ML1400 PID - Freezing the Integrator

[Old No. 7]

I'm trying to figure out if there is a relatively easy way to freeze the integrator in the Micrologix 1400. We've used some loop controllers that have a feature where you can set a value above which the integrator is frozen. This has worked well for us in the past to prevent overshoot (slow acting temperature control) and I'd like to try and implement it in the 1400.

It looks like maybe I could grab the value in PD11:0.IS and constantly write it back to PD11:0.IS when the output is greater than say 50%. This scares me a little since you cant really make any sense out of what the value means.

Anyone ever try anything like this?

 

[Ron Beaufort]

most folks turn on or off the "Limit Output CV" setting for something like this ... (it's the OL bit) ...

you'll also want to take a look at the "Output Max" setting (the CVH location) ...

 

[Old No. 7]

Ron, I don't wan't to limit the output I just want to limit (hold) the reset contribution. Unless I'm missing something, if I set CVH to 50% , then my output will never go above 50%.

Maybe another way to do this would be to dynamically change the PID reset setting so that while it is still "on", it's contribution is minimal. If I multiply the reset setting by 10 or 100 (or set it to 32767) when the output is greater than 50% and then set it back once the output drops it may accomplish what I want.

 

[Peter Nachtwey]

I'm trying to figure out if there is a relatively easy way to freeze the integrator in the Micrologix 1400. We've used some loop controllers that have a feature where you can set a value above which the integrator is frozen.

Anyone ever try anything like this?

I have tried just about everything. Usually the integrator will not over shoot or not by much if you have the system tuned right and the control output is not saturated. Ideally the integrator should wind up to the value needed to maintain the temperature set point as the error approaches zero. If you limit the integrator there may always be an error between the SP and PV. If the control output does saturate the integrator should be frozen but there are better techniques than that.

 

[Ron Beaufort]

Ron, I don't wan't to limit the output I just want to limit (hold) the reset contribution. Unless I'm missing something, if I set CVH to 50% , then my output will never go above 50%.

that would be true ...

I don't doubt that you've seen something along these lines which worked before – but I'm having a hard time imagining how that could be ...

it sounds like you're saying that at some predetermined point you want to limit JUST the Integral contribution – but at the same time allow the CV to continue to rise ... if so, would it be the Proportional contribution that keeps increasing the CV? ...

like I said – I do NOT doubt what you're saying – but I'd sure like to see a trend of how a system like that would work ... I'm an old dog – but I still like learning new tricks when something like this comes along ...

Maybe another way to do this would be to dynamically change the PID reset setting so that while it is still "on", it's contribution is minimal. If I multiply the reset setting by 10 or 100 (or set it to 32767) when the output is greater than 50% and then set it back once the output drops it may accomplish what I want.

maybe so ... good luck with it ... and please come back if (actually when) you get it to work and let us know how it turned out ...