Lab 9- PID Control



To explore PID control.


Pre-Lab: (due at the start of lab period)

1. Write simple programs to read and output analog voltages from the PLC and perform PID control of a motor speed.


Process Description:

The basic equation for a PID controller is shown below. This function will try to compensate for error in a controlled system (the difference between desired and actual output values).


The figure below shows a basic PID controller in block diagram form.


The PID calculation is effectively a calculation in the PLC. One basic method of PID control is i) read voltage, ii) do PID calculation, iii) set output voltage. (Note: it is also common to get a self contained PID card for the PLC that deals with all inputs and outputs). The ladder logic below shows a PID control function.




SoftLogix controller and FlexIO rack with an IE4XOE2 analog IO module

Computers with RS-Logix programming software


Power Supplies

Motors and drives



1. Connect the PLC to a motor driver (Ultra 100, Ultra 5000 or series 160 VFD).

2. Test the analog output control of the motor by changing output voltage values. Record these with the analog inputs from the tachometer in a table.

3. Use the PID control program to control the motor. Note: you will need to set up parameters for the PID control memory. By default the values such as scaling and alarms are all off. Note: Make sure that the PID control memory contents are reported.



1. Pre-lab and updated laboratory materials.

2. Detailed settings for all analog, PID, and VFD components. These should be done so that a technician could reprogram the equipment.



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