Using Variable Speed Drives, Servo
Motors and RS-485 Communication in
a Solar Tracking System for
Educational Purposes
“A report submitted to the School of Engineering and Energy, Murdoch
University in partial fulfilment of the requirements for the degree of
Bachelor of Engineering”
Jarrad Sibson
Supervisor: Associate Professor Graeme Cole
JUNE 2012
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Abstract
This thesis covers the development of the solar tracking system that is to be installed at the
Murdoch University South Street campus. Work had previously been completed on this
project by students of ENG454 in 2009 and by Rhyss Edwards as part of his thesis “Control
System Design and Commissioning of Photovoltaic Trough Concentrator Systems Installed
at the Murdoch University South Street Campus” in 2010. The major focus of this thesis
was to cover the communication and data transmission between the variable speed drives,
servo motors and the Labview controller over a RS-485 network. The final system is to be
used as a learning tool for engineering students.
During this thesis a large emphasis was placed on the research and documentation of the
existing tracking system. Information regarding the variable speed drives, servo motors and
RS-485 communication was gathered in order to gain an understanding of how the system
interacted. From here documents were produced regarding the installation of SEW VSD’s
and controlling the solar tracking program in order to aid future students who may work on
this project.
Robust testing of the existing system was undertaken in order to discover any deficiencies
that may be present. Re-wiring of the bench-top system took place to establish
communication between the variable speed drives and the Labview Controller.
Modifications were made to the existing Labview controller. This focussed on the structure
and sequencing of a data transmission process, from the controller to the VSD, across an
RS-485 network. Changes were made to the front panel of the controller to enhance
operator usability.
At the completion of this thesis it was discovered that the solar tracking system has a large
scope for future work to be completed on it. Further advancements to the Labview
controller will need to be made before the project is able to be commissioned.
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Acknowledgements
For their assistance and guidance throughout this project I would like to acknowledge the
following individuals:
Project Supervisor: Associate Professor Graeme R Cole, Lecturer, Murdoch University
Thesis Coordinator: Dr Gareth Lee, Lecturer, Murdoch University
Mr Will Stirling, Technical Officer, Murdoch University
Mr John Boulton, Technical Officer, Murdoch University
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Table of Contents
Abstract .................................................................................................................................... ii
Acknowledgements................................................................................................................. iii
List of Figures ........................................................................................................................... v
List of Tables .......................................................................................................................... vii
CHAPTER 1: Introduction ......................................................................................................... 1
1.1 Project Overview ............................................................................................................ 1
1.2 Project Objectives .......................................................................................................... 2
1.2.1 Research and Documentation................................................................................. 2
1.2.2 Robust Testing of Existing System .......................................................................... 2
1.2.3 Further Development of Tracking Program ............................................................ 3
1.3 Thesis Structure ............................................................................................................. 3
CHAPTER 2: System Background .............................................................................................. 4
2.1. Variable Speed Drives ................................................................................................... 4
2.1.1. Variable Speed Drive Overview.............................................................................. 4
2.1.2 Principles of Operation ........................................................................................... 4
2.1.3 Types of Variable Speed Drives ............................................................................... 5
2.1.4 Advantages of Variable Speed Drives ................................................................... 10
2.1.5 Disadvantages of Variable Speed Drives ............................................................... 12
2.2. RS-485 Communication Overview .............................................................................. 13
2.2.1. Serial Communication .......................................................................................... 13
2.2.2. RS-485 Standard................................................................................................... 15
2.3. Synchronous and Servo Motors .................................................................................. 16
2.3.1. Synchronous Motors ............................................................................................ 16
2.3.2. Principles of Operation ........................................................................................ 16
2.3.3. Types of Synchronous Motors ............................................................................. 17
2.3.2 Servo Motors......................................................................................................... 17
CHAPTER 3: System Overview................................................................................................ 19
3.1. Solar Tracking System ................................................................................................. 19
3.1.1. Motor Mount ....................................................................................................... 19
3.1.2. Chronological Tracker .......................................................................................... 20
3.2. Movidrive MDX61B ..................................................................................................... 21
3.2.1 IPOS Plus Positioning and Sequence Control: ....................................................... 23
3.2.2 IPOS Application Modules ..................................................................................... 24
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3.3. DFS Synchronous Servo Motors .................................................................................. 24
CHAPTER 4: VSD Communication .......................................................................................... 26
4.1. Telegrams .................................................................................................................... 26
4.1.1. Telegram Structure .............................................................................................. 26
4.2. Transmission Process .................................................................................................. 30
4.2.1. Character Structure.............................................................................................. 30
CHAPTER 5: Bench Top Testing Platform ............................................................................... 32
5.1 Connections and Wiring ............................................................................................... 32
5.1.1 VSD Connection..................................................................................................... 32
5.2. VSD Initialisation ......................................................................................................... 35
5.2.1 Movitools Drive Connection ................................................................................. 36
5.2.3 IPOS Program ........................................................................................................ 37
5.2.4 VSD Referencing Mode ......................................................................................... 38
5.3. Control Structure ........................................................................................................ 39
CHAPTER 6: Tracking Program Development ........................................................................ 40
6.1 Controller Overview ..................................................................................................... 40
6.1.1. Reference Mode................................................................................................... 41
6.1.2. Automatic Mode .................................................................................................. 41
6.1.3. Jog Mode .............................................................................................................. 43
6.2. Program Development ................................................................................................ 44
6.2.1 Read and Write Process ........................................................................................ 44
6.2.2 Discussion.............................................................................................................. 48
CHAPTER 7: Conclusion .......................................................................................................... 50
7.1. Project Conclusion ...................................................................................................... 50
7.2. Future Work ................................................................................................................ 51
Bibliography ........................................................................................................................... 54
Appendices............................................................................................................................. 56
List of Figures
Figure 1 - Block Diagram of a VSD ............................................................................................ 4
Figure 2 - Basic Topology of a PWM Variable Speed Drive [5] ................................................ 6
Figure 3 - Basic Topology of a CSI VSD [6]................................................................................ 8
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Figure 4 - Basic Topology of a VSI VSD [7] ............................................................................... 9
Figure 5 - Variable Torque Load [9] ....................................................................................... 11
Figure 6 - Constant Torque Load [9] ...................................................................................... 11
Figure 7 - Asynchronous Serial Data Transmission [11]......................................................... 14
Figure 8 - RS-485 Data Transmission Waveform [13] ............................................................ 15
Figure 9 - Recommended RS-485 Network Topology [14] .................................................... 16
Figure 10 - Direct Drive Motor Mount [17]............................................................................ 19
Figure 11 - Upside Down Mounting Diagram [17] ................................................................. 20
Figure 12 - Zenith and Azimuth Angles Used in a Chronological Solar Tracker [17].............. 21
Figure 13 – Connected and Operating MDX61B VSD ............................................................ 22
Figure 14 - DFS Synchronous Servo Motor ............................................................................ 25
Figure 16 - Request Telegram Structure [21]......................................................................... 27
Figure 17 - Response Telegram Structure [21] ...................................................................... 27
Figure 18 – Type Byte Structure [21] ..................................................................................... 29
Figure 19 - Character Structure.............................................................................................. 31
Figure 20 - Bench Top Testing Platform Once Reconnected and Operational ...................... 32
Figure 21 - Wiring Diagram of the RS-485 Interface [22]....................................................... 33
Figure 22 - 9 Pin D-sub Connector [23] .................................................................................. 34
Figure 23- SEW RS-485 to RS-232 Converter ......................................................................... 36
Figure 24 - SEW RS-485 to USB Converter ............................................................................. 36
Figure 25 - Example Positioning via Bus Interface ................................................................. 37
Figure 26 - Control Structure of the Solar Tracking System [17] ........................................... 39
Figure 27 - Labview Front Panel............................................................................................. 40
Figure 28 - Motor Indicators .................................................................................................. 41
Figure 29 - Tab Options in Automatic Mode.......................................................................... 41
Figure 30 - Tracking Mode Front Panel Features ................................................................... 43
Figure 31 - Initialisation State ................................................................................................ 45
Figure 32 - Send Request State .............................................................................................. 46
Figure 33 - Get Response State .............................................................................................. 46
Figure 34 - Process Message State......................................................................................... 47
Figure 35 - Do Nothing State.................................................................................................. 48
Figure 36 - Temporary Error Correction ................................................................................ 49
Figure 37 - Previous Method for Controlling Multiple VSD's ................................................. 49
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List of Tables
Table 1 - Advantages and Disadvantages of PWM VSD's [4] ................................................... 6
Table 2 - Advantages and Disadvantages of a CSI VSD [4] ....................................................... 7
Table 3 - Advantages and Disadvantages of VSI VDS's [4] ....................................................... 8
Table 4 - Advantages and Disadvantages of Flux Vector Drives ............................................ 10
Table 5 - Drive Model Numbers ............................................................................................. 22
Table 6 - Input Data for Drive 1.............................................................................................. 22
Table 7 - Output Data for Drive 1........................................................................................... 23
Table 8 - Input Data for Drive 2.............................................................................................. 23
Table 9 - Output Data for Drive 2........................................................................................... 23
Table 10 - DFS Synchronous Servo Gearmotor Data ............................................................. 25
Table 11 - Addressing Areas for RS-485 Communication [21] ............................................... 28
Table 12 - PDU Types in Cyclical Transmission [21] ............................................................... 29
Table 13 - PDU Types in Acyclical Transmission [21] ............................................................. 30
Table 14 - 9-pin D-sub Pin Assignment .................................................................................. 34
Table 15 - X13 Terminal Input List [19] .................................................................................. 35
Table 16 - Monitored Telegrams............................................................................................ 44
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CHAPTER 1: Introduction
1.1 Project Overview
When the engineering department at Murdoch University relocated to the Murdoch
Campus, the existing 80 panelled photovoltaic trough solar tracking array at the
Rockingham campus was also relocated. A decision was made to construct a 40 panelled
array utilising the old system and for two smaller solar tracking systems to be designed and
constructed at the Murdoch Campus. Instead of purchasing an off the shelf controller
which are readily available, it was decided that the two smaller systems were to be
designed utilising variable speed drives, servo motors and RS-485 communication. Variable
speed drives are readily found in many motor driven systems and in industrial applications.
They offer increased motor control and improved energy efficiency. The use of these
technologies would provide a final system that could be readily used for educational
purposes. The project was commenced by ENG454 students in 2009 and was taken over as
part of Rhyss Edwards’s thesis in 2010 [1].
This project was taken over at a stage where the tracking program was functional but
contained issues and problems that needed to be addressed. A lack of documentation
regarding connections and wiring, drive initialisation and the Labview controller was
noticed from the beginning and this caused issues in getting the system back to a functional
stage. Considerable importance was placed on research into the existing set up, with the
aim of developing relevant documentation for the installation and operation of the solar
tracking system.
The developed system will provide a working controller that can successfully transmit data
to the variable speed drives whilst providing the user a graphical interface from which they
can monitor and control the system. The program and documentation produced will enable
students and teaching staff to utilise the main components in projects other than a solar
tracking system. These can include motor driven applications where a VSD is used to
control the motors.
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1.2 Project Objectives
The major objective of this project is to further enhance the knowledge of variable speed
drives and their associated technologies as used in industrial applications. The complexity
of the solar tracking system being developed is relatively high and the knowledge and
understanding that will be gained will aid staff and students in the future use these
technologies. The project was split into three key stages; Research and Documentation,
Robust Testing and Tracking Program Development. Throughout these stages various
objectives were to be achieved. The major objectives for the project were as follows:
•
To investigate and research the major hardware components, existing system
and produce or acquire adequate documentation.
•
To perform robust testing of the existing solar tracking system using the bench
top testing platform.
•
To further develop the tracking program and fix issues that may have been
discovered in the testing stage.
1.2.1 Research and Documentation
At the beginning of this project it was noticed that insufficient documentation was
provided by the previous students who had worked on the project. Even though there was
a functioning tracking program available this had become non-operational as operating
instructions and wiring information were not available. A major emphasis was put into
researching the SEW Movidrive VSD’s, the communication methods and IPOS control. This
research provided an understanding of the existing system from which documentation was
able to be produced for future users. Information and documentation is provided on these
in further chapters.
1.2.2 Robust Testing of Existing System
The result of Rhyss Edwards’s previous work on the solar tracker was a working program
available for use. It was first important to establish communication between the VSD’s the
motor and the master computer. From here the objective was to recreate the system to
the point at which it was last operating. The aim was to effectively test the program and
the existing system to identify any problems and issues and further develop necessary
documentation.
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1.2.3 Further Development of Tracking Program
Once issues and problems had been identified in the solar tracking program it was
necessary to determine how and what relevant changes would need to be made to the
program. This would focus heavily on the communication and transmission of data
between the variable speed drives and the master computer on the RS-485 network. Front
panel usability would also be addressed so that future students would be able to operate
the tracker without problems and more easily.
1.3 Thesis Structure
This thesis is broken down into 7 chapters which provide information on the system and
the work completed on the project. The structure of the thesis is as follows:
•
CHAPTER 1: An introduction to the project and the key objectives.
•
CHAPTER 2: Provides an overview into the background and theory behind the
major components of the solar tracking system.
•
CHAPTER 3: Covers the solar tracking system, the specific hardware
components and the technologies used in the system.
•
CHAPTER 4: Describes the communication method for transmission of data in
a Movidrive variable speed drive.
•
CHAPTER 5: Explains in detail the connections and communication between
the existing bench top testing platform
•
CHAPTER 6: Outlines the development and changes made to the Labview
controller and how it functions.
•
CHAPTER 7: Covers the achievements made throughout the project and details
the future work and recommendations moving forward.
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CHAPTER 2: System Background
This chapter will provide a literature review of the hardware and communication standards
used in the development of the solar tracking system.
2.1. Variable Speed Drives
This section will provide an overview on variable speed drives, how they function, types of
VSD’s and the main advantages and disadvantages of using them.
2.1.1. Variable Speed Drive Overview
A variable speed drive (VSD), also known as a variable frequency drive (VFD) or adjustable
speed drive (ASD) is a device that, put simply, controls the speed of machinery. To be more
precise VSD’s allow us to regulate the speed or rotational force of an electric motor [2].
The speed of a motor is determined by the frequency and voltage of the supplied power. It
is however, not always ideal for motors to have a fixed speed and thus variable speed
drives allow us to control the speed as needed. This is done by altering the frequency of the
supply voltage to the motor.
Many systems that are run by motors are often designed so that they are capable of
handling peak loads. These systems however are not always operating at these peak loads
and as such can become quite energy inefficient when meeting significantly lower loads.
Through the use of VSD’s we are able to more closely match the output of the motor to the
operating load to result in energy savings. [3]
2.1.2 Principles of Operation
As has already been stated, VSD’s control the speed of motors by regulating the supply
voltage and frequency. There are three main stages to a VSD. They are:
•
Rectifier Stage
•
DC Filter Stage
•
Inverter Stage
Figure 1 - Block Diagram of a VSD
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Rectifier Stage
A power source provides 50Hz 3-phase AC power to the rectifier in the VSD. The rectifier
then converts the AC to a DC voltage. This rectifier is made up of 6 diodes known as a diode
bridge.
DC Filter Stage
The DC voltage is then passed through a DC filter via a DC bus. The filter helps to smooth
out and filter the AC component of the DC waveform [4].
Inverter Stage
The inverter circuit inverts the DC voltage back to an AC Voltage. Depending on the type of
VSD this inverter stage can vary. The types of inverter will be covered in the following
section. It is what happens in this stage that enables the speed of the motor to be
controlled.
2.1.3 Types of Variable Speed Drives
There are 4 main types of variable speed drives. They all operate on the same principle;
they all control the speed of a motor by controlling the supply power and frequency. The
four main types of VSD’s are Pulse Width Modulation (PWM), Voltage Source Inverters
(VSI), Current Source Inverters (CSI) and the relatively new Flux Vector Drive. Each type
offers certain advantages and disadvantages depending on the motor application.
Pulse Width Modulation
This is the most common technology used in VSD’S. This is largely due to the fact that it
works well with motors between 0.5 hp and 500 hp in size [5]. An AC supply voltage is fed
into the input side of the VSD which is then rectified through a diode bridge converter to a
DC voltage. The DC voltage is then passed through a filter to create and maintain a fixed DC
bus voltage [5]. This DC voltage is then fed into an inverter made up of six high-speed
electronic switches, usually power transistors or thyristors. The switches create short
pulses of differing widths and the same height as the DC bus voltage. This process describes
the term pulse width modulation. When the width of the produced pulses is varied in each
half cycle, the average output voltage produces a sine wave like output. The frequency
delivered to the motor, which in turn controls the speed, is determined by the number of
transitions from positive to negative per second of the pulse width modulated voltage. The
use of insulated gate bipolar transistors (IGBT) as the switching device is common in many
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PWM variable speed drives. They can operate with switching speeds between 2 kHz to 15
kHz [5]. The higher the frequency the less motor whine is present. This is due to the fact
that transistors operate outside the threshold of what humans are capable of hearing. It
also smooths out the output current waveform because current spikes are now removed.
The following table outlines the advantages and disadvantages of pulse width modulated
variable speed drives.
Advantages
Disadvantages
Excellent input power factor due to fixed DC
bus voltage.
Motor heating and insulation breakdown in
some applications due to high frequency
switching of transistors.
No motor cogging normally found with sixstep inverters.
Non-regenerative operation.
Highest efficiencies: 92% to 96%.
Line-side power harmonics (depending on
the application and size of the drive).
Compatibility with multi-motor applications.
Ability to ride through a 3 to 5 Hz power
loss.
Lower initial cost.
Reflects least amount of harmonics to the
power source.
Table 1 - Advantages and Disadvantages of PWM VSD's [5]
Input Converter
DC BUS (Filter)
Output Inverter
Figure 2 - Basic Topology of a PWM Variable Speed Drive
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Current Source Inverters
As the name suggests the DC input to the inverter in current source inverters is seen as a dc
current source. They are quite reliable because of their ability to limit the current and their
simple circuitry. Current source inverter based VSD’s take an AC input voltage and rectify
the voltage to DC just as the other types of VSD’s do. However, the type of rectifier used
differs. Current source inverters use either silicon controlled rectifiers (SCR’s), gate
commutated thyristors (GCT’s) or symmetrical gate commutated thyristors (SGCT’s) to
convert an AC voltage into DC [6]. A current source inverter regulates the power and
produces a variable voltage. The DC bus contains a large inductor which regulates current
ripple and also stores energy for the motor. SCR’s, GCT’s or SGCT’s are also used in the
inverter stage. The inverter then converts the DC current to AC at a variable frequency. The
CSI keeps a fixed current regardless of any voltage changes. The advantages and
disadvantages of current source inverter VSD’s can be seen in table 2 below.
Advantages
Disadvantages
Reliability due to inherent current limiting
operation.
Large power harmonic generation back into
power source.
Regenerative power capability.
Cogging below 6 Hz due to square wave
output.
Simple circuitry.
Use of large and costly inductor.
HV spikes to motor windings.
Load dependent; poor for multimotor
applications.
Poor input power factor due to SCR
converter section.
Table 2 - Advantages and Disadvantages of a CSI VSD [5]
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Figure 3 - Basic Topology of a CSI VSD
Voltage Source Inverters
Voltage source inverter VSD’s control the motor speed by regulating the output voltage
and frequency. The DC input to the inverter is a DC voltage source. Figure 4 shows the basic
circuit diagram of a voltage source inverter VSD. VSI’s use either SCR’s, GCT’s or SGCT’s
much the same as CSI’s do to rectify the input ac voltage [5]. The voltage is then input to
the DC bus, in this case a filter, to be smoothed out and is then the input to the inverter.
The voltage is always fixed regardless of the current. The VSI uses transistors, SCR’s or gate
turn off thyristors (GTO’s) in its inverter to produce a six-step output waveform [5]. Unlike
the CSI the VSI is not a current regulator but rather a voltage regulator. Voltage source
inverter VSD’s have advantages and disadvantages and these can be seen in table 3.
Advantages
Disadvantages
Basic simplicity in design.
Large power harmonic generation back into
the power source.
Applicable to multi-motor operations.
Poor input power factor due to SCR
converter section.
Operation not load dependent.
Cogging below 6 Hz due to square wave
output.
Non-regenerative operation.
Table 3 - Advantages and Disadvantages of VSI VDS's [5]
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Figure 4 - Basic Topology of a VSI VSD
Flux Vector Drive
The flux vector drive is a relatively new type of pulse width modulation variable speed
drive. It addresses the main disadvantage of AC variable speed drives, that they aren’t
capable of low rpm, high torque application. This had traditionally been the area for DC
drives. With the introduction of flux vector drives this philosophy has now changed.
Flux vector drives use a closed loop control that feeds from the motor to the VSD’s
microprocessor [7]. This enables real time data such as rotor position, speed and load levels
to be utilised in the control of the motor speed, power and torque. When the inverter is
operated in response to real time conditions it produces pulses that are outside the normal
sinusoidal range of controlling motor speed. This enables larger torque levels to be
achieved. Flux vector drives are almost exclusively used in applications that require motors
to change speed quickly. Using flux vector drives for other applications would be costly and
an overkill since conventional PWM variable speed drives are more than capable at
handling these situations [5]. The following table outlines the advantages and
disadvantages that flux vector drives have in comparison to other variable speed drives.
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Advantages
Disadvantages
Excellent control of motor speed, torque,
and power.
Higher initial cost as compared to standard
PWM drives.
Quick response to changes in load, speed,
and torque commands
Requires special motor in most cases.
Ability to provide 100% rated torque at zero
speed.
Drive setup parameters are complex.
Lower maintenance cost as compared to DC
motors and drives.
Table 4 - Advantages and Disadvantages of Flux Vector Drives
2.1.4 Advantages of Variable Speed Drives
There are numerous advantages to using variable speed drives in motor applications. These
range from greater energy efficiency to reduced system maintenance. The main advantages
of using VSD’s will be explored in the following:
Energy Savings
The reduction in energy consumption is perhaps the variable speed drive’s greatest
advantage and one of the reasons they have become so popular in modern systems. AC
motors are generally designed to operate at fixed speeds and be able of handling peak
loads. This can be quite energy inefficient as the motor will operate at the fixed speed even
when the loads are dramatically reduced. Since variable speed drives are able to control
the power supplied to the motor they can reduce the speed and thus power, which in turn
makes them much more energy efficient. In the case of variable torque loads, a much lower
torque is needed at lower speeds as opposed to higher speeds [8]. When the speed is
reduced to 50%, the required torque is also reduced to 25%, which in turn reduces the
horsepower to 12.5% of the value at full load [8]. This relationship can be seen in Figure 5.
For applications with constant torque loads the required torque is the same at high speeds
and low speeds. A drop in speed by 50% will result in a drop of power by 50% (Fig 6).
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Figure 5 - Variable Torque Load
Load Torque
Horsepower
Figure 6 - Constant Torque Load
Reduced System Maintenance
Since non VSD controlled motors are operating to handle peak loads, much more stress is
being put on the motor. The ability of the VSD to lower the speed of the motor inherently
lowers the stress.
Improved Process Control
Many variable speed drives are able to be integrated into automated process control
systems. This allows for process signals to be fed to the VSD for speed control and the
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starting and stopping of the drive [8]. It makes the process control system much more
dynamic by allowing increased functionality with programmable logic controllers and
distributed control systems.
Lower Start-up Voltage
When the motor is started the amount of in-rush current is limited by a variable speed
drive because the motor can gradually build up speed [8].
Multi-motor Capability
Depending on the type, variable speed drives can be capable of controlling multiple motors
from the one device. This decreases the initial cost of implementing VSD’s as only one drive
is required for multiple motors.
2.1.5 Disadvantages of Variable Speed Drives
Whilst there are many advantages and benefits of using variable speed drives in motor
driven systems there are also some disadvantages as follows:
Initial Cost
Compared to other speed control methods for motors, variable speed drives have a higher
initial cost. It has been stated that VSD’s offer energy savings, and this can offset this initial
cost. However for motor applications that require the motor to operate, on average, close
to the rated speed level, the energy saving is minimal and may not justify that cost [8].
Heating of Motor at Low Speeds
When operating the motors for constant torque applications at low speeds, motor heating
can become an issue. This is due to a number of factors; the first being that at low speeds a
motor cooling fan produces less air. The current to the motor also remains constant
regardless of the speed. The combination of these factors results in a greater risk of
overheating since there is less cooling air but the current remains the same.
Maintenance
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Being electronic devices, VSD’s requires additional knowledge of the device to be able to
troubleshoot or fix problems that arise. This may mean that a contractor will have to be
brought in who is specific to that product to be able to fix it.
Harmonics from Output
Since the electrical waveform generated by the VSD is not a pure sine wave, harmonics are
seen in the waveform and are supplied to the motor. These harmonics contain a current
component and with that create heat in the motor. Due to these harmonics VSD’s create
between 5 - 8% extra heat as opposed to a motor without a VSD [8].
Induced Power Line Harmonics
VSD’s can induce waveform distortion, in the form of harmonics, on the input power
supply. The way in which current is pulled off the waveform, in non-sinusoidal pulses,
means that harmonics and line distortion become an issue [8]. This can impact the rest of
the power supply in the facility and will require adequate protection on other devices.
2.2. RS-485 Communication Overview
This section provides a technical overview as well as background information on the serial
communication standard RS-485.
2.2.1. Serial Communication
Serial communication is the process of sending and receiving a byte of data (8 bits), one
data bit at a time. This differs from parallel communication which sends a byte of data
down 8 parallel channels. The advantages of serial communication are:
•
Simplicity – Serial communication needs less wires and also takes up less space.
•
Ability to be used over longer distances – Serial communication can be transmitted
over lengths up to 1200m.
•
Less Interference –Specifically crosstalk.
Serial data transfer can be done in two ways; Synchronous transmission and Asynchronous
transmission. These methods will be discussed, below.
Synchronous Transmission
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Synchronous transmission of data relies on a clock signal to synchronise the sender and
receiver. That is, there is an agreed upon time at which data is transferred. Data is
continually sent between the sender and receiver at the agreed upon intervals. Regardless
of whether or not there is data to be transmitted, a fill character is sent to ensure that data
is continually transmitting [9]. It can be more effective because only the data bits are being
sent and no start, stop or parity bits. Problems can occur when the clocks of the sender and
receiver get out of sync. This can potentially result in the transmission of corrupt data.
Asynchronous Transmission
Asynchronous data transmission relies on extra bits, added to the data to signify the
starting and stopping points of the data transmission. This can be seen in Figure 7. A start
bit is added to the start of the data byte and a stop bit at the end. A parity bit may be
added before the stop bit for error checking purposes. The start bit signifies to the receiver
that data is ready to be transmitted and the stop bit confirms that the data has been sent.
If the stop bit doesn’t appear as it should then the receiver recognises an error and
assumes the data to be corrupt.
Figure 7 - Asynchronous Serial Data Transmission
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2.2.2. RS-485 Standard
RS-485 is a serial communication standard that is based on the RS-232 standard. The
official title for the RS-485 standard is Electrical Characteristics of Balanced Voltage Digital
Interface Circuits; this is published by the ANSI Telecommunication Industry
Association/Electronic Industries Association (TIA/EIA) [10]. RS-485 is a popular
communication method in industrial process due to the fact that it can be used over long
distances and it can support multiple devices on the one bus. The RS-485 standard only
addresses the lowest level of the open system interconnection (OSI) model, the physical
layer.
Data Transmission
Data in a RS-485 network is transmitted differentially on a two-wired twisted pair bus. That
is, the transmitter and receiver are connected to two wires that have been twisted
together. The twisting is desirable, as it cancel out electromagnetic interference from
external sources. With a two wired RS-485 system the difference between the voltages of
the wires is what transmits the data. Logic 1 is defined by one polarity of the voltages and
the reverse polarity defines logic 0 [9]. It requires a minimum of a 0.2V differential to
operate and the applied voltage can be between -7V and 12V. Whilst RS-485 is known as a
two wired network it is also possible to have a four wired network. Two wired networks
only operate in half duplex mode. For this reason, the send and receive signals are sent on
the same wire and thus, only one device can transmit at any given time. With a four wired
network there are two sets of twisted pair wires allowing for both pairs to be able to
transmit and receive simultaneously. This is known as full duplex communication.
Figure 8 - RS-485 Data Transmission Waveform [11]
Network Topology
The recommended network topology for RS-485 networks is point to point series
connection nodes, a bus, or a line network topology [9]. Star and ring topologies are
Page | 15
generally not recommended due to signal reflections and termination impedance issues.
Termination resistors between the ends of the wires are generally needed to protect from
reflection from the drivers. Without termination, the resulting reflection can cause the data
to be corrupt.
Figure 9 - Recommended RS-485 Network Topology [12]
2.3. Synchronous and Servo Motors
The solar tracking system uses synchronous servo motors to drive the tilt and roll
mechanisms. This section will discuss the principles behind servo and synchronous motors.
2.3.1. Synchronous Motors
A synchronous motor is a motor that operates in synchronism with the frequency of an AC
supply current. The speed at which the shaft rotates is synchronised to this frequency [13].
This type of motor is generally used where an accurate and constant speed is needed. It
doesn’t require slip for torque to be produced and therefore can produce torque at
synchronous speeds.
2.3.2. Principles of Operation
A three phase voltage is applied to the stator windings which in turn induces a rotating
magnetic field. This magnetic field’s rotation is synchronised with the frequency of the
supply current. The rotor winding, which has its own magnetic field, tries to lock in to the
rotating magnetic field of the stator by creating a torque that rotates the rotor. If this
occurs the motor is said to be in synchronisation [13].
The synchronous speed of a synchronous motor can be determined by the following
formula:
120 ∗ Page | 16
Where ns is equal to the synchronous speed, f is equal to the frequency of the supply
current and p is the number of poles in the synchronous motor.
2.3.3. Types of Synchronous Motors
There are two types of synchronous motors: non excited motors and DC excited motors.
These two types of synchronous motor are differentiated based on the methods of
providing the rotor magnetic field.
Non Excited Motors
This type of synchronous motor has a rotor that is made of solid steel. The magnetic field of
the stator rotates and, as a result the magnetic field of the rotor (and thus the rotor) must
constantly try to catch up to the stator by rotating synchronously with it [13]. An induced
magnetic field on the poles of the rotor is created by the stator field. There are three types
of non-excited motors:
•
Permanent Magnet Motors
•
Hysteresis Motors
•
Reluctance Motors
DC excited Motors
These motors are excited by a direct current. The direct current is supplied to slip rings to
excite the motor. Current is conducted to the rotor through slip rings and brushes [13].
2.3.2 Servo Motors
A servo motor is a type of motor that uses a control loop and provides feedback to
determine the motors position [14]. The feedback information, often provided by some
form of encoder, is used to control a motors speed and position. As a control loop is
required a type of control is needed. Servo motors often utilise a proportional integral
derivative (PID) control method. The advantages of a servo motor are:
•
High intermittent torque
•
High torque to inertia ratio
•
High speeds
•
Work well for velocity control
•
Available in all sizes
•
Quiet [14]
Page | 17
The disadvantages of servo motors are as follows:
•
More expensive than stepper motors
•
Cannot work open loop since feedback is required
•
Require tuning of control loop parameters [14]
Servos are often used in conjunction with variable speed drives. They are a popular choice
for industrial applications due to their accuracy and reliability.
Page | 18
CHAPTER 3: System Overview
An existing bench top testing system was set up in the mechatronics room. This contained
two SEW Movidrive MDX61B variable speed drives, two SEW DFS synchronous servo
gearmotors and the associated cabling and connections. A designated master computer
was also present which contained the relevant software and communication adapters. This
section will discuss the hardware that was present on the existing system and the plans for
the final system.
3.1. Solar Tracking System
Although the major focus of this project was on the variable speed drives and their
communication with a computer based controller, it is important to understand how the
final system will be constructed and controlled. The solar tracker consists of a tilt
mechanism and a roll mechanism. The tilt mechanism controls the movement of the solar
tracker in a north/south orientation whereas the roll mechanism moves the array in an
east/west motion in the path of the sun. The mechanisms to control these movements
consist of a variable speed drive and a servo motor for each of the tilt and roll mechanism.
3.1.1. Motor Mount
There are various methods of mounting the motors for operation of the solar tracking
system. These included a linear actuated drive and a chain drive. However, as was
mentioned in Rhyss Edwards’s thesis [1], a decision had been made in consultation with
engineering staff members to proceed with a direct drive motor mount. This motor
mounting method can be seen in Figure 10. In order to employ this method the gear units
must be mounted upside down. However, the oil filter, drain plug and breather must be
shifted to the base of the gear unit as was discovered by Rhyss Edwards [1]. This mounting
position can be seen in Figure 11.
Figure 10 - Direct Drive Motor Mount [1]
Page | 19
Figure 11 - Upside Down Mounting Diagram [1]
3.1.2. Chronological Tracker
There are a number of controllers which utilise different methods to track the sun. These
can include a passive tracker, an active tracker and a chronological tracker. The type of
controller used in this solar tracking system is a chronological tracker. Other controller
types rely on specific renewable energy mediums in order for the trackers to operate. The
benefit of a chronological tracker is that any renewable energy medium can be used. This
means that several different types of energy mediums can potentially be used on one
array. This can be very beneficial if the solar tracker was to be used for educational
purposes such as this one. A chronological tracker tracks the sun from sun rise to sun set.
The benefit of this type of controller is that it is highly accurate if set up correctly and does
not require any feedback to determine the position of the sun. The disadvantage of a
chronological tracker is that it will track the sun even on cloudy and overcast days where
little to no sun is available. This results in minimal but some, energy inefficiency.
Method of Operation
The way a chronological tracker works is by using a solar positioning algorithm to calculate
azimuth and zenith angles from a physical location [15]. Figure 12 shows the azimuth and
zenith angles with respect to the tilt and roll references. The algorithm relies on data
relating to specific geographic location, date and time. The algorithm is relatively complex
and requires information to be calculated in regards to information about the sun. Some of
these values that are calculated include:
•
The observer local hour angle, (in degrees)
•
The geocentric sun declination, (in degrees)
•
The topocentric sun right ascension, (in degrees)
•
The true obliquity of the ecliptic, (in degrees)
•
The Earth heliocentric longitude, latitude, and radius vector (L, B, and R) [15]
Page | 20
These are some of the values associated with the solar position algorithm, the full
algorithm and associated information is contained in Appendix E
Figure 12 - Zenith and Azimuth Angles Used in a Chronological Solar Tracker [1]
3.2. Movidrive MDX61B
The VSD’s that will be mounted to control the tilt and roll motors are Movidrive MDX61B
VSD’s. The Movidrive MDX61B is a range of drive inverters (VSD) from SEW Eurodrive [16].
The benefits of these drives are that they have a wide voltage range, high overload capacity
and extensive functionality and features [17]. The specific versions of both these drives are
the standard design drives. The standard version drives come equipped with the IPOS plus
integrated positioning and control system. Figure 13 shows a connected MDX61B VSD in
full operation
Page | 21
Figure 13 – Connected and Operating MDX61B VSD
Whilst the two drives are the same model, their system capabilities differ. The model
numbers for each of the VSD’s can be seen in Table 5.
Drive 1
MDX61B-0005-5A3-4-00
Drive 2
MDX61B-0011-5A3-4-00
Table 5 - Drive Model Numbers
The capabilities of each drive, and therefore the differences are as follows, but essentially
the differences are output power, output current and the output frequency range.
Drive 1
Drive one has the following input and output data as seen in Tables 6 and 7.
Input
Value
Rated Supply Voltage
3-phase 380V-500V AC
Supply Frequency
50Hz to 60Hz
Rated Supply Current
1.8A AC
Table 6 - Input Data for Drive 1
Page | 22
Output
Value
Apparent Output Power
1.4kVA
Rated Output Current
2A AC
Output Frequency Range
0Hz to 180Hz
Table 7 - Output Data for Drive 1
Drive 2
The relevant drive input and output data can be seen in Tables 8 and 9.
Input
Value
Rated Supply Voltage
3-phase 380V-500V AC
Supply Frequency
50Hz to 60Hz
Rated Supply Current
2.8A AC
Table 8 - Input Data for Drive 2
Output
Value
Apparent Output Power
2.1kVA
Rated Output Current
3.1A AC
Output Frequency Range
0Hz to 600Hz
Table 9 - Output Data for Drive 2
As can be seen drive 2 is a more powerful unit than drive 1. It is capable of driving motors
with greater power. The recommended constant load motor power for drive 1 is 0.55kW,
whilst the recommended constant load motor power for drive 2 is double that of drive 1 at
1.1kW.
3.2.1 IPOS Plus Positioning and Sequence Control:
Movidrive MDX60B VSD’s come equipped with an integrated positioning and control
system known as IPOS Plus. The IPOS control enables highly accurate point to point
positioning capabilities [18]. The advantage of the IPOS plus system is that it takes most of
the real time control away from the master (PLC, PC). That is, the motion control is handled
by the variable speed drive itself. The master sends requests to the VSD’s on board IPOS
program and the IPOS program handles the positioning and sequencing of the motor. This
can greatly reduce the load on the master and in some cases it can completely replace it
[18].
Page | 23
IPOS programs can be created in the Movitools software package [16] using assembly
language. Movitools is SEW Eurodrive’s proprietary software that enables the user to set
up and program Movidrive VSD’s and IPOS programs. Existing application modules are also
available to download to the VSD, where the user only has to enter parameters regarding
the motor and the VSD's via a setup interface in Movitools.
3.2.2 IPOS Application Modules
There are several application modules available that are essentially pre built IPOS programs
for specific applications. They are as follows:
•
Bus Positioning
o
For applications that require the motor to move to a number of various
positions [18]. This is the suitable application module for a solar tracking
system.
•
Winding
o
Winding is used in applications where materials need to be endlessly
wound or unwound [18].
•
Flying Saw
o
This application module is used for materials that require a motor to drive
a saw to cut materials at certain lengths [18].
•
Crane Control
o
This module takes over the entire motion control in applications that
require objects to be lifted and transported in various directions [18].
•
Electronic Cam
o
Used in applications where cyclical machinery is used for complex moving
sequences [18].
•
Modulo Positioning
o
For use in applications that require a rotary movement such as hoists,
conveyer belts and rotary tables [18].
Further information can be found in the SEW manual: IPOS Plus Positioning and Sequencing
Control in Appendix C
3.3. DFS Synchronous Servo Motors
The solar tracking system utilises two 3 phase SEW DFS synchronous servo motors to
position the array, as shown in Figure 14. The benefits of these motors is that they are
extremely accurate, have a large torque range and a high overload capability. The motor
Page | 24
has a maximum output torque of 480Nm. The whole motor system consists of three main
components, the servo motor, 2 gear units and the encoder.
Encoder
Motor
Gear unit
Gear unit
Figure 14 - DFS Synchronous Servo Motor
The technical data for the synchronous servomotors can be found in the following table.
Rated Speed
3000RPM
Standstill Torque
1Nm
Standstill Current
1.65A
Max Motor Current
6.6A
Table 10 - DFS Synchronous Servo Gearmotor Data
Refer to Appendix C for more information regarding the DFS Synchronous Servo Motors.
Page | 25
CHAPTER 4: VSD Communication
A major aspect of this project revolved around understanding and establishing
communication between the variable speed drives and the programming software Labview.
This project focussed on using an RS-485 interface network to communicate between the
drives and the master computer. This section will discuss the method of communication
that SEW Movidrives utilise.
4.1. Telegrams
The variable speed drives communicate to the master through use of two types of
telegrams. These are the request and response telegrams [19]. The master first sends out a
request telegram to the VSD and then receives a response telegram back. As has been
discussed earlier RS-485 networks can only be either sending or receiving at any given time
so the VSD follows this rule.
4.1.1. Telegram Structure
The structure of each telegram is vital to enable communication with the variable speed
drive. The length of each telegram can be anywhere from 6 to 18 bytes (see figure 21 for an
example). The telegram starts with an idle time, which is a pause, and is followed by the
first byte. This first byte is the start delimiter; this signifies that data is ready to be
transferred to or from the VSD. The second byte represents the address of the slave, which
is the VSD’s address number. The third byte is the Protocol Data Unit (PDU) type and this
differentiates whether cyclical or acyclical data transmission is taking place. It also contains
information with regards to the length of the telegram. The following 2 to 14 bytes are the
Protocol Data Unit (PDU). These bytes are the actual request or response data that is being
sent in the telegram. The final byte in the telegram is a block check character (BCC) [19].
Request Telegram Structure
Figure 16 depicts the structure of an RS-485 request telegram being sent from the master
to the slave. The start delimiter for a request telegram is the hexadecimal number 02. This
signifies that it is a request telegram being sent and differentiates between the two types
of telegrams.
Page | 26
Figure 15 - Request Telegram Structure
Response Telegram Structure
Figure 17 shows the response telegram structure. The structure of the response telegram is
identical to that of the request telegram however it is the information in the data bytes
that differentiates between the two. SD2, the start delimiter for the response telegram is
the hexadecimal number 1D.
Figure 16 - Response Telegram Structure
Start Pause
The start pause is a pause before each telegram to signify that a telegram has either been
sent or is about to be received. It is important for the successful transmission of telegrams
between the master and slave. A minimum pause of 3.44ms must be adhered to by the
master before it can send a request telegram. This is to allow the variable speed drive to
definitively recognise that a request telegram is about to be received [19]. Before being
able to send a response telegram back to the master the VSD must then observe at least a
further 3.44ms before sending. Again, this is so that the master can recognise a response
Page | 27
telegram is about to start. If for some reason the master cancels a request telegram 2 start
pauses must elapse before another telegram can be sent [19].
Address Byte
The address byte represents the address of the slave regardless of the type of telegram
being sent. This allows the master to be able to specify to which VSD it wants to send a
telegram and also to distinguish which VSD the response telegram was sent. The address
range is between 0 and 255.Table 10 defines the address areas and corresponding
definitions. More information is available in the SEW manual; “Serial Communication”
available in Appendix C.
Address
0-99
100-199
253
254
255
Definition
Individual addressing with an RS-485 bus
Group addressing multicast (multicast)
Special case of group address 100: “Means not assigned to any group”,
i.e. ineffective
Local address: Only effective in conjunction with IPOS as master and the
MOVILINK command. For communication within the unit.
Universal address for peer to peer communication
Broadcast address
Table 11 - Addressing Areas for RS-485 Communication [19]
Protocol Data Unit Type Byte
The protocol data unit type byte defines the type of communication and also contains
information regarding the PDU. The type of communication can be cyclical or acyclical. As
can be seen in Figure 18 the seventh bit determines this type. A value of 0 defines cyclical
transmission and a value of 1 defines acyclical transmission. Cyclical transmission means
that the master will transmit data to the VSD in a timed cycle. If no request is sent within a
certain time, a timeout error will be recognised. Acyclical transmission is the sending of
data by the master in no defined period. That is, the master does not have to send a
telegram in a cyclical fashion.
Page | 28
Figure 17 – Type Byte Structure
The following two tables (12 and 13) show the various PDU types for cyclical and acyclical
data transmission. From these tables it can be seen that the length of a telegram can be
calculated by:
Telegram Length = PDU Length + 4
Further information on these PDU types can be found in the SEW manual: Serial
Communication, which can be found in Appendix C.
TYP Byte
PDU Name
00hex 0dec
PARAM + 1PD
01hex 1dec
1PD
02hex 2dec
PARAM + 2PD
03hex 3dec
2PD
04hex 4dec
PARAM + 3PD
05hex 5dec
3PD
06hex 6dec
PARAM + 0PD
Description
PDU Length
in Bytes
Telegram
Length in
Bytes
10
14
2
6
12
16
4
8
14
18
6
10
8
12
8 bytes parameter channel +
1 process data word
1 process data word
8 bytes parameter channel +
2 process data words
2 process data words
8 bytes parameter channel +
3 process data words
3 process data words
8 bytes parameter channel
without process data words
Table 12 - PDU Types in Cyclical Transmission [19]
Page | 29
TYP Byte
PDU Name
80hex 128dec
PARAM + 1PD
81hex 129dec
1PD
82hex 130dec
PARAM + 2PD
83hex 131dec
2PD
84hex 132dec
PARAM + 3PD
85hex 133dec
3PD
86hex 134dec
PARAM + 0PD
Description
PDU Length
in Bytes
Telegram
Length in
Bytes
10
14
2
6
12
16
4
8
14
18
6
10
8
12
8 bytes parameter channel +
1 process data word
1 process data word
8 bytes parameter channel +
2 process data words
2 process data words
8 bytes parameter channel +
3 process data words
3 process data words
8 bytes parameter channel
without process data words
Table 13 - PDU Types in Acyclical Transmission [19]
Block Check Character Byte
The final byte in the telegram is a block check character byte. This is to ensure that the VSD
or master is receiving data of integrity. The block check character is calculated by an
“exclusive or” logic operation using the parity of each of the transmitted bytes in a
telegram. The process for this can be found in the SEW Manual: Serial Communication
located in Appendix C.
4.2. Transmission Process
The transmission of the telegrams was discussed previously however this section will cover
the method of transmission at the lowest level. This covers the serial transmission of each
character in the telegram.
4.2.1. Character Structure
When using an RS-485 network with Movidrive MDX61B variable speed drives an
asynchronous serial transmission process is used to transmit data. Each character consists
of 11 bits and has the following structure:
•
1 start bit
•
8 data bits
•
1 parity bit
•
1 stop bit [19]
The first bit of the 11 bit character is a start bit. The actual data being sent is contained in
the following 8 bits. The parity bit is next and is always an even number. This means that
Page | 30
each character is of even parity. The transmission of the data is completed when the stop
bit is received. Figure 19 shows this structure.
Figure 18 - Character Structure
Page | 31
CHAPTER 5: Bench Top Testing Platform
This chapter will discuss the implementation of the hardware to form the bench top testing
platform. Although previous work had been completed on this project, there were some
issues with the existing bench top testing system which needed to be addressed.
Insufficient documentation was available for the project to be resumed from the previous
position so a complete overhaul was necessary.
Figure 19 - Bench Top Testing Platform Once Reconnected and Operational
5.1 Connections and Wiring
Previous work on the solar tracker neglected a significant amount of documentation,
particularly in wiring and connection of the variable speed drives. A large emphasis was
placed on research and documentation for future students, to aid any future progress on
this project. By carefully documenting all changes and progress, it is hoped that the issue of
inadequate documentation will not reoccur.
5.1.1 VSD Connection
The Movidrive MDX61B units had previously been mounted and connected to a three
phase power source by John Boulton. John had also connected the three-phase power to
the synchronous servo motors. An array of testing switches had been mounted to the
Page | 32
testing platform however these were no longer correctly wired in to the input terminal. The
RS-485 interface was also re-connected.
RS-485 Connection
The connection terminals for the RS-485 interface are located at terminal X13:10/11.
Terminal X13 is a set of isolated binary inputs that are used in the operation of the VSD.
The specifications for the cable required are as follows:
•
4-core twisted and shielded copper cable
•
Cross section of 0.25 to 0.75mm2
•
Cable resistance between 100-150Ω at 1MHz
The Profibus A type cable fit these criteria and was used for the connection of the RS-485
interface. The positive wire of the cable was connected to input X13:10 and the negative
wire to X13:11. The VSD’s also required that the shielding of the cable be connected to the
digital ground input at X13:9. Figure 21 shows this wiring diagram and also how multiple
VSD’s should be connected together on the RS-485 network. No external termination
resistors are needed on the bus as the VSD’s are already equipped with internal
terminating resistors [16].
Figure 20 - Wiring Diagram of the RS-485 Interface [16]
The RS-485 cable then needs to be interfaced to the master computer. A terminal block
was used to connect the positive, negative and ground wires to a 9 pin D-sub connecter (Fig
22). Only 3 of the pins are needed to be connected (pins 1, 4 and 5). These are the RxD-,
RxD+ and the common ground pins. A full list can be seen in table 13. A 120Ω terminating
Page | 33
resistor needs to be connected, on the computer side, across the bus lines. Without this
resistor signals can be reflected back to the bus and this can lead to the corruption of data.
Figure 21 - 9 Pin D-sub Connector
Pin
Number
1
2
3
4
5
6
7
8
9
Signal Name
Description
GND
CTS+
RTS+
RXD+
RXDCTSRTSTXDTXD+
Common Ground
Clear to Send +
Ready to Send +
Received Data +
Received Data Clear to Send Ready to Send Transmitted Data +
Transmitted Data -
Table 14 - 9-pin D-sub Pin Assignment
Terminal X13
Terminal X13 is a set of isolated binary inputs that are used for the operation of the
variable speed drives. An input list for terminal X13 with an enabled bus positioning IPOS
program can be seen in Table 14:
Page | 34
X13 Pin
Type
Name
1
DI00
Controller Inhibit
2
DI01
Enable
3
DI02
Fault Reset
4
DI03
Reference CAM
5
DI04
CW Limit Switch
6
DI05
CWW Limit Switch
7
DCOM
DCOM
8
VO24
VO24
9
DGND
DGND
10
RS-485 +
RS-485 +
11
RS-485 -
RS-485 -
Number
Table 15 - X13 Terminal Input List [16]
These digital inputs are involved with the start-up and operation of the connected servo
motor. Digital inputs 0 to 5 are wired into temporary switches. When implemented in the
final system research will have to be undertaken to decide and purchase the correct
switches for the field. Terminal X13 has one output that provides a 24V dc signal, this being
X13:8. This signal is utilised to power the temporary switches in the bench top testing
platform. When using this 24V signal to power binary inputs a jumper is connected
between X13:7 and X13:9. As previously discussed inputs X13:10/11 are connected to the
RS-485 cable.
5.2. VSD Initialisation
Once the connections and wiring of the drive had been established the drives were
initialised. This required direct communication to the drives using the Movidrives
proprietary software Movitools. Direct communication to the drive was achieved through
the XT port of the VSD. The XT port is a separate RS-485 interface that can be utilised by a
keypad or in this case for the connection to the Movitools software [16]. This can be done
by using either the SEW RS-485 to RS-232 converter or the RS-485 to USB converter (See
Fig 23 and 24).
Page | 35
Figure 22- SEW RS-485 to RS-232 Converter
Figure 23 - SEW RS-485 to USB Converter
5.2.1 Movitools Drive Connection
Movitools is the proprietary software provided by SEW to establish a connection and
initialise all the drive parameters. In a new project the VSD’s were added on to a network
and a choice of IPOS program and its parameters were selected and downloaded to the
drive. Movitools initially scanned the RS-485 interface until it detected a connected
variable speed drive. The process was needed to be repeated as only one drive can be
connected at a time, unless multiple SEW converters are available. Following this, the
drives were added on to the same network where each individual VSD was assigned an
address. These addresses are used in determining where a telegram is to be sent or
Page | 36
received in the network. A user guide for initialising an SEW variable speed drive has been
created and can be found in Appendix D.
5.2.3 IPOS Program
As has previously been mentioned the Movidrive MDX61B VSD’s come equipped with an
integrated positioning and control system known as IPOS Plus. It is this program that
handles the positioning and control of the connected motor. A decision had been
previously made to utilise the existing IPOS module known as “Positioning via Bus”. This
IPOS module is suitable for applications that utilise a direct drive motor mounting position.
The module is selected in Movitools and provides a setup interface that just requires
parameters regarding the motors and drives to be entered. An example of this can be seen
in Figure 25. In this process a referencing travel type was selected.
Figure 24 - Example Positioning via Bus Interface
Page | 37
5.2.4 VSD Referencing Mode
There are 9 different modes of referencing available for use within the IPOS program. A
decision needed to be made on which referencing mode was most appropriate for the solar
tracker. The reference modes are:
•
Type 0: Reference travel to zero pulse
•
In this reference mode the reference position is at the first zero pulse
detected when the reference travel is counter clockwise [18].
•
Type 1: Counter clockwise end of the reference cam
•
The motor is referenced when a reference cam signal is seen by the
VSD. The direction of travel is counter clockwise and the signal must
be a pulse, meaning a rising and then falling edge must be seen before
the motor references.
•
Type 2: Clockwise end of the reference cam
•
The reference point of the motor is determined by a reference cam
signal. The signal must be a pulse and the direction of travel for
referencing is clockwise rotation.
•
Type 3: Clockwise limit switch
•
The motor will reference upon contacting the limit switch. The
reference travel direction is clockwise.
•
Type 4: Counter clockwise limit switch
•
The motor will reference upon contacting the limit switch. The
reference travel direction is counter clockwise.
•
Type 5: No reference travel
•
•
The motor takes its machine zero from wherever it currently is.
Type 6: Reference cam flush with the clockwise limit switch
•
The reference point is the first zero pulse or falling edge to the left of
the reference cam [18].
•
Type 7: Reference cam flush with the counter clockwise limit switch
•
The reference point is the first zero pulse or falling edge to the right of
the reference cam [18].
•
Type 8: Without enable
•
Referencing can take place when the drive is not enabled. The
reference point is the motors current position [18].
Page | 38
Further information can be found in the SEW manual; IPOS Positioning and Sequencing
Control. This can be found in Appendix C.
Discussion
For the solar tracking system it was decided that reference type 1, referencing to the
counter clockwise end of the reference cam, be used. Since a signal is used to determine
the reference point and it requires a rising and falling edge, a magnetic reed switch would
be appropriate for the motor to reference machine zero. The benefit of this is that the
motor can reference to a true zero which requires no offset for the motor. Referencing to
the limit switches would also be appropriate however it would require an offset to be
determined for machine zero. Further research would then be required to determine the
most appropriate referencing method out of types 1, 2, 3 and 4.
5.3. Control Structure
The control structure of the overall system can be seen in Figure 26. It displays the
hierarchical structure of the overall system. A computer running the Labview program will
control the requests made to each of the VSD drives via the RS-485 communication
interface. The computer will need to have a National Instruments RS-485 card installed to
be able to communicate via Labview over RS-485 to the variable speed drives. The
computer will be mounted on site near the final constructed system. A decision will need to
be made on the computer as factors such as heat and dust could affect the operating
capabilities of the computer.
MS
Windows
Computer
Labview
Controller
1 VI
Controller
2 VI
Field RS-485
Link 1
Link 2
VSDs
Tilt 1
Roll 1
Tilt 2
Roll 2
Figure 25 - Control Structure of the Solar Tracking System
Page | 39
CHAPTER 6: Tracking Program Development
An existing tracking program developed by Rhyss Edwards was previously used as the
controller for the solar tracking system. This Labview program has been further modified to
improve the transmission of data from the Labview program to the variable speed drives.
An overview of the program and the changes made to it will be discussed in this Chapter.
6.1 Controller Overview
There are 3 main modes of the Labview controller. They are the reference mode, automatic
mode and jog mode. Each of these modes can be selected via a tab on the front panel of
the Labview program. Upon start-up of the program the controller automatically enters the
reference mode. Once referenced the operator has the choice of whether to control the
system automatically or manually via the jog mode. Figure 27 shows the front panel of the
Labview Program.
Figure 26 - Labview Front Panel
Page | 40
6.1.1. Reference Mode
In the reference mode the program sends a request to the VSD to move the motors in a
counter clockwise direction at a designated reference speed. The program then waits until
a reference signal has been passed through the inverter which stops the motor and
designates the position as machine zero. This signal is activated via a switch wired in to
terminal X13:4 on the VSD’s. Once referenced the IPOS reference indicator will be activated
on the front panel (see Fig 28).
Figure 27 - Motor Indicators
6.1.2. Automatic Mode
The automatic mode can be accessed via the tab on the front panel. Within this mode (see
Fig 29) there is the option of having the tracking system be in tracking mode, manual stow
mode and cleaning mode. The manual stow position moves the array into the stow position
if the operator needs to stow the array during the day. The cleaning mode moves the array
into a position conducive to cleaning. The tracking mode is used for tracking of the sun.
Figure 28 - Tab Options in Automatic Mode
Tracking Mode
The tracking mode front panel (see Fig 30) gives the operator the ability to change the
speed of the motors, the update delay, and the tilt and roll offset values. Figure 30 shows
Page | 41
the front panel of the tracking mode. The program updates the set point information
depending on the value in the update delay. Within the tracking mode there are 5 possible
states that the program can enter depending on various conditions. These states are as
follows:
•
Normal – When in the normal state all set point values are held until the next
update.
•
Update – Updates the set point for both motors based on the solar tracking
algorithm.
•
Software Limits – Motors are moved into the stow position when the software
limits are reached by the motor. Waits until the set points for each motor are
back within the limits.
•
Night Stow – When the sunset time is reached the array moves in to the stow
position until the following days sunrise.
•
Wind Stow – Moves the array into the stow position and waits until the wind
speed is below that of the allowed limit.
Page | 42
Figure 29 - Tracking Mode Front Panel Features
6.1.3. Jog Mode
This mode allows the operator to manually jog the motors backwards and forwards as a
method of manual control. A bug has been identified that does not allow the motors to
reverse once the motors have been referenced and automatic mode has been operating.
Instead the motors only move forwards. An investigation into this issue was undertaken.
Jog Backwards Error
An investigation was carried out to try and determine the cause of the issue. The program
disabled one of the VSD’s so that only one was being controlled. The request telegrams
were then monitored to determine which byte was sending the wrong information. Once
the motor was referenced the motor was jogged backwards and the telegrams were
monitored and documented. The tracker was then put into automatic mode and then back
Page | 43
into jog mode. The motor was jogged backwards, which due to the error moved the motor
forwards, and the request telegram monitored. The hypothesis was that the telegram
where the bug was present would have a byte that differed, from when the motor jogged
backwards correctly. This was thought to be the cause of the problem. This however
proved to be untrue as the telegrams always remained the same. No fix was able to be
found. However, it appears that the issue may be occurring within the IPOS program rather
than the programming in Labview. The telegrams for each are as follows:
Telegram When Motor Jogs Backwards
Telegram When Error is Present
02 01 05 0D 06 03 E8 04 3C DE
02 01 05 0D 06 03 E8 04 3C DE
Table 16 - Monitored Telegrams
6.2. Program Development
A major focus was on improving the data transfer between the Labview program and the
VSD. This was due to the previous method lacking adequate sequencing and structuring of
the data transfer process. This involved developing a more sequenced and structured
method of transmitting the data from the controller to the roll and tilt VSD’s. The previous
method used was structured incorrectly. Instead of sending a telegram to ask for
information from the drive first, the program would first read whatever bytes were at the
drive. The new method first sends a request telegram to the VSD and then waits for a
response. This structure is more appropriate for RS-485 transmission. The RS-485 read and
write operations are now integrated in a state structure. There are 5 states that control the
read and write processes these are:
•
Initialisation
•
Send Request
•
Get Response
•
Process Message
•
Do Nothing
6.2.1 Read and Write Process
The 5 states have been designed in order to provide a specific sequence for the telegrams
to be sent and received. This will lower the chance of telegrams with corrupt data being
transmitted. The program will not enter the next state unless all conditions in the previous
Page | 44
state have been met. This ensures that the correct telegram is being delivered to the
correct VSD.
Initialisation
This state (see Fig 31) initialises the values in the shift registers so previous values aren’t
fed into the program. Once initialised, the program moves into the “Send Request” state.
Figure 30 - Initialisation State
Send Request
Once initialisation has taken place the program enters the “Send Request” state. A request
telegram is generated from the information in the tracking section. The generate output
subVI then compiles the information and sends the request as a string to the VISA write
module. This module opens the port and transmits the telegram over the RS-485 network
to the variable speed drive. Once complete, the program moves into the “Get Response”
state.
Page | 45
Figure 31 - Send Request State
Get Response
In this state the program initialises the VISA read block to accept 10 bytes from the port. A
light on the front panel will be displayed on the front panel to signify that there are bytes at
the port. This is useful in the debugging process as it can determine whether information is
being received. The bytes are then converted from an integer to a string. From here the
program confirms that 10 bytes have been received before it can move in to the next state.
The converted string is then displayed in the advanced tab on the front panel for error
checking purposes.
Figure 32 - Get Response State
Page | 46
Process Message
This state processes the response telegram from the VSD to retrieve the motor position
and the operating status information of the motor so it can be displayed to the operator on
the front panel. The telegram must first be processed in the Check Telegram subVI. This
determines whether the telegram adheres to the correct structure that was explained in
Chapter 4. Within this subVI a Block Check Character (BCC) check is also computed to
determine the telegrams validity. If the telegram fails either the telegram structure or BCC
check then the program will return to the “Send Request” state in order to repeat the
process and receive a valid telegram.
Figure 33 - Process Message State
Do Nothing
Despite the name of this state being called “Do Nothing” it does actually have a purpose.
The program will only enter this state once the successful sending and receiving of a
telegram has occurred. It signifies the end of the sending and receiving process and enables
the program to move back into the “Send Request” state to send further data. It is also the
state where the program decides which VSD is to be the sender and receiver for the
following loop. Since the loop time is 100ms the program sends and receives telegrams to
both VSD’s every 200ms.
Page | 47
Figure 34 - Do Nothing State
6.2.2 Discussion
The new method of sending and receiving data has streamlined the process and given the
program a more suitable structure for this process. Whilst the program is fully functional a
temporary fix has been included for one minor issue. When a response telegram is being
processed it must pass the telegram structure and BCC check. Failing this the program
would be sent back to the “Send Request” state. This in theory works fine. However, due to
the program not being able to switch the VSD it becomes out of sync with the telegram and
the error becomes permanent. The temporary fix was to implement a count function that
counts the amount of times the errors occur and when it reaches 10 the program will enter
the “Do Nothing” state where the VSD is switched (See Fig 36). This currently fixes the
telegram error and resynchronises the program. A fix for this was unable to be completed.
However, finding a different position for when the program switches communication
between VSD’s could be one solution. A count method should remain in the error case and
a message displayed if the error occurs 10 times in a row.
Page | 48
Figure 35 - Temporary Error Correction
The new modified send and receive process has now also improved the way in which the
master communicates to multiple drives. The previous method used the loop counter to
determine which drive a telegram was to be sent to and received by. Every loop it would
alternate between the two drives. The issue with this method was that it was possible for
only part of a telegram to be received in one loop and then the program would switch
VSD’s to communicate with. This method can be seen in Figure 37. The new process waits
until the whole message has been received from the VSD before it will switch
communication to the drives. This lowers the chance of corrupt data being transmitted
over the RS-485 network.
Figure 36 - Previous Method for Controlling Multiple VSD's
Page | 49
CHAPTER 7: Conclusion
7.1. Project Conclusion
The use of variable speed drives and its associated technologies has become increasingly
popular in industrial applications. This can be attributed to the many advantages they offer
to motor driven systems. These advantages include; the ability to operate a motor in a wide
range of speeds and the ability to increase energy efficiency in motor driven applications
[2]. This technology has been implemented in the design of a solar tracking system for use
at Murdoch University.
Work on the development of the solar tracking system had previously been completed by
the students of ENG454 in 2009 and further developments were made by Rhyss Edwards in
his thesis “Control System Design and Commissioning of Photovoltaic Trough Concentrator
Systems Installed at the Murdoch University South Street Campus” in 2010 [1]. This work
had left the project with a functional, albeit not totally robust, Labview controller. Thus,
further development was needed.
The project focussed on three key stages; research and documentation, robust testing and
further development on the tracking program. From very early on it was apparent that the
existing system lacked suitable documentation in regards to the system set up and key
operational instructions. The majority of the early stages of the project were focussed on
the research of the existing tracking system and its key components. These components
included; variable speed drives, servo motors and RS-485 communication technologies. A
review of the specific SEW Movidrive MDX61B was undertaken to ensure that correct
connections and wiring procedures could be completed to advance to the next stage of the
thesis. Key documents were gathered on the information researched and documentation
has been produced for the initialisation of the variable speed drives and correct operational
procedures. These documents have been included in the Appendices. Once enough
knowledge had been gained the robust testing of the bench top testing platform was
commenced.
The primary task of the robust testing phase was to replicate the position that Rhyss
Edwards had achieved in his work on the project in 2010. This process was complicated by
the lack of documentation. Establishing direct communication of the variable speed drives
was achieved using the proprietary software SEW Movitools and allowed an IPOS control
program to be configured and downloaded on to the drives. First control of the motors was
Page | 50
achieved here using this software. Following this, the existing Labview controller was used
to establish communication, manual and automatic control of the tilt and roll motors.
Several programming issues such as the method of data transmission and multiple motor
control arose throughout this stage which led to the improvements and further
developments to the solar tacker program in the final stage.
Improvements were made to the existing Labview tracking program throughout the
development stage. In the testing phase it had become apparent that there were issues
regarding the transfer of data to the variable speed drive on the RS-485 network. To
address these issues the RS-485 read and write operations were re-developed within the
Labview program. A more sequenced and structured read and write stage was
implemented resulting in a more accurate method of data transmission. Greater error
checking methods were also employed in the read and write process to ensure that only
reliable data was being passed to and from the VSD. Minor changes were made to the front
panel of the program to make it more user friendly for future students.
A greater knowledge of variable speed drives, Rs-485 communication and more specifically
the transmission data across this interface has been attained and recorded throughout this
project. This will aid staff and students if these technologies are used in other projects or
applications in the engineering course. The solar tracking program is fully operational albeit
for some minor bugs, but these are far less than when the project was started. The goals
that were set out to be attained throughout this project have been achieved. The project
has a large scope for improvements and future work, which can be undertaken to further
develop the solar tracking system.
7.2. Future Work
There is potential for advancements to be made to the solar tracking system due to the
complex nature of the system. Future works that should be undertaken on this project are
as follows:
Labview Tracking Program
The provided Labview program provided is currently fully functional. However further
improvements should be made to the program. These include:
Page | 51
•
Finding a solution to the reference mode reverse problem. This problem was
first documented by Rhyss Edwards in 2010. Testing was undertaken in this
project to try and discover the issue. Telegrams being sent to the VSD’s were
monitored and no difference in the telegrams was noticed in reference mode
compared to non-referenced. More information can be found in Chapter 6.
•
Telegram Structure Error and BCC check error. Currently when these errors
occur, albeit rarely, the state of the transmission process becomes out of sync
with the VSD. A temporary solution has been employed however; it currently
does not address the errors appropriately. This method is unsuitable for final
implementation so a solution is required. Further details are available in
Chapter 6.
•
Implementing hardware limit switches. Previously hardware limit switches
were not enabled within the VSD’s. This has been corrected and test limit
switches are now functional. An error reset method needs to be employed in
Labview so the error can be cleared from the VSD via the front panel of the
controller.
Referencing Method
Currently the reference method existing on the variable speed drives is “counter clockwise
end of the reference cam”. There are 8 other methods of referencing available in the IPOS
control program. Referencing via limit switches has been tested and successfully used in
these tests. Further research will have to be done to decide which reference method best
suits the solar tracker for use in the final tracking program.
Anemometer
Procurement of an anemometer is advised for use on the final constructed system. Wind
damage could be possible if the final system has no way of stowing if dangerous winds
were to come. So an anemometer is required to detect wind speed. It has been decided
that the condition for determining when the tracking system shall be stowed will be when a
wind gust of 70km/h is detected for 2 seconds [1]. This was documented by Rhyss Edwards
in the previous work and is currently the method used on the large array tracking system at
Murdoch University [1].
The options for implementing the anemometer in the Labview program will have to be
further explored. There is a terminal for analog inputs and also a terminal of digital inputs
Page | 52
on the Movidrive MDX61B VSD. The analog inputs are available for determining a set point
for the motors. Since this set point method is not employed by the current tracking
program, further research will be needed in order to determine whether the analog input
can be utilised for other devices. Failing this method the currently unused digital inputs on
terminal X13 can possibly be used. Once again further research will be required to explore
this option. If this is possible, staff and a technical officer will need to be involved so that a
design for the anemometer can be decided. The anemometer will need to output a digital
signal when the wind speed is above the current limit threshold.
Field Implementation
A motor mount design was decided upon by Rhyss Edwards in consultation with Murdoch
staff in 2010. John Boulton was heavily involved in this process and consultation with him
will be necessary to mount the roll motors. Following this a renewable energy medium will
need to be decided upon. Ideas were put forward by Rhyss Edwards these included solar
panels, solar troughs or solar thermal systems [1]. After consultation with Graeme Cole a
decision has yet to be made. He indicated the possibility to use a variety of renewable
energy mediums on the one array to increase the value as a teaching tool. The decision will
need to include engineering staff as the final system will be a teaching tool for use in the
Industrial Computer Systems and Renewable Engineering streams.
Further to that limit switches will need to be chosen and procured for use as the CW and
CCW limits. If the reference cam reference method is chosen a switch for this reference
point will also be decided. It is possible that this could be a magnetic reed switch so the
motor can pass it once to reference (a rising and falling edge is needed). Since the limit
switches are used by the VSD’s the option of low voltage limit switches is possible [1].
Further components and plans need to be decided upon as noted by Rhyss Edwards in
2010. These are:
•
Field cabinet selection
•
Sourcing a Field PC
•
Wiring Diagrams
•
Field box design [1]
These items will need to be sourced and designed so the final solar tracking system can be
implemented at the Murdoch University campus.
Page | 53
Bibliography
[1]
Rhyss Edwards. (2010, November) Control System Design and Commissioning of
Photovoltaic Trough Concentrator Systems Installed at the Murdoch University South
Street Campus. Thesis.
[2]
ABB. (2008, November) ABB. [Online].
http://www.abb.com/cawp/db0003db002698/a5bd0fc25708f141c12571f10040fd37.aspx
[3]
Office of Energy Efficiency Canada. (2009, April) Natural Resources Canda. [Online].
http://oee.nrcan.gc.ca/industrial/equipment/variable-frequency-drives/10251
[4]
Dave Polka. Joliet Technologies. [Online].
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[5]
Solomon S Turkel. (1999, March) EC&M. [Online].
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[6]
Aaron Vander Meulen and John Mauron. (2010, August) Eaton. [Online].
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Dave Polka. Joliet Technologies. [Online].
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[8]
Process Automation Control. Process Automation Control. [Online].
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[9]
Belden. (2012) Belden - EIA-485. [Online].
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[10]
Manny: Zhang, Jing Soltero and Chris Cockril. (2010, May) Texas Instruments. [Online].
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[11]
Roy Ovesen. (2007, January) Wikipedia. [Online]. http://en.wikipedia.org/wiki/File:RS485_waveform.svg
[12]
Lammert Bies. (2011, August) Lammert Bies. [Online].
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Johnson Electric. (2012) Johnson Electric. [Online].
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Ibrahim Reda and Andreas Afshin. (2008, January) Solar Position Algorithm for. PDF.
[16]
SEW EURODRIVE. (2010, January) Movidrive MDX61B Operating Instructions. PDF.
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SEW Eurodrive. (2011) SEW Eurodrive. [Online].
http://www.seweurodrive.com/produkt/movidrive-b-drive-inverter.htm
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SEW Eurodrive. (2009, November) IPOS Positioning and Sequence Control. PDF.
[19]
SEW EURODRIVE. (2001, November) Serial Communication Manual. PDF.
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ABB. (2011, May) ABB Australia. [Online]. www.abbaustralia.com.au
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Douglas B Weber. (2010, April) Motion Systems Design. [Online].
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Phuc Nguyen. (2009, July) Connexions. [Online]. http://cnx.org/content/m28695/latest/
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Glenn Johnson. (2009, May) Process Online. [Online].
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SCADA environment for Environmental Purposes. Thesis.
Page | 55
Appendices
Please see the CD for the following appendices.
Appendix
A – Solar Tracking Program
•
Labview Controller
•
Previous Version
B – Labview Programs
•
Control of One VSD
•
Movidrive Read and Write
C – SEW Manuals
•
Movidrive MDX61B
•
Servo Motors
D – Documentation
•
Installing an IPOS program
•
Labview Solar Tracker User Manual
•
Input/Output List
E – SPA
•
Solar Tracking Algorithm Manual
F – Progress Report and Proposal
Page | 56
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