PID instruction in Allen Bradley PLC [Proportional/Integral/Derivative] Closed Loop Control

For  Processor SLC 5/02SLC 5/03SLC 5/04SLC 5/05 


MicroLogix 1200 and MicroLogix 1500 (A special PID file replaces the old integer file control block.)

Description of PID in PLC

 This output instruction is used to control physical properties such as temperature, pressure, liquid level, or flow rate of process loops.

The PID instruction normally controls a closed loop using inputs from an analog input module and providing an output to an analog output module as a response to effectively hold a process variable at a desired set point.

The PID equation controls the process by sending an output signal to the actuator. The greater the error between the setpoint and the process variable input, the greater the output signal, and vice versa. An additional value (feed forward or bias) can be added to the control output as an offset. The result of the PID calculation (control variable) will drive the process variable you are controlling toward the set point.

The PID instruction can be operated in the timed mode or the STI mode. In the timed mode, the instruction updates its output periodically at a user-selectable rate. In the STI mode, the instruction should be placed in an STI interrupt subroutine. It then updates its output every time the STI subroutine is scanned. The STI time interval and the PID loop update rate must be the same in order for the equation to execute properly.
Important: The integer control block can no longer be used with SoftLogix 5 version 2.0 and higher. 

Entering Parameters in PID Instruction

PID File - (MicroLogix 1200 and 1500 only) Specify a PID file. If you have not already defined a PD filetype among your data files, it will be created for you. The file length is fixed at 23 words. The PD file replaces the old integer file control block.

Control Block Length - (Not valid with MicroLogix 1200 or 1500) Specify an integer file, for example N7:0. The file length is fixed at 23 words. 

Process Variable PV - The element address that stores the process input value. This address can be the location of the analog input word where the value of the input A/D is stored. You can also enter an integer address if you choose to pre-scale your input value to the range 0-16383.

Control Variable CV - The element address that stores the PID output. The output value ranges from 0-16383, with 16383 being the 100% "ON" value. This is normally an integer address, so that you can scale the PID output range to the particular analog range your application requires.

Setup Screen - Double click Setup Screen on the instruction to bring up a display that prompts you for other parameters you must enter to fully program the PID instruction.
Clicking Setup Screen on the PID instruction displays a dialog that allows you to enter additional parameters. These parameters are described here.

                                                                                                                                 Source:Rockwell Software 

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