PID for Flow Control

Vic

Member
Join Date
Jun 2002
Posts
246
I have tuned numerous PID loops in position controllers so I understand the basics of PID control but I am far from an expert. In positioning loops, when the target is in position, the error is zero and there is zero output to the servo valve.

I have an upcoming application where I have to use a PID loop to control flow. The controller is a SCL 5/05 with a VFD controlling the pump and a flow meter for feedback. My question is that in this situation, when the SP is near maximum, the PID output must be near maximum also to get the pump running at the required speed. If the error is close to zero, does the PID control use the I term alone to give the output or is some other method required to get the output to near maximum.

Thanks
 
This is the PID algorithm used by the SLC. As you can see, if feed forward is not used, and the error (E) is zero, and the PV is steady [D(PV)/df = 0], then the Output will be carried by the integral term. Something to keep in mind, while in Manual Mode, the instruction will adjust the integral sum so that the output is "bumpless" when switching to Auto.
 
So your answer is yes.

Integral term removes steady state error.

If just PD you would have an asymptotic response, never reaching 0 error.
 
Yeah, without knowing the exact application, especially in terms of accuracy, it may be better to simply to use some form of regression for prediction.

This depends on the presence of disturbance though. Being that if there were significant changes to the system your pump is supplying (ie pressure) this method wouldn't work nicely.
 

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