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Old February 19th, 2003, 12:13 PM   #3
Allen Nelson
United States

Allen Nelson is offline
Join Date: Apr 2002
Location: West Chester, PA
Posts: 1,368
Re: how often should I trigger the PID?

Originally posted by Ron Beaufort
Once I had determined the “acceptable” tuning values for Kc, Ti, and Td which I have just quoted - I entered these into the controller AND DID NOT ALTER THEM in any way as the rest of the tests proceeded. Specifically, after the “find the period experiment” had been accomplished in graph #1 - the values for Kc, Ti, and Td were set and then subsequently left unchanged.
Interesting Ron, but look at the PID equations:


The PLC doesn't reaaly "know" the value of dt in the above equations. It doesn't measure the elapsed time, it just takes the value that you enter in the Loop Update Time setting for the dt , and calcuates everything from there.

So by making a three-fold increase in dt without changing Ti and Td, you've effectively de-tuned the loop.

Try the experiment again, but this time, when you go from dt = 0.5 to = 1.5 sec (a three-fold increase), change Ti from 0.31 to 0.93 (a three-fold increase), and Td from 0.03 to 0.09.

This should keep the relative contribution from each of the components the same. I'm interested in seeing the results.
Attached Images
File Type: gif pid.gif (5.1 KB, 1873 views)

Last edited by Allen Nelson; February 19th, 2003 at 12:28 PM.
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