PID control using Modicon Compact PLC


Thread Starter

Craig Palmer

I am using the PID2 instruction in a Compact PLC with a 984-E275 CPU.

I am specifically interested in anyone's experience with how to use the tracking function when switching the loop from auto to manual or vice-versa.

The problem I have is that when I put the loop into auto by turning on the “top” input of the PID instruction, then depending on the setpoint and the PV the output (CV) “jumps” a large amount before it starts to control normally.

The process I am controlling cannot handle this large change in the output.

I know that the “middle” input may have something to do with this but the documentation in the manual I have is not very helpful in understanding how to use this.

Any help on this matter would be appreciated.

Trevor Houtstra

Are you using ConCept V2.5 software to program your PLC? If so, setpoint tracking is inherent in the PID function blocks.

When you say that your are "turning on" the top input, are you setting the MAN input to a 1 (TRUE), because the PID block is in manual when the MAN input is TRUE.
I'm not sure if this is the best approach, but it works for me. When the loop in in manual, just SUB the PV into the setpoint. Then when you transfer to auto, there is no bump. Going back the other way, SUB the Mv into where you control your manual output while in auto so there is no bump in the other direction.

Andrew Ward
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