Setup of PID Loops in Accord Builder
A PID Loop Controller is used to control an Analog Device, based on a desired setpoint and feedback (in a loop fashion) from an Analog Instrument, also called the process variable.
An example is where Heat is transferred by allowing a quantity of steam to pass through a control valve. Initially the Temperature in the system (process variable) read by the Analog Input Temperature Transmitter will be low. As the system heats up the Control Valve will begin to close. The ideal is to maintain the Control Valve Analog Device at an output which maintains the temperature Process Variable at the desired Setpoint.
The difference between the Setpoint and the Process Variable is called the Error. The Loop controller allows the system to provide a low steady-state Error for different Setpoints.
PID stands for Proportional, Integral and Derivative control. The three terms refer to portions of the controller loop which provide the output value from the controller which is sent to the Analog device being controlled.
Proportional Control, also called gain, is a straight multiplier factor on the Error
Integral Control, is based on the average Error over recent time period.
Derivative Control is based on the rate of change of the Output and (Error)
A controller only using Proportional control (P) would always generate an output which would cause the Process Variable to oscillate. Use of the I and D terms allows the user to generate controller action which will provide the required system response in terms of the output not oscillating at steady state. The process of modifying P, I and D terms to achieve smooth non-oscillating response is known as “tuning the loop”. Many systems just use P and I control and leave the derivative D term at 0.
Following insertion in a unit object each PID Loop must be configured with settings that will determine its operation and its description in documentation. During insertion the PID Loop is given a name and short description. These may be changed at any time.
There are two tabs in the PID Loop properties configuration section. The name and analog device of the PID Loop are given in the Current Item heading. The PID loop is always associated with the analog output.
The following aspects of the PID Loop must be configured as required:
AI Source: This is the Analog Input or Variable that is used as the Process Variable. It must be already configured.
P: Proportional, or Gain, Factor.
I : Integral Factor.
D: Derivative Factor.
Upper Limit Output: Maximum Range value for the Loop. Usually same as the Analog Device.
Lower Limit Output: Minimum Range value for the Loop. Usually same as the Analog Device.
Sample time: This is the interval between successive processing of the PID loop. This is set up in the PLC hardware to be 100 milliseconds. All PID loops are processed from OB35 in Simatic S7 PLC. The Hardware configuration will have OB35 interrupt time set to 100 milliseconds.
Steady State Deadband: A Deadband value around the Setpoint within which the Loop is deemed to be in steady state. If the loop achieves this state for 100 seconds (given sampling time of 100ms) then the Integral component is stored when the “Store Integral when in Steady State” is enabled.
Example- If the Steady State Hysteresis is 2.0 for a Loop with setpoint of 80.0 and then the Steady State is achieved when the Process Variable is between 78.0 and 82.0 for 100 seconds.
Store Integral when in Steady State: This allows the integral component of the PID output to be stored when the Loop error is less than the Steady State Deadband. On Loop Restart this component is used for I component, giving a faster restart response.
Integral Reset Value: Integral component value used when loop is starting up. This is not read if there is a stored Value from “Store Integral when in Steady State”.
Response Time Lag: A factor for how quickly the loop is to react. It is used to set the rate of decay of Derivative term effect. The larger the value the slower the loop will act.
Derivative Deadband: The Derivative Deadband is the minimum amount of an error change that will cause a change to the derivative component of the PID calculation. It is a Deadband on the error with respect to the Derivative Component of the PID loop.
These settings are also available in Accord HMI PID Controls for tuning.
The following aspects of the device may be filled in if required:
Description: This describes the Loop in more detail, if required.