion. The function dss is the counterpart of ss for descriptor state-space models. Specifically, sys = dss(A,B,C,D,E) creates a continuous-time DSS model with matrix data A,B,C,D,E. For example, consider the dynamical model 1-16 Creating LTI Models dω J -------- + Fω = T dt y=ω with vector ω of angular velocities. If the inertia matrix J is poorly conditioned with respect to inversion, you can specify this system as a descriptor model by sys = dss( F,eye(n),eye(n),0,J) % n = length of vector ω Frequency Response Data (FRD) Models In some instances, you may only have sampled frequency response data, rather than a transfer function or state-space model for the system you want to analyze or control. For information on frequency response analysis of linear systems, see Chapter 8 of [1]. For example, suppose the frequency response function for the SISO system you want to model is G(w). Suppose, in addition, that you perform an experiment to evaluate G(w) at a fixed set of frequencies, w 1, w 2, …, w n. You can do this by driving the system with a sequence of sinusoids at each of these frequencies, as depicted below. sin w i t G(w) = yi ( t ) Here w i is the input frequency of each sinusoid, i = 1 ... n, and G(w) = G ( w ) exp ( j ∠G ( w ) ) . The steady state output response of this system satisfies y i ( t ) = G ( wi ) sin ( w i t + ∠G ( wi ) ) ; i = 1…n A frequency response data (FRD) object is a model form you can use to store frequency response data (complex frequency response, along with a corresponding vector of frequency points) that you obtain either through simulations or experimentally. In this example, the frequency response data is obtained from the set of response pairs: { (G ( w i ),wi) }, i = 1…n . 1-17 1 LTI Models Once you store your data in an FRD model, you can treat it as an LTI model, and manipulate an FRD model in most of the same ways you manipulate TF, SS, and ZPK models. The basic syntax for creating a SISO FRD model is sys = frd(response,frequencies,units) where • frequencies is a real vector of length Nf. • response is a vector of length Nf of complex frequency response values for these frequencies. • units is an optional string for the units of frequency: either 'rad/s' (default) or 'Hz'. For example, the MAT-file LTIexamples.mat contains a frequency vector freq, and a corresponding complex frequency response data vector respG. To load this frequency-domain data and construct an FRD model, type load LTIexamples sys = frd(respG,freq) Continuous-time frequency response with 1 output and 1 input at 5 frequency points. From input 1 to: Frequency(rad/s) output 1 ----------------------1 0.812505 0.000312i 2 0.092593 0.462963i 4 0.075781 0.001625i 5 0.043735 0.000390i The syntax for creating a MIMO FRD model is the same as for the SISO case, except that response is a p-by-m-by-Nf multidimensional array, where p is the number of outputs, m is the number of inputs, and Nf is the number of frequency data points (the length of frequency). 1-18 Creating LTI Models The following table summarizes the complex-valued response data format for FRD models. Data Format for the Argument response in FRD Models Model Form Response Data Format SISO model Vector of length Nf for which response(i) is the frequency response at the frequency frequency(i) MIMO model with Ny outputs and Nu inputs Ny-by-Nu-by-Nf multidimensional array for which response(i,j,k) specifies the frequency response from input j to output i at frequency frequency(k) S1-by-...-by-Sn array of models with Ny outputs and Nu inputs Ny-by-Nu-by-S1-by-...-by-Sn multidimensional array, for which response(i,j,k,:) specifies the array of frequency response data from input j to output i at frequency frequency(k) Discrete-Time Models Creating discrete-time models is very much like creating continuous-time models, except that you must also specify a sampling period or sample time for discrete-time models. The sample time value should be scalar and expressed in seconds. You can also use the value –1 to leave the sample time unspecified. To specify discrete-time LTI models using tf, zpk, ss, or frd, simply append the desired sample time value Ts to the list of inputs. sys1 sys2 sys3 sys4 = = = = tf(num,den,Ts) zpk(z,p,k,Ts) ss(a,b,c,d,Ts) frd(response,frequency,Ts) For example, h = tf([1 1],[1 0.5],0.1) creates the discrete-time transfer function h ( z ) = ( z – 1 ) ⁄ ( z – 0.5 ) with sample time 0.1 seconds, and sys = ss(A,B,C,D,0.5) specifies the discrete-time state-space model 1-19 1 LTI Models x [ n + 1 ] = Ax [ n ] + Bu [ n ] y [ n ] = Cx [ n ] + Du [ n ] with sampling period 0.5 second. The vectors x [ n ], u [ n ], y [ n ] denote the values of the state, input, and output vectors at the nth sample. By convention, the sample time of continuous-time models is Ts = 0. Setting Ts = 1 leaves the sample time of a discrete-time model unspecified. For example, h = tf([1 0.2],[1 0.3], 1) produces Transfer function: z 0.2 ------z + 0.3 Sampling time: unspecified Note Do not simply omit Ts in this case. This would make h a continuous-time transfer function. If you forget to specify the sample time when creating your model, you can still set it to the correct value by reassigning the LTI property Ts. See “Sample Time” on page 1-33 for more information on setting this property. Discrete-Time TF and ZPK Models You can specify discrete-time TF and ZPK models using tf and zpk as indicated above. Alternatively, it is often convenient to specify such models by 1 Defining the variable z as a particular discrete-time TF or ZPK model with the appropriate sample time 2 Entering your TF or ZPK model directly as a rational expression in z. This approach parallels the procedure for specifying continuous-time TF or ZPK models using rational expressions. This procedure is described in “SISO 1-20 Creating LTI Models Transfer Function Models” on page 1-8 and “SISO Zero-Pole-Gain Models” on page 1-12. For example, z = tf('z', 0.1); H = (z+2)/(z^2 + 0.6*z + 0.9); creates the same TF model as H = tf([1 2], [1 0.6 0.9], 0.1); Similarly, z = zpk('z', 0.1); H = [z/(z+0.1)/(z+0.2) ; (z^2+0.2*z+0.1)/(z^2+0.2*z+0.01)] produces the single-input, two-output ZPK model Zero/pole/gain from input to output... z #1: --------------(z+0.1) (z+0.2) #2: (z^2 + 0.2z + 0.1) -----------------(z+0.1)^2 Sampling time: 0.1 Note that • The syntax z = tf('z') is equivalent to z = tf('z', 1) and leaves the sample time unspecified. The same applies to z = zpk('z'). • Once you have defined z as indicated above, any rational expressions in z creates a discrete-time model of the same type and with the same sample time as z. Discrete Transfer Functions in DSP Format In digital signal processing (DSP), it is customary to write discrete transfer functions as rational expressions in z –1 and to order the numerator and denominator coefficients in ascending powers of z – 1. For example, the numerator and denominator of 1-21 1 LTI Models 1 + 0.5z – 1 H ( z – 1 ) = --------------------------------------1 + 2z – 1 + 3z – 2 would be specified as the row vectors [1 0.5] and [1 2 3], respectively. When the numerator and denominator have different degrees, this convention clashes with the “descending powers of z ” convention assumed by tf (see “Transfer Function Models” on page 1-8, or tf). For example, h = tf([1 0.5],[1 2 3]) produces the transfer function z + 0.5 ---------------------------2 z + 2z + 3 which differs from H ( z – 1 ) by a factor z . To avoid such convention clashes, the Control System Toolbox offers a separate function filt dedicated to the DSP-like specification of transfer functions. Its syntax is h = filt(num,den) for discrete transfer functions with unspecified sample time, and h = filt(num,den,Ts) to further specify the sample time Ts. This function creates TF objects just like tf, but expects num and den to list the numerator and denominator coefficients in ascending powers of z – 1 . For example, typing h = filt([1 0.5],[1 2 3]) produces Transfer function: 1 + 0.5 z^ 1 ------------------1 + 2 z^ 1 + 3 z^ 2 Sampling time: unspecified You can also use filt to specify MIMO transfer functions in z – 1. Just as for tf, the input arguments num and den are then cell arrays of row vectors of appropriate dimensions (see “Transfer Function Models” on page 1-8 for 1-22 Creating LTI Models details). Note that each row vector should comply with the “ascending powers of z – 1 ” convention. Data Retrieval The functions tf, zpk, ss, and frd pack the model data and sample time in a single LTI object. Conversely, the following commands provide convenient data retrieval for any type of TF, SS, or ZPK model sys, or FRD model sysfr. [num,den,Ts] = tfdata(sys) % Ts = sample time [z,p,k,Ts] = zpkdata(sys) [a,b,c,d,Ts] = ssdata(sys) [a,b,c,d,e,Ts] = dssdata(sys) [response,frequency,Ts] = frdata(sysfr) Note that • sys can be any type of LTI object, except an FRD model • sysfr, the input argument to frdata, can only be an FRD model You can use any variable names you want in the output argument list of any of these functions. The ones listed here correspond to the model property names described in Tables 2-2 – 2.5. The output arguments num and den assigned to tfdata, and z and p assigned to zpkdata, are cell arrays, even in the SISO case. These cell arrays have as many rows as outputs, as many columns as inputs, and their ijth entry specifies the transfer function from the jth input to the ith output. For example, H = [tf([1 1],[1 2 10]) , tf(1,[1 0])] creates the one-output/two-input transfer function s–1 H ( s ) = -----------------------------2 s + 2s + 10 1 --s Typing [num,den] = tfdata(H); num{1,1}, den{1,1} displays the coefficients of the numerator and denominator of the first input channel. 1-23 1 LTI Models ans = 0 ans = 1 1 1 2 10 Note that the same result is obtained using H.num{1,1}, H.den{1,1} See “Direct Property Referencing Using Dot Notation” on page 1-32 for more information about this syntax. To obtain the numerator and denominator of SISO systems directly as row vectors, use the syntax [num,den,Ts] = tfdata(sys,'v') For example, typing sys = tf([1 3],[1 2 5]); [num,den] = tfdata(sys,'v') produces num = 0 1 3 2 5 den = 1 Similarly, [z,p,k,Ts] = zpkdata(sys,'v') returns the zeros, z, and the poles, p, as vectors for SISO systems. 1-24 LTI Properties LTI Properties The previous section shows how to create LTI objects that encapsulate the model data and sample time. You also have the option to attribute additional information, such as the input names or notes on the model history, to LTI objects. This section gives a complete overview of the LTI properties, i.e., the various pieces of information that can be attached to the TF, ZPK, SS, and FRD objects. Type help ltiprops for online help on available LTI properties. From a data structure standpoint, the LTI properties are the various fields in the TF, ZPK, SS, and FRD objects. These fields have names (the property names) and are assigned values (the property values). We distinguish between generic properties, common to all four types of LTI objects, and model-specific properties that pertain only to one particular type of model. This section contains the following topics: • “Generic LTI Properties” on page 1-25 • “Model-Specific Properties” on page 1-27 • “Setting LTI Properties” on page 1-29 • “Accessing Property Values Using get” on page 1-31 • “Direct Property Referencing Using Dot Notation” on page 1-32 • “Additional Insight into LTI Properties” on page 1-33 Generic LTI Properties The generic properties are those shared by all four types of LTI models (TF, ZPK, SS, and FRD objects). They are listed in the table below. Property Name Description Property Value ioDelay I/O delay(s) Matrix InputDelay Input delay(s) Vector InputGroup Input channel groups Structure InputName Input channel names Cell vector of strings Notes Notes on the model history Text 1-25 1 LTI Models Property Name Description Property Value OutputDelay Output delay(s) Vector OutputGroup Output channel groups Structure OutputName Output channel names Cell vector of strings Ts Sample time Scalar Userdata Additional data Arbitrary The sample time property Ts keeps track of the sample time (in seconds) of discrete-time systems. By convention, Ts is 0 (zero) for continuous-time systems, and Ts is 1 for discrete-time systems with unspecified sample time. Ts is always a scalar, even for MIMO systems. The InputDelay, OutputDelay, and ioDelay properties allow you to specify time delays in the input or output channels, or for each input/output pair. Their default value is zero (no delay). See “Time Delays” on page 1-42 for details on modeling delays. The InputName and OutputName properties enable you to give names to the individual input and output channels. The value of each of these properties is a cell vector of strings with as many cells as inputs or outputs. For example, the OutputName property is set to { 'temperature' ; 'pressure' } for a system with two outputs labeled temperature and pressure. The default value is a cell of empty strings. Using the InputGroup and OutputGroup properties of LTI objects, you can create different groups of input or output channels, and assign names to the groups. For example, you may want to designate the first four inputs of a five-input model as controls, and the last input as noise. See “Input Groups and Output Groups” on page 1-36 for more information. Finally, Notes and Userdata are available to store additional information on the model. The Notes property is dedicated to any text you want to supply with your model, while the Userdata property can accommodate arbitrary user-supplied data. They are both empty by default. 1-26 LTI Properties For more detailed information on how to use LTI properties, see “Additional Insight into LTI Properties” on page 1-33. Model-Specific Properties The remaining LTI properties are specific to one of the four model types (TF, ZPK, SS, or FRD). For single LTI models, these are summarized in the following four tables. The property values differ for LTI arrays. See set for more information on these values. TF-Specific Properties Property Name Description Property Value den Denominator(s) Real cell array of row vectors num Numerator(s) Real cell array of row vectors Variable Transfer function variable String 's', 'p', 'z', 'q', or 'z^ 1' ZPK-Specific Properties Property Name Description Property Value k Gains Two-dimensional real matrix p Poles Cell array of column vectors Variable Transfer function variable String 's', 'p', 'z', 'q', or Zeros Cell array of column vectors z 'z^ 1' 1-27 1 LTI Models SS-Specific Properties Property Name Description Property Value a State matrix A 2-D real matrix b Input-to-state matrix B 2-D real matrix c State-to-output matrix C 2-D real matrix d Feedthrough matrix D 2-D real matrix e Descriptor E matrix 2-D real matrix Nx Number of states Scalar integer StateName State names Cell vector of strings FRD-Specific Properties Property Name Description Property Value Frequency Frequency data points Real-valued vector ResponseData Frequency response Complex-valued multidimensional array Units Units for frequency String 'rad/s' or 'Hz' Most of these properties are dedicated to storing the model data. Note that the E matrix is set to [] (the empty matrix) for standard state-space models, a storage-efficient shorthand for the true value E = I . The Variable property is only an attribute of TF and ZPK objects. This property defines the frequency variable of transfer functions. The default values are 's' (Laplace variable s ) in continuous time and 'z' (Z-transform variable z ) in discrete time. Alternative choices include 'p' (equivalent to s ) and 'q' or 'z^ 1' for the reciprocal q = z –1 of the z variable. The influence of the variable choice is mostly limited to the display of TF or ZPK models. One exception is the specification of discrete-time transfer functions with tf (see tf for details). 1-28 LTI Properties Note that tf produces the same result as filt when the Variable property is set to 'z^ 1' or 'q'. Finally, the StateName property is analogous to the InputName and OutputName properties and keeps track of the state names in state-space models. Setting LTI Properties There are three ways to specify LTI property values: • You can set properties when creating LTI models with tf, zpk, ss, or frd. • You can set or modify the properties of an existing LTI model with set. • You can also set property values using structure-like assignments. This section discusses the first two options. See “Direct Property Referencing Using Dot Notation” on page 1-32 for details on the third option. The function set for LTI objects follows the same syntax as its Handle Graphics® counterpart. Specifically, each property is updated by a pair of arguments PropertyName,PropertyValue where • PropertyName is a string specifying the property name. You can type the property name without regard for the case (upper or lower) of the letters in the name. Actually, you need only type any abbreviation of the property name that uniquely identifies the property. For example, 'user' is sufficient to refer to the Userdata property. • PropertyValue is the value to assign to the property (see set for details on admissible property values). 1-29 1 LTI Models As an illustration, consider the following simple SISO model for a heating system with an input delay of 0.3 seconds, an input called “energy,” and an output called “temperature.” energy e – 0.3s 1 -----------s+1 temperature delay A Simple Heater Model You can use a TF object to represent this delay system, and specify the time delay, the input and output names, and the model history by setting the corresponding LTI properties. You can either set these properties directly when you create the LTI model with tf, or by using the set command. For example, you can specify the delay directly when you create the model, and then use the set command to assign InputName, OutputName, and Notes to sys. sys = tf(1,[1 1],'Inputdelay',0.3); set(sys,'inputname','energy','outputname','temperature',... 'notes','A simple heater model') Finally, you can also use the set command to obtain a listing of all settable properties for a given LTI model type, along with valid values for these properties. For the transfer function sys created above set(sys) produces num: Ny-by-Nu cell of row vectors (Nu = no. of inputs) den: Ny-by-Nu cell of row vectors (Ny = no. of outputs) Variable: [ 's' | 'p' | 'z' | 'z^-1' | 'q' ] Ts: scalar InputDelay: Nu-by-1 vector OutputDelay: Ny-by-1 vector ioDelay: Ny-by-Nu array (I/O delays) InputName: Nu-by-1 cell array of strings OutputName: Ny-by-1 cell array of strings InputGroup: M-by-2 structure of M input groups 1-30 LTI Properties OutputGroup: P-by-2 structure of P output groups Notes: array or cell array of strings UserData: arbitrary Accessing Property Values Using get You access the property values of an LTI model sys with get. The syntax is PropertyValue = get(sys,PropertyName) where the string PropertyName is either the full property name, or any abbreviation with enough characters to identify the property uniquely. For example, typing h = tf(100,[1 5 100],'inputname','voltage',... 'outputn','current',... 'notes','A simple circuit') get(h,'notes') produces ans = 'A simple circuit' To display all of the properties of an LTI model sys (and their values), use the syntax get(sys). In this example, get(h) produces num: {[0 0 100]} den: {[1 5 100]} Variable: 's' Ts: 0 ioDelay: 0 InputDelay: 0 OutputDelay: 0 InputName: {'voltage'} OutputName: {'current'} InputGroup: [1x1 struct] OutputGroup: [1x1 struct] Notes: {'A simple circuit'} 1-31 1 LTI Models UserData: [] Notice that default (output) values have been assigned to any LTI properties in this list that you have not specified. Finally, you can also access property values using direct structure-like referencing. This topic is explained in the next section. Direct Property Referencing Using Dot Notation An alternative way to query/modify property values is by structure-like referencing. Recall that LTI objects are basic MATLAB structures except for the additional flag that marks them as TF, ZPK, SS, or FRD objects (see “LTI Objects” on page 1-3). The field names for LTI objects are the property names, so you can retrieve or modify property values with the structure-like syntax. PropertyValue = sys.PropertyName% gets property value sys.PropertyName = PropertyValue% sets property value These commands are respectively equivalent to PropertyValue = get(sys,'PropertyName') set(sys,'PropertyName',PropertyValue) For example, type sys = ss(1,2,3,4,'InputName','u'); sys.a and you get the value of the property “a” for the state-space model sys. ans = 1 Similarly, sys.a = 1; resets the state transition matrix for sys to –1. Unlike standard MATLAB structures, you do not need to type the entire field name or use uppercase characters. You only need to type the minimum number of characters sufficient to identify the property name uniquely. Thus either of the commands sys.InputName 1-32 LTI Properties sys.inputn produces ans = 'u' Any valid syntax for structures extends to LTI objects. For example, given the TF model h ( p ) = 1 ⁄ p h = tf(1,[1,0],'variable','p'); you can reset the numerator to p + 2 by typing h.num{1} = [1 2]; or equivalently, with h.num{1}(2) = 2; Additional Insight into LTI Properties By reading this section, you can learn more about using the Ts, InputName, OutputName, InputGroup, and OutputGroup LTI properties through a set of examples. For basic information on Notes and Userdata, see “Generic LTI Properties” on page 1-25. For detailed information on the use of InputDelay, OutputDelay, and ioDelay, see “Time Delays” on page 1-42. Sample Time The sample time property Ts is used to specify the sampling period (in seconds) for either discrete-time or discretized continuous-time LTI models. Suppose you want to specify z H ( z ) = ---------------------------2 2z + z + 1 as a discrete-time transfer function model with a sampling period of 0.5 seconds. To do this, type h = tf([1 0],[2 1 1],0.5); This sets the Ts property to the value 0.5, as is confirmed by h.Ts 1-33 1 LTI Models ans = 0.5000 For continuous-time models, the sample time property Ts is 0 by convention. For example, type h = tf(1,[1 0]); get(h,'Ts') ans = 0 To leave the sample time of a discrete-time LTI model unspecified, set Ts to – 1. For example, h = tf(1,[1 1], 1) produces Transfer function: 1 ----z 1 Sampling time: unspecified The same result is obtained by using the Variable property. h = tf(1,[1 1],'var','z') In operations that combine several discrete-time models, all specified sample times must be identical, and the resulting discrete-time model inherits this common sample time. The sample time of the resultant model is unspecified if all operands have unspecified sample times. With this inheritance rule for Ts, the following two models are equivalent. tf(0.1,[1 1],0.1) + tf(1,[1 0.5], 1) and tf(0.1,[1 1],0.1) + tf(1,[1 0.5],0.1) Note that tf(0.1,[1 1-34 1],0.1) + tf(1,[1 0.5],0.5) LTI Properties returns an error message. ??? Error using ==> lti/plus In SYS1+SYS2, both models must have the same sample time. Caution. Resetting the sample time of a continuous-time LTI model sys from zero to a nonzero value does not discretize the original model sys. The command set(sys,'Ts',0.1) only affects the Ts property and does not alter the remaining model data. Use c2d and d2c to perform continuous-to-discrete and discrete-to-continuous conversions. For example, use sysd = c2d(sys,0.1) to discretize a continuous system sys at a 10Hz sampling rate. Use d2d to change the sample time of a discrete-time system and resample it. Input Names and Output Names You can use the InputName and OutputName properties (in short, I/O names) to assign names to any or all of the input and output channels in your LTI model. For example, you can create a SISO model with input thrust, output velocity, and transfer function H ( p ) = 1 ⁄ ( p + 10 ) by typing h = tf(1,[1 10]); set(h,'inputname','thrust','outputname','velocity',... 'variable','p') Equivalently, you can set these properties directly by typing h = tf(1,[1 10],'inputname','thrust',... 'outputname','velocity',... 'variable','p') This produces Transfer function from input "thrust" to output "velocity": 1 -----p + 10 1-35 1 LTI Models Note how the display reflects the input and output names and the variable selection. In the MIMO case, use cell vectors of strings to specify input or output channel names. For example, type num = {3 , [1 2]}; den = {[1 10] , [1 0]}; H = tf(num,den); % H(s) has one output and two inputs set(H,'inputname',{'temperature' ; 'pressure'}) The specified input names appear in the display of H. Transfer function from input "temperature" to output: 3 -----s + 10 Transfer function from input "pressure" to output: s + 2 ----s To leave certain names undefined, use the empty string '' as in H = tf(num,den,'inputname',{ 'temperature' ; '' }) Input Groups and Output Groups In many applications, you may want to create several (distinct or intersecting) groups of input or output channels and name these groups. For example, you may want to label one set of input channels as noise and another set as controls. To see how input and output groups (I/O groups) work, 1 Create a random state-space model with one state, three inputs, and three outputs. 2 Assign the first two inputs to a group named controls, the first output to a group named temperature, and the last two outputs to a group named measurements. Note that both InputGroup and OutputGroup are structures. 1-36 LTI Properties To do this, type h = rss(1,3,3); h.InputGroup.controls=[1 2]; h.OutputGroup.temperature = [1]; h.OutputGroup.measurements = [2 3]; h and MATLAB returns a state-space model of the following form. a = x1 x2 x3 x1 -2.809 -2.432 1.125 x2 1.967 -2.042 1.655 x3 -1.82 0.8313 -1.017 b = x1 x2 x3 u1 -0.7829 0 -0.2512 u2 0.4801 0.6682 -0.07832 y1 y2 y3 x1 -0.01179 0.9131 0.05594 u3 0 2.309 0.5246 c = x2 0 0.4855 -0.005005 x3 -0.2762 1.276 1.863 d = y1 y2 y3 u1 -0.5226 0.1034 -0.8076 Input groups: Name controls u2 0 0 0 u3 0 0.2617 0 Channels 1,2 1-37 1 LTI Models Output groups: Name temperature measurements Channels 1 2,3 Continuous-time model. Similarly, you can add or delete channels from an existing input or output group by redefining the group members, For example, h.OutputGroup.temperature=[1 2] adds the second output to the temperature group. To delete a channel from a group, just respecify it. For example, h.OutputGroup.temperature=[1] restores the original temperature group by deleting output #2 from the group. 1-38 Model Conversion Model Conversion There are four LTI model formats you can use with the Control System Toolbox: TF, ZPK, SS, and FRD. This section shows how to convert models from one format to the other. The following topics are discussed in this section: • “Explicit Conversion” on page 1-39 • “Automatic Conversion” on page 1-40 • “Caution About Model Conversions” on page 1-40 Explicit Conversion Model conversions are performed by tf, ss, zpk, and frd. Given any TF, SS, or ZPK model sys, the syntax for conversion to another model type is sys = tf(sys) % Conversion to TF sys = zpk(sys) % Conversion to ZPK sys = ss(sys) % Conversion to SS sys = frd(sys,frequency) % Conversion to FRD Notice that FRD models can’t be converted to the other model types. In addition, you must also include a vector of frequencies (frequency) as an input argument when converting to an FRD model. For example, you can convert the state-space model sys = ss( 2,1,1,3) to a zero-pole-gain model by typing zpk(sys) to which MATLAB responds Zero/pole/gain: 3 (s+2.333) ----------(s+2) 1-39 1 LTI Models Note that the transfer function of a state-space model with data ( A, B, C, D ) is –1 H ( s ) = D + C ( sI – A ) B for continuous-time models, and –1 H ( z ) = D + C ( zI – A ) B for discrete-time models. Automatic Conversion Some algorithms operate only on one type of LTI model. For example, the algorithm for zero-order-hold discretization with c2d can only be performed on state-space models. Similarly, commands like tfdata expect one particular type of LTI models (TF). For convenience, such commands automatically convert LTI models to the appropriate or required model type. For example, in sys = ss(0,1,1,0) [num,den] = tfdata(sys) tfdata first converts the state-space model sys to an equivalent transfer function in order to return numerator and denominator data. Note that conversions to state-space models are not uniquely defined. For this reason, automatic conversions to state space are disabled when the result depends on the choice of state coordinates, for example, in commands like initial or kalman. Caution About Model Conversions When manipulating or converting LTI models, keep in mind that • The three LTI model types TF, ZPK, and SS, are not equally well-suited for numerical computations. In particular, the accuracy of computations using high-order transfer functions is often poor. Therefore, it is often preferable to work with the state-space representation. In addition, it is often beneficial to balance and scale state-space models using ssbal. You get this type of balancing automatically when you convert any TF or ZPK model to state space using ss. 1-40 Model Conversion • Conversions to the transfer function representation using tf may incur a loss of accuracy. As a result, the transfer function poles may noticeably differ from the poles of the original zero-pole-gain or state-space model. • Conversions to state space are not uniquely defined in the SISO case, nor are they guaranteed to produce a minimal realization in the MIMO case. For a given state-space model sys, ss(tf(sys)) may return a model with different state-space matrices, or even a different number of states in the MIMO case. Therefore, if possible, it is best to avoid converting back and forth between state-space and other model types. 1-41 1 LTI Models Time Delays Using the ioDelay, InputDelay, and OutputDelay properties of LTI objects, you can specify delays in both continuous- and discrete-time LTI models. With these properties, you can, for example, represent • LTI models with independent delays for each input/output pair. For example, the continuous-time model with transfer function H(s ) = e – 0.1s 10 2 --s e e – 0.3s – 0.2s s+1 --------------s + 10 s–1 -----------s+5 • State-space models with delayed inputs and/or delayed outputs. For example, x· ( t ) = Ax ( t ) + Bu ( t – τ ) y ( t ) = Cx ( t – θ ) + Du ( t – ( θ + τ ) ) where τ is the time delay between the input u ( t ) and the state vector x ( t ) , and θ is the time delay between x ( t ) and the output y ( t ) . You can assign the delay properties ioDelay, InputDelay, and OutputDelay either when first creating your model with the tf, zpk, ss, or frd constructors, or later with the set command (see “LTI Properties and Methods” on page 1-4 for details). This section contains the following topics: • “Functions that Support Time Delays” on page 1-43 • “Specifying Input/Output Delays” on page 1-43 • “Specifying Delays on the Inputs or Outputs” on page 1-47 • “Specifying Delays in Discrete-Time Models” on page 1-49 • “Retrieving Information About Delays” on page 1-51 • “Padé Approximation of Time Delays” on page 1-51 1-42 Time Delays Functions that Support Time Delays Most analysis commands support time delays, including • All time and frequency response commands • Conversions between model types • Continuous-to-discrete conversions (c2d) • Horizontal and vertical concatenation • Series, parallel, and feedback interconnections of discrete-time models with delays • Interconnections of continuous-time delay systems as long as the resulting transfer function from input j to output i is of the form exp ( – sτ ij ) h ij ( s ) where h ij ( s ) is a rational function of s • Padé approximation of time delays (pade) Specifying Input/Output Delays Using the ioDelay property, you can specify frequency-domain models with independent delays in each entry of the transfer function. In continuous time, such models have a transfer function of the form e – sτ 11 H( s) = h 11 ( s ) ... e – sτ 1m : e – sτ p1 h p1 ( s ) ... e h 1m ( s ) : – sτ p m = [ exp ( – sτ ij ) h ij ( s ) ] h pm ( s ) where the h ij ’s are rational functions of s , and τ ij is the time delay between input j and output i . See “Specifying Delays in Discrete-Time Models” on page 1-49 for details on the discrete-time counterpart. We collectively refer to the scalars τ ij as the I/O delays. The syntax to create H ( s ) above is H = tf(num,den,'ioDelay',Tau) or H = zpk(z,p,k,'ioDelay',Tau) where 1-43 1 LTI Models • num, den (respectively, z, p, k) specify the rational part [ h ij ( s ) ] of the transfer function H ( s ) • Tau is the matrix of time delays for each I/O pair. That is, Tau(i,j) specifies the I/O delay τ ij in seconds. Note that Tau and H ( s ) should have the same row and column dimensions. You can also use the ioDelay property in conjunction with state-space models, as in sys = ss(A,B,C,D,'ioDelay',Tau) This creates the LTI model with the following transfer function. H(s ) = exp ( – sτ ij ) r ij ( s ) Here r ij ( s ) is the ( i, j ) entry of –1 R ( s ) = D + C ( sI – A ) B Note State-space models with I/O delays have only a frequency-domain interpretation. They cannot, in general, be described by state-space equations with delayed inputs and outputs. Distillation Column Example This example is adapted from [2] and illustrates the use of I/O delays in process modeling. The process of interest is the distillation column depicted by the figure below. This column is used to separate a mix of methanol and water (the feed) into bottom products (mostly water) and a methanol-saturated distillate. 1-44 Time Delays Enriched vapor Cooling water Condensate Distillate Feed Reflux Reboiler Vapor Steam flow Bottom liquid Bottom products Distillation Column Schematically, the distillation process functions as follows: • Steam flows into the reboiler and vaporizes the bottom liquid. This vapor is reinjected into the column and mixes with the feed • Methanol, being more volatile than water, tends to concentrate in the vapor moving upward. Meanwhile, water tends to flow downward and accumulate as the bottom liquid • The vapor exiting at the top of the column is condensed by a flow of cooling water. Part of this condensed vapor is extracted as the distillate, and the rest of the condensate (the reflux) is sent back to the column. • Part of the bottom liquid is collected from the reboiler as bottom products (waste). The regulated output variables are • Percentage X D of methanol in the distillate • Percentage X B of methanol in the bottom products. 1-45 1 LTI Models The goal is to maximize X D by adjusting the reflux flow rate R and the steam flow rate S in the reboiler. To obtain a linearized model around the steady-state operating conditions, the transient responses to pulses in steam and reflux flow are fitted by first-order plus delay models. The resulting transfer function model is – 1s XD ( s ) XB ( s ) = – 3s 12.8e – 18.9e ------------------------ ------------------------16.7e + 1 21.0s + 1 R ( s ) – 7s – 3s S ( s ) 6.6e – 19.4e ------------------------ ------------------------10.9s + 1 14.4s + 1 Note the different time delays for each input/output pair. You can specify this MIMO transfer function by typing H = tf({12.8 18.9;6.6 19.4},... {[16.7 1] [21 1];[10.9 1] [14.4 1]},... 'iodelay',[1 3;7 3],... 'inputname',{'R' , 'S'},... 'outputname',{'Xd' , 'Xb'}) The resulting TF model is displayed as Transfer function from input "R" to output... 12.8 Xd: exp( 1*s) * ---------16.7 s + 1 Xb: 6.6 exp( 7*s) * ---------10.9 s + 1 Transfer function from input "S" to output... 18.9 Xd: exp( 3*s) * -------21 s + 1 Xb: 1-46 19.4 exp( 3*s) * ---------14.4 s + 1 Time Delays Specifying Delays on the Inputs or Outputs While ideal for frequency-domain models with I/O delays, the ioDelay property is inadequate to capture delayed inputs or outputs in state-space models. For example, the two models · ( M 1 ) x ( t ) = – x ( t ) + u ( t – 0.1 ) y( t) = x( t) · ( M2 ) z ( t ) = –z ( t ) + u( t ) y ( t ) = z ( t – 0.1 ) share the same transfer function – 0.1s e h ( s ) = ---------------s+1 As a result, they cannot be distinguished using the ioDelay property (the I/O delay value is 0.1 seconds in both cases). Yet, these two models have different state trajectories since x ( t ) and z ( t ) are related by z ( t ) = x ( t – 0.1 ) Note that the 0.1 second delay is on the input in the first model, and on the output in the second model. InputDelay and OutputDelay Properties When the state trajectory is of interest, you should use the InputDelay and OutputDelay properties to distinguish between delays on the inputs and delays on the outputs in state-space models. For example, you can accurately specify the two models above by M1 = ss( 1,1,1,0,'inputdelay',0.1) M2 = ss( 1,1,1,0,'outputdelay',0.1) In the MIMO case, you can specify a different delay for each input (or output) channel by assigning a vector value to InputDelay (or OutputDelay). For example, sys = ss(A,[B1 B2],[C1;C2],[D11 D12;D21 D22]) sys.inputdelay = [0.1 0] sys.outputdelay = [0.2 0.3] creates the two-input, two-output model 1-47 1 LTI Models x· ( t ) = Ax ( t ) + B 1 u 1 ( t – 0.1 ) + B 2 u 2 ( t ) y 1 ( t + 0.2 ) = C 1 x ( t ) + D 11 u 1 ( t – 0.1 ) + D 12 u 2 ( t ) y 2 ( t + 0.3 ) = C 2 x ( t ) + D 21 u 1 ( t – 0.1 ) + D 22 u 2 ( t ) You can also use the InputDelay and OutputDelay properties to conveniently specify input or output delays in TF, ZPK, or FRD models. For example, you can create the transfer function 1 --s H(s ) = 2 -----------s+1 e – 0.1s by typing s = tf('s'); H = [1/s ; 2/(s+1)]; H.inputdelay = 0.1 % rational part The resulting model is displayed as Transfer function from input to output... 1 #1: exp( 0.1*s) * s #2: 2 exp( 0.1*s) * ----s + 1 By comparison, to produce an equivalent transfer function using the ioDelay property, you would need to type H = [1/s ; 2/(s+1)]; H.iodelay = [0.1 ; 0.1]; Notice that the 0.1 second delay is repeated twice in the I/O delay matrix. More generally, for a TF, ZPK, or FRD model with input delays [ α 1, ..., α m ] and output delays [ β 1, ..., β p ] , the equivalent I/O delay matrix is 1-48 Time Delays α1 + β1 α2 + β1 α1 + β2 α2 + β2 : α1 + βp : α2 + βp ... αm + β1 αm + β2 ... : αm + βp Specifying Delays in Discrete-Time Models You can also use the ioDelay, InputDelay, and OutputDelay properties to specify delays in discrete-time LTI models. You specify time delays in discrete-time models with integer multiples of the sampling period. The integer k you supply for the time delay of a discrete-time model specifies a time delay –k of k sampling periods. Such a delay contributes a factor z to the transfer function. For example, h = tf(1,[1 0.5 0.2],0.1,'inputdelay',3) produces the discrete-time transfer function Transfer function: 1 z^( 3) * ----------------z^2 + 0.5 z + 0.2 Sampling time: 0.1 Notice the z^( 3) factor reflecting the three-sampling-period delay on the input. Mapping Discrete-Time Delays to Poles at the Origin Since discrete-time delays are equivalent to additional poles at z = 0 , they can be easily absorbed into the transfer function denominator or the state-space equations. For example, the transfer function of the delayed integrator y[ k + 1] = y[ k ] + u[ k – 2 ] is 1-49 1 LTI Models –2 z H ( z ) = -----------z–1 You can specify this model either as the first-order transfer function 1 ⁄ ( z – 1 ) with a delay of two sampling periods on the input Ts = 1; % sampling period H1 = tf(1,[1 1],Ts,'inputdelay',2) or directly as a third-order transfer function: H2 = tf(1,[1 1 0 0],Ts) % 1/(z^3 z^2) While these two models are mathematically equivalent, H1 is a more efficient representation both in terms of storage and subsequent computations. When necessary, you can map all discrete-time delays to poles at the origin using the command delay2z. For example, H2 = delay2z(H1) absorbs the input delay in H1 into the transfer function denominator to produce the third-order transfer function Transfer function: 1 --------z^3 z^2 Sampling time: 1 Note that H2.inputdelay now returns 0 (zero). 1-50 Time Delays Retrieving Information About Delays There are several ways to retrieve time delay information from a given LTI model sys: • Use property display commands to inspect the values of the ioDelay, InputDelay, and OutputDelay properties. For example, sys.iodelay get(sys,'inputdelay') • Use the helper function hasdelay to determine if sys has any delay at all. The syntax is hasdelay(sys) which returns 1 (true) if sys has any delay, and 0 (false) otherwise • Use the function totaldelay to determine the total delay between each input and each output (cumulative contribution of the ioDelay, InputDelay, and OutputDelay properties). Type help totaldelay or see the Reference pages for details. Padé Approximation of Time Delays The function pade computes rational approximations of time delays in continuous-time LTI models. The syntax is sysx = pade(sys,n) where sys is a continuous-time model with delays, and the integer n specifies the Padé approximation order. The resulting LTI model sysx is of the same type as sys, but is delay free. For models with multiple delays or a mix of input, output, and I/O delays, you can use the syntax sysx = pade(sys,ni,no,nio) where the vectors ni and no, and the matrix nio specify independent approximation orders for each input, output, and I/O delay, respectively. Set ni=[] if there are no input delays, and similarly for no and nio. For example, consider the “Distillation Column Example” on page 1-44. The two-input, two-output transfer function in this example is 1-51 1 LTI Models – 1s H(s ) = – 3s – 18.9e 12.8e ------------------------ ------------------------16.7e + 1 21.0s + 1 – 7s – 3s 6.6e – 19.4e ------------------------ ------------------------10.9s + 1 14.4s + 1 To compute a Padé approximation of H(s) using • A first-order approximation for the 1 second and 3 second delays • A second-order approximation for the 7 second delay, type pade(H,[],[],[1 1;2 1]) where H is the TF representation of H ( s ) defined in the distillation column example. This command produces a rational transfer function. Transfer function from input "R" to output... 12.8 s + 25.6 Xd: --------------------16.7 s^2 + 34.4 s + 2 Xb: 6.6 s^2 5.657 s + 1.616 --------------------------------------10.9 s^3 + 10.34 s^2 + 3.527 s + 0.2449 Transfer function from input "S" to output... 18.9 s 12.6 Xd: ---------------------21 s^2 + 15 s + 0.6667 Xb: 1-52 19.4 s 12.93 -------------------------14.4 s^2 + 10.6 s + 0.6667 Simulink Block for LTI Systems Simulink Block for LTI Systems You can incorporate LTI objects into Simulink® diagrams using the LTI System block shown below. Double-click on the block in your Simulink diagram to display or modify model information. The LTI System block can be accessed either by typing ltiblock at the MATLAB prompt or by selecting Control System Toolbox from the Blocksets and Toolboxes section of the main Simulink library. The LTI System block consists of the dialog box shown on the right in the figure above. In the editable text box labeled LTI system variable, enter either the variable name of an LTI object located in the MATLAB workspace (for example, sys) or a MATLAB expression that evaluates to an LTI object (for example, tf(1,[1 1])). The LTI System block accepts both continuous and discrete LTI objects in either transfer function, zero-pole-gain, or state-space form. Simulink converts the model to its state-space equivalent prior to initializing the simulation. Use the editable text box labeled Initial states to enter an initial state vector for state-space models. The concept of “initial state” is not well-defined for 1-53 1 LTI Models transfer functions or zero-pole-gain models, as it depends on the choice of state coordinates used by the realization algorithm. As a result, you cannot enter nonzero initial states when you supply TF or ZPK models to LTI blocks in a Simulink diagram. Note For MIMO systems, the input delays stored in the LTI object must be either all positive or all zero. Also, you cannot use LTI blocks for FRD models or LTI arrays. 1-54 References References [1] Dorf, R.C., and R.H. Bishop, Modern Control Systems, Addison-Wesley, Menlo Park, CA, 1998. [2] Wood, R.K., and M.W. Berry, “Terminal Composition Control of a Binary Distillation Column,” Chemical Engineering Science, 28 (1973), pp. 1707-1717. 1-55 1 LTI Models 1-56 2 Operations on LTI Models Overview (p. 2-2) A brief discussion of LTI operations Precedence and Property Inheritance (p. 2-3) How to perform operations on sets of LTI models that contain models in different formats Extracting and Modifying Subsystems (p. 2-5) Inspect individual or groups of I/O pairs of large models Arithmetic Operations on LTI Models (p. 2-11) How to use overloaded operations to perform arithmetic on LTI models Model Interconnection Functions (p. 2-16) Creating new models by interconnecting existing models; for example, in series or in parallel Continuous/Discrete Conversions of LTI Models (p. 2-20) Converting between continuous- and discrete-time LTI systems Resampling of Discrete-Time Models (p. 2-26) Changing the sample rate of a discrete-time LTI model 2 Operations on LTI Models Overview You can perform basic matrix operations such as addition, multiplication, or concatenation on LTI models. Such operations are “overloaded,” which means that they use the same syntax as they do for matrices, but are adapted to apply to LTI objects. These overloaded operations and their interpretation in this context are discussed in this chapter. You can read about discretization methods in this chapter as well. These operations can be applied to LTI models of different types. As a result, before discussing operations on LTI models, we discuss model type precedence and how LTI model properties are inherited when models are combined using these operations. To read about how you can apply these operations to arrays of LTI models, see “Operations on LTI Arrays” on page 4-25. To read about functions for analyzing LTI models, see Chapter 3, “Model Analysis Tools.” 2-2 Precedence and Property Inheritance Precedence and Property Inheritance You can apply operations to LTI models of different types. The resulting type is then determined by the rules discussed in “Precedence Rules” on page 1-5. For example, if sys1 is a transfer function and sys2 is a state-space model, then the result of their addition sys = sys1 + sys2 is a state-space model, since state-space models have precedence over transfer function models. To supersede the precedence rules and force the result of an operation to be a given type, for example, a transfer function (TF), you can either • Convert all operands to TF before performing the operation • Convert the result to TF after performing the operation Suppose, in the above example, you want to compute the transfer function of sys. You can either use a priori conversion of the second operand sys = sys1 + tf(sys2); or a posteriori conversion of the result sys = tf(sys1 + sys2) Note These alternatives are not equivalent numerically; computations are carried out on transfer functions in the first case, and on state-space models in the second case. Another issue is property inheritance, that is, how the operand property values are passed on to the result of the operation. While inheritance is partly operation-dependent, some general rules are summarized below: • In operations combining discrete-time LTI models, all models must have identical or unspecified (sys.Ts = 1) sample times. Models resulting from such operations inherit the specified sample time, if there is one. • Most operations ignore the Notes and Userdata properties. 2-3 2 Operations on LTI Models • In general, when two LTI models sys1 and sys2 are combined using operations such as +, *, [,], [;], append, and feedback, the resulting model inherits its I/O names and I/O groups from sys1 and sys2. However, conflicting I/O names or I/O groups are not inherited. For example, the InputName property for sys1 + sys2 is left unspecified if sys1 and sys2 have different InputName property values. • A model resulting from operations on TF or ZPK models inherits its Variable property value from the operands. Conflicts are resolved according the following rules: - For continuous-time models, 'p' has precedence over 's'. - For discrete-time models, 'z^ 1' has precedence over 'q' and 'z', while 'q' has precedence over 'z'. 2-4 Extracting and Modifying Subsystems Extracting and Modifying Subsystems Subsystems relate subsets of the inputs and outputs of a system. The transfer matrix of a subsystem is a submatrix of the system transfer matrix. The following sections describe these topics: • “Basic Subsystem Concepts” on page 2-5 • “Referencing FRD Models Through Frequencies” on page 2-7 • “Referencing Channels by Name” on page 2-8 • “Resizing LTI Systems” on page 2-9 Basic Subsystem Concepts For example, if sys is a system with two inputs, three outputs, and I/O relation y = Hu then H ( 3, 1 ) gives the relation between the first input and third output. y 3 = H ( 3,1 ) u 1 Accordingly, use matrix-like subindexing to extract this subsystem. SubSys = sys(3,1) The resulting subsystem SubSys is an LTI model of the same type as sys, with its sample time, time delay, I/O name, and I/O group property values inherited from sys. For example, if sys has an input group named controls consisting of channels one, two, and three, then SubSys also has an input group named controls with the first channel of SubSys assigned to it. If sys is a state-space model with matrices a, b, c, d, the subsystem sys(3,1) is a state-space model with data a, b(:,1), c(3,:), d(3,1). Note the following rules when extracting subystems: • In the expression sys(3,1), the first index selects the output channel while the second index selects the input channel. • When extracting a subsystem from a given state-space model, the resulting state-space model may not be minimal. Use the command sminreal to eliminate unnecessary states in the subsystem. 2-5 2 Operations on LTI Models You can use similar syntax to modify the LTI model sys. For example, sys(3,1) = NewSubSys redefines the I/O relation between the first input and third output, provided NewSubSys is a SISO LTI model. Rules for Modifying LTI Model Subsystems The following rules apply when modifying LTI model subsystems: • sys, the LTI model that has had a portion reassigned, retains its original model type (TF, ZPK, SS, or FRD) regardless of the model type of NewSubSys. • Subsystem assignment does not reassign any I/O names or I/O group names of NewSubSys that are already assigned to NewSubSys. • Reassigning parts of a MIMO state-space model generally increases its order. • If NewSubSys is an FRD model, then sys must also be an FRD model. Furthermore, their frequencies must match. Other standard matrix subindexing extends to LTI objects as well. For example, sys(3,1:2) extracts the subsystem mapping the first two inputs to the third output. sys(:,1) selects the first input and all outputs, and sys([1 3],:) extracts a subsystem with the same inputs, but only the first and third outputs. For example, consider the two-input/two-output transfer function 1 ----------------s + 0.1 T(s) = s–1 ---------------------------s 2 + 2s + 2 2-6 0 1 --s Extracting and Modifying Subsystems To extract the transfer function T 11 ( s ) from the first input to the first output, type T(1,1) Transfer function: 1 ------s + 0.1 Next reassign T 11 ( s ) to 1 ⁄ ( s + 0.5 ) and modify the second input channel of T by typing T(1,1) = tf(1,[1 0.5]); T(:,2) = [ 1 ; tf(0.4,[1 0]) ] Transfer function from input 1 to output... 1 #1: ------s + 0.5 #2: s 1 ------------s^2 + 2 s + 2 Transfer function from input 2 to output... #1: 1 #2: 0.4 --s Referencing FRD Models Through Frequencies You can extract subsystems from FRD models, as you do with other LTI model types, by indexing into input and output (I/O) dimensions. You can also extract subsystems by indexing into the frequencies of an FRD model. To index into the frequencies of an FRD model, use the string 'Frequency' (or any abbreviation, such as, 'freq', as long as it does not conflict with existing I/O channel or group names) as a keyword. There are two ways you can specify FRD models using frequencies: 2-7 2 Operations on LTI Models • Using integers to index into the frequency vector of the FRD model • Using a Boolean (logical) expression to specify desired frequency points in an FRD model For example, if sys is an FRD model with five frequencies, (e.g., sys.Frequency=[1 1.1 1.2 1.3 1.4]), then you can create a new FRD model sys2 by indexing into the frequencies of sys as follows. sys2 = sys('frequency', 2:3); sys2.Frequency ans = 1.1000 1.2000 displays the second and third entries in the frequency vector. Similarly, you can use logical indexing into the frequencies. sys2 = sys('frequency',sys.Frequency >1.0 & sys.Frequency <1.15); sys2.freq ans = 1.1000 You can also combine model extraction through frequencies with indexing into the I/O dimensions. For example, if sys is an FRD model with two inputs, two outputs, and frequency vector [2.1 4.2 5.3], with sys.Units specified in rad/s, then sys2 = sys(1,2,'freq',1) specifies sys2 as a SISO FRD model, with one frequency data point, 2.1 rad/s. Referencing Channels by Name You can also extract subsystems using I/O group or channel names. For example, if sys has an input group named noise, consisting of channels two, four, and five, then sys(1,'noise') is equivalent to sys(1,[2 4 5]) 2-8 Extracting and Modifying Subsystems Similarly, if pressure is the name assigned to an output channel of the LTI model sys, then sys('pressure',1) = tf(1, [1 1]) reassigns the subsystem from the first input of sys to the output labeled pressure. You can reference a set of channels by input or output name by using a cell array of strings for the names. For example, if sys has one output channel named pressure and one named temperature, then these two output channels can be referenced using sys({'pressure','temperature'}) Resizing LTI Systems Resizing a system consists of adding or deleting inputs and/or outputs. To delete the first two inputs, simply type sys(:,1:2) = [] In deletions, at least one of the row/column indexes should be the colon (:) selector. To perform input/output augmentation, you can proceed by concatenation or subassignment. Given a system sys with a single input, you can add a second input using sys = [sys,h]; or, equivalently, using sys(:,2) = h; where h is any LTI model with one input, and the same number of outputs as sys. There is an important difference between these two options: while concatenation obeys the “Precedence Rules” on page 1-5, subsystem assignment does not alter the model type. So, if sys and h are TF and SS objects, respectively, the first statement produces a state-space model, and the second statement produces a transfer function. For state-space models, both concatenation and subsystem assignment increase the model order because they assume that sys and h have independent states. If you intend to keep the same state matrix and only 2-9 2 Operations on LTI Models update the input-to-state or state-to-output relations, use set instead and modify the corresponding state-space data directly. For example, sys = ss(a,b1,c,d1) set(sys,'b',[b1 b2],'d',[d1 d2]) adds a second input to the state-space model sys by appending the B and D matrices. You should simultaneously modify both matrices with a single set command. Indeed, the statements sys.b = [b1 b2] and set(sys,'b',[b1 b2]) cause an error because they create invalid intermediate models in which the B and D matrices have inconsistent column dimensions. 2-10 Arithmetic Operations on LTI Models Arithmetic Operations on LTI Models You can apply almost all arithmetic operations to LTI models, including those shown below. Operation Description + Addition – Subtraction * Multiplication / Right matrix divide \ Left matrix divide inv Matrix inversion ' Pertransposition .' Transposition ^ Powers of an LTI model (as in s^2) This following sections discuss these operations in detail: • “Addition and Subtraction” on page 2-11 • “Multiplication” on page 2-13 • “Inversion and Related Operations” on page 2-13 • “Transposition” on page 2-14 • “Pertransposition” on page 2-14 • “Concatenation of LTI Models” on page 2-17 • “Feedback and Other Interconnection Functions” on page 2-18 Addition and Subtraction Adding LTI models is equivalent to connecting them in parallel. Specifically, the LTI model sys = sys1 + sys2 2-11 2 Operations on LTI Models represents the parallel interconnection shown below. sys1 y1 + u y + sys2 y2 sys If sys1 and sys2 are two state-space models with data A 1, B 1, C 1, D 1 and A 2, B 2, C 2, D 2 , the state-space data associated with sys1 + sys2 is A1 0 , 0 A2 B1 , C1 C2 , B2 D1 + D2 Scalar addition is also supported and behaves as follows: if sys1 is MIMO and sys2 is SISO, sys1 + sys2 produces a system with the same dimensions as sys1 whose ijth entry is sys1(i,j) + sys2. Similarly, the subtraction of two LTI models sys = sys1 sys2 is depicted by the following block diagram. sys1 y1 + u – sys2 y2 sys 2-12 y Arithmetic Operations on LTI Models Multiplication Multiplication of two LTI models connects them in series. Specifically, sys = sys1 * sys2 returns an LTI model sys for the series interconnection shown below. v u sys2 sys1 y Notice the reverse orders of sys1 and sys2 in the multiplication and block diagram. This is consistent with the way transfer matrices are combined in a series connection: if sys1 and sys2 have transfer matrices H 1 and H 2 , then y = H1 v = H 1 ( H2 u ) = ( H1 × H2 ) u For state-space models sys1 and sys2 with data A 1, B 1, C 1, D 1 and A 2, B 2, C 2, D 2 , the state-space data associated with sys1*sys2 is A1 B1 C 2 0 A2 B1 D2 , B2 , C1 D1 C2 , D1 D2 Finally, if sys1 is MIMO and sys2 is SISO, then sys1*sys2 or sys2*sys1 is interpreted as an entry-by-entry scalar multiplication and produces a system with the same dimensions as sys1, whose ijth entry is sys1(i,j)*sys2. Inversion and Related Operations Inversion of LTI models amounts to inverting the following input/output relationship. y = Hu → –1 u = H y This operation is defined only for square systems (that is, systems with as many inputs as outputs) and is performed using inv(sys) The resulting inverse model is of the same type as sys. Related operations include 2-13 2 Operations on LTI Models • Left division sys1\sys2, which is equivalent to inv(sys1)*sys2 • Right division sys1/sys2, which is equivalent to sys1*inv(sys2) For a state-space model sys with data A, B, C, D , inv(sys) is defined only when D is a square invertible matrix, in which case its state-space data is –1 A – BD C , BD –1 , –1 –D C , D –1 Transposition You can transpose an LTI model sys using sys.' This is a literal operation with the following effect: • For TF models (with input arguments, num and den), the cell arrays num and den are transposed. • For ZPK models (with input arguments, z, p, and k), the cell arrays z and p and the matrix k are transposed. • For SS models (with model data A, B, C, D ), transposition produces the state-space model AT, CT, BT, DT. • For FRD models (with complex frequency response matrix Response), the matrix of frequency response data at each frequency is transposed. Pertransposition For a continuous-time system with transfer function H ( s ) , the pertransposed system has the transfer function G ( s ) = [ H ( –s ) ]T The discrete-time counterpart is –1 G(z ) = [ H( z ) ] T Pertransposition of an LTI model sys is performed using sys' You can use pertransposition to obtain the Hermitian (conjugate) transpose of the frequency response of a given system. The frequency response of the 2-14 Arithmetic Operations on LTI Models pertranspose of H ( s ), G ( s ) = [ H ( – s ) ] T , is the Hermitian transpose of the frequency response of H ( s ): G ( jw ) = H ( jw ) H . To obtain the Hermitian transpose of the frequency response of a system sys over a frequency range specified by the vector w, type freqresp(sys', w); 2-15 2 Operations on LTI Models Model Interconnection Functions The Control System Toolbox provides a number of functions to help with the model building process. These include model interconnection functions to perform I/O concatenation ([,], [;], and append), general parallel and series connections (parallel and series), and feedback connections (feedback and lft). These functions are useful to model open- and closed-loop systems. Interconnection Operator Description [,] Concatenates horizontally [;] Concatenates vertically append Appends models in a block diagonal configuration augstate Augments the output by appending states connect Forms an SS model from a block diagonal LTI object for an arbitrary interconnection matrix feedback Forms the feedback interconnection of two models lft Produces the LFT interconnection (Redheffer Star product) of two models parallel Forms the generalized parallel connection of two models series Forms the generalized series connection of two models The next sections cover these topics: • “Concatenation of LTI Models” on page 2-17 • “Feedback and Other Interconnection Functions” on page 2-18 2-16 Model Interconnection Functions Concatenation of LTI Models LTI model concatenation is done in a manner similar to the way you concatenate matrices in MATLAB, using sys = [sys1 , sys2]% horizontal concatenation sys = [sys1 ; sys2]% vertical concatenation sys = append(sys1,sys2)% block diagonal appending In I/O terms, horizontal and vertical concatenation have the following block-diagram interpretations (with H 1 and H 2 denoting the transfer matrices of sys1 and sys2). H1 u1 H1 y1 H2 y2 + + u2 y u H2 y = H1 , H 2 u1 y1 u2 y2 Horizontal Concatenation = H1 H2 u Vertical Concatenation You can use concatenation as an easy way to create MIMO transfer functions or zero-pole-gain models. For example, H = [ tf(1,[1 0]) 1 ; 0 tf([1 1],[1 1]) ] specifies 1 --H( s) = s 0 1 s–1 -----------s+1 2-17 2 Operations on LTI Models Use append(sys1,sys2) to specify the block-decoupled LTI model interconnection. u1 u2 sys1 y1 sys1 0 0 sys2 sys2 y2 Appended Models Transfer Function See append for more information on this function. Feedback and Other Interconnection Functions The following LTI model interconnection functions are useful for specifying closed- and open-loop model configurations: • feedback puts two LTI models with compatible dimensions in a feedback configuration. • series connects two LTI models in series. • parallel connects two LTI models in parallel. • lft performs the Redheffer star product on two LTI models. • connect works with append to apply an arbitrary interconnection scheme to a set of LTI models. 2-18 Model Interconnection Functions For example, if sys1 has m inputs and p outputs, while sys2 has p inputs and m outputs, then the negative feedback configuration of these two LTI models + u sys1 y sys2 is realized with feedback(sys1,sys2) This specifies the LTI model with m inputs and p outputs whose I/O map is ( I + sys1 ⋅ sys2 ) –1 sys1 See the reference pages online for more information on feedback, series, parallel, lft, and connect. 2-19 2 Operations on LTI Models Continuous/Discrete Conversions of LTI Models The function c2d discretizes continuous-time TF, SS, or ZPK models. Conversely, d2c converts discrete-time TF, SS, or ZPK models to continuous time. Several discretization/interpolation methods are supported, including zero-order hold (ZOH), first-order hold (FOH), Tustin approximation with or without frequency prewarping, and matched poles and zeros. The syntax sysd = c2d(sysc,Ts); sysc = d2c(sysd); % Ts = sampling period in seconds performs ZOH conversions by default. To use alternative conversion schemes, specify the desired method as an extra string input: sysd = c2d(sysc,Ts,'foh');% use first-order hold sysc = d2c(sysd,'tustin');% use Tustin approximation The conversion methods and their limitations are discussed in the next sections: • “Zero-Order Hold” on page 2-20 • “First-Order Hold” on page 2-22 • “Tustin Approximation” on page 2-23 • “Tustin with Frequency Prewarping” on page 2-23 • “Matched Poles and Zeros” on page 2-23 • “Discretization of Systems with Delays” on page 2-24 Zero-Order Hold Zero-order hold (ZOH) devices convert sampled signals to continuous-time signals for analyzing sampled continuous-time systems. The zero-order-hold 2-20 Continuous/Discrete Conversions of LTI Models discretization H d ( z ) of a continuous-time LTI model H ( s ) is depicted in the following block diagram. y(t) u(t) u[k] ZOH H( s) y[k] Ts Hd( z ) The ZOH device generates a continuous input signal u(t) by holding each sample value u[k] constant over one sample period. u(t) = u[k] , kT s ≤ t ≤ ( k + 1 )T s The signal u ( t ) is then fed to the continuous system H ( s ) , and the resulting output y ( t ) is sampled every T s seconds to produce y [ k ] . Conversely, given a discrete system H d ( z ) , the d2c conversion produces a continuous system H ( s ) whose ZOH discretization coincides with H d ( z ) . This inverse operation has the following limitations: • d2c cannot operate on LTI models with poles at z = 0 when the ZOH is used. • Negative real poles in the z domain are mapped to pairs of complex poles in the s domain. As a result, the d2c conversion of a discrete system with negative real poles produces a continuous system with higher order. The next example illustrates the behavior of d2c with real negative poles. Consider the following discrete-time ZPK model. hd = zpk([], 0.5,1,0.1) Zero/pole/gain: 1 ------(z+0.5) Sampling time: 0.1 Use d2c to convert this model to continuous-time hc = d2c(hd) 2-21 2 Operations on LTI Models and you get a second-order model. Zero/pole/gain: 4.621 (s+149.3) --------------------(s^2 + 13.86s + 1035) Discretize the model again c2d(hc,0.1) and you get back the original discrete-time system (up to canceling the pole/zero pair at z=–0.5): Zero/pole/gain: (z+0.5) --------(z+0.5)^2 Sampling time: 0.1 First-Order Hold First-order hold (FOH) differs from ZOH by the underlying hold mechanism. To turn the input samples u [ k ] into a continuous input u ( t ) , FOH uses linear interpolation between samples. t – kT s u ( t ) = u [ k ] + ------------------ ( u [ k + 1 ] – u [ k ] ) , Ts kTs ≤ t ≤ ( k + 1 )T s This method is generally more accurate than ZOH for systems driven by smooth inputs. Due to causality constraints, this option is only available for c2d conversions, and not d2c conversions. Note This FOH method differs from standard causal FOH and is more appropriately called triangle approximation (see [2], p. 151). It is also known as ramp-invariant approximation because it is distortion-free for ramp inputs. 2-22 Continuous/Discrete Conversions of LTI Models Tustin Approximation The Tustin or bilinear approximation uses the approximation z = e sT s 1 + sT s ⁄ 2 ≈ -------------------------1 – sT s ⁄ 2 to relate s-domain and z-domain transfer functions. In c2d conversions, the discretization H d ( z ) of a continuous transfer function H ( s ) is derived by H d ( z ) = H ( s' ) , where 2 z–1 s' = ------ -----------Ts z + 1 Similarly, the d2c conversion relies on the inverse correspondence H ( s ) = H d ( z' ), where 1 + sT s ⁄ 2 z' = ------------------------1 – sT s ⁄ 2 Tustin with Frequency Prewarping This variation of the Tustin approximation uses the correspondence z–1 ω s' = --------------------------------- -----------tan ( ωT s ⁄ 2 ) z + 1 H d ( z ) = H ( s' ) , This change of variable ensures the matching of the continuous- and discrete-time frequency responses at the frequency ω . H ( jω ) = H d ( e jωT s ) Matched Poles and Zeros The matched pole-zero method applies only to SISO systems. The continuous and discretized systems have matching DC gains and their poles and zeros correspond in the transformation z = e sT s See [2], p. 147 for more details. 2-23 2 Operations on LTI Models Discretization of Systems with Delays You can also use c2d to discretize SISO or MIMO continuous-time models with time delays. If Ts is the sampling period used for discretization, • A delay of tau seconds in the continuous-time model is mapped to a delay of k sampling periods in the discretized model, where k = fix(tau/Ts). • The residual fractional delay tau k*Ts is absorbed into the coefficients of the discretized model (for the zero-order-hold and first-order-hold methods only). For example, to discretize the transfer function H(s ) = e – 0.25s 10 ------------------------------2 s + 3s + 10 using zero-order hold on the input, and a 10 Hz sampling rate, type h = tf(10,[1 3 10],'inputdelay',0.25); hd = c2d(h,0.1) This produces the discrete-time transfer function Transfer function: 0.01187 z^2 + 0.06408 z + 0.009721 z^( 2) * ---------------------------------z^3 1.655 z^2 + 0.7408 z Sampling time: 0.1 Here the input delay in H ( s ) amounts to 2.5 times the sampling period of 0.1 seconds. Accordingly, the discretized model hd inherits an input delay of two sampling periods, as confirmed by the value of hd.inputdelay. The residual half-period delay is factored into the coefficients of hd by the discretization algorithm. The step responses of the continuous and discretized models are compared in the figure below. This plot was produced by the command 2-24 Continuous/Discrete Conversions of LTI Models step(h,'--',hd,'-') Step Response 1.4 1.2 Amplitude 1 0.8 0.6 0.4 0.2 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Time (sec.) Note The Tustin and matched pole/zero methods are accurate only for delays that are integer multiples of the sampling period. It is therefore preferable to use the zoh and foh discretization methods for models with delays. 2-25 2 Operations on LTI Models Resampling of Discrete-Time Models You can resample a discrete-time TF, SS, or ZPK model sys1 by typing sys2 = d2d(sys1,Ts) The new sampling period Ts does not have to be an integer multiple of the original sampling period. For example, typing h1 = tf([1 0.4],[1 h2 = d2d(h1,0.25); 0.7],0.1); resamples h1 at the sampling period of 0.25 seconds, rather than 0.1 seconds. You can compare the step responses of h1 and h2 by typing step(h1,'--',h2,'-') The resulting plot is shown on the figure below (h1 is the dashed line). 2-26 References References [1] Åström, K.J., and B. Wittenmark, Computer-Controlled Systems: Theory and Design, Prentice-Hall, 1990, pp. 48–52. [2] Franklin, G.F., J.D. Powell, and M.L. Workman, Digital Control of Dynamic Systems, Second Edition, Addison-Wesley, 1990. 2-27 2 Operations on LTI Models 2-28 3 Model Analysis Tools General Model Characteristics (p. 3-2) Tests you can use to determine characteristics such as the number of I/O pairs, the sample time if your system is discrete, and so on Model Dynamics (p. 3-4) How to determine system poles and zeros, natural frequency and damping, and other model dynamics State-Space Realizations (p. 3-7) Analyzing state-space characteristics, such as controllability and observability 3 Model Analysis Tools General Model Characteristics General model characteristics include the model type, I/O dimensions, and continuous or discrete nature. Related commands are listed in the table below. These commands operate on continuous- or discrete-time LTI models or arrays of LTI models of any type. General Model Characteristics Commands class Display model type ('tf', 'zpk', 'ss', or 'frd'). hasdelay Test true if LTI model has any type of delay. isa Test true if LTI model is of specified class. isct Test true for continuous-time models. isdt Test true for discrete-time models. isempty Test true for empty LTI models. isproper Test true for proper LTI models. issiso Test true for SISO models. ndims Display the number of model/array dimensions. reshape Change the shape of an LTI array. size Output/input/array dimensions. Used with special syntax, size also returns the number of state dimensions for state-space models, and the number of frequencies in an FRD model. This example illustrates the use of some of these commands. See the related reference pages for more details. H = tf({1 [1 1]},{[1 0.1] [1 2 10]}) Transfer function from input 1 to output: 1 ------s + 0.1 3-2 General Model Characteristics Transfer function from input 2 to output: s 1 -------------s^2 + 2 s + 10 class(H) ans = tf size(H) Transfer function with 2 input(s) and 1 output(s). [ny,nu] = size(H)% Note: ny = number of outputs ny = 1 nu = 2 isct(H)% Is this system continuous? ans = 1 isdt(H)% Is this system discrete? ans = 0 3-3 3 Model Analysis Tools Model Dynamics The Control System Toolbox offers commands to determine the system poles, zeros, DC gain, norms, etc. You can apply these commands to single LTI models or LTI arrays. The following table gives an overview of these commands. Model Dynamics covar Covariance of response to white noise. damp Natural frequency and damping of system poles. dcgain Low-frequency (DC) gain. dsort Sort discrete-time poles by magnitude. esort Sort continuous-time poles by real part. norm Norms of LTI systems ( H 2 and L ∞ ). pole, eig System poles. pzmap Pole/zero map. zero System transmission zeros. With the exception of L ∞ norm, these commands are not supported for FRD models. Here is an example of model analysis using some of these commands. h = tf([4 8.4 30.8 60],[1 4.12 17.4 30.8 60]) Transfer function: 4 s^3 + 8.4 s^2 + 30.8 s + 60 --------------------------------------s^4 + 4.12 s^3 + 17.4 s^2 + 30.8 s + 60 pole(h) ans = 1.7971 + 2.2137i 1.7971 2.2137i 3-4 Model Dynamics 0.2629 + 2.7039i 0.2629 2.7039i zero(h) ans = 0.0500 + 2.7382i 0.0500 2.7382i 2.0000 dcgain(h) ans = 1 [ninf,fpeak] = norm(h,inf)% peak gain of freq. response ninf = 1.3402 % peak gain fpeak = 1.8537 % frequency where gain peaks These functions also operate on LTI arrays and return arrays. For example, the poles of a three dimensional LTI array sysarray are obtained as follows. sysarray = tf(rss(2,1,1,3)) Model sysarray(:,:,1,1) ======================= Transfer function: -0.6201 s - 1.905 --------------------s^2 + 5.672 s + 7.405 Model sysarray(:,:,2,1) ======================= Transfer function: 0.4282 s^2 + 0.3706 s + 0.04264 ------------------------------s^2 + 1.056 s + 0.1719 Model sysarray(:,:,3,1) 3-5 3 Model Analysis Tools ======================= Transfer function: 0.621 s + 0.7567 --------------------s^2 + 2.942 s + 2.113 3x1 array of continuous-time transfer functions. pole(sysarray) ans(:,:,1) = -3.6337 -2.0379 ans(:,:,2) = -0.8549 -0.2011 ans(:,:,3) = -1.6968 -1.2452 3-6 State-Space Realizations State-Space Realizations The following functions are useful to analyze, perform state coordinate transformations on, and derive canonical state-space realizations for single state-space LTI models or LTI arrays of state-space models. State-Space Realizations canon Canonical state-space realizations. ctrb Controllability matrix. ctrbf Controllability staircase form. gram Controllability and observability gramians. obsv Observability matrix. obsvf Observability staircase form. ss2ss State coordinate transformation. ssbal Diagonal balancing of state-space realizations. The function ssbal uses a simple diagonal similarity transformation –1 –1 ( A, B, C ) → ( T AT, T B, CT ) to balance the state-space data the norm of the matrix. –1 T AT CT ( A, B, C ) . This is accomplished by reducing –1 T B 0 Such balancing usually improves the numerical conditioning of subsequent state-space computations. Note that conversions to state-space using ss produce balanced realizations of transfer functions and zero-pole-gain models. By contrast, the canonical realizations produced by canon, ctrbf, or obsvf are often badly scaled, sensitive to perturbations of the data, and poorly suited for state-space computations. Consequently, it is wise to use them only for analysis purposes and not in control design algorithms. 3-7 3 Model Analysis Tools 3-8 4 Arrays of LTI Models The Concept of an LTI Array (p. 4-2) What is an LTI array? Dimensions, Size, and Shape of an LTI How to understand the dimensions of complex LTI Array (p. 4-7) Arrays. Building LTI Arrays (p. 4-12) Constructing LTI Arrays from collections of LTI models. Indexing into LTI Arrays (p. 4-21) How to access elements of an LTI Array. Operations on LTI Arrays (p. 4-25) Building LTI Arrays from smaller ones. 4 Arrays of LTI Models The Concept of an LTI Array LTI Arrays are arrays that store an LTI models as one element in the array. Since an array in MATLAB can be treated as a single variable, so can an LTI Array. This means that you can operate on, say, 100 LTI models by one operations on a single variable—making LTI Arrays a powerful tool if you have large numbers of LTI models to consider in your analysis and design process. Because LTI Arrays are multidimensional in nature, there is a large set of topics that relate to them. This section discusses the these topics: • “When to Use an LTI Array” on page 4-2 • “When to Collect a Set of Models in an LTI Array” on page 4-3 • “Restrictions for LTI Models Collected in an Array” on page 4-3 • “Where to Find Information on LTI Arrays” on page 4-3 • “Visualizing LTI Arrays” on page 4-4 • “Higher Dimensional Arrays of LTI Models” on page 4-6 When to Use an LTI Array In many applications, it is useful to consider collections of linear, time invariant (LTI) models. For example, you may want to consider a model with a single parameter that varies, such as sys1 = tf(1, [1 1 1]); sys2 = tf(1, [1 1 2]); sys3 = tf(1, [1 1 3]); and so on. A convenient way to store and analyze a collection like this is to use LTI arrays. Continuing this example, you can create this LTI array and store all three transfer functions in one variable. sys_ltia = (sys1, sys2, sys3); You can use the LTI array sys_ltia just like you would use, for example, sys1. You can use LTI arrays to collect a set of LTI models into a single MATLAB variable. You then use this variable to manipulate or analyze the entire collection of models in a vectorized fashion. You access the individual models in the collection through indexing rather than by individual model names. 4-2 The Concept of an LTI Array LTI arrays extend the concept of single LTI models in a similar way to how multidimensional arrays extend two-dimensional matrices in MATLAB (see Multidimensional Arrays in the MATLAB documentation). When to Collect a Set of Models in an LTI Array You can use LTI arrays to represent • A set of LTI models arising from the linearization of a nonlinear system at several operating points • A collection of transfer functions that depend on one or more parameters • A set of LTI models arising from several system identification experiments applied to one plant • A set of gain-scheduled LTI controllers • A list of LTI models you want to collect together under the same name Restrictions for LTI Models Collected in an Array For each model in an LTI array, the following properties must be the same: • The number of inputs and outputs • The sample time, for discrete-time models • The I/O names and I/O groups Note You cannot specify Simulink LTI blocks with LTI arrays. Where to Find Information on LTI Arrays The next two sections give examples that illustrate how to visualize an LTI array, its dimensions, and size. To read about how to build an LTI array, go to “Building LTI Arrays” on page 4-12. The remainder of the chapter is devoted to indexing and operations on LTI Arrays. You can also apply the analysis functions in the Control System Toolbox to LTI arrays. See Chapter 5, “Model Analysis Tools,” for more information on these functions. You can also view response plots of LTI arrays with the LTI Viewer. 4-3 4 Arrays of LTI Models Visualizing LTI Arrays To visualize the concept of an LTI array, consider the set of five transfer function models shown below. In this example, each model has two inputs and two outputs. They differ by parameter variations in the individual model components. 1.1 -----------s+1 0 1.3 ----------------s + 1.1 0 1.11 ----------------s + 1.2 0 1.15 ----------------s + 1.3 0 1.09 ----------------s + 1.4 0 0 1 -----------s+5 0 1 ----------------s + 5.2 0 1 ----------------s + 5.4 0 1 ----------------s + 5.6 0 1 ----------------s + 5.8 Five LTI Models to be Collected in an LTI Array This LTI array embodies a 1.09 ----------------0 1-by-5 list of models. 1.15 s + 1.4 ----------------0 1 s + 1.3 0 ---------------s + 5.8 1.11 1 ----------------- 00 ---------------s + 1.2 s + 5.6 1.3 1 ----------------0 ----------------0 1.1s + 1.1 s + 5.4 ------------ 0 1 s+1 0 ----------------1 s + 5.2 0 -----------Each element of the LTI array s+5 is a model. An LTI Array Containing These Five Models 4-4 The Concept of an LTI Array Just as you might collect a set of two-by-two matrices in a multidimensional array, you can collect this set of five transfer function models as a list in an LTI array under one variable name, say, sys. Each element of the LTI array is an LTI model. Individual models in the LTI array sys are accessed via indexing. The general form for the syntax you use to access data in an LTI array is sysa(Outputs,Inputs,Models) The first index selects the output channels. The second index selects the input channels. The remaining indices select particular models in the LTI array by their array coordinates. For example, you can access the third model in sys with sys(:,:,3). The following illustrates how you can use indexing to select models or their components from sys. 1.09 ----------------0 1.15 s + -1.4 0 ---------------s + 1.3 1.11 1 ----------------- 00 ---------------s + 1.2 1s + 5.8 ----------------1.3 0 ----------------0 1s + 5.6 s + 1.1 1.1 ---------------------------- 0 0 s + 5.4 s+1 1 ----------------0 1 s + 5.2 0 -----------s+5 1.11 ----------------s + 1.2 0 0 1 ----------------s + 5.4 sysa(2,2,3) selects the (2,2) entry of the third model in the array. sysa(:,:,3) selects the third model in the array. Using Indices to Select Models and Their Components See “Indexing into LTI Arrays” on page 4-21 for more information on indexing. 4-5 4 Arrays of LTI Models Higher Dimensional Arrays of LTI Models You can also collect a set of models in a two-dimensional array. The following diagram illustrates a 2-by-3 array of six, two-output, one-input models called m2d. m2d(:,:,1,3) Each entry in this 2-by-3 array of models is a two-output, one-input transfer function. 3.42 -------------------s + 2.84 7.29 m2d(:,:,1,1) m2d(:,:,2,1) 3.36 ----------------s + 2.9 7.23 m2d(:,:,1,2) m2d(:,:,2,2) 3.4 -------------------s + 2.86 7.27 m2d(:,:,1,3) m2d(:,:,2,3) m2d(:,:,1,3) extracts the model in the (1,3) position of the array. 3.45 -------------------s + 2.81 7.32 m2d: A 2-by-3 Array of Two-Output, One-Input Models More generally, you can organize models into a 3-D or higher-dimensional array, in much the same way you arrange numerical data into multidimensional arrays (see Multidimensional Arrays in the MATLAB documentation). 4-6 Dimensions, Size, and Shape of an LTI Array Dimensions, Size, and Shape of an LTI Array To rapidly manipulate LTI Arrays, you must understand their structure. This structure is discussed in the following sections: • “I/O and Array Dimensions of LTI Arrays” on page 4-7 • “Accessing the Dimensions of an LTI Array Using size and ndims” on page 4-9 • “Using reshape to Rearrange an LTI Array” on page 4-11 I/O and Array Dimensions of LTI Arrays The dimensions and size of a single LTI model are determined by the output and input channels. An array of LTI models has additional quantities that determine its dimensions, size, and shape. There are two sets of dimensions associated with LTI arrays: • The I/O dimensions — The output dimension and input dimension common to all models in the LTI array • The array dimensions — The dimensions of the array of models itself The size of the LTI array is determined by • The lengths of the I/O dimensions — The number of outputs (or inputs) common to all models in the LTI array • The length of each array dimension — The number of models along that array dimension 4-7 4 Arrays of LTI Models . The next figure illustrates the concepts of dimension and size for the LTI array m2d, a 2-by-3 array of one-input, two-output transfer function models. ray di m ens ion is 2 The length of the second array dimension is 3. m2d(:,:,1,2) m2d(:,:,1,3) m2d(:,:,2,1) m2d(:,:,2,2) m2d(:,:,2,3) T he le n gt h of th e fi rs t ar m2d(:,:,1,1) 3.4 -------------------s + 2.86 7.27 3.45 -------------------s + 2.81 7.32 m2d(:,:,2,3) Output dimension (length 2) 3.36 ----------------s + 2.9 7.23 3.45 -------------------s + 2.81 7.32 Input dimension (length 1) Dimensions and Size of m2d, an LTI Array You can load this sample LTI array into your workspace by typing load LTIexamples size(m2d) 2x3 array of continuous-time transfer functions Each transfer function has 2 outputs and 1 input. The I/O dimensions correspond to the row and column dimensions of the transfer matrix. The two I/O dimensions are both of length 1 for SISO models. For MIMO models the lengths of these dimensions are given by the number of outputs and inputs of the model. Five related quantities are pertinent to understanding the array dimensions: 4-8 Dimensions, Size, and Shape of an LTI Array • N, the number of models in the LTI array • K, the number of array dimensions • S 1 S 2 …S K, the list of lengths of the array dimensions - S i is the number of models along the i th dimension. • S 1 – by – S 2 – by – … – by – S K , the configuration of the models in the array - The configuration determines the shape of the array. - The product of these integers S 1 × S 2 × … × S K is N. In the example model m2d, • The length of the output dimension, the first I/O dimension, is 2, since there are two output channels in each model. • The length of the input dimension, the second I/O dimension, is 1, since there is only one input channel in each model. • N, the number of models in the LTI array, is 6. • K, the number of array dimensions, is 2. • The array dimension lengths are [2 3]. • The array configuration is 2-by-3. Accessing the Dimensions of an LTI Array Using size and ndims You can access the dimensions and shape of an LTI array using • size to determine the lengths of each of the dimensions associated with an LTI array • ndims to determine the total number of dimensions in an LTI array When applied to an LTI array, size returns [Ny Nu S1 S2 ... Sk] where • Ny is the number of outputs common to all models in the LTI array. • Nu is the number of inputs common to all models in the LTI array. • S1 S2 ... Sk are the lengths of the array dimensions of a k-dimensional array of models. Si is the number of models along the ith array dimension. 4-9 4 Arrays of LTI Models Note the following when using the size function: • By convention, a single LTI model is treated as a 1-by-1 array of models. For single LTI models, size returns only the I/O dimensions [Ny Nu]. • For LTI arrays, size always returns at least two array dimensions. For example, the size of a 2-by-1 LTI array in [Ny Nu 2 1] • size ignores trailing singleton dimensions beyond the second array dimension. For example, size returns [Ny Nu 2 3] for a 2-by-3-by-1-by-1 LTI array of models with Ny outputs and Nu inputs. The function ndims returns the total number of dimensions in an LTI array: • 2, for single LTI models • 2 + p, for LTI arrays, where p (greater than 2) is the number of array dimensions Note that ndims (sys) = length(size(sys)) To see how these work on the sample 2-by-3 LTI array m2d of two-output, one-input models, type load LTIexamples s = size(m2d) s = 2 1 2 3 Notice that size returns a vector whose entries correspond to the length of each of the four dimensions of m2d: two outputs and one input in a 2-by-3 array of models. Type ndims(m2d) ans = 4 to see that there are indeed four dimensions attributed to this LTI array. 4-10 Dimensions, Size, and Shape of an LTI Array Using reshape to Rearrange an LTI Array Use reshape to reorganize the arrangement (array configuration) of the models of an existing LTI array. For example, to arrange the models in an LTI Array sys as a w 1 × … × w p array, type reshape(sys,w1,...,wp) where w1,...,wp are any set of integers whose product is N, the number of models in sys. You can reshape the LTI array m2d into a 3-by-2, a 6-by-1, or a 1-by-6 array using reshape. For example, type load LTIexamples sys = reshape(m2d,6,1); size(sys) 6x1 array of continuous-time transfer functions Each transfer function has 2 outputs and 1 inputs. s = size(sys) s = 2 1 6 1 4-11 4 Arrays of LTI Models Building LTI Arrays There are several ways to build LTI arrays: • Using a for loop to assign each model in the array • Using stack to concatenate LTI models into an LTI array • Using tf, zpk, ss, or frd In addition, you can use the command rss to generate LTI arrays of random state-space models. The following sections discuss these topics: • “Building LTI Arrays Using for Loops” on page 4-12 • “Building LTI Arrays Using the stack Function” on page 4-15 • “Building LTI Arrays Using tf, zpk, ss, and frd” on page 4-17 • “Generating Random LTI Arrays Using rss” on page 4-19 Building LTI Arrays Using for Loops Consider the following second-order SISO transfer function that depends on two parameters, ζ and ω ω2 H ( s ) = --------------------------------------2 s + 2ζωs + ω 2 Suppose, based on measured input and output data, you estimate confidence intervals [ω 1,ω 2] , and [ζ 1,ζ 2] for each of the parameters, ω and ζ. All of the possible combinations of the confidence limits for these model parameter values give rise to a set of four SISO models. 4-12 Building LTI Arrays ω1 ω2 ω 12 ω 12 ζ 1 H 11 ( s ) = -------------------------------------------- H 12 ( s ) = -------------------------------------------s 2 + 2ζ 1 ω 1 s + ω 12 s 2 + 2ζ 2 ω 1 s + ω 12 ω 22 ω 22 ζ 2 H ( s ) = -------------------------------------------- H 22 ( s ) = -------------------------------------------21 s 2 + 2ζ 2 ω 2 s + ω 22 s 2 + 2ζ 1 ω 2 s + ω 22 Four LTI Models Depending on Two Parameters You can arrange these four models in a 2-by-2 array of SISO transfer functions called H. ω1 ω2 ζ1 H(:,:,1,1) H(:,:,1,2) ζ2 H(:,:,2,1) H(:,:,2,2) Each entry of this 2-by-2 array is a SISO transfer function model. The LTI Array H Here, for i,j ∈ { 1, 2 } , H(:,:,i,j) represents the transfer function ω j2 --------------------------------------------s 2 + 2ζ i ω j s + ω j2 corresponding to the parameter values ζ = ζ i and ω = ω j . The first two colon indices ( : ) select all I/O channels from the I/O dimensions of H. The third index of H refers to the first array dimension ( ζ), while the fourth index is for the second array dimension (ω). 4-13 4 Arrays of LTI Models Suppose the limits of the ranges of values for ζ and ω are [0.66,0.76] and [1.2,1.5], respectively. Enter these at the command line. zeta = [0.66,0.75]; w = [1.2,1.5]; Since the four models have the same parametric structure, it’s convenient to use two nested for loops to construct the LTI array. for i = 1:2 for j = 1:2 H(:,:,i,j) = tf(w(j)^2,[1 2*zeta(i)*w(j) w(j)^2]); end end H now contains the four models in a 2-by-2 array. For example, to display the transfer function in the (1,2) position of the array, type H(:,:,1,2) Transfer function: 2.25 ------------------s^2 + 1.98 s + 2.25 4-14 Building LTI Arrays For the purposes of efficient computation, you can initialize an LTI array to zero, and then reassign the entire array to the values you want to specify. The general syntax for zero assignment of LTI arrays is Lengths of the output/input dimensions Lengths of the array dimensions sysa = tf(zeros(Ny,Nu,S1,...,SK)) sysa = zpk(zeros(Ny,Nu,S1,...,SK)) sysa = ss(zeros(Ny,Nu,S1,...,SK,Nx)) sysa = frd(zeros(Ny,Nu,Nf,S1,...,SK)) The number of frequency vectors in the FRD The maximum number of states in any model in the LTI array To initialize H in the above example to zero, type H = tf(zeros(1,1,2,2)); before you implement the nested for loops. Building LTI Arrays Using the stack Function Another way to build LTI arrays is using the function stack. This function operates on single LTI models as well as LTI arrays. It concatenates a list of LTI arrays or single LTI models only along the array dimension. The general syntax for stack is stack(Arraydim,sys1,sys2...) where • Arraydim is the array dimension along which to concatenate the LTI models or arrays. • sys1, sys2, ... are the LTI models or LTI arrays to be concatenated. 4-15 4 Arrays of LTI Models When you concatenate several models or LTI arrays along the jth array dimension, such as in stack(j,sys1,sys2,...,sysn) • The lengths of the I/O dimensions of sys1,...,sysn must all match. • The lengths of all but the jth array dimension of sys1,...,sysn must match. For example, if two TF models sys1 and sys2 have the same number of inputs and outputs, sys = stack(1,sys1,sys2) concatenates them into a 2-by-1 array of models. There are two principles that you should keep in mind: • stack only concatenates along an array dimension, not an I/O dimension. • To concatenate LTI models or LTI arrays along an input or output dimension, use the bracket notation ([,] [;]). See “Model Interconnection Functions” on page 2-16 for more information on the use of bracket notation to concatenate models. See also “Special Cases for Operations on LTI Arrays” on page 4-28 for some examples of this type of concatenation of LTI arrays. Here’s an example of how to build the LTI array H using the function stack. % Set up the parameter vectors. zeta = [0.66,0.75]; w = [1.2,1.5]; % Specify the four % H11 = tf(w(1)^2,[1 H12 = tf(w(2)^2,[1 H21 = tf(w(1)^2,[1 H22 = tf(w(2)^2,[1 individual models with those parameters. 2*zeta(1)*w(1) 2*zeta(1)*w(2) 2*zeta(2)*w(1) 2*zeta(2)*w(2) w(1)^2]); w(2)^2]); w(1)^2]); w(2)^2]); % Set up the LTI array using stack. COL1 = stack(1,H11,H21); % The first column of the 2-by-2 array COL2 = stack(1,H12,H22); % The second column of the 2-by-2 array H = stack(2, COL1, COL2); % Concatenate the two columns of models. 4-16 Building LTI Arrays Notice that this result is very different from the single MIMO LTI model returned by H = [H11,H12;H21,H22]; Accessing LTI Arrays of Variable Order For arrays of state-space models with variable order, you cannot use the dot operator (e.g., sys.a) to access arrays. Use the syntax [a,b,c,d] = ssdata(sys,'cell') to extract the state-space matrices of each model as separate cells in the cell arrays a, b, c, and d. Building LTI Arrays Using tf, zpk, ss, and frd You can also build LTI arrays using the tf, zpk, ss, and frd constructors. You do this by using multidimensional arrays in the input arguments for these functions. Specifying Arrays of TF Models tf For TF models, use sys = tf(num,den) where • Both num and den are multidimensional cell arrays the same size as sys (see “Accessing the Dimensions of an LTI Array Using size and ndims” on page 4-9). • sys(i,j,n1,...,nK) is the (i, j) entry of the transfer matrix for the model located in the ( n 1, …, n K ) position of the array. • num(i,j,n1,...,nK) is a row vector representing the numerator polynomial of sys(i,j,n1,...,nK). • den(i,j,n1,...,nK) is a row vector representing denominator polynomial of sys(i,j,n1,...,nK). See “MIMO Transfer Function Models” on page 1-10 for related information on the specification of single TF models. 4-17 4 Arrays of LTI Models Specifying Arrays of ZPK Models Using zpk For ZPK models, use sys = zpk(zeros,poles,gains) where • Both zeros and poles are multidimensional cell arrays whose cell entries contain the vectors of zeros and poles for each I/O pair of each model in the LTI array. • gains is a multidimensional array containing the scalar gains for each I/O pair of each model in the array. • The dimensions (and their lengths) of zeros, poles, and gains, determine those of the LTI array, sys. Specifying Arrays of SS Models Using ss To specify arrays of SS models, use sys = ss(a,b,c,d) where a, b, c, and d are real- or complex-valued multidimensional arrays of appropriate dimensions. All models in the resulting array of SS models have the same number of states, outputs, and inputs. Note You cannot use the ss constructor to build an array of state-space models with different numbers of states. Use stack to build such LTI arrays. The Size of LTI Array Data for SS Models The size of the model data for arrays of state-space models is summarized in the following table. Data 4-18 Size (Data) a N s N s S 1S 2 …S K b N s N u S 1 S 2 …S K Building LTI Arrays Data Size (Data) c Ny Ns S 1 S2 … S K d N y N u S 1 S 2 …S K where • N s is the maximum of the number of states in each model in the array. • N u is the number of inputs in each model. • N y is the number of outputs in each model. • S 1, S 2, …, S K are the lengths of the array dimensions. Specifying Arrays of FRD Models Using frd To specify a K-dimensional array of p-output, m-input FRD models for which S 1, S 2, …, S K are the lengths of the array dimensions, use sys = frd(response,frequency,units) where • frequency is a real vector of n frequency data points common to all FRD models in the LTI array. • response is a p-by-m-by-n-by- S 1 -by- …-by- S K complex-valued multidimensional array. • units is the optional string specifying 'rad/s' or 'Hz'. Note that for specifying an LTI array of SISO FRD models, response can also be a multidimensional array of 1-by-n matrices whose remaining dimensions determine the array dimensions of the FRD. Generating Random LTI Arrays Using rss A convenient way to generate arrays of state-space models with the same number of states in each model is to use rss. The syntax is rss(N,P,M,sdim1,...,sdimk) where • N is the number of states of each model in the LTI array. 4-19 4 Arrays of LTI Models • P is the number of outputs of each model in the LTI array. • M is the number of inputs of each model in the LTI array. • sdim1,...,sdimk are the lengths of the array dimensions. For example, to create a 4-by-2 array of random state-space models with three states, one output, and one input, type sys = rss(3,2,1,4,2); size(sys) 4x2 array of continuous-time state-space models Each model has 2 outputs, 1 input, and 3 states. 4-20 Indexing into LTI Arrays Indexing into LTI Arrays You can index into LTI arrays in much the same way as you would for multidimensional arrays to • Access models • Extract subsystems • Reassign parts of an LTI array • Delete parts of an LTI array When you index into an LTI array sys, the indices should be organized according to the following format sys(Outputs, Inputs, n 1, …, n K ) where • Outputs are indices that select output channels. • Inputs are indices that select input channels. • n 1, …, n K are indices into the array dimensions that select one model or a subset of models in the LTI array. Note on Indexing into LTI Arrays of FRD models. For FRD models, the array indices can be followed by the keyword 'frequency' and some expression selecting a subset of the frequency points as in sys (outputs, inputs, n1,...,nk, 'frequency', SelectedFreqs) See “Referencing FRD Models Through Frequencies” on page 2-7 for details on frequency point selection in FRD models. The next sections discuss these topics: • “Accessing Particular Models in an LTI Array” on page 4-22 • “Extracting LTI Arrays of Subsystems” on page 4-23 • “Reassigning Parts of an LTI Array” on page 4-23 • “Deleting Parts of an LTI Array” on page 4-24 • “Example: Addition of Two LTI Arrays” on page 4-26 • “Dimension Requirements” on page 4-27 • “Special Cases for Operations on LTI Arrays” on page 4-28 4-21 4 Arrays of LTI Models Accessing Particular Models in an LTI Array To access any given model in an LTI array, • Use colon arguments (:,:) for the first two indices to select all I/O channels. • The remaining indices specify the model coordinates within the array. For example, if sys is a 5-by-2 array of state-space models defined by sys = rss(4,3,2,5,2); you can access (and display) the model located in the (3,2) position of the array sys by typing sys(:,:,3,2) If sys is a 5-by-2 array of 3-output, 2-input FRD models, with frequency vector [1,2,3,4,5], then you can access the response data corresponding to the middle frequency (3 rad/s), of the model in the (3,1) position by typing sys(:,:,3,1,'frequency',3.0) To access all frequencies of this model in the array, you can simply type sys(:,:,3,1) Single Index Referencing of Array Dimensions You can also access models using single index referencing of the array dimensions. For example, in the 5-by-2 LTI array sys above, you can also access the model located in the (3,2) position by typing sys(:,:,8) since this model is in the eighth position if you were to list the 10 models in the array by successively scanning through its entries along each of its columns. For more information on single index referencing, see “Advanced Indexing” under “M-File Programming” in the MATLAB online documentation. 4-22 Indexing into LTI Arrays Extracting LTI Arrays of Subsystems To select a particular subset of I/O channels from all the models in an LTI array, use the syntax described in “Extracting and Modifying Subsystems” on page 2-5. For example, sys = rss(4,3,2,5,2); A = sys(1, [1 2]) or equivalently, A = sys(1,[1 2],:,:) selects the first two input channels, and the first output channel in each model of the LTI array A, and returns the resulting 5-by-2 array of one-output, two-input subsystems. You can also combine model selection with I/O selection within an LTI array. For example, to access both, • The state-space model in the (3,2) array position • Only the portion of that model relating the second input to the first output type sys(1,2,3,2) To access the subsystem from all inputs to the first two output channels of this same array entry, type sys(1:2,:,3,2) Reassigning Parts of an LTI Array You can reassign entire models or portions of models in an LTI array. For example, sys = rss(4,3,2,5,2); % 5X2 array of state-space models H = rss(4,1,1,5,2); % 5X2 array of SISO models sys(1,2) = H reassigns the subsystem from input two to output one, for all models in the LTI array sys. This SISO subsystem of each model in the LTI array is replaced with the LTI array H of SISO models. This one-line assignment command is equivalent to the following 10-step nested for loop. 4-23 4 Arrays of LTI Models for k = 1:5 for j = 1:2 sys(1,2,k,j) = H(:,:,k,j); end end Notice that you don’t have to use the array dimensions with this assignment. This is because I/O selection applies to all models in the array when the array indices are omitted. Similarly, the commands sys(:,:,3,2) = sys(:,:,4,1); sys(1,2,3,2) = 0; reassign the entire model in the (3,2) position of the LTI array sys and the (1,2) subsystem of this model, respectively. Deleting Parts of an LTI Array You can use indexing to delete any part of an LTI array by reassigning it to be empty ([]). For instance, sys = rss(4,3,2,5,2); sys(1,:) = []; size(sys) 5x2 array of continuous-time state-space models Each model has 2 outputs, 2 inputs, and 4 states. deletes the first output channel from every model of this LTI array. Similarly, sys(:,:,[3 4],:) = [] deletes the third and fourth rows of this two-dimensional array of models. 4-24 Operations on LTI Arrays Operations on LTI Arrays Using LTI arrays, you can apply almost all of the basic model operations that work on single LTI models to entire sets of models at once. These basic operations include • The arithmetic operations: +, , *, /,\,',.' • The functions: concatenation along I/O dimensions ([,], [;]), feedback, append, series, parallel, and lft When you apply any of these operations to two (or more) LTI arrays (for example, sys1 and sys2), the operation is implemented on a model-by-model basis. Therefore, the kth model of the resulting LTI array is derived from the application of the given operation to the kth model of sys1 and the kth model of sys2. For example, if sys1 and sys2 are two LTI arrays and sysa = op(sys1,sys2) then the kth model in the resulting LTI array sys is obtained by adding the kth models in sys1 to the kth model in sys2 sysa(:,:,k) = sys1(:,:,k) + sys2(:,:,k) You can also apply any of the response plotting functions such as step, bode, and nyquist to LTI arrays. These plotting functions are also applied on a model by model basis. 4-25 4 Arrays of LTI Models Example: Addition of Two LTI Arrays The following diagram illustrates the addition of two 3-by-1 LTI arrays sys1+sys2. sys2(:,:,3) sys1(:,:,3 1 -----------s+2 + sys1(:,:,2) 1 ----------------s + 2.5 sys1 2s + 6.5 --------------------------------2 s + 6.5s + 9 = sys2(:,:,2) + sys1(:,:,1 1 ----------------s + 2.9 1 ---------------s + 4.5 sys(:,:,3) 2.1 -----------s+4 sys(:,:,2) = 3.15 + 9.25 -----------------------------------2 s + 6.5s + 10 sys(:,:,1) sys2(:,:,1) + 1.5 ----------------s + 3.9 = + sys2 = 2.5s + 8.25 -------------------------------------------2 s + 6.8s + 11.31 The Addition of Two LTI Arrays The summation of these LTI arrays sysa = sys1+sys2 is equivalent to the following model-by-model summation: for k = 1:3 sysa(:,:,k)=sys1(:,:,k) + sys2(:,:,k) end 4-26 sysa Operations on LTI Arrays Note that • Each model in sys1 and sys2 must have the same number of inputs and outputs. This is required for the addition of two LTI arrays. • The lengths of the array dimensions of sys1 and sys2 must match. The following sections describe these topics: • “Dimension Requirements” on page 4-27 • “Special Cases for Operations on LTI Arrays” on page 4-28 • “Other Operations on LTI Arrays” on page 4-30 Dimension Requirements In general, when you apply any of these basic operations to two or more LTI arrays, • The I/O dimensions of each of the LTI arrays must be compatible with the requirements of the operation. • The lengths of array dimensions must match. The I/O dimensions of each model in the resulting LTI array are determined by the operation being performed. See Chapter 3, “Operations on LTI Models,” for requirements on the I/O dimensions for the various operations. For example, if sys1 and sys2 are both 1-by-3 arrays of LTI models with two inputs and two outputs, and sys3 is a 1-by-3 array of LTI models with two outputs and 1 input, then sys1 + sys2 is an LTI array with the same dimensions as sys1 and sys2. sys1 * sys3 is a 1-by-3 array of LTI models with two outputs and one input, and [sys1,sys3] is a 1-by-3 array of LTI models with two outputs and three inputs. 4-27 4 Arrays of LTI Models Special Cases for Operations on LTI Arrays There are some special cases in coding operations on LTI arrays. Consider sysa = op(sys1,sys2) where op is a symbol for the operation being applied. sys1 is an LTI array, and sysa (the result of the operation) is an LTI array with the same array dimensions as sys1. You can use shortcuts for coding sysa = op(sys1,sys2) in the following cases: • For operations that apply to LTI arrays, sys2 does not have to be an array. It can be a single LTI model (or a gain matrix) whose I/O dimensions satisfy the compatibility requirements for op (with those of each of the models in sys1). In this case, op applies sys2 to each model in sys1, and the kth model in sys satisfies sysa(:,:,k) = op(sys1(:,:,k),sys2) • For arithmetic operations, such as +, *, /, and \, sys2 can be either a single SISO model, or an LTI array of SISO models, even when sys1 is an LTI array of MIMO models. This special case relies on scalar expansion capabilities for arithmetic operations in MATLAB. - When sys2 is a single SISO LTI model (or a scalar gain), op applies sys2 to sys1 on an entry-by-entry basis. The ijth entry in the kth model in sysa satisfies sysa(i,j,k) = op(sys1(i,j,k),sys2) - When sys2 is an LTI array of SISO models (or a multidimensional array of scalar gains), op applies sys2 to sys1 on an entry-by-entry basis for each model in sysa. sysa(i,j,k) = op(sys1(i,j,k),sys2(:,:,k)) Examples of Operations on LTI Arrays with Single LTI Models Suppose you want to create an LTI array containing three models, where, for τ in the set { 1.1, 1.2, 1.3 } , each model H τ ( s ) has the form 4-28 Operations on LTI Arrays 1 ----------s +τ Hτ ( s ) = 0 1 --s –1 You can do this efficiently by first setting up an LTI array h containing the SISO models 1 ⁄ ( s + τ ) and then using concatenation to form the LTI array H of MIMO LTI models H τ ( s ), τ ∈ { 1.1, 1.2, 1.3 }. To do this, type tau = [1.1 1.2 1.3]; for i=1:3 % Form LTI array h of SISO models. h(:,:,i)=tf(1,[1 tau]); end H = [h 0; 1 tf(1,[1 0])]; %Concatenation: array h & single models size(H) 3x1 array of continuous-time transfer functions Each transfer function has 2 output(s) and 2 input(s). Similarly, you can use append to perform the diagonal appending of each model in the SISO LTI array h with a fixed single (SISO or MIMO) LTI model. S = append(h,tf(1,[1 3])); % Append a single model to h. specifies an LTI array S in which each model has the form 1 ----------s +τ Sτ ( s )= 0 0 1 -----------s+3 You can also combine an LTI array of MIMO models and a single MIMO LTI model using arithmetic operations. For example, if h is the LTI array of three SISO models defined above, [h,h] + [tf(1,[1 0]);tf(1,[1 5])] adds the single one-output, two-input LTI model [1/s 1/(s + 5)] to every model in the 3-by-1 LTI array of one-output, two-input models [h,h]. The result is a new 3-by-2 array of models. 4-29 4 Arrays of LTI Models Examples: Arithmetic Operations on LTI Arrays and SISO Models Using the LTI array of one-output, two-input state-space models [h,h], defined in the previous example, tf(1,[1 3]) + [h,h] adds a single SISO transfer function model to each entry in each model of the LTI array of MIMO models [h,h]. Finally, G = rand(1,1,3,1); sysa = G + [h,h] adds the array of scalars to each entry of each MIMO model in the LTI array [h,h] on a model-by-model basis. This last command is equivalent to the following for loop. hh = [h,h]; for k = 1:3 sysa(:,:,k) = G(1,1,k) + hh(:,:,k); end Other Operations on LTI Arrays You can also apply the analysis functions, such as bode, nyquist, and step, to LTI arrays. 4-30 5 Preliminaries for Customization Terminology (p. 5-2) The terms used in customizing tools, viewers, and response plots The Property and Preferences Hierarchy (p. 5-3) How to understand the scope of any property or preference setting 5 Preliminaries for Customization Terminology The Control System Toolbox provides editors that allow you to set properties and preferences in the SISO Design Tool, the LTI Viewer, and in any response plots that you create from the MATLAB prompt. Properties refer to settings that are specific to an individual response plot. This includes the following: • Axes labels, and limits • Data units and scales • Plot styles, such as grids, fonts, and axes foreground colors • Plot characteristics, such as rise time, peak response, and gain and phase margins Preferences refers to properties that persist either • Within a single session for a specific instance of an LTI Viewer or a SISO Design Tool • Across Control System Toolbox sessions The former are called tool preferences, the latter toolbox preferences. 5-2 The Property and Preferences Hierarchy The Property and Preferences Hierarchy This diagram explains the hierarchy from properties, which are local, to toolbox preferences, which are global and persist from session to session. Toolbox Preferences Persist across sessions User Preferences Inheritance Saved to disk Not saved Tool Preferences Specific to an instance of a tool to disk SISO Design DesignTool Tool SISO Inheritance LTI Viewer Not saved Plot Properties Specific to an instance of a plot to disk Response Response Plot Plot Response Response Plot Plot Response Response Plot Plot 5-3 5 Preliminaries for Customization 5-4 6 Setting Toolbox Preferences Toolbox Preferences Editor (p. 6-2) An editor that allow you to set plot preferences that persist from session to session Units Pane (p. 6-3) Setting frequency, magnitude, and phase units Style Pane (p. 6-4) How to toggle grid visibility, set foreground colors, and so on Characteristics Pane (p. 6-5) Settings for response characteristics and phase wrapping SISO Tool Pane (p. 6-6) Settings for the SISO Design Tool 6 Setting Toolbox Preferences Toolbox Preferences Editor The Toolbox Preferences editor allows you to set plot preferences that will persist from session to session. This is the highest level shown in “The Property and Preferences Hierarchy” on page 5-3. Opening the Toolbox Preferences Editor To open the Toolbox Preferences Editor, select Toolbox Preferences under the File menu of the LTI Viewer or the SISO Design Tool. Alternatively, you can type ctrlpref at the MATLAB prompt. Note To get help on panes in the Control System Toolbox Preferences editor, you can click on the tabs. . The Control System Toolbox Preferences Editor 6-2 Units Pane Units Pane Note To get help on panes in the Control System Toolbox Preferences editor, click on the tabs. Use the Units pane to set preferences for the following: • Frequency — Radians per second (rad/s) or Hertz (Hz) • Magnitude — Decibels (dB) or absolute value (abs) • Phase — Degrees or radians For frequency and magnitude axes, you can select logarithmic or linear scales. 6-3 6 Setting Toolbox Preferences Style Pane Note Click on the tabs to get help on panes in the Control System Toolbox Preferences editor. Use the Style pane to toggle grid visibility and set font preferences and axes foreground colors for all plots you create using the Control System Toolbox. This figure shows the Style pane. You have the following choices: • Grid — Activate grids by default in new plots • Font preferences — Set the font size, weight (bold), and angle (italic) • Colors — Specify the color vector to use for the axes foreground, which includes the X-Y axes, grid lines, and tick labels. Use a three-element vector to represent red, green, and blue (RGB) values. Vector element values can range from 0 to 1. If you do not want to specify RGB values numerically, click the Select button to open the Select Colors window. See “Select colors” on page 7-11 for more information. 6-4 Characteristics Pane Characteristics Pane Note Click on the tabs to get help on panes in the Control System Toolbox Preferences editor. The Characteristics pane has selections for response characteristics and phase wrapping. This figure shows the Characteristics pane with default settings. The following are the available options for the Characteristics pane: • Response Characteristics - Specify settling time tolerance — You can set the threshold of the settling time calculation to any percentage from 0 to 100%. The default is 2%. - Specify rise time boundaries — The standard definition of rise time is the time it takes the signal to go from 10% to 90% of the final value. You can choose any percentages you like (from 0% to 100%), provided that the first value is smaller than the second. • Phase Wrapping — By default, the phase is not wrapped. Wrap the phrase by clearing this box. If the phase is wrapped, all phase values are shifted such that their equivalent value displays in the range [-180°, 180°). 6-5 6 Setting Toolbox Preferences SISO Tool Pane Note Click on the tabs below to get help on panes in the Control System Toolbox Preferences editor. The SISO Tool pane has settings for the SISO Design Tool. This figure shows the SISO Tool pane with default settings. You can make the following selections: • Compensator Format — You can select either the time-constant format or the zero/pole/gain format. The time-constant format is ( 1 + Tz 1 s ) dcgain × ---------------------------- … ( 1 + Tp1 s ) where Tz1, Tz2, ..., are the zero time constants, and Tp1, Tp2, ..., are the pole time constants. The zero/pole/gain format is a variation on the time-constant format. 6-6 SISO Tool Pane ( s + z1 ) K × -------------------( s + p1 ) In this case, the gain is compensator gain; z1, z2, ... and p1, p2, ..., are the zero and pole locations, respectively. • Bode Options — By default, the SISO Design Tool shows the plant and sensor poles and zeros as blue x’s and o’s, respectively. Clear this box to eliminate the plant’s poles and zeros from the Bode plot. Note that the compensator poles and zeros (in red) will still appear. 6-7 6 Setting Toolbox Preferences 6-8 7 Setting Tool Preferences Introduction (p. 7-2) A quick comparison of setting preferences in the LTI Viewer and SISO Design Tool LTI Viewer Preferences Editor (p. 7-3) How to set units, fonts, response plot characteristics, and so on in the LTI Viewer SISO Design Tool Preferences Editor (p. 7-8) How to set units, fonts, response plot characteristics, and so on in the SISO Design Tool 7 Setting Tool Preferences Introduction Both the LTI Viewer and the SISO Design Tool have Tool Preferences Editors. These editors comprise the middle layer of “The Property and Preferences Hierarchy” on page 5-3. Both editors allow you to set default characteristics for specific instances of LTI Viewers and SISO Design Tools. If you open a new instance of either, each defaults to the characteristics specified in the Toolbox Preferences editor. 7-2 LTI Viewer Preferences Editor LTI Viewer Preferences Editor Select LTI Viewer Preferences under the Edit menu of the LTI Viewer to open the LTI Viewer Preferences editor, which is a tool for customizing various LTI Viewer properties, including units, fonts, and various other viewer characteristics. This figure shows the editor open to its first pane. Note Click on the tabs to get help on LTI Viewer Preference editor. The LTI Viewer Preferences Editor 7-3 7 Setting Tool Preferences Units Pane Note Click on the pane tabs below to get help on LTI Viewer Preference editor panes. You can select the following on the Units pane: • Frequency — Radians per second (rad/s) or Hertz (Hz) • Magnitude — Decibels (dB) or absolute value (abs) • Phase — Degrees or radians For frequency and magnitude axes, you can select logarithmic or linear scales. 7-4 LTI Viewer Preferences Editor Style Pane Note Click on the tabs to get help on LTI Viewer Preference editor. Use the Style pane to toggle grid visibility and set font preferences and axes foreground colors for all plots in the LTI Viewer. This figure shows the Style pane. You have the following choices: • Grid — Activate grids for all plots in the LTI Viewer • Fonts — Set the font size, weight (bold), and angle (italic) • Colors — Specify the color vector to use for the axes foreground, which includes the X-Y axes, grid lines, and tick labels. Use a three-element vector to represent red, green, and blue (RGB) values. Vector element values can range from 0 to 1. • If you do not want to specify the RGB values numerically, press the Select button to open the Select Colors window. See “Select colors” on page 7-11 for more information. 7-5 7 Setting Tool Preferences Characteristics Pane Note Click on the tabs to get help on LTI Viewer Preference editor. The Characteristics pane, shown below, has selections for response characteristics and phase wrapping. The following choices are available: • Response Characteristics - Specify settling time tolerance — You can set the threshold of the settling time calculation to any percentage from 0 to 100%. The default is 2%. - Specify rise time boundaries — The standard definition of rise time is the time it takes the signal to go from 10% to 90% of the final value. You can choose any percentages you like (from 0% to 100%), provided that the first value is smaller than the second. • Phase Wrapping — By default, the phase is not wrapped. Wrap the phrase by clearing this box. If the phase is wrapped, all phase values are shifted such that their equivalent value displays in the range [-180°, 180°). 7-6 LTI Viewer Preferences Editor Parameters Pane Note Click on the tabs to get help on LTI Viewer Preference editor. Use the Parameters pane, shown below, to specify input vectors for time and frequency simulation. The defaults are to generate time and frequency vectors for your plots automatically. You can, however, override the defaults as follows: • Time Vector - Define stop time — Specify the final time value for your simulation - Define vector — Specify the time vector manually using equal-sized time steps • Frequency Vector - Define range — Specify the bandwidth of your response. Whether it’s in rad/s or Hz depends on the selection you made in the Units pane. - Define vector — Specify the vector for your frequency values. Any real, positive, strictly monotonically increasing vector is valid. 7-7 7 Setting Tool Preferences SISO Design Tool Preferences Editor Note Click on the tabs to get help on SISO Tool Preference editor. To open the SISO Tool Preferences editor, select SISO Tool Preferences from the Edit menu of the SISO Design Tool. This window opens. The SISO Tool Preferences Editor 7-8 SISO Design Tool Preferences Editor Units Pane Note Click on the pane tabs below to get help on SISO Tool Preference editor panes. The Units pane has settings for the following units: • Frequency — Radians per second (rad/s) or Hertz (Hz) • Magnitude — Decibels (dB) or absolute value (abs) • Phase — Degrees or radians For frequency and magnitude axes, you can select logarithmic or linear scales. 7-9 7 Setting Tool Preferences Style Pane Note Click on the tabs to get help on SISO Tool Preference editor. Use the Style pane to toggle grid visibility and set font preferences and axes foreground colors for all plots in the SISO Design Tool. This figure shows the Style pane. Click on the Grids, Fonts, and Colors panels for help contents. Grids Panel Select the box to activate grids for all plots in the SISO Design Tool Fonts Panel Set the font size, weight (bold), and angle (italic) by using the menus and check boxes. Colors Panel Specify the color vector to use for the axes foreground, which includes the X-Y axes, grid lines, and tick labels. Use a three-element vector to represent red, green, and blue (RGB) values. Vector element values can range from 0 to 1. 7-10 SISO Design Tool Preferences Editor Select colors. Click the Select button to open the Select Color window for the axes foreground. You can use this window to choose axes foreground colors without having to set RGB (red-green-blue) values numerically. To make your selections, click on the colored rectangles and press OK. If you want a broader range of colors, click the Define Custom Colors button. This extends the Select Color window, as shown in this figure. You can pick colors from the color spectrum located in the upper right corner of the window. To select a custom color, follow these steps: 7-11 7 Setting Tool Preferences 1 Place your cursor at a point in the color spectrum that has a color you want to define. 2 Left-click. Notice that the hue, saturation, luminescence (lum.), red, green, and blue fields specify the numerical values for the selected color. 3 Press Add to Custom Colors. This adds the selected color to the row of boxes labeled Custom Color. You can now use this color just like the basic colors. Options Pane Note Click on the tabs to get help on SISO Tool Preference editor. The Options pane, shown below, has selections for compensator format and Bode diagrams. You can make the following selections: 7-12 SISO Design Tool Preferences Editor • Compensator Format — Select the time constant, natural frequency, or zero/pole/gain format. The time constant format is a factorization of the compenator transfer function of the form ( 1 + Tz 1 s ) dcgain × ---------------------------- … ( 1 + Tp 1 s ) where Tz1, Tz2, ..., are the zero time constants, and Tp1, Tp2, ..., are the pole time constants. The natural frequency format is ( 1 + s ⁄ ωz1 ) dcgain × ------------------------------------ … ( 1 + s ⁄ ( ω p1 ) ) where ωz1,ωz2, ... and ωp1, ωp2, ..., are the natural frequencies of the zeros and poles, respectively. The zero/pole/gain format is ( s + z1 ) K × -------------------( s + p1 ) where z1, z2, ... and p1, p2, ..., are the zero and pole locations, respectively. • Bode Options — By default, the SISO Design Tool shows the plant and sensor poles and zeros as blue x’s and o’s, respectively. Clear this check box to eliminate the plant’s poles and zeros from the Bode plot. Note that the compensator poles and zeros (in red) will still appear. 7-13 7 Setting Tool Preferences Line Colors Pane Note Click on the pane tabs below to get help on SISO Tool Preference editor. The Line Colors pane, shown below, has selections for specify the colors of the lines in the response plots of the SISO Design Tool. Click on the Select button for help on choosing colors. To change the colors of plot lines associated with parts of your model, specify a three-element vector to represent red, green, and blue (RGB) values. Vector element values can range from 0 to 1. If you do not want to specify the RGB values numerically, click the Select button to open the Select Color window. See “Select colors” on page 7-11 for more information. 7-14 8 Customizing Response Plot Properties Introduction (p. 8-2) An overview of setting response plot properties Response Plots Property Editor (p. 8-4) An editor for setting axes labels and limits, fonts, grids, and so on Property Editing for Subplots (p. 8-11) How to edit individual plots in multi-plot figure window Customizing Plots Inside the SISO Design Tool (p. 8-12) Special considerations for editing root locus, Bode plots, and Nichols plots in the SISO Design Tool 8 Customizing Response Plot Properties Introduction The lowest level of “The Property and Preferences Hierarchy” on page 5-3 is setting response plot properties. This means that any property you set for a given plot will only affect that plot. Once you have created a response plot, there are two ways to open the Property Editor: • Double-click in the plot region. • Select Properties from the right-click menu. Before looking at the Property Editor, open a step response plot using these commands. load ltiexamples step(sys_dc) 8-2 Introduction This creates a step plot. Select Properties from the right-click window. Note that when you open the Property Editor, a black dashed box appears around the step response, as this figure shows. The dashed line indicates that the Property Editor is active for this plot. A SISO System Step Response 8-3 8 Customizing Response Plot Properties Response Plots Property Editor Note Click on the tabs to get help on panes in the Property Editor. This figure shows the Property Editor dialog box for a step response. The Property Editor for the Step Response In general, you can change the following properties of response plots: • Labels — Titles and X- and Y-labels • Limits — Numerical ranges of the X and Y axes • Units — Where applicable (e.g., rad/s to Hertz). If you cannot customize units, as is the case with step responses, the Property Editor will display that no units ar available for the selected plot. • Style — Show a grid and adjust font properties, such as font size, bold and italics • Characteristics — Where applicable, these include peak response, settling time, phase and gain margins, etc. Plot characteristics change with each plot response type. The Property Editor displays only the characteristics that make sense for the selected response plot. For example, phase and gain margins are not available for step responses. As you make changes in the Property Editor, they display immediately in the response plot. Conversely, if you make changes in a plot using right-click 8-4 Response Plots Property Editor menus, the Property Editor for that plot automatically updates. The Property Editor and its associated plot are dynamically linked. Labels Pane Note Click on the tabs below to get help on the Property Editor. To specify new text for plot titles and axis labels, type the new string in the field next to the label you want to change. Note that the label changes immediately as you type, so you can see how the new text looks as you are typing. 8-5 8 Customizing Response Plot Properties Limits Pane Note Click on the tabs to get help on the Property Editor. The Control System Toolbox selects default values for the axes limits to make sure that the maximum and minimum x and y values are displayed. If you want to override the default settings, change the values in the Limits fields. The Auto-Scale box automatically clears if you click on a different field. The new limits appear immediately in the response plot. To reestablish the default values, select the Auto-Scale box again. 8-6 Response Plots Property Editor Units Pane Note Click on the tabs to get help on the Property Editor. You can use the Units pane to change units in your response plot. The contents of this pane depend on the response plot associated with the editor. Note that for step and impulse responses, there are no alternate units available (only time and amplitude are possible in the toolbox). This table lists the options available for the other response objects. Use the menus to toggle between units. Optional Unit Conversions for Response Plots Response Plot Unit Conversions Bode and Bode Magnitude Frequency in rad/s or Hertz (Hz) using logarithmic or linear scale Magnitude in decibels (dB) or the absolute value Phase in degrees or radians Impulse None 8-7 8 Customizing Response Plot Properties Optional Unit Conversions for Response Plots Response Plot Unit Conversions Nichols Chart and Nyquist Diagram Frequency in rad/s or Hertz Magnitude in decibels or the absolute value Phase in degrees or radians Pole/Zero Map Frequency in rad/s or Hertz Singular Values Frequency in rad/s or Hertz using logarithmic or linear scale Magnitude in decibels or the absolute value Step None Style Pane Note Click on the tabs to get help on the Property Editor. Use the Style pane to toggle grid visibility and set font preferences and axes foreground colors for response plots. You have the following choices: 8-8 Response Plots Property Editor • Grid — Activate grids by default in new plots • Fonts — Set the font size, weight (bold), and angle (italic) • Colors — Specify the color vector to use for the axes foreground, which includes the X-Y axes, grid lines, and tick labels. Use a three-element vector to represent red, green, and blue (RGB) values. Vector element values can range from 0 to 1. If you do not want to specify RGB values numerically, click the Select button to open the Select Color window. See “Select colors” on page 7-11 for more information. Characteristics Pane Note Click on the tabs to get help on the Property Editor. The Characteristics pane allows you to customize response characteristics for plots. Each response plot has its own set of characteristics; the table below lists 8-9 8 Customizing Response Plot Properties them. Use the check boxes to activate the feature and the fields to specify rise or settling time percentages. Response Characteristic Options for Response Plots 8-10 Plot Customizable Feature Bode Diagram Show peak response Show minimum stability margins Show all stability margins Unwrap phase (default is wrapped) Bode Magnitude Show peak response Impulse Show peak response Show settling time within xx% (specify the percentage) Nichols Chart Show peak response Show minimum stability margins Show all stability margins Unwrap phase (default is wrapped) Nyquist Diagram Show peak response Show minimum stability margins Show all stability margins Pole/Zero Map None Sigma Show peak response Step Show peak response Show settling time within xx% (specify the percentage) Show rise time from xx to yy% (specify the percentages) Show steady state Property Editing for Subplots Property Editing for Subplots If you create more than one plot in a single figure window, you can edit each plot individually. For example, the following code creates a figure with two plots, a step and an impulse response with two randomly selected systems. subplot(2,1,1) step(rss(2,1)) subplot(2,1,2) impulse(rss(1,1)) After the figure window appears, double-click in the upper (step response) plot to activate the Property Editor. You will see a dashed line appear around the step response, indicating that it is the active plot for the editor. To switch to the lower (impulse response) plot, just click once in the impulse response plot region. The dashed box switches to the impulse response, and the Property Editor updates as well. 8-11 8 Customizing Response Plot Properties Customizing Plots Inside the SISO Design Tool Customizing plots inside the SISO Design Tool is similar to how you customize any response plot. The Control System Toolbox provides the following property editors specific to the SISO Design Tool: • “Root Locus Property Editor” • “Open-Loop Bode Property Editor” on page 8-18 • “Open-Loop Nichols Property Editor” on page 8-22 • “Prefilter Bode Property Editor” on page 8-25 You can use each of these property editors to create the customized plots within the SISO Design tool. Root Locus Property Editor There are three ways to open the Property Editor for root locus plots: • Double-click in the root locus away from the curve • Select Properties from the right-click menu • Select Root Locus and then Properties from Edit in the menu bar Note Click on the tabs to get help on the Root Locus Property Editor. 8-12 Customizing Plots Inside the SISO Design Tool This figure shows the Property Editor: Root Locus window. Labels Pane Note Click on the tabs to get help on the Root Locus Property Editor. 8-13 8 Customizing Response Plot Properties You can use the Label pane to specify plot titles and axis labels. To specify a new label, type the string in the appropriate field. The root locus plot automatically updates. Limits Pane Note Click on the pane tabs below to get help on panes in the Root Locus Property Editor. The SISO Design Tool specifies default values for the real and imaginary axes ranges to make sure that all the poles and zeros in your model appear in the root locus plot. Use the Limits pane, shown below, to override the default settings. To change the limits, specify the new limits in the real and imaginary axes Limits fields. The Auto-Scale check box automatically clears once you click in a different field. Your root locus diagram updates immediately. If you want to reapply the default limits, select the Auto-Scale check boxes again. 8-14 Customizing Plots Inside the SISO Design Tool The Limit Stack panel provides support for storing and retrieving custom limit specifications. There are four buttons available: — Add the current limits to the stack — Retrieve the previous stack entry — Retrieve the next stack entry — Remove the current limits from the stack Using these buttons, you can store and retrieve any number of saved custom axes limits. 8-15 8 Customizing Response Plot Properties Options Pane Note Click on the tabs to get help on the Root Locus Property Editor. The Options pane contains settings for adding a grid and changing the plot’s aspect ratio. This figure shows the Options pane. Select Show grid to display a grid on the root locus. If you have damping ratio constraints on your root locus, selecting Display damping ratios as % peak overshoot displays the damping ratio values along the grid lines. This figure shows both options activated for an imported model, Gservo. If you want to verify these settings, type load ltiexamples 8-16 Customizing Plots Inside the SISO Design Tool at the MATLAB prompt and import Gservo from the workspace into your SISO Design Tool. Displaying Damping Ratio Values The numbers displayed on the root locus gridlines are the damping ratios as a percentage of the overshoot values. If you select the Equal check box in the Aspect Ratio panel, the x and y-axes are set to equal limit values. 8-17 8 Customizing Response Plot Properties Open-Loop Bode Property Editor The Property Editor for open-loop Bode diagrams is identical to the one for root locus, with one exception, the Options pane. Also, note the prefilter and open-loop Bode diagram property editors are identical. As is the case with the root locus Property Editor, there are three ways to open the Bode diagram property editor: • Double-click in the Bode magnitude or phase plot away from the curve • Select Properties from the right-click menu • Select Open-Loop Bode and then Properties from Edit in the menu bar Note Click on the tabs to get help on the Open-Loop Bode Property editor. This figure shows the Property Editor: Open-Loop Bode editor. 8-18 Customizing Plots Inside the SISO Design Tool Labels Pane Note Click on the tabs to get help on the Open-Loop Bode Property editor. You can use the Label pane to specify plot titles and axis labels. To specify a new label, type the string in the appropriate field. The Bode diagram automatically updates. 8-19 8 Customizing Response Plot Properties Limits Pane Note Click on the tabs to get help on the Open-Loop Bode Property editor. The Control System Toolbox sets default limits for the frequency, magnitude, and phase scales for your plots. Use the Limits pane to override the default values. To change the limits, specify the new values in the Limits fields for frequency, magnitude, and phase. The Auto-Scale check box automatically deactivates once you click in a different field. The Bode diagram updates immediately. To restore the default settings, select the Auto-Scale boxes again. 8-20 Customizing Plots Inside the SISO Design Tool Options Pane Note Click on the tabs to get help on the Open-Loop Bode Property editor. This figure shows the Options pane for Bode diagrams. The following options are available from this pane: • Grid — Select Show grid to display grid lines. • Magnitude/Phase — There are three radio buttons; you can toggle between the following displays: - Show magnitude & phase - Show magnitude only - Show phase only • Response Characteristics — Select Show stability margins to display the phase and gain margins on your Bode diagram. The margins appear as brown stems, and the Bode diagram displays the numerical values of the margins in one of the bottom corners of the gain and phase plots. The Bode diagram in “Displaying Damping Ratio Values” on page 8-17 has the stability margins displayed. 8-21 8 Customizing Response Plot Properties Open-Loop Nichols Property Editor As is the case with the root locus Property Editor, there are three ways to open the Nichols plot property editor: • Double-click in the Nichols plot away from the curve • Select Properties from the right-click menu • Select Open-Loop Nichols and then Properties from Edit in the menu bar Note Click on the tabs to get help on the Open-Loop Nichols Property editor. This figure shows the Property Editor: Open-Loop Nichols editor. 8-22 Customizing Plots Inside the SISO Design Tool Labels Pane Note Click on the tabs to get help on the Open-Loop Nichols Property editor. You can use the Label pane to specify plot titles and axis labels. To specify a new label, type the string in the appropriate field. The Nichols plot automatically updates. 8-23 8 Customizing Response Plot Properties Limits Pane Note Click on the tabs to get help on the Open-Loop Nichols Property editor. The Control System Toolbox sets default limits for the frequency, magnitude, and phase scales for your plots. Use the Limits pane to override the default values. To change the limits, specify the new values in the Limits fields for open-loop phase and/or gain. The Auto-Scale check box automatically deactivates once you click in a different field. The Nichols plot updates immediately. To restore the default settings, select the Auto-Scale boxes again. 8-24 Customizing Plots Inside the SISO Design Tool Options Pane Note Click on the tabs to get help on the Open-Loop Nichols Property editor. This figure shows the Options pane for Bode diagrams. The following options are available from this pane: • Grid — Select Show grid to display grid lines. • Response Characteristics — Select Show stability margins to display the phase and gain margins on your Nichols plot. Prefilter Bode Property Editor The Prefilter Bode Property editor is identical to the Open-Loop Bode diagram property editor. There are three ways to open the prefilter editor: • Double-click in the prefilter Bode magnitude or phase plot away from the curve • Select Properties from the right-click menu • Select Prefilter Bode and then Properties from Edit in the menu bar See “Open-Loop Bode Property Editor” on page 8-18 for a description of the features of this editor. 8-25 8 Customizing Response Plot Properties 8-26 9 Design Case Studies Yaw Damper for a 747 Jet Transport (p. 9-2) Illustrating the classical design process (jetdemo.m) Hard-Disk Read/Write Head Controller Using linear quadratic Gaussian techniques to regulate the beam thickness in a steel rolling mill (diskdemo.m) (p. 9-20) LQG Regulation: Rolling Mill Example Illustrating classical digital controller design (p. 9-31) (milldemo.m) Kalman Filtering (p. 9-50) Kalman filtering that illustrates both steady-state and time-varying Kalman filter design and simulation (kalmdemo.m) 9 Design Case Studies Yaw Damper for a 747 Jet Transport This case study demonstrates the tools for classical control design by stepping through the design of a yaw damper for a 747 jet transport aircraft. The jet model during cruise flight at MACH = 0.8 and H = 40,000 ft. is A = [-0.0558 0.5980 -3.0500 0 B = [ 0.0729 -4.7500 .15300 0 C = [0 0 1 0 D = [0 0 0 0]; -0.9968 -0.1150 0.3880 0.0805 0.0802 -0.0318 -0.4650 1.0000 0.0415 0 0 0]; 0.0000 0.00775 0.1430 0]; 0 0 0 1]; The following commands specify this state-space model as an LTI object and attach names to the states, inputs, and outputs. states = {'beta' 'yaw' 'roll' 'phi'}; inputs = {'rudder' 'aileron'}; outputs = {'yaw' 'bank angle'}; sys = ss(A,B,C,D,'statename',states,... 'inputname',inputs,... 'outputname',outputs); You can display the LTI model sys by typing sys. MATLAB responds with a = beta yaw roll phi 9-2 beta -0.0558 0.598 -3.05 0 yaw -0.9968 -0.115 0.388 0.0805 roll 0.0802 -0.0318 -0.465 1 phi 0.0415 0 0 0 Yaw Damper for a 747 Jet Transport b = beta yaw roll phi rudder 0.0729 -4.75 0.153 0 yaw bank angle beta 0 0 aileron 0 0.00775 0.143 0 c = yaw 1 0 roll 0 0 phi 0 1 d = yaw bank angle rudder 0 0 aileron 0 0 Continuous-time model. The model has two inputs and two outputs. The units are radians for beta (sideslip angle) and phi (bank angle) and radians/s for yaw (yaw rate) and roll (roll rate). The rudder and aileron deflections are in radians as well. 9-3 9 Design Case Studies Computing Open-Loop Eigenvalues Compute the open-loop eigenvalues and plot them in the s -plane. damp(sys) Eigenvalue -7.28e-003 -5.63e-001 -3.29e-002 + 9.47e-001i -3.29e-002 - 9.47e-001i Damping Freq. (rad/s) 1.00e+000 1.00e+000 3.48e-002 3.48e-002 7.28e-003 5.63e-001 9.47e-001 9.47e-001 pzmap(sys) This model has one pair of lightly damped poles. They correspond to what is called the “Dutch roll mode.” Suppose you want to design a compensator that increases the damping of these poles, so that the resulting complex poles have a damping ratio ζ > 0.35 with 9-4 Yaw Damper for a 747 Jet Transport natural frequency ω n < 1 rad/s. You can do this using the Control System toolbox analysis tools. Open-Loop Analysis First, perform some open-loop analysis to determine possible control strategies. Start with the time response (you could use step or impulse here). impulse(sys) The impulse response confirms that the system is lightly damped. But the time frame is much too long because the passengers and the pilot are more concerned about the behavior during the first few seconds rather than the first few minutes. Next look at the response over a smaller time frame of 20 seconds. 9-5 9 Design Case Studies impulse(sys,20) Look at the plot from aileron (input 2) to bank angle (output 2). To show only this plot, right-click and choose I/O Selector, then click on the (2,2) entry. The I/O Selector should look like this. 9-6 Yaw Damper for a 747 Jet Transport The new figure is shown below. The aircraft is oscillating around a nonzero bank angle. Thus, the aircraft is turning in response to an aileron impulse. This behavior will prove important later in this case study. Typically, yaw dampers are designed using the yaw rate as sensed output and the rudder as control input. Look at the corresponding frequency response. sys11=sys('yaw','rudder') % Select I/O pair. 9-7 9 Design Case Studies bode(sys11) From this Bode diagram, you can see that the rudder has significant effect around the lightly damped Dutch roll mode (that is, near ω = 1 rad/s). 9-8 Yaw Damper for a 747 Jet Transport Root Locus Design A reasonable design objective is to provide a damping ration ζ > 0.35 with a natural frequency ω n < 1.0 rad/s. Since the simplest compensator is a static gain, first try to determine appropriate gain values using the root locus technique. % Plot the root locus for the rudder to yaw channel rlocus(sys11) This is the root locus for negative feedback and shows that the system goes unstable almost immediately. If, instead, you use positive feedback, you may be able to keep the system stable. rlocus(-sys11) 9-9 9 Design Case Studies sgrid This looks better. By using simple feedback, you can achieve a damping ratio of ζ = 0.45 . Click on the blue curve and move the data marker to track the gain and damping values. To achieve a 0.45 damping ratio, the gain should be about 2.85. This figure shows the data marker with similar values. 9-10 Yaw Damper for a 747 Jet Transport Next, close the SISO feedback loop. K = 2.85; cl11 = feedback(sys11,-K); % Note: feedback assumes negative % feedback by default 9-11 9 Design Case Studies Plot the closed-loop impulse response for a duration of 20 seconds, and compare it to the open-loop impulse response. impulse(sys11,'b--',cl11,'r',20) The closed-loop response settles quickly and does not oscillate much, particularly when compared to the open-loop response. Now close the loop on the full MIMO model and see how the response from the aileron looks. The feedback loop involves input 1 and output 1 of the plant (use feedback with index vectors selecting this input/output pair). At the MATLAB prompt, type cloop = feedback(sys,-K,1,1); damp(cloop) % closed-loop poles Eigenvalue Damping -3.42e-001 -2.97e-001 + 6.06e-001i 9-12 Freq. (rad/s) 1.00e+000 4.40e-001 3.42e-001 6.75e-001 Yaw Damper for a 747 Jet Transport -2.97e-001 - 6.06e-001i -1.05e+000 4.40e-001 1.00e+000 6.75e-001 1.05e+000 Plot the MIMO impulse response. impulse(sys,'b--',cloop,'r',20) The yaw rate response is now well damped, but look at the plot from aileron (input 2) to bank angle (output 2). When you move the aileron, the system no longer continues to bank like a normal aircraft. You have over-stabilized the spiral mode. The spiral mode is typically a very slow mode and allows the aircraft to bank and turn without constant aileron input. Pilots are used to this behavior and will not like your design if it does not allow them to fly normally. This design has moved the spiral mode so that it has a faster frequency. 9-13 9 Design Case Studies Washout Filter Design What you need to do is make sure the spiral mode does not move further into the left-half plane when you close the loop. One way flight control designers have addressed this problem is to use a washout filter kH ( s ) where s H ( s ) = -----------s+a The washout filter places a zero at the origin, which constrains the spiral mode pole to remain near the origin. We choose a = 0.2 for a time constant of five seconds and use the root locus technique to select the filter gain H. First specify the fixed part s ⁄ ( s + a ) of the washout by H = zpk(0,-0.2,1); Connect the washout in series with the design model sys11 (relation between input 1 and output 1) to obtain the open-loop model oloop = H * sys11; and draw another root locus for this open-loop model. rlocus(-oloop) 9-14 Yaw Damper for a 747 Jet Transport sgrid Create and drag a data marker around the upper curve to locate the maximum damping, which is about ζ = 0.3 . 9-15 9 Design Case Studies This figure shows a data marker at the maximum damping ratio; the gain is approximately 2.07. Look at the closed-loop response from rudder to yaw rate. K = 2.07; cl11 = feedback(oloop,-K); 9-16 Yaw Damper for a 747 Jet Transport impulse(cl11,20) The response settles nicely but has less damping than your previous design. Finally, you can verify that the washout filter has fixed the spiral mode problem. First form the complete washout filter kH ( s ) (washout + gain). WOF = -K * H; Then close the loop around the first I/O pair of the MIMO model sys and simulate the impulse response. cloop = feedback(sys,WOF,1,1); % Final closed-loop impulse response 9-17 9 Design Case Studies impulse(sys,'b--',cloop,'r',20) 9-18 Yaw Damper for a 747 Jet Transport The bank angle response (output 2) due to an aileron impulse (input 2) now has the desired nearly constant behavior over this short time frame. To inspect the response more closely, use the I/O Selector in the right-click menu to select the (2,2) I/O pair. Although you did not quite meet the damping specification, your design has increased the damping of the system substantially and now allows the pilot to fly the aircraft normally. 9-19 9 Design Case Studies Hard-Disk Read/Write Head Controller Hard Disk Drive Disk Platen Ω Disk Drive Motor Read/Write Head l θ Solenoid This case study demonstrates the ability to perform classical digital control design by going through the design of a computer hard-disk read/write head position controller. Deriving the Model Using Newton’s law, a simple model for the read/write head is the differential equation 2 dθ d θ J ---------- + C ------- + Kθ = K i i 2 dt dt where J is the inertia of the head assembly, C is the viscous damping coefficient of the bearings, K is the return spring constant, K i is the motor torque constant, θ is the angular position of the head, and i is the input current. Taking the Laplace transform, the transfer function from i to θ is Ki H ( s ) = --------------------------------2 Js + Cs + K 9-20 Hard-Disk Read/Write Head Controller 2 Using the values J = 0.01 kg m , C = 0.004 Nm/(rad/s), K = 10 Nm/rad, and K i = 0.05 Nm/rad, form the transfer function description of this system. At the MATLAB prompt, type J = num den H = .01; C = 0.004; K = 10; Ki = .05; = Ki; = [J C K]; tf(num,den) MATLAB responds with Transfer function: 0.05 ----------------------0.01 s^2 + 0.004 s + 10 Model Discretization The task here is to design a digital controller that provides accurate positioning of the read/write head. The design is performed in the digital domain. First, discretize the continuous plant. Because our plant will be equipped with a digital-to-analog converter (with a zero-order hold) connected to its input, use c2d with the 'zoh' discretization method. Type Ts = 0.005; % sampling period = 0.005 second Hd = c2d(H,Ts,'zoh') Transfer function: 6.233e-05 z + 6.229e-05 ----------------------z^2 - 1.973 z + 0.998 Sampling time: 0.005 You can compare the Bode plots of the continuous and discretized models with 9-21 9 Design Case Studies bode(H,'-',Hd,'--') To analyze the discrete system, plot its step response, type step(Hd) 9-22 Hard-Disk Read/Write Head Controller The system oscillates quite a bit. This is probably due to very light damping. You can check this by computing the open-loop poles. Type % Open-loop poles of discrete model damp(Hd) Eigenvalue 9.87e-01 + 1.57e-01i 9.87e-01 - 1.57e-01i Magnitude 9.99e-01 9.99e-01 Equiv. Damping 6.32e-03 6.32e-03 Equiv. Freq. 3.16e+01 3.16e+01 The poles have very light equivalent damping and are near the unit circle. You need to design a compensator that increases the damping of these poles. 9-23 9 Design Case Studies Adding a Compensator Gain The simplest compensator is just a gain, so try the root locus technique to select an appropriate feedback gain. rlocus(Hd) As shown in the root locus, the poles quickly leave the unit circle and go unstable. You need to introduce some lead or a compensator with some zeros. Adding a Lead Network Try the compensator z+a D ( z ) = ----------z+b with a = – 0.85 and b = 0 . 9-24 Hard-Disk Read/Write Head Controller The corresponding open-loop model u D( z) Hd( z ) Compensator Plant y is obtained by the series connection D = zpk(0.85,0,1,Ts) oloop = Hd * D Now see how this compensator modifies the open-loop frequency response. bode(Hd,'--',oloop,'-') The plant response is the dashed line and the open-loop response with the compensator is the solid line. 9-25 9 Design Case Studies The plot above shows that the compensator has shifted up the phase plot (added lead) in the frequency range ω > 10 rad/s. Now try the root locus again with the plant and compensator as open loop. rlocus(oloop) zgrid Open the Property Editor by right-clicking in the plot away from the curve. On the Limits page, set the x- and y-axis limits from -1 to 1.01. This figure shows the result. This time, the poles stay within the unit circle for some time (the lines drawn by zgrid show the damping ratios from ζ = 0 to 1 in steps of 0.1). Use a data marker to find the point on the curve where the gain equals 4.111e+03. 9-26 Hard-Disk Read/Write Head Controller This figure shows the data marker at the correct location. Design Analysis To analyze this design, form the closed-loop system and plot the closed-loop step response. K = 4.11e+03; cloop = feedback(oloop,K); 9-27 9 Design Case Studies step(cloop) This response depends on your closed loop set point. The one shown here is relatively fast and settles in about 0.07 seconds. Therefore, this closed loop disk drive system has a seek time of about 0.07 seconds. This is slow by today's standards, but you also started with a very lightly damped system. Now look at the robustness of your design. The most common classical robustness criteria are the gain and phase margins. Use the function margin to determine these margins. With output arguments, margin returns the gain and phase margins as well as the corresponding crossover frequencies. Without output argument, margin plots the Bode response and displays the margins graphically. To compute the margins, first form the unity-feedback open loop by connecting the compensator D ( z ) , plant model, and feedback gain k in series. olk = K * oloop; 9-28 Hard-Disk Read/Write Head Controller oloop u + D(z) – Plant y k Next apply margin to this open-loop model. Type [Gm,Pm,Wcg,Wcp] = margin(olk); Margins = [Gm Wcg Pm Wcp] Margins = 3.7987 296.7978 43.2031 106.2462 To obtain the gain margin in dB, type 20*log10(Gm) ans = 11.5926 You can also display the margins graphically by typing margin(olk) 9-29 9 Design Case Studies The command produces the plot shown below. This design is robust and can tolerate a 11 dB gain increase or a 40 degree phase lag in the open-loop system without going unstable. By continuing this design process, you may be able to find a compensator that stabilizes the open-loop system and allows you to reduce the seek time. 9-30 LQG Regulation: Rolling Mill Example LQG Regulation: Rolling Mill Example This case study demonstrates the use of the LQG design tools in a process control application. The goal is to regulate the horizontal and vertical thickness of the beam produced by a hot steel rolling mill. This example is adapted from [1]. The full plant model is MIMO and the example shows the advantage of direct MIMO LQG design over separate SISO designs for each axis. Type milldemo at the command line to run this demonstration interactively. Process and Disturbance Models The rolling mill is used to shape rectangular beams of hot metal. The desired outgoing shape is sketched below. Rolling cylinders y x Shaped beam 9-31 9 Design Case Studies This shape is impressed by two pairs of rolling cylinders (one per axis) positioned by hydraulic actuators. The gap between the two cylinders is called the roll gap. Rolling mill stand Incoming beam Shaped beam x-axis Rolling cylinders The objective is to maintain the beam thickness along the x- and y-axes within the quality assurance tolerances. Variations in output thickness can arise from the following: • Variations in the thickness/hardness of the incoming beam • Eccentricity in the rolling cylinders Feedback control is necessary to reduce the effect of these disturbances. Because the roll gap cannot be measured close to the mill stand, the rolling force is used instead for feedback. The input thickness disturbance is modeled as a low pass filter driven by white noise. The eccentricity disturbance is approximately periodic and its frequency is a function of the rolling speed. A reasonable model for this disturbance is a second-order bandpass filter driven by white noise. 9-32 LQG Regulation: Rolling Mill Example This leads to the following generic model for each axis of the rolling process. Open-loop Model for x- or y-axis H( s) u + hydraulic actuator f1 – + + δ force-to-gap gain Fe ( s ) we gx eccentricity model Fi ( s ) wi f2 + f + input disturbance model u δ f w i, w e command thickness gap (in mm) incremental rolling force driving white noise for disturbance models The measured rolling force variation f is a combination of the incremental force delivered by the hydraulic actuator and of the disturbance forces due to eccentricity and input thickness variation. Note that • The outputs of H ( s ), F e ( s ) , and F i ( s ) are the incremental forces delivered by each component. • An increase in hydraulic or eccentricity force reduces the output thickness gap δ . • An increase in input thickness increases this gap. The model data for each axis is summarized below. 9-33 9 Design Case Studies Model Data for the x-Axis 8 2.4 × 10 H x ( s ) = -----------------------------------2 2 s + 72s + 90 4 10 F ix ( s ) = ------------------s + 0.05 4 3 × 10 s F ex ( s ) = ----------------------------------------2 2 s + 0.125s + 6 g x = 10 –6 Model Data for the y-Axis 8 7.8 × 10 H y ( s ) = ------------------------------------2 2 s + 71s + 88 4 2 × 10 F iy ( s ) = -------------------s + 0.05 5 10 s F ey ( s ) = -------------------------------------------2 2 s + 0.19s + 9.4 g y = 0.5 × 10 –6 LQG Design for the x-Axis As a first approximation, ignore the cross-coupling between the x- and y-axes and treat each axis independently. That is, design one SISO LQG regulator for each axis. The design objective is to reduce the thickness variations δ x and δ y due to eccentricity and input thickness disturbances. Start with the x -axis. First specify the model components as transfer function objects. % Hydraulic actuator (with input "u-x") Hx = tf(2.4e8,[1 72 90^2],'inputname','u-x') 9-34 LQG Regulation: Rolling Mill Example % Input thickness/hardness disturbance model Fix = tf(1e4,[1 0.05],'inputn','w-ix') % Rolling eccentricity model Fex = tf([3e4 0],[1 0.125 6^2],'inputn','w-ex') % Gain from force to thickness gap gx = 1e-6; Next build the open-loop model shown in “Open-loop Model for x- or y-axis” above. You could use the function connect for this purpose, but it is easier to build this model by elementary append and series connections. % I/O map from inputs to forces f1 and f2 Px = append([ss(Hx) Fex],Fix) % Add static gain from f1,f2 to outputs Px = [-gx gx;1 1] * Px x-gap and x-force % Give names to the outputs: set(Px,'outputn',{'x-gap' 'x-force'}) Note To obtain minimal state-space realizations, always convert transfer function models to state space before connecting them. Combining transfer functions and then converting to state space may produce nonminimal state-space models. The variable Px now contains an open-loop state-space model complete with input and output names. Px.inputname ans = 'u-x' 'w-ex' 'w-ix' 9-35 9 Design Case Studies Px.outputname ans = 'x-gap' 'x-force' The second output 'x-force' is the rolling force measurement. The LQG regulator will use this measurement to drive the hydraulic actuator and reduce disturbance-induced thickness variations δ x . The LQG design involves two steps: 1 Design a full-state-feedback gain that minimizes an LQ performance measure of the form J ( ux ) = ∞ 2 ∫0 qδx + rux dt 2 2 Design a Kalman filter that estimates the state vector given the force measurements 'x-force'. The performance criterion J ( u x ) penalizes low and high frequencies equally. Because low-frequency variations are of primary concern, eliminate the high-frequency content of δ x with the low-pass filter 30 ⁄ ( s + 30 ) and use the filtered value in the LQ performance criterion. lpf = tf(30,[1 30]) % Connect low-pass filter to first output of Px Pxdes = append(lpf,1) * Px set(Pxdes,'outputn',{'x-gap*' 'x-force'}) % Design the state-feedback gain using LQRY and q=1, r=1e-4 kx = lqry(Pxdes(1,1),1,1e-4) 9-36 LQG Regulation: Rolling Mill Example Note lqry expects all inputs to be commands and all outputs to be measurements. Here the command 'u-x' and the measurement 'x-gap*' (filtered gap) are the first input and first output of Pxdes. Hence, use the syntax Pxdes(1,1) to specify just the I/O relation between 'u-x' and 'x-gap*'. Next, design the Kalman estimator with the function kalman. The process noise wx = w ex w ix has unit covariance by construction. Set the measurement noise covariance to 1000 to limit the high frequency gain, and keep only the measured output 'x-force' for estimator design. estx = kalman(Pxdes(2,:),eye(2),1000) Finally, connect the state-feedback gain kx and state estimator estx to form the LQG regulator. Regx = lqgreg(estx,kx) This completes the LQG design for the x -axis. Look at the regulator Bode response between 0.1 and 1000 rad/s. 9-37 9 Design Case Studies bode(Regx,{0.1 1000}) The phase response has an interesting physical interpretation. First, consider an increase in input thickness. This low-frequency disturbance boosts both output thickness and rolling force. Because the regulator phase is approximately 0o at low frequencies, the feedback loop then adequately reacts by increasing the hydraulic force to offset the thickness increase. Now consider the effect of eccentricity. Eccentricity causes fluctuations in the roll gap (gap between the rolling cylinders). When the roll gap is minimal, the rolling force increases and the beam thickness diminishes. The hydraulic force must then be reduced (negative force feedback) to restore the desired thickness. This is exactly what the LQG regulator does as its phase drops to -180o near the natural frequency of the eccentricity disturbance (6 rad/s). 9-38 LQG Regulation: Rolling Mill Example Next, compare the open- and closed-loop responses from disturbance to thickness gap. Use feedback to close the loop. To help specify the feedback connection, look at the I/O names of the plant Px and regulator Regx. Px.inputname ans = 'u-x' 'w-ex' 'w-ix' Regx.outputname ans = 'u-x' Px.outputname ans = 'x-gap' 'x-force' Regx.inputname ans = 'x-force' This indicates that you must connect the first input and second output of Px to the regulator. clx = feedback(Px,Regx,1,2,+1) % Note: +1 for positive feedback You are now ready to compare the open- and closed-loop Bode responses from disturbance to thickness gap. 9-39 9 Design Case Studies bode(Px(1,2:3),'--',clx(1,2:3),'-',{0.1 100}) The dashed lines show the open-loop response. Note that the peak gain of the eccentricity-to-gap response and the low-frequency gain of the input-thickness-to-gap response have been reduced by about 20 dB. Finally, use lsim to simulate the open- and closed-loop time responses to the white noise inputs w ex and w ix . Choose dt=0.01 as sampling period for the simulation, and derive equivalent discrete white noise inputs for this sampling rate. dt = 0.01 t = 0:dt:50 % time samples % Generate unit-covariance driving noise wx = [w-ex;w-ix]. % Equivalent discrete covariance is 1/dt wx = sqrt(1/dt) * randn(2,length(t)) 9-40 LQG Regulation: Rolling Mill Example lsim(Px(1,2:3),':',clx(1,2:3),'-',wx,t) The dotted lines correspond to the open-loop response. In this simulation, the LQG regulation reduces the peak thickness variation by a factor 4. 9-41 9 Design Case Studies LQG Design for the y-Axis The LQG design for the y -axis (regulation of the y thickness) follows the exact same steps as for the x -axis. % Specify model components Hy = tf(7.8e8,[1 71 88^2],'inputn','u-y') Fiy = tf(2e4,[1 0.05],'inputn','w-iy') Fey = tf([1e5 0],[1 0.19 9.4^2],'inputn','w-ey') gy = 0.5e-6 % force-to-gap gain % Build open-loop model Py = append([ss(Hy) Fey],Fiy) Py = [-gy gy;1 1] * Py set(Py,'outputn',{'y-gap' 'y-force'}) % State-feedback gain design Pydes = append(lpf,1) * Py % Add low-freq. weigthing set(Pydes,'outputn',{'y-gap*' 'y-force'}) ky = lqry(Pydes(1,1),1,1e-4) % Kalman estimator design esty = kalman(Pydes(2,:),eye(2),1e3) % Form SISO LQG regulator for y-axis and close the loop Regy = lqgreg(esty,ky) cly = feedback(Py,Regy,1,2,+1) Compare the open- and closed-loop response to the white noise input disturbances. dt = 0.01 t = 0:dt:50 wy = sqrt(1/dt) * randn(2,length(t)) 9-42 LQG Regulation: Rolling Mill Example lsim(Py(1,2:3),':',cly(1,2:3),'-',wy,t) The dotted lines correspond to the open-loop response. The simulation results are comparable to those for the x -axis. Cross-Coupling Between Axes The x / y thickness regulation, is a MIMO problem. So far you have treated each axis separately and closed one SISO loop at a time. This design is valid as long as the two axes are fairly decoupled. Unfortunately, this rolling mill process exhibits some degree of cross-coupling between axes. Physically, an increase in hydraulic force along the x -axis compresses the material, which in turn boosts the repelling force on the y -axis cylinders. The result is an increase in y -thickness and an equivalent (relative) decrease in hydraulic force along the y-axis. 9-43 9 Design Case Studies The figure below shows the coupling. Coupling Between the x- and y-axes + δx + ux x-axis w ex model gx w ix – + g xy g yx + – uy fx fy y-axis w ey w iy model gy + + δy g xy = 0.1 g yx = 0.4 Accordingly, the thickness gaps and rolling forces are related to the outputs δ x, f x, ... of the x- and y-axis models by 9-44 LQG Regulation: Rolling Mill Example δx δy fx 0 0 gyx g x δx 0 1 g xy g y 0 δy 0 0 1 – g yx fx 0 0 – g xy 1 fy fy = 1 cross-coupling matrix Let’s see how the previous “decoupled” LQG design fares when cross-coupling is taken into account. To build the two-axes model, shown in “Coupling Between the x- and y-axes” above, append the models Px and Py for the x - and y-axes. P = append(Px,Py) For convenience, reorder the inputs and outputs so that the commands and thickness gaps appear first. P = P([1 3 2 4],[1 4 2 3 5 6]) P.outputname ans = 'x-gap' 'y-gap' 'x-force' 'y-force' Finally, place the cross-coupling matrix in series with the outputs. gxy = 0.1; gyx = 0.4; CCmat = [eye(2) [0 gyx*gx;gxy*gy 0] ; zeros(2) [1 -gyx;-gxy 1]] Pc = CCmat * P Pc.outputname = P.outputname To simulate the closed-loop response, also form the closed-loop model by feedin = 1:2 % first two inputs of Pc are the commands feedout = 3:4 % last two outputs of Pc are the measurements cl = feedback(Pc,append(Regx,Regy),feedin,feedout,+1) You are now ready to simulate the open- and closed-loop responses to the driving white noises wx (for the x-axis) and wy (for the y-axis). 9-45 9 Design Case Studies wxy = [wx ; wy] lsim(Pc(1:2,3:6),':',cl(1:2,3:6),'-',wxy,t) The response reveals a severe deterioration in regulation performance along the x-axis (the peak thickness variation is about four times larger than in the simulation without cross-coupling). Hence, designing for one loop at a time is inadequate for this level of cross-coupling, and you must perform a joint-axis MIMO design to correctly handle coupling effects. 9-46 LQG Regulation: Rolling Mill Example MIMO LQG Design Start with the complete two-axis state-space model Pc derived above. The model inputs and outputs are Pc.inputname ans = 'u-x' 'u-y' 'w-ex' 'w-ix' 'w_ey' 'w_iy' P.outputname ans = 'x-gap' 'y-gap' 'x-force' 'y-force' As earlier, add low-pass filters in series with the 'x-gap' and 'y-gap' outputs to penalize only low-frequency thickness variations. Pdes = append(lpf,lpf,eye(2)) * Pc Pdes.outputn = Pc.outputn Next, design the LQ gain and state estimator as before (there are now two commands and two measurements). k = lqry(Pdes(1:2,1:2),eye(2),1e-4*eye(2)) est = kalman(Pdes(3:4,:),eye(4),1e3*eye(2)) RegMIMO = lqgreg(est,k) % LQ gain % Kalman estimator % form MIMO LQG regulator 9-47 9 Design Case Studies The resulting LQG regulator RegMIMO has two inputs and two outputs. RegMIMO.inputname ans = 'x-force' 'y-force' RegMIMO.outputname ans = 'u-x' 'u-y' Plot its singular value response (principal gains). sigma(RegMIMO) 9-48 LQG Regulation: Rolling Mill Example Next, plot the open- and closed-loop time responses to the white noise inputs (using the MIMO LQG regulator for feedback). % Form the closed-loop model cl = feedback(Pc,RegMIMO,1:2,3:4,+1); % Simulate with lsim using same noise inputs lsim(Pc(1:2,3:6),':',cl(1:2,3:6),'-',wxy,t) The MIMO design is a clear improvement over the separate SISO designs for each axis. In particular, the level of x / y thickness variation is now comparable to that obtained in the decoupled case. This example illustrates the benefits of direct MIMO design for multivariable systems. 9-49 9 Design Case Studies Kalman Filtering This final case study illustrates the use of the Control System Toolbox for Kalman filter design and simulation. Both steady-state and time-varying Kalman filters are considered. Consider the discrete plant x [ n + 1 ] = Ax [ n ] + B ( u [ n ] + w [ n ] ) y [ n ] = Cx [ n ] with additive Gaussian noise w [ n ] on the input u [ n ] and data A = [1.1269 1.0000 0 -0.4940 0 1.0000 0.1129 0 0]; B = [-0.3832 0.5919 0.5191]; C = [1 0 0]; Our goal is to design a Kalman filter that estimates the output y [ n ] given the inputs u [ n ] and the noisy output measurements y v [ n ] = Cx [ n ] + v [ n ] where v [ n ] is some Gaussian white noise. Discrete Kalman Filter The equations of the steady-state Kalman filter for this problem are given as follows. • Measurement update x̂ [ n n ] = x̂ [ n n – 1 ] + M ( y v [ n ] – Cx̂ [ n n – 1 ] ) • Time update x̂ [ n + 1 n ] = Ax̂ [ n n ] + Bu [ n ] 9-50 Kalman Filtering In these equations, • x̂ [ n n – 1 ] is the estimate of x [ n ] given past measurements up to y v [ n – 1 ] • x̂ [ n n ] is the updated estimate based on the last measurement y v [ n ] Given the current estimate x̂ [ n n ] , the time update predicts the state value at the next sample n + 1 (one-step-ahead predictor). The measurement update then adjusts this prediction based on the new measurement y v [ n + 1 ] . The correction term is a function of the innovation, that is, the discrepancy. y v [ n + 1 ] – Cx̂ [ n + 1 n ] = C ( x [ n + 1 ] – x̂ [ n + 1 n ] ) between the measured and predicted values of y [ n + 1 ] . The innovation gain M is chosen to minimize the steady-state covariance of the estimation error given the noise covariances T E ( w [ n ]w [ n ] ) = Q , T E ( v [ n ]v [ n ] ) = R You can combine the time and measurement update equations into one state-space model (the Kalman filter). x̂ [ n + 1 n ] = A ( I – MC ) x̂ [ n n – 1 ] + B AM u[n] yv [ n ] ŷ [ n n ] = C ( I – MC ) x̂ [ n n – 1 ] + CM y v [ n ] This filter generates an optimal estimate ŷ [ n n ] of y [ n ] . Note that the filter state is x̂ [ n n – 1 ] . Steady-State Design You can design the steady-state Kalman filter described above with the function kalman. First specify the plant model with the process noise. x [ n + 1 ] = Ax [ n ] + Bu [ n ] + Bw [ n ] y [ n ] = Cx [ n ] (state equation) (measurement equation) This is done by % Note: set sample time to -1 to mark model as discrete Plant = ss(A,[B B],C,0,-1,'inputname',{'u' 'w'},... 9-51 9 Design Case Studies 'outputname','y'); Assuming that Q = R = 1 , you can now design the discrete Kalman filter by Q = 1; R = 1; [kalmf,L,P,M] = kalman(Plant,Q,R); This returns a state-space model kalmf of the filter as well as the innovation gain M M = 3.7980e-01 8.1732e-02 -2.5704e-01 The inputs of kalmf are u and y v , and its outputs are the plant output and state estimates y e = ŷ [ n n ] and x̂ [ n n ] . u ye kalmf yv x̂ [ n n ] Kalman filter Because you are interested in the output estimate y e , keep only the first output of kalmf. Type kalmf = kalmf(1,:); kalmf a = x1_e x2_e x3_e x1_e 0.7683 0.6202 -0.081732 x2_e -0.494 0 1 x1_e u -0.3832 y 0.3586 b = 9-52 x3_e 0.1129 0 0 Kalman Filtering x2_e x3_e 0.5919 0.5191 0.3798 0.081732 y_e x1_e 0.6202 x2_e 0 y_e u 0 y 0.3798 c = x3_e 0 d = I/O groups: Group name KnownInput Measurement OutputEstimate I/O I I O Channel(s) 1 2 1 Sampling time: unspecified Discrete-time model. To see how the filter works, generate some input data and random noise and compare the filtered response y e with the true response y . You can either generate each response separately, or generate both together. To simulate each response separately, use lsim with the plant alone first, and then with the plant and filter hooked up together. The joint simulation alternative is detailed next. The block diagram below shows how to generate both true and filtered outputs. u Plant Process noise y yv Kalman filter ye Sensor noise y 9-53 9 Design Case Studies You can construct a state-space model of this block diagram with the functions parallel and feedback. First build a complete plant model with u, w, v as inputs y and y v (measurements) as outputs. a b c d P = = = = = A; [B B 0*B]; [C;C]; [0 0 0;0 0 1]; ss(a,b,c,d,-1,'inputname',{'u' 'w' 'v'},... 'outputname',{'y' 'yv'}); Then use parallel to form the following parallel connection. w y v P yv u Kalman yv filter ye sys = parallel(P,kalmf,1,1,[],[]) Finally, close the sensor loop by connecting the plant output y v to the filter input y v with positive feedback. % Close loop around input #4 and output #2 SimModel = feedback(sys,1,4,2,1) % Delete yv from I/O list SimModel = SimModel([1 3],[1 2 3]) The resulting simulation model has w, v, u as inputs and y, y e as outputs. SimModel.inputname ans = 'w' 9-54 Kalman Filtering 'v' 'u' SimModel.outputname ans = 'y' 'y_e' You are now ready to simulate the filter behavior. Generate a sinusoidal input u and process and measurement noise vectors w and v . t = [0:100]'; u = sin(t/5); n = length(t) randn('seed',0) w = sqrt(Q)*randn(n,1); v = sqrt(R)*randn(n,1); Now simulate with lsim. [out,x] = lsim(SimModel,[w,v,u]); y = out(:,1); ye = out(:,2); yv = y + v; % true response % filtered response % measured response and compare the true and filtered responses graphically. subplot(211), plot(t,y,'--',t,ye,'-'), xlabel('No. of samples'), ylabel('Output') title('Kalman filter response') subplot(212), plot(t,y-yv,'-.',t,y-ye,'-'), xlabel('No. of samples'), ylabel('Error') 9-55 9 Design Case Studies The first plot shows the true response y (dashed line) and the filtered output y e (solid line). The second plot compares the measurement error (dash-dot) with the estimation error (solid). This plot shows that the noise level has been significantly reduced. This is confirmed by the following error covariance computations. MeasErr = y-yv; MeasErrCov = sum(MeasErr.*MeasErr)/length(MeasErr); EstErr = y-ye; EstErrCov = sum(EstErr.*EstErr)/length(EstErr); The error covariance before filtering (measurement error) is MeasErrCov MeasErrCov = 1.1138 while the error covariance after filtering (estimation error) is only 9-56 Kalman Filtering EstErrCov EstErrCov = 0.2722 Time-Varying Kalman Filter The time-varying Kalman filter is a generalization of the steady-state filter for time-varying systems or LTI systems with nonstationary noise covariance. Given the plant state and measurement equations x [ n + 1 ] = Ax [ n ] + Bu [ n ] + Gw [ n ] y v [ n ] = Cx [ n ] + v [ n ] the time-varying Kalman filter is given by the recursions • Measurement update x̂ [ n n ] = x̂ [ n n – 1 ] + M [ n ] ( y v [ n ] – Cx̂ [ n n – 1 ] ) T –1 T M [ n ] = P [ n n – 1 ]C ( R [ n ] + CP [ n n – 1 ]C ) P [ n n ] = ( I – M [ n ]C ) P [ n n – 1 ] • Time update x̂ [ n + 1 n ] = Ax̂ [ n n ] + Bu [ n ] T P [ n + 1 n ] = AP [ n n ]A + GQ [ n ]G T with x̂ [ n n – 1 ] and x̂ [ n n ] as defined in “Discrete Kalman Filter” and in the following. T Q [ n ] = E ( w [ n ]w [ n ] ) T R [ n ] = E ( v [ n ]v [ n ] ) T P[ n n] = E({ x [ n] – x [ n n ] }{ x [ n] – x [ n n ]} ) T P[ n n – 1] = E( { x[ n] – x[ n n – 1] }{ x [n ] – x [n n – 1 ]} ) 9-57 9 Design Case Studies For simplicity, we have dropped the subscripts indicating the time dependence of the state-space matrices. Given initial conditions x [ 1 0 ] and P [ 1 0 ] , you can iterate these equations to perform the filtering. Note that you must update both the state estimates x [ n . ] and error covariance matrices P [ n . ] at each time sample. Time-Varying Design Although the Control System Toolbox does not offer specific commands to perform time-varying Kalman filtering, it is easy to implement the filter recursions in MATLAB. This section shows how to do this for the stationary plant considered above. First generate noisy output measurements % Use process noise w and measurement noise v generated above sys = ss(A,B,C,0,-1); y = lsim(sys,u+w); % w = process noise yv = y + v; % v = measurement noise Given the initial conditions x[ 1 0] = 0 , P [ 1 0 ] = BQB T you can implement the time-varying filter with the following for loop. P = B*Q*B'; % Initial error covariance x = zeros(3,1); % Initial condition on the state ye = zeros(length(t),1); ycov = zeros(length(t),1); for i=1:length(t) % Measurement update Mn = P*C'/(C*P*C'+R); x = x + Mn*(yv(i)-C*x); P = (eye(3)-Mn*C)*P; ye(i) = C*x; errcov(i) = C*P*C'; % Time update 9-58 % x[n|n] % P[n|n] Kalman Filtering x = A*x + B*u(i); P = A*P*A' + B*Q*B'; end % x[n+1|n] % P[n+1|n] You can now compare the true and estimated output graphically. subplot(211), plot(t,y,'--',t,ye,'-') title('Time-varying Kalman filter response') xlabel('No. of samples'), ylabel('Output') subplot(212), plot(t,y-yv,'-.',t,y-ye,'-') xlabel('No. of samples'), ylabel('Output') The first plot shows the true response y (dashed line) and the filtered response y e (solid line). The second plot compares the measurement error (dash-dot) with the estimation error (solid). The time-varying filter also estimates the covariance errcov of the estimation error y – y e at each sample. Plot it to see if your filter reached steady state (as you expect with stationary input noise). 9-59 9 Design Case Studies subplot(211) plot(t,errcov), ylabel('Error covar') From this covariance plot, you can see that the output covariance did indeed reach a steady state in about five samples. From then on, your time-varying filter has the same performance as the steady-state version. Compare with the estimation error covariance derived from the experimental data. Type EstErr = y-ye; EstErrCov = sum(EstErr.*EstErr)/length(EstErr) EstErrCov = 0.2718 This value is smaller than the theoretical value errcov and close to the value obtained for the steady-state design. Finally, note that the final value M [ n ] and the steady-state value M of the innovation gain matrix coincide. 9-60 Kalman Filtering Mn, M Mn = 0.3798 0.0817 -0.2570 M = 0.3798 0.0817 -0.2570 References [1] Grimble, M.J., Robust Industrial Control: Optimal Design Approach for Polynomial Systems, Prentice Hall, 1994, p. 261 and pp. 443-456. 9-61 9 Design Case Studies 9-62 10 Reliable Computations Introduction (p. 10-2) The general problem of achieving reliable computations Conditioning and Numerical Stability (p. 10-4) The key concepts of the conditioning of problems and the stability of algorithms Choice of LTI Model (p. 10-8) The implications of LTI Model format choice Scaling (p. 10-15) How to scale matrices to improve their condition numbers Summary (p. 10-17) The important points to remember References (p. 10-18) Relevant control and numerics literature 10 Reliable Computations Introduction When working with low-order SISO models (less than five states), computers are usually quite forgiving and insensitive to numerical problems. You generally won’t encounter any numerical difficulties and MATLAB will give you accurate answers regardless of the model or conversion method you choose. For high order SISO models and MIMO models, however, the finite-precision arithmetic of a computer is not so forgiving and you must exercise caution. In general, to get a numerically accurate answer from a computer, you need • A well-conditioned problem • An algorithm that is numerically stable in finite-precision arithmetic • A good software implementation of the algorithm A problem is said to be well-conditioned if small changes in the data cause only small corresponding changes in the solution. If small changes in the data have the potential to induce large changes in the solution, the problem is said to be ill-conditioned. An algorithm is numerically stable if it does not introduce any more sensitivity to perturbation than is already inherent in the problem. Many numerical linear algebra algorithms can be shown to be backward stable; i.e., the computed solution can be shown to be (near) the exact solution of a slightly perturbed original problem. The solution of a slightly perturbed original problem will be close to the true solution if the problem is well-conditioned. Thus, a stable algorithm cannot be expected to solve an ill-conditioned problem any more accurately than the data warrant, but an unstable algorithm can produce poor solutions even to well-conditioned problems. For further details and references to the literature see [5]. While most of the tools in the Control System Toolbox use reliable algorithms, some of the tools do not use stable algorithms and some solve ill-conditioned problems. These unreliable tools work quite well on some problems (low-order systems) but can encounter numerical difficulties, often severe, when pushed on higher-order problems. These tools are provided because • They are quite useful for low-order systems, which form the bulk of real-world engineering problems. • Many control engineers think in terms of these tools. • A more reliable alternative tool is usually available in this toolbox. 10-2 Introduction • They are convenient for pedagogical purposes. At the same time, it is important to appreciate the limitations of computer analyses. By following a few guidelines, you can avoid certain tools and models when they are likely to get you into trouble. The following sections try to illustrate, through examples, some of the numerical pitfalls to be avoided. We also encourage you to get the most out of the good algorithms by ensuring, if possible, that your models give rise to problems that are well-conditioned. 10-3 10 Reliable Computations Conditioning and Numerical Stability Two of the key concepts in numerical analysis are the conditioning of problems and the stability of algorithms. Conditioning Consider the linear system Ax = b given by A = 0.7800 0.9130 0.5630 0.6590 b = 0.2170 0.2540 The true solution is x = [1, -1]' and you can calculate it approximately using MATLAB. x = A\b x = 1.0000 -1.0000 format long, x x = 0.99999999991008 -0.99999999987542 Of course, in real problems you almost never have the luxury of knowing the true solution. This problem is very ill-conditioned. To see this, add a small perturbation to A E = 0.0010 -0.0020 0.0010 -0.0010 and solve the perturbed system ( A + E )x = b xe = (A+E)\b xe = -5.0000 7.3085 10-4 Conditioning and Numerical Stability Notice how much the small change in the data is magnified in the solution. One way to measure the magnification factor is by means of the quantity A A –1 called the condition number of A with respect to inversion. The condition number determines the loss in precision due to roundoff errors in Gaussian elimination and can be used to estimate the accuracy of results obtained from matrix inversion and linear equation solution. It arises naturally in –1 perturbation theories that compare the perturbed solution ( A + E ) b with the –1 true solution A b . In MATLAB, the function cond calculates the condition number in 2-norm. cond(A) is the ratio of the largest singular value of A to the smallest. Try it for the example above. The usual rule is that the exponent log10(cond(A)) on the condition number indicates the number of decimal places that the computer can lose to roundoff errors. IEEE standard double precision numbers have about 16 decimal digits of accuracy, so if a matrix has a condition number of 1010, you can expect only six digits to be accurate in the answer. If the condition number is much greater than 1/sqrt(eps), caution is advised for subsequent computations. For IEEE arithmetic, the machine precision, eps, is about 2.2 × 10 -16, and 1/sqrt(eps) = 6.7 × 10 8. Another important aspect of conditioning is that, in general, residuals are reliable indicators of accuracy only if the problem is well-conditioned. To illustrate, try computing the residual vector r = Ax – b for the two candidate solutions x = [0.999 -1.001]' and x = [0.341 -0.087]'. Notice that the second, while clearly a much less accurate solution, gives a far smaller residual. The conclusion is that residuals are unreliable indicators of relative solution accuracy for ill-conditioned problems. This is a good reason to be concerned with computing or estimating accurately the condition of your problem. Another simple example of an ill-conditioned problem is the n -by- n matrix with ones on the first upper-diagonal. A = diag(ones(1,n-1),1); This matrix has n eigenvalues at 0. Now consider a small perturbation of the –n data consisting of adding the number 2 to the first element in the last ( n th) 10-5 10 Reliable Computations row of A. This perturbed matrix has n distinct eigenvalues λ 1, ..., λ n with λ k = 1 ⁄ 2 exp ( j2πk ⁄ n ) . Thus, you can see that this small perturbation in the n data has been magnified by a factor on the order of 2 to result in a rather large perturbation in the solution (the eigenvalues of A). Further details and related examples are to be found in [7]. It is important to realize that a matrix can be ill-conditioned with respect to inversion but have a well-conditioned eigenproblem, and vice versa. For example, consider an upper triangular matrix of ones (zeros below the diagonal) given by A = triu(ones(n)); This matrix is ill-conditioned with respect to its eigenproblem (try small perturbations in A(n,1) for, say, n=20), but is well-conditioned with respect to inversion (check its condition number). On the other hand, the matrix A = 1 1 1 1+δ has a well-conditioned eigenproblem, but is ill-conditioned with respect to inversion for small δ . Numerical Stability Numerical stability is somewhat more difficult to illustrate meaningfully. Consult the references in [5], [6], and [7] for further details. Here is one small example to illustrate the difference between stability and conditioning. Gaussian elimination with no pivoting for solving the linear system Ax = b is known to be numerically unstable. Consider A = 0.001 1.000 1.000 – 1.000 b = 1.000 0.000 All computations are carried out in three-significant-figure decimal arithmetic. –1 The true answer x = A b is approximately x = 0.999 0.999 10-6 Conditioning and Numerical Stability Using row 1 as the pivot row (i.e., subtracting 1000 times row 1 from row 2) you arrive at the equivalent triangular system. 0.001 1.000 x 1 0 – 1000 x 2 = 1.000 – 1000 Note that the coefficient multiplying x 2 in the second equation should be – 1001, but because of roundoff, becomes – 1000 . As a result, the second equation yields x 2 = 1.000 , a good approximation, but now back-substitution in the first equation 0.001x 1 = 1.000 – ( 1.000 ) ( 1.000 ) yields x 1 = 0.000 . This extremely bad approximation of x 1 is the result of numerical instability. The problem itself can be shown to be quite well-conditioned. Of course, MATLAB implements Gaussian elimination with pivoting. 10-7 10 Reliable Computations Choice of LTI Model Now turn to the implications of the results in the last section on the linear modeling techniques used for control engineering. The Control System Toolbox includes the following types of LTI models that are applicable to discussions of computational reliability: • State space • Transfer function, polynomial form • Transfer function, factored zero-pole-gain form The following subsections show that state space is most preferable for numerical computations. State Space The state-space representation is the most reliable LTI model to use for computer analysis. This is one of the reasons for the popularity of “modern” state-space control theory. Stable computer algorithms for eigenvalues, frequency response, time response, and other properties of the ( A, B, C, D ) quadruple are known [5] and implemented in this toolbox. The state-space model is also the most natural model in the MATLAB matrix environment. Even with state-space models, however, accurate results are not guaranteed, because of the problems of finite-word-length computer arithmetic discussed in the last section. A well-conditioned problem is usually a prerequisite for obtaining accurate results and makes it important to have reasonable scaling of the data. Scaling is discussed further in the “Scaling” section later in this chapter. Transfer Function Transfer function models, when expressed in terms of expanded polynomials, tend to be inherently ill-conditioned representations of LTI systems. For systems of order greater than 10, or with very large/small polynomial coefficients, difficulties can be encountered with functions like roots, conv, bode, step, or conversion functions like ss or zpk. A major difficulty is the extreme sensitivity of the roots of a polynomial to its coefficients. This example is adapted from Wilkinson, [6] as an illustration. Consider the transfer function 10-8 Choice of LTI Model 1 1 H ( s ) = ------------------------------------------------------------- = ----------------------------------------------------------20 19 ( s + 1 ) ( s + 2 )... ( s + 20 ) s + 210s + ... + 20! The A matrix of the companion realization of H ( s ) is A = 0 0 : 0 – 20! 1 0 : 0 . 0 1 . ... ... ... ... . . . 0 0 : 1 – 210 Despite the benign looking poles of the system (at -1,-2,..., -20) you are faced 18 with a rather large range in the elements of A , from 1 to 20! ≈ 2.4 × 10 . But 19 the difficulties don’t stop here. Suppose the coefficient of s in the transfer – 23 – 23 –7 function (or A ( n, n ) ) is perturbed from 210 to 210 + 2 (2 ≈ 1.2 × 10 ). Then, computed on a VAX (IEEE arithmetic has enough mantissa for only n = 17 ), the poles of the perturbed transfer function (equivalently, the eigenvalues of A ) are eig(A)' ans = Columns 1 through 7 -19.9998 -19.0019 -17.9916 -17.0217 -15.9594 -15.0516 -13.9504 Columns 8 through 14 -13.0369 -11.9805 -11.0081 -9.9976 -9.0005 -7.9999 Columns 15 through 20 -6.0000 -5.0000 -4.0000 -3.0000 -2.0000 -1.0000 -7.0000 The problem here is not roundoff. Rather, high-order polynomials are simply intrinsically very sensitive, even when the zeros are well separated. In this –9 case, a relative perturbation of the order of 10 induced relative –2 perturbations of the order of 10 in some roots. But some of the roots changed very little. This is true in general. Different roots have different sensitivities to different perturbations. Computed roots may then be quite meaningless for a polynomial, particularly high-order, with imprecisely known coefficients. 10-9 10 Reliable Computations Finding all the roots of a polynomial (equivalently, the poles of a transfer function or the eigenvalues of a matrix in controllable or observable canonical form) is often an intrinsically sensitive problem. For a clear and detailed treatment of the subject, including the tricky numerical problem of deflation, consult [6]. It is therefore preferable to work with the factored form of polynomials when available. To compute a state-space model of the transfer function H ( s ) defined above, for example, you could expand the denominator of H , convert the transfer function model to state space, and extract the state-space data by H1 = tf(1,poly(1:20)) H1ss = ss(H1) [a1,b1,c1] = ssdata(H1) However, you should rather keep the denominator in factored form and work with the zero-pole-gain representation of H ( s ) . H2 = zpk([],1:20,1) H2ss = ss(H2) [a2,b2,c2] = ssdata(H2) Indeed, the resulting state matrix a2 is better conditioned. [cond(a1) cond(a2)] ans = 2.7681e+03 8.8753e+01 and the conversion from zero-pole-gain to state space incurs no loss of accuracy in the poles. format long e [sort(eig(a1)) sort(eig(a2))] ans = 9.999999999998792e-01 2.000000000001984e+00 3.000000000475623e+00 3.999999981263996e+00 5.000000270433721e+00 5.999998194359617e+00 7.000004542844700e+00 10-10 1.000000000000000e+00 2.000000000000000e+00 3.000000000000000e+00 4.000000000000000e+00 5.000000000000000e+00 6.000000000000000e+00 7.000000000000000e+00 Choice of LTI Model 8.000013753274901e+00 8.999848908317270e+00 1.000059459550623e+01 1.099854678336595e+01 1.200255822210095e+01 1.299647702454549e+01 1.400406940833612e+01 1.499604787386921e+01 1.600304396718421e+01 1.699828695210055e+01 1.800062935148728e+01 1.899986934359322e+01 2.000001082693916e+01 8.000000000000000e+00 9.000000000000000e+00 1.000000000000000e+01 1.100000000000000e+01 1.200000000000000e+01 1.300000000000000e+01 1.400000000000000e+01 1.500000000000000e+01 1.600000000000000e+01 1.700000000000000e+01 1.800000000000000e+01 1.900000000000000e+01 2.000000000000000e+01 There is another difficulty with transfer function models when realized in state-space form with ss. They may give rise to badly conditioned eigenvector matrices, even if the eigenvalues are well separated. For example, consider the normal matrix A = [5 4 1 1 4 5 1 1 1 1 4 2 1 1 2 4] Its eigenvectors and eigenvalues are given as follows. [v,d] = eig(A) v = 0.7071 -0.7071 0.0000 -0.0000 -0.0000 0.0000 0.7071 -0.7071 -0.3162 -0.3162 0.6325 0.6325 0.6325 0.6325 0.3162 0.3162 1.0000 0 0 0 0 2.0000 0 0 0 0 5.0000 0 0 0 0 10.0000 d = The condition number (with respect to inversion) of the eigenvector matrix is 10-11 10 Reliable Computations cond(v) ans = 1.000 Now convert a state-space model with the above A matrix to transfer function form, and back again to state-space form. b = [1 ; 1 ; 0 ; -1]; c = [0 0 2 1]; H = tf(ss(A,b,c,0)); [Ac,bc,cc] = ssdata(H) % Transfer function % Convert back to state space The new A matrix is Ac = 18.0000 16.0000 0 0 -6.0625 0 4.0000 0 2.8125 0 0 1.0000 -1.5625 0 0 0 Note that Ac is not a standard companion matrix and has already been balanced as part of the ss conversion (see ssbal for details). Note also that the eigenvectors have changed. [vc,dc] = eig(Ac) vc = -0.5017 -0.8026 -0.3211 -0.0321 0.2353 0.7531 0.6025 0.1205 0.0510 0.4077 0.8154 0.4077 0.0109 0.1741 0.6963 0.6963 dc = 10.0000 0 0 0 0 5.0000 0 0 0 0 2.0000 0 0 0 0 1.0000 The condition number of the new eigenvector matrix cond(vc) 10-12 Choice of LTI Model ans = 34.5825 is thirty times larger. The phenomenon illustrated above is not unusual. Matrices in companion form or controllable/observable canonical form (like Ac) typically have worse-conditioned eigensystems than matrices in general state-space form (like A). This means that their eigenvalues and eigenvectors are more sensitive to perturbation. The problem generally gets far worse for higher-order systems. Working with high-order transfer function models and converting them back and forth to state space is numerically risky. In summary, the main numerical problems to be aware of in dealing with transfer function models (and hence, calculations involving polynomials) are • The potentially large range of numbers leads to ill-conditioned problems, especially when such models are linked together giving high-order polynomials. • The pole locations are very sensitive to the coefficients of the denominator polynomial. • The balanced companion form produced by ss, while better than the standard companion form, often results in ill-conditioned eigenproblems, especially with higher-order systems. The above statements hold even for systems with distinct poles, but are particularly relevant when poles are multiple. Zero-Pole-Gain Models The third major representation used for LTI models in MATLAB is the factored, or zero-pole-gain (ZPK) representation. It is sometimes very convenient to describe a model in this way although most major design methodologies tend to be oriented towards either transfer functions or state-space. In contrast to polynomials, the ZPK representation of systems can be more reliable. At the very least, the ZPK representation tends to avoid the extraordinary arithmetic range difficulties of polynomial coefficients, as illustrated in the “Transfer Function” section. The transformation from state 10-13 10 Reliable Computations space to zero-pole-gain is stable, although the handling of infinite zeros can sometimes be tricky, and repeated roots can cause problems. If possible, avoid repeated switching between different model representations. As discussed in the previous sections, when transformations between models are not numerically stable, roundoff errors are amplified. 10-14 Scaling Scaling State space is the preferred model for LTI systems, especially with higher order models. Even with state-space models, however, accurate results are not guaranteed, because of the finite-word-length arithmetic of the computer. A well-conditioned problem is usually a prerequisite for obtaining accurate results. You should generally normalize or scale the ( A, B, C, D ) matrices of a system to improve their conditioning. An example of a poorly scaled problem might be a dynamic system where two states in the state vector have units of light years and millimeters. You would expect the A matrix to contain both very large and very small numbers. Matrices containing numbers widely spread in value are often poorly conditioned both with respect to inversion and with respect to their eigenproblems, and inaccurate results can ensue. Normalization also allows meaningful statements to be made about the degree of controllability and observability of the various inputs and outputs. A set of ( A, B, C, D ) matrices can be normalized using diagonal scaling matrices N u , N x , and N y to scale u, x, and y. u = Nu un x = Nx xn y = Ny yn so the normalized system is x· n = A n x n + B n u n yn = Cn xn + Dn u n where –1 A n = N x AN x –1 C n = N y CNx –1 B n = N x BN u –1 D n = N y DN u Choose the diagonal scaling matrices according to some appropriate normalization procedure. One criterion is to choose the maximum range of each of the input, state, and output variables. This method originated in the days of analog simulation computers when u n , x n , and y n were forced to be between ±10 Volts. A second method is to form scaling matrices where the diagonal 10-15 10 Reliable Computations entries are the smallest deviations that are significant to each variable. An excellent discussion of scaling is given in the introduction to the LINPACK Users’ Guide, [1]. Choose scaling based upon physical insight to the problem at hand. If you choose not to scale, and for many small problems scaling is not necessary, be aware that this choice affects the accuracy of your answers. Finally, note that the function ssbal performs automatic scaling of the state vector. Specifically, it seeks to minimize the norm of –1 –1 N x AN x Nx B CN x 0 by using diagonal scaling matrices N x . Such diagonal scaling is an economical way to compress the numerical range and improve the conditioning of subsequent state-space computations. 10-16 Summary Summary This chapter has described numerous things that can go wrong when performing numerical computations. You won’t encounter most of these difficulties when you solve practical lower-order problems. The problems described here pertain to all computer analysis packages. MATLAB has some of the best algorithms available, and, where possible, notifies you when there are difficulties. The important points to remember are • State-space models are, in general, the most reliable models for subsequent computations. • Scaling model data can improve the accuracy of your results. • Numerical computing is a tricky business, and virtually all computer tools can fail under certain conditions. 10-17 10 Reliable Computations References [1] Dongarra, J.J., J.R. Bunch, C.B. Moler, and G.W. Stewart, LINPACK User’s Guide, SIAM Publications, Philadelphia, PA, 1978. [2] Franklin, G.F., and J.D. Powell, Digital Control of Dynamic Systems, Addison-Wesley, 1980. [3] Kailath, T., Linear Systems, Prentice-Hall, 1980. [4] Laub, A.J., “Numerical Linear Algebra Aspects of Control Design Computations,” IEEE Transactions on Automatic Control, Vol. AC-30, No. 2, February 1985, pp. 97-108. [5] Wilkinson, J.H., Rounding Errors in Algebraic Processes, Prentice-Hall, 1963. [6] Wilkinson, J.H., The Algebraic Eigenvalue Problem, Oxford University Press, 1965. 10-18 11 SISO Design Tool Overview (p. 11-2) A brief introduction to the SISO Design Tool, a graphical user interface for designing prefilters and compensators for SISO systems Importing and Exporting Models (p. 11-4) How to move models in and out of the SISO Design Tool Tuning Compensators (p. 11-7) Designing compensators using root-locus, Bode diagram, and Nichols plot techniques Using the Right-Click Menu to Design Compensators (p. 11-12) A quick way to add dynamics to your compensator Viewing Loop Responses and System Data (p. 11-16) How to plot various responses and view system information Storing and Retrieving Designs (p. 11-19) How to save your compensator designs Creating and Editing Design Constraints (p. 11-20) Using design constraints to help you meet design specifications Customizing the SISO Design Tool (p. 11-33) Customize the look and feel of the SISO Design Tool 11 SISO Design Tool Overview The SISO Design Tool is a graphical user interface (GUI) that allows you to analyze and tune SISO feedback control systems. Using the SISO Design Tool, you can graphically tune the gains and dynamics of the compensator (C) and prefilter (F) using a mix of root locus and loop shaping techniques. For a quick discussion of basic tasks you can do with the SISO Design Tool, see the following: • “Importing and Exporting Models” on page 11-4 • “Tuning Compensators” on page 11-7 - “Root-Locus” on page 11-8 - “Open-Loop Bode Diagram” on page 11-9 - “Open-Loop Nichols Plot” on page 11-10 - “Prefilter Bode Diagram” on page 11-11 • “Using the Right-Click Menu to Design Compensators” on page 11-12 • “Viewing Loop Responses and System Data” on page 11-16 • “Viewing Loop Responses and System Data” on page 11-16 • “Storing and Retrieving Designs” on page 11-19 • “Customizing the SISO Design Tool” on page 11-33 In addition, there is an extensive discussion of how to use the SISO Design Tool in Designing Compensators, Chapter 4 in Getting Started with the Control System Toolbox. See SISO Design Tool for a description of all the features available. For example, you can use the root locus view to stabilize the feedback loop and enforce some minimum damping, and use the Bode diagrams to adjust the bandwidth, check the gain and phase margins, or add a notch filter for disturbance rejection. You can also bring up an open-loop Nichols view or Bode diagram of the prefilter (F) by selecting these items from the View menu. All views are dynamically linked; for example, if you change the gain in the root locus, it immediately affects the Bode diagrams as well. 11-2 Overview Current Compensator panel Root Locus Feedback structure Open-loop Bode diagram Status panel The SISO Design Tool is designed to work closely with the LTI Viewer, allowing you to rapidly iterate on your design and immediately see the results in the LTI Viewer. When you make a change in your compensator, the LTI Viewer associated with your SISO Design Tool automatically updates the response plots that you have chosen. By default, the SISO Design Tool displays the root locus and open-loop Bode diagrams for your imported systems. You can also bring up an open-loop Nichols view or prefilter Bode diagram by selecting these items in the View menu. Imported systems can include any elements of the feedback structure diagram located to the right of the Current Compensator panel. You cannot change imported plant (G) or sensor (H) models, but you can use the SISO Design Tool for designing a new (or modifying an existing) prefilter (F) or compensator (C) for your imported plant and sensor configuration. 11-3 11 SISO Design Tool Importing and Exporting Models The SISO Design Tool provides graphical user interfaces to facilitate importing and exporting of linear models. Importing Models To import models into the SISO Design Tool, select Import under the File menu. This opens the Import System Data window. To import a model, • Specify whether you want to import a SISO model from the MATLAB workspace, a MAT-file, or from a Simulink model. The window lists the available models for each format under SISO Models. • Click on the desired model. • Click a right arrow to specify whether you want to import the model as the plant (G), Sensor (H), Prefilter (F), or Compensator (C). Click the OK button. Alternatively, you can directly import a model into the SISO Design Tool using the sisotool function, as in sisotool(modelname) In this case, modelname is imported as the plant (G). See the sisotool function for more information. 11-4 Importing and Exporting Models Exporting Models Use Export in the File menu to open the SISO Tool Export window. Selecting Models to Export The SISO Tool Export window not only lists all the models displayed in your SISO Design Tool, but also contains various transfer functions associated with standard control analysis techniques. These include open and closed loop transfer functions, input and output sensitivity functions, and the state-space model of the overall feedback loop. To select a model for export, left-click on the row containing the component name. To specify a different export name, double-click on the Export As cell for the component. This makes the name in the cell editable. You can export models back to the MATLAB workspace or to disk. In the latter case, the models are saved in a MAT-file. Exporting to Workspace To export models to the MATLAB workspace, simply click Export to Workspace. 11-5 11 SISO Design Tool Exporting to Disk If you choose Export to Disk, this window opens. The Export to Disk window provides a default file name. If you want to change the name, specify the new name for your model(s) and click Save. Your models are stored in a MAT-file. Exporting Multiple Models There are two ways to export multiple models: • If the models are adjacent in the model selection list, hold down the Shift key while selecting the models with your mouse. • If the models are nonadjacent, hold down the Ctrl key and select the models by left-clicking. 11-6 Tuning Compensators Tuning Compensators The SISO Design Tool simplifies the task of designing compensators. Regardless of which views you have open — root-locus, Bode diagrams, or Nichols plots — there are three ways to alter compensator designs: • Interactive graphics let you tune compensator gains and adjust dynamics (poles and zeros). • Right-click menus allow you to add/remove dynamics and customize the view (for example, add a grid, zoom in/out, add design constraints, or customize plot properties). • The Edit Compensator window is a GUI with fields for keyboard entry of gain values and pole/zero locations. You can perform any of these tasks in the root locus, open-loop and prefilter Bode diagrams, or Nichols plots in the SISO Design Tool. Once you’ve added dynamics to your compensator, you can dynamically update pole and zero locations by dragging them. The SISO Design Tool is designed so that a change in any one view is automatically reflected in other views in the GUI. In particular, the Current Compensator panel always reflects the current compensator design. The next sections discuss some of the ways you can tune compensators in different SISO Design Tool views. You can tune compensators in any of the views in the SISO Design Tool. These views include the following: • “Root-Locus” on page 11-8 • “Open-Loop Bode Diagram” on page 11-9 • “Open-Loop Nichols Plot” on page 11-10 • “Prefilter Bode Diagram” on page 11-11 11-7 11 SISO Design Tool Root-Locus You can tune your compensator using root-locus techniques. This figure shows an imported compensator and plant model; use the right-click menus and interactive graphics features to add, adjust, and remove compensator dynamics. Move any red square to change the compensator gain. Use the right-click menu to add poles and zeros to your compensator design. You can move the compensator poles and zeros (red x’s and o’s, respectively) with your mouse. The blue x’s and o’s represent plant poles and zeros. You cannot move them. The Status panel displays tips on how to use the SISO Design Tool. See Root Locus Design in Getting Started with the Control System Toolbox for an example of how to use root locus design techniques. 11-8 Tuning Compensators Open-Loop Bode Diagram The SISO Design Tool supports the open-loop Bode diagram view of your system. You can use the right-click menu and interactive graphics features to add, adjust, and remove compensator dynamics. This figure shows some of the features of the open-loop Bode diagram. Drag the bode magnitude curve up and down to adjust the compensator gain. You can move compensator poles and zeros (red x’s and o’s, respectively) with your mouse. The blue x’s and o’s represent plant poles and zeros. You cannot move them. The Status panel displays tips on how to use the SISO Design Tool. You can adjust notch filter parameters by moving them with your mouse. Use the right-click menu to add poles and zeros to your compensator. See Bode Diagram Design in Getting Started with the Control System Toolbox for an example of how to use Bode diagram design techniques. 11-9 11 SISO Design Tool Open-Loop Nichols Plot An alternative method for compensator design is the open-loop Nichols plot. You can use the right-click menu and interactive graphics features to add, adjust, and remove compensator dynamics. This figure shows some of the features of the open-loop Nichols plot. Use the right-click menu to add poles and zeros to your compensator. You can move compensator poles and zeros (red x’s and o’s, respectively) with your mouse. The brown stems display the gain and phase margins. The blue x’s and o’s represent plant poles and zeros. You cannot move them. The Status panel displays tips on how to use the SISO Design Tool. See Nichols Plot Design in Getting Started with the Control System Toolbox for an example of how to use Nichols plot design techniques. 11-10 Tuning Compensators Prefilter Bode Diagram The SISO Design Tool supports the prefilter Bode diagram view. You can add dynamics to your prefilter design using the right-click menu, and you can adjust dynamics by dragging poles and zeros with your mouse. This figure shows some of the features. The green curve is the prefilter Bode magnitude. Move it up and down to adjust the prefilter gain. Use the right-click menu to add poles and zeros to your prefilter design. The Status panel displays tips on how to use the SISO Design Tool. The magenta curve is the closed-loop response from the prefilter input to the plant output. The green x is a prefilter pole. Use your mouse to move this pole to a new location. 11-11 11 SISO Design Tool Using the Right-Click Menu to Design Compensators The quickest way to design compensators is to use the right-click menu. Ths SISO Design Tool includes right-click menus for all the views available in the tool. These views include the root-locus, open-loop Bode diagrams, Nichols plot, and the prefilter Bode diagrams. The menu items in each of these views are identical. The design constraints differ, however, depending on which view you are accessing the menus from. You can use the right-click menu to design a compensator by adding poles, zeros, lead, lag, and notch filters. . Adding Dynamics to Your Compensator The Add menu options give you the ability to add dynamics to your compensator design, including poles, zeros, lead and lag networks, and notch filters. This figure shows the Add submenu. 11-12 Using the Right-Click Menu to Design Compensators The following pole/zero configurations are available: • Real Pole • Complex Pole • Integrator (a pole at the origin) • Real Zero • Complex Zero • Differentiator (a zero at the origin) • Lead • Lag • Notch In all but the integrator and differentiator, once you select the configuration, your cursor changes to an ‘x’. To add the item to your compensator design, place the x at the desired location on the plot and left-click your mouse. You will see the root locus design automatically update to include the new compensator dynamics. The notch filter has three adjustable parameters. For a discussion about how to add and adjust notch filters, see “Adding a Notch Filter” in Getting Started with the Control System Toolbox. Example: Adding a Complex Pair of Poles This example shows you how to add a complex pair of poles to the open-loop Bode diagram. First, type load ltiexamples sisotool('bode',sys_dc) 11-13 11 SISO Design Tool at the MATLAB prompt. This opens the SISO Design Tool with the DC motor example loaded and the open-loop Bode diagram displayed. After selecting Add Pole/Zero and then Complex Pole from the right-click menu, use the mouse cursor to specify the frequency of the complex pole pair. To add a complex pair of poles, 1 Select Add Pole/Zero and then Complex Pole from the right-click menu 2 Place the mouse cursor where you want the pole to be located 3 Left-click to add the pole 11-14 Using the Right-Click Menu to Design Compensators Your SISO Design Tool should look similar to this. This ‘x’ represents the added poles. In the case of Bode diagrams, when you place a complex pole, the default damping value is 1, which means you have a double real pole. To change the damping, grab the red ‘x’ by left-clicking on it and drag it upward with your mouse. You will see damping ratio change in the Status Panel at the bottom of the SISO Design Tool. 11-15 11 SISO Design Tool Viewing Loop Responses and System Data The SISO Design Tool provides support for viewing loop responses for your system. To see available frequency and time domain responses, pull down the Analysis menu. Choose the response you want to see; an LTI Viewer opens with the view plotted. For example, if you select Response to Step Command, you will see a closed-loop step response of your system. Viewing Specialized Loop Responses If you choose Other Loop Responses from the Analysis menu, the Response Plot Setup window opens. Loop diagram Plots and Contents of Plots Response Plot Setup Window The following sections describe the main components of the Response Plot Setup window. 11-16 Viewing Loop Responses and System Data Loop Diagram At the top of the Response Plot Setup window is a loop diagram. This block diagram shows the feedback structure of your system. The diagram in “Response Plot Setup Window” on page 11-16 shows the default configuration; the compensator is in the forward path. If your system has the compensator in the feedback path, this window correctly displays the alternate feedback structure. Note that the window lists two transfer functions next to the loop diagram: • Loop transfer — This is defined as the compensator (C), the plant (G), and the sensor (H) multiplied together (CGH). If you haven’t defined a sensor, its default value is 1. 1 • Sensitivity function — This is defined as ------------- , where L is the loop transfer 1+L function. Some of the open- and closed-loop responses use these definitions. See “Contents of Plots” on page 11-17 for more information. Plots You can have up to six plots in one LTI Viewer. By default, the Response Plot Setup window specifies one step response plot. To add a plot, start by selecting “2. None” from the list of plots and then specify a new plot type in the Change to field. You can choose any of the plots available in the LTI Viewer. Select “None” to remove a plot. Contents of Plots Once you have selected a plot type, you can include several open- and closed-loop transfer functions to be displayed in that plot. You can plot open-loop responses for each of the components of your system, including your compensator (C), plant (G), prefilter (F), or sensor (H). In addition, loop transfer and sensitivity transfer functions are available. Their definitions are listed in the Response Plot Setup window. See the block diagram in “Response Plot Setup Window” on page 11-16 for definitions of the input/output points for closed-loop responses. 11-17 11 SISO Design Tool Viewing System Data You can view data about your model by selecting System Data under the View menu. The System Data window displays the poles and zeros of your imported plant and sensor models. Press Show Transfer Function to see the associated transfer function. For example, this picture shows the Gservo plant’s transfer function. 11-18 Storing and Retrieving Designs Storing and Retrieving Designs The SISO Design Tool provides a graphical user interface (GUI's) for storing and retrieving compensator designs. Each design consists of a pair (C, F) of compensator and prefilter models To open the Compensator Design Archive window, select Store/Retrieve from the Compensators menu. You can use this window both to store and retrieve compensator designs. To store a design, specify the name you want to save it under and press Store. To retrieve any of the prefilter and/or compensator designs that you have created during a SISO Design Tool session, right-clicking on the Design Name you want to retrieve. Press Retrieve and the design is sent back to the SISO Design Tool. 11-19 11 SISO Design Tool Creating and Editing Design Constraints When designing compensators, it is common to have design specifications that call for specific settling times, damping ratios, and other characteristics. The SISO Design Tool provides design constraints that can help make the task of meeting design specifications easier. The New Constraint window, which allows you to create design constraints, automatically changes to reflect which constraints are available for the view in which you are working. Select Design Constraints and then New to open the New Constraint window, which is shown below. Since each view has a different set of constraint types, click on the following links to go to the appropriate descriptions: • Root locus • Open-loop Bode diagram and prefilter Bode diagram (same) • Nichols plot Design Constraints for the Root Locus For the root locus, you have the following constraint types: • “Settling Time” • “Percent Overshoot” • “Damping Ratio” • “Natural Frequency” Use the Constraint Type menu to select a design constraint. In each case, to specify the constraint, enter the value in the Constraint Parameters panel. You can select any or all of them, or have more than one of each. 11-20 Creating and Editing Design Constraints Settling Time If you specify a settling time in the continuous-time root locus, a vertical line appears on the root locus plot at the pole locations associated with the value provided (using a first-order approximation). In the discrete-time case, the constraint is a curved line. Percent Overshoot Specifying percent overshoot in the continuous-time root locus causes two rays, starting at the root locus origin, to appear. These rays are the locus of poles associated with the percent value (using a second-order approximation). In the discrete-time case, In the discrete-time case, the constraint appears as two curves originating at (1,0) and meeting on the real axis in the left-hand plane. Note that the percent overshoot (p.o.) constraint can be expressed in terms of the damping ratio, as in this equation. p.o. = 100e – πζ ⁄ 1 – ζ 2 where ζ is the damping ratio. Damping Ratio Specifying a damping ratio in the continuous-time root locus causes two rays, starting at the root locus origin, to appear. These rays are the locus of poles associated with the damping ratio. In the discrete-time case, the constraint appears as curved lines originating at (1,0) and meeting on the real axis in the left-hand plane. Natural Frequency If you specify a natural frequency, a semicircle centered around the root locus origin appears. The radius equals the natural frequency. 11-21 11 SISO Design Tool Example: Adding Damping Ratio Constraints This example add a damping ratio of 0.707 inequality constraint. First, type load ltiexamples sisotool(sys_dc) at the MATLAB prompt. This opens the SISO Design Tool with the DC motor example imported. From the root locus right-click menu, select Design Constraints and then New to open the New Constraint window. To add the constraint, select Damping Ratio as the constraint type. The default damping ratio is 0.707. The SISO Design Tool should now look similar to this figure. Damping Ratio Constraints in the Root Locus The two rays centered at (0,0) represent the damping ratio constraint. The dark edge is the region boundary, and the shaded area outlines the exclusion region. 11-22 Creating and Editing Design Constraints This figure explains what this means for this constraint. Damping ζ < 0.707 Damping ζ > 0.707 You can, for example, use this design constraint to ensure that the closed-loop poles, represented by the red squares, have some minimum damping. Try adjusting the gain until the damping ratio of the closed-loop poles is 0.7. Design Constraints for Open-Loop and Prefilter Bode Diagrams For both the open-loop and prefilter Bode diagrams, you have the following options: • “Upper Gain Limit” • “Lower Gain Limit” Specifying any of these constraint types causes lines to appear in the Bode magnitude curve. To specify an upper or lower gain limit, enter the frequency range, the magnitude limit, and/or the slope in decibels per decade, in the appropriate fields of the Constraint Parameters panel. You can have as many gain limit constraints as you like in your Bode magnitude plots. Upper Gain Limit You can specify an upper gain limit, which appears as a straight line on the Bode magnitude curve. You must select frequency limits, the upper gain limit in decibels, and the slope in dB/decade. Lower Gain Limit Specify the lower gain limit in the same fashion as the upper gain limit. 11-23 11 SISO Design Tool Example: Adding Upper Gain Limits This example shows you how to add two upper gain limit constraints to the open-loop Bode diagram. First, type load ltiexamples sisotool('bode',Gservo) at the MATLAB prompt. This opens the SISO Design Tool with the servomechanism model loaded. Use the right-click menu to add a grid. First, add an upper gain limit constraint of 0 dB from 10 rad/s to 100 rad/s. 11-24 Creating and Editing Design Constraints This figure shows the New Constraint editor with the correct parameters. Your SISO Design Tool should now look like this. Now, to constrain the rolloff, open the New Constraint editor and add an upper gain limit from 100 rad/s to 1000 rad/s with a slope of -20 db/decade. 11-25 11 SISO Design Tool This figure shows the result. With these constraints in place, you can see how much you can increase the compensator gain and still meet design specifications. Note that you can change the constraints by moving them with your mouse. See “Editing Constraints” on page 11-29 for more information. Design Constraints for Open-Loop Nichols Plots For open-loop Nichols plots, you have the following design constraint options: • “Phase Margin” on page 11-27 • “Gain Margin” on page 11-27 • “Closed-Loop Peak Gain” on page 11-27 Specifying any of these constraint types causes lines or curves to appear in the Nichols plot. In each case, to specify the constraint, enter the value in the Constraint Parameters panel. You can select any or all of them, or have more than one of each. 11-26 Creating and Editing Design Constraints Phase Margin Specify a minimum phase amount at a given location. For example, you can require a minimum of 30 degrees at the -180 degree crossover. The phase margin specified should be a number greater than 0. The location must be a -180 plus a multiple of 360 degrees. If you enter an invalid location point, the closed valid location is selected. Gain Margin Specify a gain margin at a given location. For example, you can require a minimum of 20 dB at the -180 degree crossover. The location must be -180 plus a multiple of 360 degrees. If you enter an invalid location point, the closed valid location is selected. Closed-Loop Peak Gain Specify a peak closed-loop gain at a given location. The specified value can be positive or negative in dB. The constraint follows the curves of the Nichols plot grid, so it is recommended that you have the grid on when using this feature. Example: Adding a Closed-Loop Peak Gain Constraint This example shows how to add a closed-loop peak gain constraint to the Nichols plot. First, type load ltiexamples sisotool('nichols',Gservo) 11-27 11 SISO Design Tool This opens the SISO Design Tool with Gservo imported as the plant. Use the right-click menu to add a grid, as this figure shows. To add closed-loop peak gain of 1 dB at -180 degrees, open the New Constraint editor and select Closed-Loop Peak Gain from the pull-down menu. Set the peak gain field to 1 dB. The figure shows the resulting design constraint; use Zoom X-Y to zoom in on the plot for clarity. 11-28 Creating and Editing Design Constraints As long as the curve is outside the gray region, the closed-loop gain is guaranteed to be less than 1 dB. Note that this is equivalent, up to second order, to specifying the peak overshoot in the time domain. In this case, a 1 dB closed-loop peak gain corresponds to an overshoot of 15%. Editing Constraints To edit an existing constraint, left-click on the constraint itself to select it. Two black squares appear on the constraint when it is selected, and your mouse cursor turns into a large black cross (+). In general, there are two ways to adjust a constraint: • Click on the constraint and drag it. This does not change the shape of the constraint. That is, the adjustment is strictly a translation of the constraint. • Grab a black square and drag it. In this case, you can rotate, expand, and/or contract the constraint. For example, in Bode diagrams you can move an upper gain limit by clicking on it and moving it anywhere in the plot region. As long as you haven’t grabbed 11-29 11 SISO Design Tool a black square, the length and slope of the gain limit will not change as you move the line. On the other hand, you can change the slope of the upper gain limit by grabbing one of the black squares and rotating the line. In all cases, the Status panel at the bottom of the SISO Design Tool displays the constraint values as they change. This figure shows the process of editing an upper gain limit in the open-loop Bode diagram. Rotate the black square to change the slope of the upper limit constraint. You can also stretch or shrink the constraint by dragging the black square. Grab the gray line itself to move it up/down or left/right. The Status bar displays the updated constraint values. An alternative way to adjust a constraint is to select Design Constraints and then Edit from the right-click menu. The Edit Constraints window opens. 11-30 Creating and Editing Design Constraints To adjust a constraint, select the constraint by clicking on it and change the values in the fields of the Constraint parameters panel. If you have additional constraints in, for example, the Bode diagram, you can edit them directly from this window by selecting Open-Loop Bode from the Editor menu. Deleting Constraints To delete a constraint, place your cursor directly over the constraint itself. You cursor changes into a large ‘x’. Right-click to open a menu containing Edit and Delete. Select Delete from the menu list; this eliminates the constraints. You can also delete constraints by left-clicking on the constraint and then pressing the BackSpace or Delete key on your keyboard. Finally, you can delete constraints by selecting Undo Add Constraint from the Edit menu, or pressing Ctrl+Z if adding constraints was the last action you took. 11-31 11 SISO Design Tool Current Compensator The Current Compensator panel shows the structure of the compensator you are designing. The default compensator structure is a unity gain with no dynamics. Once you add poles and/or zeros, the Current Compensator panel displays the compensator in zero/pole/gain format. This picture shows a Current Compensator panel with Gcl1 entered as the compensator. You can change the gain of the compensator by changing the number in the text field. If you want to change the poles and zeros of the compensator, click on the window to open the Edit Compensator window. If you have a discrete time system, the Current Compensator panel display changes. This figure shows the Current Compensator panel with Gcl1 discretized with a time step of 0.001 second. Here, w is the z-transform shifted by -1 and scaled by the sample time; see the definition to the right of the transfer function. This is done to simplify the representation; note that the coefficients are a close match to those shown for the continuous time representation. If you see either NumC or DenC in place of a polynomial, it means that the numerator or denominator of the transfer function is too large to fit in the panel. Try stretching the SISO Design Tool horizontally to see the complete transfer function. 11-32 Customizing the SISO Design Tool Customizing the SISO Design Tool The SISO Design Tool provides a graphical user interface (GUI), called the SISO Tool Preferences window, for customizing units, linestyles, axes foreground and linestyle colors, fonts, compensator format, and Bode plot options. Any options you set in this window apply to all the plots of the current instance of the SISO Design Tool. If you open another instance of the SISO Design Tool, it inherits its options from the Toolbox Preferences Editor. For more information, see the SISO Design Tool Preferences Editor. 11-33 11 SISO Design Tool 11-34 12 LTI Viewer Basic LTI Viewer Tasks (p. 12-2) A brief overview of the basic functionality of the LTI Viewer Using the Right-Click Menu (p. 12-4) How to use the right-click menu, a quick way to navigate the LTI Viewer Importing and Exporting Models (p. 12-9) How to move models in and out of the LTI Viewer Selecting Response Types (p. 12-11) How to add or change existing views Analyzing MIMO Models (p. 12-14) Different ways of viewing multiple-input, multiple output LTI models Customizing the LTI Viewer (p. 12-17) Specifying units, grids, axes, and so on 12 LTI Viewer Basic LTI Viewer Tasks LTI Viewer is a graphical user interface (GUI) that simplifies the analysis of linear, time-invariant systems. You use the LTI Viewer to view and compare the response plots of SISO and MIMO systems, or of several linear models at the same time. You can generate time and frequency response plots to inspect key response parameters, such as rise time, maximum overshoot, and stability margins. The easiest way to work with the LTI Viewer is to use the right-click menus. For example, type load ltiexamples ltiview(sys_dc) at the MATLAB prompt. The default plot is a step response. Use the right-click menu to manipulate this plot. The LTI Viewer can display up to seven different plot types simultaneously, including step, impulse, Bode (magnitude and phase or magnitude only), Nyquist, Nichols, sigma, pole/zero, and I/O pole/zero. 12-2 Basic LTI Viewer Tasks See ltiview for help on the function that opens an LTI Viewer. For examples of how to use the LTI Viewer, see Analyzing Models in Getting Started with the Control System Toolbox. 12-3 12 LTI Viewer Using the Right-Click Menu The quickest way to manipulate views in the LTI Viewer is use the right-click menu. You can access several LTI Viewer controls and options, including • Plot Type — Changes the plot type • Systems — Selects or deselects any of the models loaded in the LTI Viewer • Characteristics — Displays key response characteristics and parameters • Grid — Adds grids to your plot • Properties — Opens the Property Editor, where you can customize plot attributes In addition to right-click menus, all response plots include data markers. These allow you to scan the plot data, identify key data, and determine the source system for a given plot. Setting Characteristics of Response Plots The Characteristics menu changes for each plot response type. Characteristics refers to response plot information, such as peak response, or, in some cases, rise time and settling time. The next sections describe the menu items for each of the eight plot types. Step Response Step plots the model’s response to a step input. You can display the following information in the step response: • Peak Response — The largest deviation from the steady-state value of the step response 12-4 Using the Right-Click Menu • Settling Time — The time required for the step response to decline and stay at 5% of its final value • Rise Time — The time required for the step response to rise from 10% to 90% of its final value • Steady-State — The final value for the step response Note You can change the definitions of settling time and rise time using the Characteristics pane of the Control System Toolbox editor, the LTI Viewer editor, or the Property editor. Impulse Response Impulse Response plots the model’s response to an impulse. The LTI Viewer can display the following information in the impulse response: • Peak Response — The maximum positive deviation from the steady-state value of the impulse response • Settling Time — The time required for the step response to decline and stay at 5% of its final value Bode Diagram Bode plots the open-loop Bode phase and magnitude diagrams for the model. 12-5 12 LTI Viewer The LTI Viewer can display the following information in the Bode diagram: • Peak Response — The maximum value of the Bode magnitude plot over the specified region • Stability Margins (Minimum Crossing) — The minimum phase and gain margins. The gain margin is defined to the gain (in dB) when the phase first crosses -180°. The phase margin is the distance, in degrees, of the phase from -180° when the gain magnitude is 0 dB. • Stability Margins (All Crossings) — Display all stability margins Bode Magnitude Bode Magnitude plots the Bode magnitude diagram for the model. The LTI Viewer can display the following information in the Bode magnitude diagram: • Peak Response, which is the maximum value of the Bode magnitude in decibels (dB), over the specified range of the diagram. • Stability (Minimum Crossing) — The minimum gain margins. The gain margin is defined to the gain (in dB) when the phase first crosses -180°. • Stability (All Crossings) — Display all gain stability margins Nyquist Diagrams Nyquist plots the Nyquist diagram for the model. 12-6 Using the Right-Click Menu The LTI Viewer can display the following types of information in the Nyquist diagram: • Peak Response — The maximum value of the Nyquist diagram over the specified region • Stability (Minimum Crossing) — The minimum gain and phase margins for the Nyquist diagram. The gain margin is the distance from the origin to the phase crossover of the Nyquist curve. The phase crossover is where the curve meets the real axis. The phase margin is the angle subtended by the real axis and the gain crossover on the circle of radius 1. • Stability (All Crossings) — Display all gain stability margins Nichols Charts Nichols plots the Nichols Chart for the model. The LTI Viewer can display the following types of information in the Nichols chart: • Peak Response — The maximum value of the Nichols chart in the plotted region. • Stability (Minimum Crossing) — The minimum gain and phase margins for the Nichols chart. • Stability (All Crossings) — Display all gain stability margins 12-7 12 LTI Viewer Singular Values Singular Values plots the singular values for the model. The LTI Viewer can display the Peak Response, which is the largest magnitude of the Singular Values curve over the plotted region. Pole/Zero and I/O Pole/Zero Pole/Zero plots the poles and zeros of the model with ‘x’ for poles and ‘o’ for zeros. I/O Pole/Zero plots the poles and zeros of I/O pairs. There are no Characteristics available for pole-zero plots. 12-8 Importing and Exporting Models Importing and Exporting Models To import models into the LTI Viewer, select Import under the Edit menu. This opens the LTI Browser, shown below. Use the LTI Browser to import LTI models into or from the LTI Viewer workspace. To import a model, • Click on the desired model in the LTI Browser List. To perform multiple selections, - Hold the Ctrl key and click on nonadjacent models. - Hold the Shift key while clicking to select multiple adjacent models. • Click the OK or Apply button. Note that the LTI Browser lists only the LTI models in the MATLAB workspace. Alternatively, you can directly import a model into the LTI Viewer using the ltiview function, as in ltiview({'step', 'bode'}, modelname) See the ltiview function for more information. 12-9 12 LTI Viewer Exporting Models Use Export in the File menu to open the LTI Viewer Export window, shown below. The LTI Viewer Export window lists all the models with responses currently displayed in your LTI Viewer. You can export models back to the MATLAB workspace or to disk. In the latter case, the Control System Toolbox saves the files as MAT-files. To export single or multiple models, follow the steps described in the importing models section above. If you choose Export to Disk, this window opens. Choose a name for your model(s) and click Save. Your models are stored in a MAT-file. 12-10 Selecting Response Types Selecting Response Types There are two methods for selecting response plots in the LTI Viewer: • Selecting Plot Type from the right-click menus • Opening the Plot Configurations window Right-Click Menu: Plot Type If you have a plot open in the LTI Viewer, you can switch to any other response plot available by selecting Plot Type from the right-click menu. To change the response plot, select the new plot type from the Plot Type submenu. The LTI Viewer automatically displays the new response plot. Plot Configurations Window The Plot Type feature of the right-click menu works on existing plots, but you can also add plots to an LTI Viewer by using the Plot Configurations window. By default, the LTI Viewer opens with a closed-loop step response. To reconfigure an open viewer, select Plot Configuration in the Edit menu. 12-11 12 LTI Viewer Use the radio buttons to select the number of plots you want displayed in your LTI Viewer. For each plot, select a response type from the menus located on the right-hand side of the window. It’s possible to configure a single LTI Viewer to contain up to six response plots. 12-12 Selecting Response Types Available response plots include step, impulse, Bode (magnitude and phase, or magnitude only), Nyquist, Nichols, sigma, pole/zero maps, and I/O pole/zero maps. 12-13 12 LTI Viewer Analyzing MIMO Models If you import a MIMO system, or an LTI array containing multiple linear models, you can use special features of the right-click menu to group the response plots by input/output (I/O) pairs or select individual plots for display. For example, if you randomly generate a 3-input, 3-output MIMO system, sys_mimo=rss(3,3,3); and open an LTI Viewer, ltiview(sys_mimo); the default is an unwrapped set of 9 plots, one from each input to each output. 12-14 Analyzing MIMO Models I/O Grouping for MIMO Models You can group this by inputs, by outputs, or both by selecting I/O Grouping and then Inputs, Outputs, or All, respectively, from the right-click menu. For example, if you select Outputs, the LTI Viewer reconfigures into 3 plots, one for each input. Selecting None returns to the default configuration, where all I/O pairs are displayed individually. 12-15 12 LTI Viewer Selecting I/O Pairs Another way to organize MIMO system information is to choose I/O Selector from the right-click menu, which opens the I/O Selector window. This window automatically configures to the number of I/O pairs in your MIMO system. You can select • Any individual plot (only one at a time) by clicking on a button • Any row or column by clicking on Y(*) or U(*) • All of the plots by clicking [all] Using these options, you can inspect individual I/O pairs, or look at particular I/O channels in detail. 12-16 Customizing the LTI Viewer Customizing the LTI Viewer The LTI Viewer has a tool preferences editor, which allows you to set default characteristics for specific instances of LTI Viewers. If you open a new instance of either, each defaults to the characteristics specified in the Toolbox Preferences editor. LTI Viewer Preferences Editor Select Viewer Preferences in the Edit menu of the LTI Viewer to open the LTI Viewer Preferences editor. This figure shows the editor open to its first pane. The LTI Viewer Preferences editor contains four panes: • Units — Convert between various units, including rad/s and Hertz • Style — Customize grids, fonts, and colors • Characteristics — Specify response plot characteristics, such as settling time tolerance • Parameters — Set time and frequency ranges, stop times, and time step size If you want to customize the settings for all instances of LTI Viewers, see the Toolbox Preferences editor. 12-17 12 LTI Viewer 12-18 Index A addition of LTI models 2-11 scalar 2-12 adjoint See pertransposition append 2-16, 2-17, 4-29 array dimensions 4-7 arrays See LTI arrays B balancing realizations 3-7 building LTI arrays 4-12 C canonical realizations 3-7 cell array 1-11, 1-14 classical control 9-2, 9-20 closed loop See feedback compensator adding a notch filter 11-12 adding an integrator 11-12 adding dynamics 11-12 tuning 11-7 concatenation, model 1-10 horizontal 2-17 LTI arrays 4-15 state-space model order, effects on 2-9 vertical 2-17 conditioning, state-space models 10-4 connection feedback 9-11 parallel 2-12, 9-54 series 2-13, 9-14 constructor functions LTI objects 1-4 continuous-time 3-2 conversion, model automatic 1-40 between model types 1-39, 2-3 continuous to discrete (c2d) 2-20 discrete to continuous (d2c) 1-35, 2-20 with negative real poles 2-21 FRD model, to 1-39 resampling 2-26 SS model, to 1-39 state-space, to 1-41 TF model, to 1-39 ZPK model, to 1-39 covariance error 9-56, 9-60 customizing plots 8-1 customizing subplots 8-11 D d2d 2-26 delays combining 1-51 discrete-time models 1-49 discretization 2-24 I/O 1-25, 1-42, 1-43 information, retrieving 1-51 input 1-25, 1-42, 1-47 output 1-26, 1-42, 1-47 Padé approximation 1-51 supported functionality 1-43 deletion parts of LTI arrays 4-24 parts of LTI models 2-9 Index-1 Index denominator property 1-27 specification 1-8, 1-11, 1-21 value 1-23 descriptor systems See state-space models, descriptor design classical 9-2, 9-20 Kalman estimator 9-36, 9-50, 9-57 LQG 9-31 regulators 9-31 robustness 9-28 root locus 9-9, 9-24 digital filter filt 1-22 specification 1-21 dimensions array 4-7 I/O 4-7 discrete-time models 3-2 control design 9-20 Kalman estimator 9-50 resampling 2-26 See also LTI models discretization 1-35, 2-20, 9-21 delay systems 2-24 first-order hold 2-22 matched poles/zeros 2-23 Tustin method 2-23 zero-order hold 2-20 dual See transposition Index-2 E error covariance 9-56, 9-60 extraction LTI arrays, in 4-23 LTI models, in 2-5 F feedback 9-11 feedthrough gain 1-28 filt 1-22 filtering See Kalman estimator first-order hold (FOH) 2-22 with delays 2-24 FRD (frequency response data) objects 1-4, 1-17 conversion to 1-39 frequencies indexing by 2-7 referencing by 2-7 uses 1-3 frequency response 1-17 G gain 1-11 feedthrough 1-28 property LTI properties gain 1-27 gain margins 9-28 get 1-31 group See I/O groups Index H K hasdelay 1-51 Kalman filtering 9-50 Kalman estimator continuous 9-36 discrete 9-50 time-varying 9-57 I I/O concatenation 2-16 delays 1-25, 1-42, 1-43 dimensions 3-2 LTI arrays 4-7 groups 1-25 referencing models by group name 2-8 names 1-25, 1-26, 1-36 conflicts, naming 2-4 referencing models by 2-8 relation 2-5 indexing into LTI arrays 4-21 single index access 4-22 inheritance 2-3 input 1-2 delays 1-25, 1-42, 1-47 groups 1-25 names 1-25 number of inputs 3-2 InputDelay See delays InputGroup 1-25, 1-25, 1-26 conflicts, naming 2-4 See also I/O groups InputName 1-33, 1-35 conflicts, naming 2-4 See also I/O names inversion model 2-13 ioDelayMatrix See delay L LQG (linear quadratic-gaussian) method continuous LQ regulator 9-36 cost function 9-36 design 9-31, 9-47 LQ-optimal gain 9-36 regulator 9-31 LTI (linear time-invariant) 1-2 LTI arrays accessing models 4-22 analysis functions 4-30 array dimensions 4-7 building 4-15 building LTI arrays 4-12 building with rss 4-19 building with tf,zpk,ss, and frd 4-17 concatenation 4-15 conversion, model See conversion deleting parts of 4-24 dimensions, size, and shape 4-7 extracting subsystems 4-23 indexing into 4-21, 4-22 interconnection functions 4-25 model dimensions 4-7 operations on 4-25 dimension requirements 4-27 special cases 4-28 Index-3 Index LTI arrays, continued reassigning parts of 4-23 size 4-7 stack 4-15 LTI models addition 2-11 scalar 2-12 building 2-16 characteristics 3-2 concatenation effects on model order 2-9 horizontal 2-17 vertical 2-17 continuous 3-2 conversion 1-39, 2-3 continuous/discrete 2-20 See also conversion, model creating 1-8 discrete 1-19, 3-2 discretization, matched poles/zeros 2-23 empty 1-12, 3-2 functions, analysis 3-4 I/O group or channel name, referencing by 2-8 interconnection functions 2-16 inversion 2-13 model data, accessing 1-23 modifying 2-5 multiplication 2-13 operations 2-1 precedence rules 2-3 See also operations proper transfer function 3-2 resizing 2-9 subsystem, modifying 2-9 subtraction 2-12 type 3-2 Index-4 LTI objects 1-25, 1-32 constructing 1-4 methods 1-4 properties See LTI properties See also LTI models LTI properties 1-4, 1-25, 1-33 accessing property values (get) 1-31, 1-32 displaying properties 1-31 generic properties 1-25 I/O groups See I/O, groups I/O names See I/O, names inheritance 2-3 model-specific properties 1-27 online help (ltiprops) 1-25 property names 1-25, 1-29 property values 1-25, 1-30 setting 1-29 sample time 2-3 variable property 2-4 LTI Viewer configuring plots 12-11 customization 12-17 I/O grouping 12-15 importing/exporting models 12-9 MIMO models 12-14 overview 12-2 right-click menu 12-4 selecting I/O pairs 12-16 selecting response types 12-11 LTI Viewer Preferences Editor 7-3 Index M map, I/O 2-5 margins gain and phase 9-28 methods 1-4 MIMO 1-2, 2-17 model building 2-16 feedback connection 9-11 parallel connection 2-12, 9-54 series connection 2-13, 9-14 model dynamics, function list 3-4 modeling See model building multiplication 2-13 scalar 2-13 multiplication 2-13 overloaded 1-4 pertransposition 2-14 precedence 2-3 resizing 2-9 subsystem, extraction 2-5 subtraction 2-12 transposition 2-14 output 1-2 delays 1-26, 1-42, 1-47 groups 1-26 names 1-26 number of outputs 3-2 OutputDelay See delays OutputGroup 1-26 N Notes 1-26 numerator property 1-27 specification 1-8, 1-11, 1-21 value 1-23 numerical stability 10-6 O object-oriented programming 1-4 objects See LTI objects operations on LTI models addition 2-11 append 2-17 arithmetic 2-11 concatenation 1-10, 2-9, 2-17 extracting a subsystem 1-6 inversion 2-13 group names, conflicts 2-4 See also I/O, groups OutputName 1-33 conflicts, naming 2-4 See also I/O, names P Padé approximation (pade) 1-51 parallel connection 2-12, 9-54 pertransposition 2-14 phase margins 9-28 plot customization 8-1 poles 1-12 property 1-27 precedence rules 1-5, 2-3 proper transfer function 3-2 properties sample time 2-3 See LTI properties variable 2-4 Index-5 Index Property Editor 8-4 R realization state coordinate transformation 3-7 realizations 3-7 balanced 3-7 canonical 3-7 regulation 9-31 resampling 2-26 response, I/O 2-5 robustness 9-28 root locus design 9-9, 9-24 See also Root Locus Design GUI rss building an LTI array with 4-19 S sample time 1-19, 1-26, 1-33, 2-3 accessing 1-23 resampling 2-26 setting 1-35 unspecified 1-26 scaling 10-15 series connection 2-13, 9-14 set 1-29 SISO 1-2, 3-2 SISO Design Tool Bode diagram techniques 11-9 customizing 11-33 customizing plots 8-12 design constraints 11-20 designing prefilters 11-11 exporting multiple models 11-6 Index-6 Nichols plot techniques 11-10 overview 11-2 root locus right-click menu 11-12 root-locus techniques 11-8 storing/retrieving designs 11-19 tuning compensators 11-7 viewing loop responses 11-16 viewing specialized loop responses 11-16 viewing system data 11-16, 11-18 SISO Tool Preferences Editor 7-8 ss 1-14 SS models 2-14 SS objects See state-space models stability numerical 10-6 stack 4-15 state 1-14 matrix 1-28 names 1-28 transformation 3-7 vector 1-2 state-space models 1-2, 1-4, 10-8 balancing 3-7 conditioning 10-4 conversion to 1-39 See also conversion descriptor 1-16, 1-23 matrices 1-15 model data 1-15 quick data retrieval 1-23 realizations 3-7 scaling 10-15 specification 1-14 ss 1-14 transfer functions of 1-40 subplot customization 8-11 Index subsystem 1-6, 2-5 subsystem operations on LTI models subsystem, modifying 2-9 subtraction 2-12 transposition 2-14 triangle approximation 2-22 Ts See sample time Tustin approximation 2-23 with frequency prewarping 2-23 T Td See delays tf 1-9 U Userdata 1-26 TF objects See transfer functions tfdata output, form of 1-23 time delays See delays time-varying Kalman filter 9-57 Toolbox Preferences Editor 6-2 totaldelay 1-51 transfer functions 1-2, 1-4, 10-8 constructing with rational expressions 1-9 conversion to 1-39 denominator 1-8 discrete-time 1-19, 1-21 DSP convention 1-21 filt 1-22 MIMO 1-10, 2-17 numerator 1-8 quick data retrieval 1-23 specification 1-8 static gain 1-11 tf 1-9 TF object, display for 1-9 variable property 1-27, 2-4 V variable property 2-4 Z zero-order hold (ZOH) 2-20, 9-21 with delays 2-24 zero-pole-gain (ZPK) models 1-2, 1-4, 10-13 conversion to 1-39 MIMO 1-13, 2-17 quick data retrieval 1-23 specification 1-12 zpk 1-12 zeros 1-12 property 1-27 zpk 1-12 ZPK objects See zero-pole-gain (ZPK) models zpkdata output, form of 1-23 Index-7 Index Index-8
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