Help... PID is oscillating and I can't get it to level out

lunenburger · Mar 10, 2015

I am using a PID in a controllogix and trying to get a level under control.
Here is the graph and PID settings:

I have changed the values and the oscillation changes but I cannot seem to get rid of it.
It is a very slow loop, 30 min oscillation.
And changing the setpoint is not an option since we are in production.
Any help would be greatly appreciated.....

jamesau · Mar 10, 2015

Well, your integral gain is WAY too high if this level loop behaves as a typical one (integrating process).
You might start by cutting that term back to .02 or less and adjust the proportional gain only. Some would say you don't need the integral term at all (set to zero) but I always like a smattering of it for level loops.

dginbuffalo · Mar 10, 2015

I'd say cut your gain in half too and observe the result. Maybe even less than half. Your going to have to let it stabilize a few cycles though to get the true result. When the level is low, your "pouring it on" too much and overshooting setpoint, and just the opposite when your level is high. There's a lot of factors involved, but this is what I would try.

lunenburger · Mar 10, 2015

Yes, Integral of 0.2 worked much better.
Really starting to level out the valve reaction.
Original Integral was at 2 and i increased it from there.......wrong direction....

Thanks for the help

CapinWinky · Mar 10, 2015

Yeah, integral causes the output to increase the longer you are below the set point and vice versa and by that very nature, causes oscillation. That's why there is the derivative term that reduces output the faster you approach the set point, it pumps the brakes when you get close to stop overshoot.

EDIT: I mention that because you don't have anything for D, making it just a PI loop.

Peter Nachtwey · Mar 10, 2015

CapinWinky said:
Yeah, integral causes the output to increase the longer you are below the set point and vice versa and by that very nature, causes oscillation. That's why there is the derivative term that reduces output the faster you approach the set point, it pumps the brakes when you get close to stop overshoot.

EDIT: I mention that because you don't have anything for D, making it just a PI loop.

A simple tank level control doesn't require a PID, only a PI is required if an exact level must be maintained. One can get by with just a P gain if the level can be in a proportional band.

lunenburger · Mar 12, 2015

Thanks for the suggestions, big difference in control...

lunenburger · Mar 13, 2015

Also, this isn't a typical tank level. It is a fryer 2'deep x 10'wide x 20'long frying 1200Lbs/min of product. Operators want level to be maintained within 0.2 of an inch!!!

Plus the level is measured with a pressure transmitter, oil is "bubbly" so level measurement is somewhat erratic.

Thanks for all the help.... it was greatly appreciated

Help... PID is oscillating and I can't get it to level out

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