Micrologix 1100 PID output limiting

Hello all,

I don't get the chance to use the Micrologix much anymore and have a question about PID output limiting. I am limiting in the PID, the low CV to 66%. This is a minimum speed a vacuum pump can run at. When i go to auto, it "appears" the PID takes awhile to respond. My guess is it's building up it's output to 66% and then surpassing. Is this a logical guess? I am considering one shotting 66% into the CV when i initially turn the PID to auto. Any suggestions?

In the PID configure screen you should be able to set the Min CV to 66.


Will.

You need to set the "output Limit bit" (PD??:?/OL)

Then the output will never go lower then your 66% value.

If you need it to go lower then other logic will have to be written to lower the value, say in manual mode.

See pdf

Thanks guys- I probably wasn't real clear in my original post. I am limiting to 66% and have output limit bit turned on. It seems though, when the PID is in manual and I turn it to auto while "way" under setpoint, it doesn't do anything for 40-60 seconds, and then it climbs above the 66% point. I would think when switched to auto, it would immediately start climbing above 66% since that's the low limit?

I would try using the bias and setting it to 66% just so you don't need to wait for the integrator to wind up.

Peter Nachtwey said:

I would try using the bias and setting it to 66% just so you don't need to wait for the integrator to wind up.

Click to expand...

Thanks Peter- I may try that. I was one shotting the CV to 10,812 (66% of 16,383) when the PID switches to Auto. It seemed to work in the shop but of course, the customer says it's not in the field...