# PID Problem? Or Something



## oliquir (Jan 13, 2011)

maybe vfd has lost some pid parameters. do you read 55 psi on the vfd or on the transmitter.


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## Peewee0413 (Oct 18, 2012)

Transmitters. I was thinking transmitter, and changes them out already. One thing that makes no sense is it not trying to zero the error % in the pid. Even if a pump is told to meet an unreachable pressure, motor should run balls to the wall to try and make 0% error. Am I not right? Note: both pumps are not linked logically in any way...which is why I say program.... can someone explain when and why a loop goes into overide mode?


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## KennyW (Aug 31, 2013)

Peewee0413 said:


> Transmitters. I was thinking transmitter, and changes them out already. One thing that makes no sense is it not trying to zero the error % in the pid. Even if a pump is told to meet an unreachable pressure, motor should run balls to the wall to try and make 0% error. Am I not right? Note: both pumps are not linked logically in any way...which is why I say program.... can someone explain when and why a loop goes into overide mode?


You said before that the CV is going to 100%. If so then the loop is indeed trying to go balls to the wall to reach the setpoint. If the actual drive output does not agree with the Loop CV then that is an issue. When all is well the Loop CV is linked to the drive command/speed reference directly.


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## Peewee0413 (Oct 18, 2012)

I see on the vfd reference speed 60 hz. But it is not running. It does not turn on until it drops more than 10 psi below SP. When it turns on, it tries to maintain 10 below SP. When its less than 10 psi from the SP it turns off. Really short On/Off cycle, which makes me worry about equipment damage.


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## Peewee0413 (Oct 18, 2012)




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## Peewee0413 (Oct 18, 2012)




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## Peewee0413 (Oct 18, 2012)

The first pic was prior to our current issue, which was intermittent. The second picture was while it was acting normal..


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## Peewee0413 (Oct 18, 2012)

I believe the drops in the yellow cv was when it went in overide mode for a split second before going back in program. Note i think the pics are of two separate pumps , but they both did the same thing, I just grabbed two random photos taken the same day.


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## KennyW (Aug 31, 2013)

A loop goes into override when the override input's value is lower than the CV. I don't know what drive or controller you're using but you should check to see if there's a value i. The override input parameter. Easy way to bypass it into put 100 into it.


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## KennyW (Aug 31, 2013)

A loop goes into override when the override input's value is lower than the CV. I don't know what drive or controller you're using but you should check to see if there's a value i. The override input parameter. Easy way to bypass it into put 100 into it.


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## sparky970 (Mar 19, 2008)

Can you achieve the SP by running in manual?


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## don_resqcapt19 (Jul 18, 2010)

Peewee0413 said:


> I see on the vfd reference speed 60 hz. But it is not running. ...


It looks like you need to be looking at the run command and not the reference signal.


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## Peewee0413 (Oct 18, 2012)

sparky970 said:


> Can you achieve the SP by running in manual?


Yes.


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## Peewee0413 (Oct 18, 2012)

We also use device net.


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## Peewee0413 (Oct 18, 2012)

Talked to Allen Bradley and they said if our programming company fails to fix it, they can send someone out free of charge to "assess"...


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## Semi-Ret Electrician (Nov 10, 2011)

Peewee0413 said:


> Transmitters. I was thinking transmitter, and changes them out already. One thing that makes no sense is it not trying to zero the error % in the pid. Even if a pump is told to meet an unreachable pressure, motor should run balls to the wall to try and make 0% error. Am I not right? Note: both pumps are not linked logically in any way...which is why I say program.... can someone explain when and why a loop goes into overide mode?


When measuring DP with two discrete pressure transmitters you're asking for all sorts of problems, i.e. induced noise, ground loops, bias errors, transmitter errors etc. You should really just use one DP transmitter. 

If slamming more than one pump on at the same time to control the same variable you just make the problem worse. If you designate a "lead" pump and others that kick in if the 1st can't handle it, you would be much better off. 

Also, check your PID parameters. If the proportional band "P" is too narrow you're running On-Off control and just not taking advantage of your VFD and might as well just have a DP pressure switch and a simple on-off contactor controlling the pump(s). 

Too much Intergral "I" and you will drive your controller into "reset wind-up" resulting in output away from set-point, and take forever to recover.

If you use too much derivative "D" control will be very poor. I only use it for temperature applications. 

As previously stated, if you can't control it manually (or measure it) all bets are off.


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## sparky970 (Mar 19, 2008)

Semi-Ret Electrician said:


> When measuring DP with two discrete pressure transmitters you're asking for all sorts of problems, i.e. induced noise, ground loops, bias errors, transmitter errors etc. You should really just use one DP transmitter.
> 
> If slamming more than one pump on at the same time to control the same variable you just make the problem worse. If you designate a "lead" pump and others that kick in if the 1st can't handle it, you would be much better off.
> 
> ...



For that application described, I wouldn't use any derivative.


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## Peewee0413 (Oct 18, 2012)

No DP .....Two completely different systems. .not lead lag. more like potable and process water pumps. I know how to tune a loop. The process engineer doesn't so his butthole quivers anytime I touch the PID. Thought I screwed it up or something. Called the programmer now everything is default from start up and still jacked.


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## Semi-Ret Electrician (Nov 10, 2011)

You probably know this but:

How do you shut the system down? And, start the system up?

If you just cut power off to the pump(s) but leave the controller powered up (and in auto) you will get Integral or Reset wind-up. Which will give you full (or no) output for a long time, depending on if it's a pump up or down system.

You must start up in manual get the system stable, then switch to auto.


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## Peewee0413 (Oct 18, 2012)

Shuts down if u try a transition from man-auto. ..some reason in auto PV stay 10 below SP. The cycle continues afterwards.


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