Controllogix PID overshooting CV High Limit

offroadunleashed · Oct 29, 2018

Hello,

I have a PID that I am using for an oxygen valve that regulates the pressure. Often when the header pressure is too low, the output will reach 123%. My CV High Limit is set to 100%. Why would this exceed the limit?

1756-L73 Processor V23.12

Mispeld · Oct 29, 2018

Based on those screen shots, it appears that your .CV is scaled 0 to 1000, corresponding to 0 to 100% output. So a CV value of 123 will be 12.3% output of the loop.

offroadunleashed · Oct 29, 2018

Mispeld said:
Based on those screen shots, it appears that your .CV is scaled 0 to 1000, corresponding to 0 to 100% output. So a CV value of 123 will be 12.3% output of the loop.

Yes, but the Output % is reaching 123%.

Mispeld · Oct 29, 2018

offroadunleashed said:
Yes, but the Output % is reaching 123%.

According to the Allen-Bradley documentation (see attached), output limiting is the last thing to happen before conversion to CV. I don't have a good explanation unless external logic is writing to .OUT and the PID is not being scanned.

offroadunleashed · Oct 29, 2018

I think I've figured out my issue. I was retrieving the Setpoint from the Rosemount valve via an OF8H. For whatever reason the extra 23% is only the feedback from the valve. Ive since changed it to the PID output. Thanks for the help.

Controllogix PID overshooting CV High Limit

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