is it possible to create a reverse acting PID in a Scadapack?

Hey guys,

I'm working on creating a PID loop that will modulate a valve on a water filter, and the idea is to close the valve until the flow through the filter matches a setpoint. I just realized that if the flow is too low, it'll require the PID loop to close the valve, not open it so it'll be a reverse not direct output. However, I don't see any way to do a reverse control on the PIDA block. Does anyone have a suggestion on how to create this sort of control loop?

Just found the answer for anyone who might come across this post. Positive proportional gain values affect direct action, and negative proportional gain values reverse the action.

defcon.klaxon said:

Positive proportional gain values affect direct action, and negative proportional gain values reverse the action.

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Who'da thought ? ? ?

I think ( not 100% sure ) is to reverse the setpoint and process variable inputs to the PID instruction.

It would be confusing for someone following you though. Lots of documentation.

I haven't tried this but I would think inverting the output scaling value
High Scale Value = 0
Low scale Value = 100

From what I've done on PID in all my air handling projects, there is a drop down in the configuration tab that lets you either choose PV - SP or SP - PV. This also changes the update method in the Integral configuration in the PID. So almost all of my PID's were PV - SP like for heat, fans, water valves. So the ones that were "reversed" were the chilled water valves in a cooling routine. Upon setpoint drop below PV the valve would close. If SP got above PV, the valve would open. Heat and fan speed opposite. Hope this helps.

Mickey said:

I think ( not 100% sure ) is to reverse the setpoint and process variable inputs to the PID instruction.

It would be confusing for someone following you though. Lots of documentation.

Click to expand...

It should work.


Or subtracting from 100% PID's output. But then also manual mode needs subtracking from 100% so that valve is positioned to right place

danw said:

Who'da thought ? ? ?

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Same than on Siemens FB41 PID. So Siemens step7 programmers