Limiting CV while maintaining controller in program mode

[Jim ManleyModeratorKarma: 262Rank: Jedi]

Background

We use steam for various tasks around the distillery.  Due to some things beyond our control, there are times when we have to limit the steam going to one piece of equipment while another piece of equipment is running.  At this point, this is being done by putting the PIDE in Override mode which disables the PIDE’s ability to do its control thing.  It also requires an operator to be on hand to manage temperatures, etc., which can lead to inconsistency in batches.

Question: Is there a way to limit the output of CV while still allowing the PIDE to control the system?  The CV signal is a 4-20mA analog to a control valve.

Would it be possible to create a second PIDE that sets CV=0% to 4mA and CV=100% to 8mA then set up controller logic that selects between the two versions of the PIDE based on which equipment is running?

Thanks,

Jim

[Jim ManleyModeratorKarma: 262Rank: Jedi]

Turns out that I can get away with running the equipment in question while limiting the steam input to 25% all the time.  I did this by setting the CV High limit to 25%.

[AnonymousKarma: 0Rank: Padawan]

Be careful when you use the CVlimit what i have found is when using limits the error still builds so if your PV is below (or above) the SP as soon as you release the limit it will jump to the new limit range. For example if you limit it to 25% and at some point in the process you send the limit to 75% if the PV is below the SP the CV will jump to 75%. This works in the opposite as well if you are limiting it to 25% closed when you send the limit to 0% the CV will follow. I’ve talked to AB tech support about this as well they were as puzzled as I was. For applications that I’ve used the CV limits in I’ve I ensure that I put in a 1 second timer that upon sending a new limit I place the loop in manual while the timer times and send the previous limit to the CVProg.

[Jim ManleyModeratorKarma: 262Rank: Jedi]

Thanks Titus.

This  particular PIDE, under normal operating conditions, would not be set above 25%.  But it’s good to know that if I ever need to change that, I’ll need to be aware of the condition you described above.

Jim

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