Loop Update Time & Cascade Control Loops

williamlove

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Join Date
Jul 2007
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I ask you answer if you know precisely what PID loop update time in a Rockwell processor is and if you are experienced with cascade loops. Thank you!

Assume a powerful ControlLogix with almost no logic loading it down so we can disregard overkill or wasted resources for this discussion.
Assume the loop update time of the PID is always correctly matched to the scan time of the logic.
Assume the loop update time is always at least five times slower than the rate at which the PV is updated with a new reading from the analog input.

Question 1. Is there ever any reason not to scan a PID very often (i.e. low loop update time)? I am not asking if it improves the control, I am asking does it ever hurt the control. If yes, please explain.

Question 2. In a cascade control loop, should the loop update time of the inner and outer loops be the same?

I ask this because we have a debate going about how you make sure the outer loop is “slow” compared to the inner loop. The literature says to make sure the loop dynamics of the outer loop are slower than the loop dynamics of the inner loop. I say this has nothing to do with loop update time and I say the loop update time should be as fast as possible under the assumptions I led with. But our engineer thinks the outer and inner loops of the cascade loop should be different (with timers making sure the loop scan rate matches the loop update time). I disagree but he is adamant ;)

In your reply I’d appreciate it if you’d refer to questions 1 and 2 because I am hungry for precise and specific answers to those. But any additional helpful information is sought. Thank you.
 
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