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PID Loop - Pause I&D Calculation


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I have an application where the PV moves very fast. When a known disturbance or an anticipated disturbance occurs, I would like to pause the PID loop calculation and lock by CV where it is. I see that Unilogic has a Pause I&D calculation that seems to be the feature to use in this sort of situation but what I am wondering is whether or not I would have better results if I simply de-energized the PID Run Block and then re-energized it when the process is expected to be stable again. 


In other words, what is the difference in the calculation between simply removing the PID Run command and Pausing the I&D calculation?

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  • 3 weeks later...



When deactivating the PID run block, the CV will remian at the last laculated value, unless you store something else to it.  No calculations will be made and the CV will not react to the PV.


The pause I&D calculations will still allow the PID loop to run with only a proportional value - so the PID will still calculate a proportional value for teh CV relevant to where the PV is located.  However, it will not calculate an integral and derivative value.  So if there is a large disturbance to the system that the PID loop will not be able to regulate or allow the system to get to the set-point (until that disturbance is removed), pausing the I&D calculations helps because it does not generate integral error and go into an integral overflow fault. 


Hope this helps.

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