PIDE switchover

[dginbuffalo]

Hello,
I have 2 PIDE's controlling the same valve. One, opens the valve to a setpoint pressure. it switches to the other PIDE to close the valve if motor goes in overcurrent. I have both PIDE's going into a SEL that picks which output to use. Here's the question: i am using valve feedback position into CVPrevious for a bumpless transfer feature. if I enable a different PID with the actual valve position loaded into CVPrevious, is that where the CV will start from? in otherwards, i don't want one calculating while the other is controlling-Or even if it does- i want it to start at the present valve position. thoughts??

 

[jamesau]

Sounds like that is what you want to do. AB has some elaboration of it here: http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/logix-wp008_-en-p.pdf

What you're trying to do is commonly called constraint control and is rather straightforward with PID functions that have 'external reset feedback' capability. I haven't used AB's PIDE to do this but it seems perfectly suited for it.

Good luck.

 

[dginbuffalo]

Hey thanks! that's a great document!

 

[jamesau]

Also, if the operator is to have manual control over the valve, it would seem you'd program a third input to the ESEL; this would be the 'manual output value'. It would seem InsUsed input parameter would remain 2 (In1 from pressure PIDE, In2 from overcurrent PIDE). You'd have to play with Selectormode and other parameters to get nice bumpless transfer from auto to manual and back again, as well as coerce the 'manual output value' to Out output parameter when the loop is in auto (for bumpless auto-to-manual bumpless transfer).

See http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm006_-en-p.pdf and forgive me any imprecise use of terminology in the above description.