Compactlogix PID Help

I am in need of help with a compactlogix PID on a winder that winds felt.

This winder was built by a one man shop machine builder which is no longer in buisness. My understanding is that the gentleman recently passed away.

We just got this in from a supplier that also makes felt for us and we terminated the contract so the winder is ours since we had it built.

The tension seems to have a very wild swing up and down and the load cells come to a summing board and the give 0-10 volts to the compactlogix analog card on channel 2. Channel 2 is the PV so it goes right into the PID and it changes very rapidly and I am thinking that if we can filter it to smooth it out before it goes into the PID it may help as our other felt winders are that way?

I have 2 questions.

1 how can i build a filter to smooth the PV? I have searched and found many times discussed on the forum and I think I am more confused now than before I searched. can anyone give me a example?

2 what do I need to adjust in the PID to slow down it's response? It is much more responsive than it needs to be and is causing hunting.This is the PID instruction and not the PIDE.

In the program they are moving a value of 10 to the pid max output tag so it is limited to 10% output and this is not enough to keep the felt tight.

Why would you limit a PID like this? the drive motor is 0-60hz should the PID output not be 0-100% which scales to 0-60hz on the drive with a 100% output being 60hz on the drive?

Hi Tim,

The original designer might have limited the drive output as a very crude way to control these wild swings, instead of tackling the underlying problems.

I'm not sure how much you can "play" with the machine without ending up with a big mess of felt - but as a start, lets check a couple of things:

Where is the PID executed? Is it in its own periodic task? if it is, the loop update time (.UPD) should match the periodic task execution rate. funnily enough when i had a quick look at the manual, they even mentioned winders / tensioners as an application with a very fast (10ms) loop update time.

As a starting point, have a look at the controller gains - .KP, .KI and .KD - post those back here if you can. a quick look over the Compactlogix PID instruction in the manual should help you identify these - have a look on page 511 in this doc
http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm003_-en-p.pdf