Is a PIDE instruction enough?

[Peter Nachtwey]

I have tried to tune the PIDE with a lot of methods (Logix AutoTune, RSTune, test and error, Ziegler-Nichols, using just P, just PI, etc) without good results.

Something that I am missing or definitely PIDE isntruction never will work here?

The PIDE instruction will work.
First I like Daba's suggestion about checking for the proper installation of the flow meter where there is no turbulent flow.


RS-Tune etc will not work if they are only meant to tune SOPDT type temperatures. Each type of system is different and there are different formulas/models/equations that are required for each. How do you know if RSTune has the right set of formulas.


Forget about ZN. Something really should be done about those people that keep spreading this bad info around.


I don't like Daba's remove the high and low and sort idea. A low pass filter should work. If there are extremely high or low readings then there is something wrong with the sensor. Fix it, don't cover it up.


Have you tried using feed forwards? This assumes the pumps speed is proportional to the flow. If the flow is to remain constant then a simple bias would do. Using the bias or feed forwards will make a good estimate of the required pump speed for a certain flow. This should be accurate do within 5% but this means the PID only needs to correct for the last 5% reducing the error.


I find it easy to get the feed forwards correct within 5% but normally I am withing 2% or less.

 

[daba]

The PIDE instruction will work.
First I like Daba's suggestion about checking for the proper installation of the flow meter where there is no turbulent flow.

Of course it may NOT be the position of the flowmeter causing the varying flow....

Pipework design on the suction side of the pump is critical. Vortexing in the outlet of the supply vessel could be an issue, as the pump draws air or gas, the flowrate immediately drops. And that vortexing will lead to cavitation on the pump impellers, shortening its life dramatically.

When vortexing into the pump body occurs, reducing the flowrate of the fluid, then increasing the pump speed, to attempt to bring the flowrate up, is totally the wrong thing to do, as it makes the situation worse ! And those 10 second accel and decel ramps will be a total hindrance in a PID control situation where you may be changing the speed reference every 100mS. I have achieved excellent flow control loops by removing all ramping from drives, not have time-terms competing with each other.

Is the NPSH correct for this system ?

I would run the pump at a range of manual speeds and Trend the flowmeter readings. This will SHOW what is going on here. Another thing I do is LISTEN to the pump and pipework.

Clearly something is amiss with this system, so the question has to be asked - has it been "designed", or just "made"