Vision PID FB + Autotune

Vision PID FB + Autotune
PID FB + Autotune
28/9/05
Table Of Contents
PID FB........................................................................................................................................................................ 1
FB Operations......................................................................................................................................................... 1
PID Autotune.......................................................................................................................................................... 1
Examples ................................................................................................................................................................ 2
PID Configuration ...................................................................................................................................................... 3
PID Function Parameters........................................................................................................................................ 3
Run Auto-Tune........................................................................................................................................................... 7
Auto-tune Parameters ............................................................................................................................................. 7
Run PID...................................................................................................................................................................... 9
Pause Integral & Derivative Calculation .................................................................................................................. 11
Read Control Components ....................................................................................................................................... 13
Error Integral ............................................................................................................................................................ 15
General Background: How PID Works .................................................................................................................... 17
About PID and Process Control............................................................................................................................ 17
Inside the PID Function........................................................................................................................................ 18
Proportional Band ........................................................................................................................................... 18
Integral Action ................................................................................................................................................ 19
Derivative Action............................................................................................................................................ 20
PID Status Messages ................................................................................................................................................ 21
PID Tips ................................................................................................................................................................... 23
Background: the Proportional Band and PV ........................................................................................................ 23
PID Analog Input Tips ......................................................................................................................................... 23
PID Range Settings: PV Low and PV High limits.......................................................................................... 23
PID via Digital Output (Contactor, Solenoid Valve, SSR)................................................................................... 25
Manual Loop Tuning............................................................................................................................................ 25
Index......................................................................................................................................................................... 27
i
PID FB
The PID FB enables you to use system feedback to continuously control a dynamic process. The purpose of
PID control is to keep a process running as close as possible to a desired Set Point. VisiLogic's PID FB includes
auto-tune.
In order to ensure proper PID function, you should use Autotune.
During the Autotune process, the PID function collects certain essential data. Unitronics' proprietary PID
algorithm uses this data to run smooth, accurate PID.
FB Operations
PID Configuration
Run Auto Tune
Run PID
Pause Integral & Derivative Calculation
Error Integral
Read Control Components
PID Autotune
Auto-tuning loops enables the system to set the parameters for PID.
The picture below shows the elements of a basic PID application with Auto-tune.
1
PID FB + Autotune
After Auto-tune runs, the P, I, and D values are automatically written to the Configuration parameters and the
Auto-tune vector is also filled with the Auto-tune parameters.
Note ♦
Note that, once you have run Auto-tune, you can back up the P, I, and D values, the sample time
(ST), and the 32 MI-long Auto-tune vector into a Data Table. You can then transfer these
values to another Vision controlling an identical system, in order to run PID without tuning the
loop.
Examples
PID Tips can help you with certain PID applications, such as those using SSR switches.
Sample applications may be found in the VisiLogic Examples folder. This folder contains field-tested VisiLogic
(.vlp) sample applications. You can open this folder via the Help Menu.
The folder is typically located at: C:\ProgramFiles\Unitronics\VisiLogic\Examples\Verx.xx, where x.xx
indicates the version of VisiLogic.
2
PID Configuration
To place a PID Configuration:
1.
Select PID Configuration from the FBs menu, then place the function in the net; the PID parameter box
opens.
2.
The Select Operand and Address box opens; prompting you to link operands to the PID parameters.
Note ♦
To enable PID, values must be provided for:
ƒ Set Point
ƒ Input Range:PV Low Limit & PV High Limit
ƒ Output Range:CV Low Limit & CV High Limit
These values are used to Auto-tune the loop.
After Auto-tune runs, the P, I, D and Sample Time values are automatically written to the Configuration
parameters.
PID Function Parameters
Parameters:
Inputs
Type
Function
SP:
Set Point
MI
SP is the target value for the process. In a heating system, this is the temperature value set for
the system. Note that the Set Point and Process value must be given in the same type of units
(degrees Celsius, bars, meters per second, etc.)
PV:
Process Value
MI
PV is the feedback from the process. PV is output from the process and input to the PID
function. In a heating system, the temperature measured by a temperature sensor provides the
PV.
Kp:
Proportional
Band
MI
Use this parameter to define the proportional band, in units of 0.1%. The proportional band is
a percentage of the total Process Value (PV). It is a range defined around the Set Point.
When the PV is within this range, the PID function is active.
Ti:
Integral Time
MI
Use this parameter to define the integral time, in units of 1 second. Integral action responds to
the rate of change in the controller’s CV output relative to the change in Error. The integral
time you set is the amount of time, as calculated by the controller, required to bring the
process to Set Point.
3
PID FB + Autotune
Note that integral (wind up) error may be initialized via the Force Error Integral operation.
Td:
Derivative Time
MI
Use this parameter to define the derivative time, in units of 1 second. Derivative action
responds to the rate and direction of change in the Error. This means that a fast change in
error causes a strong response from the controller. The derivative action ‘anticipates’ the
PV’s value in relation to the Set Point and adjusts the CV accordingly, thus shortening the
PID function’s response time.
ST:
Sample Time
MI
Use this parameter to define the intervals between PID function updates, in units of 10mSecs.
Action:
0: Heat, 1: Cool
MB
Select Off to activate Reverse Action (control type = heating), ON to activate Direct Action (
control type = cooling ).
Input Range:
Process Value
Low limit
MI
Use this parameter to define the lower limit for the Process Value.
Input Range:
Process Value
High limit
MI
Use this parameter to define the upper limit for the Process Value.
Output Range:
Control Value
Low limit
MI
Use this parameter to define the lower limit for the Control Value.
Output Range:
Control Value
High limit
MI
Use this parameter to define the upper limit for the Control Value.
Parameters:
Outputs
Type
Function
CV:
Control Value
MI
CV is the output from the PID function. CV is output from the PID function and input to
the process. Note that this output signal may be an analog or time-proportional variable
value.
Status Messages
Initialized to 0 when
Configuration is
activated.
MI
Value
Message
>=0
FB status OK
<0
4
-1
Proportion band zero.
-2
Input range is invalid (PV input).
-3
Output range is invalid (CV output).
-4
Integral has reached maximum of 100,000. PID will not allow the
Integral value to increase any further.
-5
Error in Auto Tune vector addresses, ex., the vector exceeds the final
address in the MI data type.
PID Configuration
Auto-tune
parameters
Note ♦
MI
-6
Set Point less then Input low range or Set Point more then Input high
range.
-7 to-10
Auto tune error.
-11
Noise is more then 5% of Input Range.
The start of a 32 MI-long Auto-tune vector that contains the Auto-tuned
parameters.
Note that, once you have run Auto-tune, you can back up the P, I, and D values, the sample time
(ST), and the 32 MI-long Auto-tune vector into a Data Table. You can then transfer these
values to another Vision controlling an identical system, in order to run PID without tuning the
loop.
5
Run Auto-Tune
The Run Auto-tune operation uses the Configuration's parameters:
ƒ
ƒ
ƒ
Set Point
Input Range:PV Low Limit & PV High Limit
Output Range:CV Low Limit & CV High Limit
These values are used to Auto-tune the loop. After Auto-tune is run, the Auto-tune MB turns ON, and all of the
Auto-tune parameters are written into the Autotune Parameter MI vector that is defined in the PID
Configuration.
Note ♦
Note that, once you have run Auto-tune, you can back up the P, I, and D values, the sample time
(ST), and the 32 MI-long Auto-tune vector into a Data Table. You can then transfer these
values to another Vision controlling an identical system, in order to run PID without tuning the
loop.
In order to Auto-tune the loop, the PID Run operation must be suspended.
Auto-tune Parameters
Parameters:
Inputs
Type
Function
Stage
#
The number of Stages aids the system to determine accurate Auto-tune parameters. The
Default is 3. The higher the number of stages, the longer the Auto-tune time, however
choosing a lower Stage may result in less accurate Auto-tune parameters.
Auto-tune Done
MB
After Auto-tune is run, the Auto-tune MB turns ON, and all of the Auto-tune parameters are
written into the Auto-tune Parameter MI vector that is defined in the PID Configuration.
7
Run PID
In order to run a PID loop, the Run operation must be included in the application following the PID
Configuration. In order to Auto-tune the loop, the PID Run must be suspended.
9
Pause Integral & Derivative Calculation
If conditions require, you can suspend this value and prevent it from changing. This may prove useful, for
example, in a temperature application, when an opened oven door can cause a temporary temperature drop.
11
Read Control Components
This function enables you to scale down very large PID control values to smaller, more logical values. The
current functions factors the PID control values by a value in an MI, then stores the values in the output MIs.
Parameters
Type
Function
Resolution Factor
#
This is the value used to factor the PID control values.
Control Value:
Proportional Output
MI
Stores the factored Proportional Output.
Control Value:
Integral Output
MI
Stores the factored Integral Output.
Control Value:
Derivative Output
MI
Stores the factored Derivative Output.
13
Error Integral
You can read and write to the Integral Value.
Read Error Integral
Use this operation to store the current error in the linked ML.
Force Error Integral
Use this to initialize or change the error value while the application is running. You can erase wind up error by
writing '0' into the linked register.
15
General Background: How PID Works
The PID function uses system feedback to continuously control a dynamic process. The purpose of PID control
is to keep a process running as close as possible to a desired Set Point.
About PID and Process Control
A common type of control is On-Off control. Many heating systems work on this principle. The heater is off
when the temperature is above the Set Point, and turns on when the temperature is below the Set Point. The lag
in the system response time causes the temperature to overshoot and oscillate around the Set Point.
PID control enables you to minimize overshoot and damp the resulting oscillations.
PID enables your controller to automatically regulate your process by:
1.
Taking the output signal from the process, called the Process Variable (PV),
2.
Comparing this output value with the process Set Point. The difference between the output Process
Variable and the Set Point is called the Error signal.
3.
Using the Error signal to regulate the controller output signal, called the Control Variable (CV), to keep
the process running at the Set Point. Note that this output signal may be an analog or time-proportional
variable value.
In the figure below, a system is regulated according to temperature.
17
PID FB + Autotune
Inside the PID Function
The PID function is based on 3 actions, Proportional, Integral, and Derivative. The PID output is the combined
output of all 3 actions.
All of the PID functions are activated by changes in the process Error, the difference between the Process Value
and the process Set Point value (E = SP – PV).
Proportional Band
The proportional band is a range defined around the Set Point. It is expressed as a percentage of the total
Process Value (PV). When the PV is within this range, the PID function is active.
Note ♦ The proportional band may exceed 100%. In this case, PID control is applied over the entire system
range.
Proportional Action
Proportional action begins after the PV enters the proportional band; at this point, the Error is 100%. The action
outputs a value that is in direct linear proportion to the size of the Error value.
A broad proportional band causes a more gradual initial response from the controller. Typically, Set Point
overshoot is low; but when the system stabilizes, oscillations around the Set Point tend to be greater.
18
General background How PID Works
A narrow band causes a rapid response that typically overshoots the Set Point by a greater margin. However,
the system does tend to stabilize closer to the set point. Note that a proportional band set at 0.0% actually forces
the controller into On-Off mode.
The drawback of proportional control is that it can cause the system to stabilize below set point. This occurs
because when the system is at set point, Error is zero and the control value output is therefore pegged at zero as
well. The majority of systems require continuous power to run at set point. This is achieved by integrating
integral and derivative control into the system.
Direct and Reverse Action
Direct action causes the output to change in the same direction as the change in Error, meaning that a positive
change in Error causes a positive change in the proportional band’s output. Reverse action creates an inverse
change in the output, meaning that a positive change in Error causes a negative change in output.
Integral Action
Integral action responds to the rate of change in the controller’s CV output relative to the change in Error. The
integral time you set is the amount of time, as calculated by the controller, required to bring the process to Set
Point. Note that if you set a short integral time, the function will respond very quickly and may overshoot the
Set Point. Setting a larger integral time value will cause a slower response. Integral time is sometimes called
Reset.
The controller’s CV output may reach and remain at 100%, a condition called saturation. This may occur, for
example, if the process is unable to reach Set Point. This causes the Error signal to remain stuck in either the
positive or negative range. In this situation, the integral action will grow larger and larger as the Error
accumulates over time. This is called integral "wind up", which can cause the controller to overshoot the set
point by a wide margin.
This situation can be prevented by setting an MB to clear the accumulated Integral error when saturation is
occurs.
19
PID FB + Autotune
Derivative Action
Derivative action responds to the rate and direction of change in the Error. This means that a fast change in
error causes a strong response from the controller.
The derivative action ‘anticipates’ the PV’s value in relation to the Set Point and adjusts the controller’s CV
output accordingly, thus shortening the PID function’s response time.
20
PID Status Messages
PID error indications are given in the Status Messages MI in the PID configuration.
Status Messages
Initialized to 0 when
Configuration is
activated.
MI
Value
Message
0
FB status OK
1, 2, 3
Auto-tune in progress
4
PID running
5, 6
Setpoint change in progress
7
Integral-wind up
8
integral-wind down
9
Pause mode, Integral and Derivative values are not currently being
calculated
10, 11
CV exceeds proportional band, no calculation performed
12, 13
AT parameter mismatch
Note that this means that PID will run without Auto-tune. The user
may either rewrite the PID values to the 32-MI long Auto-tune vector,
or may re-run Auto-tune
-1
Proportion band zero.
-2
Input range is invalid (PV input).
-3
Output range is invalid (CV output).
-4
Integral Overflow has reached maximum of 100,000. PID will not
allow the Integral value to increase any further.
-5
Error in 32-MI long Auto Tune vector addresses, ex., the vector
exceeds the final address in the MI data type.
-6
Set Point less than Input low range or Set Point more than Input high
range.
-7 to-10
Auto tune error, failed to calculate PID parameters
-11
Noise is more than 5% of Input Range.
21
PID Tips
Background: the Proportional Band and PV
The proportional band is a range defined around the Set Point. It is expressed as a percentage of the total Process
Value (PV). When the PV is within this range, the PID function is active.
Note ♦
♦
The proportional band may exceed 100%. In this case, PID control is applied over the entire
system range.
The PV and Setpoint must be the same unit type.
You can set PV limits by assigning Power up values as shown below.
Note ♦
♦
A broad proportional band increases the stability of the system, but also increases fluctuations
during the stable phase.
A proportional band that is too narrow will cause the system to react as though to ON-OFF
control, and greatly overshoot and undershoot the setpoint.
You can increase the proportional band or the integral time to decrease overshooting and
stabilize the system.
PID Analog Input Tips
PID Range Settings: PV Low and PV High limits.
If your PID PV is based on an analog input using current or voltage, you can use the analog range units to set the
PV low and PV high limits. If this is done, you must also 'normalize' the setpoint to this range.
23
PID FB + Autotune
In general, it is recommended
that you linearize the ranges to
Engineering Units (EU). In this
case use the analog reading
ranges for X1 and X2, and use
engineering units for Y1 and
Y2.
Note ♦
Input resolution
Reading Range
10 bit, (0 to 10V, 0-20mA)
0-1023 units
10 bit (4-20mA)
205 to 1023
12 bit (0 to 10V, 0-20mA)
0-4095
12 bit (-20mA)
818-4095
14 bit (0 to 10V, 0-20mA)
0-16383 unit
14 bit (4-20mA )
3277-16383
If you are using a PT100 or Thermocouple input, the values do not have to be linearized.
This is because the input value is already in degrees Celsius or Fahrenheit ( 0.1° resolution)
according to the Hardware Configuration.
Example: If the MI that is linked to a temperature input, set to Celsius, contains the value 385,
the temperature reading is 38.5°C.
Example: Temperature, Thermocouple type 'J'
Example: Linearizing to bars (pressure EU)
Assume that you are using a 14-bit input, 4-20mA with a pressure transducer with a range of 0-10 bars. Set
Linearization parameters as shown below.
Although you can set the PV limits to the input range, this may not produce accurate results.
24
PID Tips
Example 1: Assume that the 14-bit pressure transducer mentioned above is in a system with range limits of 0-5
bars. If you set the PV Low to 0 and the PV high to 1000 (10.00 bars), this is the range that will be used by the
PID function. PID will work properly.
However, you can achieve better PID control by setting the PV Low to 0 and the PV High to 600 ( 6 bars). Since
the error is a function of the PID working band, Example 2 will run with approximately 40% greater accuracy
than Example 1.
Example 2: Thermocouple type 'J' has as range of -200 to 760°C. Assume that this is the input for a PID system
with an ambient temperature that can reach a low of 10°C to a maximum setpoint of 250°C. If you set the PV
Low to 0 and the PV High to 300°C, PID control will function with approximately 3 times greater accuracy than
if you set the PV range to cover the entire -200 to 760 input range.
PID via Digital Output (Contactor, Solenoid Valve, SSR)
You can use the PWM (Pulse Width Modulation) FB to control a PID system. PWM FB enables you to control
the ratio between the ON and OFF status of a selected MB (Duty Cycle) within the defined cycle time, which is
given as ticks of 2.5 milliseconds. The ratio is given as an ON pulse percentage on the range of 0-1000 (0100%).
In order to control PID with the PWM FB, you must set the PID CV range to 0-1000 (0-100.0%).
Since the PWM cycle time is set in ticks of 2.5 ms, e.g. 1 s = 400. If the output is a relay/contactor/solenoid
valve, the recommended cycle time range is between 2000 and 12000 (5 to 30 sec).
Example: As the PWM output MB pulses ON and OFF, the pulse ON time is proportional to the CV. If the
cycle time is = 4000 (10 sec), and the CV is 100 (10.0%), the output bit will be ON for 1 second and OFF for 9
seconds, thereby supplying 10% of the energy to the system.
If the switch is an SSR, the recommended cycle time range is between 200 and 1000 (0.5 to 2.5 sec). Transistor
outputs are preferable.
If you use an SSR, you can use a Unitronics PLC that supports a high-speed output. In Hardware Configuration,
set the Frequency (F=1/CT) parameter's Power-up Value to 5 to 10Hz ( which is 0.2 -0.1s cycle time). Link the
PWM Duty Cycle MI to the CV. Note that in order to activate the output pulses, you must SET the RUN MB
linked to the HSO in Hardware Configuration.
Manual Loop Tuning
In certain cases, you may already have the PID values required by you application. However, you should still
perform Autotune in order to ensure proper PID function.
This is because during the Autotune process, the PID function collects certain essential data. Unitronics'
proprietary PID algorithm uses this data to run smooth, accurate PID.
25
Index
A
Auto tune ................................................1, 3, 7, 9, 23
C
configure...................................................................3
CV ........................................1, 3, 7, 9, 11, 13, 15, 23
E
Error .................................................................21, 23
F
FB .........................................................11, 13, 15, 23
P
PID............................1, 3, 7, 9, 11, 13, 15, 17, 21, 23
S
Status messages ..................................................... 21
27
28
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