I could not find any thread discussing this particular scenerio. Please forward me there if one exists.

We have an existing program in AB RSL5000 that uses the "ESEL" block to choose between 5 different PID loops. One PID always operates as Primary. As soon any of the other loops have a lower output than the one in operation, that loop takes over as the override control. We use the "current CV" to be passed into the "CVPrevious" in "PIDE" block with "CVSetPrevious" tag enabled.

I am trying to replicate this using Step 7 and I was wondering if anyone has performed a similar task or have any suggestions on it. I have comeup with few ideas but nothing seems to be simple enough to implement like it was in RSL5000.

Any suggestions will be appreciated.

Thank you.

 

That sounds like a lot of PID loops to be doing a min select from.

Anyway, the usual trick is to apply an output high limit to the PID loops that you are not using. The output high limit is the value of the currently selected PID + a few percent. So ....

Currently Selected output = min(PID1, PID2, PID3, PID4, PID5)
Output Hi Limit for other PIDs = Currently selected output + 2 %
Output Hi Limit for Current PID = 100%

Rob
www[.]lymac.co.nz

 

Thanks Rob, I will try this out and let you know how it works out for my application.