Sensor Averaging for PID loop

Hello All,

I have a PID loop on a PLC 5/80E PLC that I have been using to control room temperature. Just recently, they've installed a 2nd temperature transmitter at the opposing end of the room.
So what I want to do is average the (2) temperature values as my PV for the loop. The problem is that the (2) transmitters have different ranges (ie; -18 to 38 C and -29 to 60 C). I'm thinking about averaging the (2) and/or automatically switching to one tx if the other fails. But I think to do this, I have to change the Engineering Units Min/Max of the PID loop every time that I switch transmitters.

Example:
Tx1 , PID Range = -18 to 38 C
Tx2, PID Range = -29 to 60 c
Averaging, PID Range = 0 to 100 C

Is there a better way of doing this?

Any problem you may have with ranges relate tot he PID loop set point and not to the range of the transmitter.

Before I continue, the best thing to do is to get 2 transmitters with the same range. If you can't do that you can scale the values in the PLC to their actual range, do the averaging and then de-scale the results to the PID loop's set-point range.

The point is that as long as you're not using raw units (ie. 0-4095) you can manipulate the temperature values because they represent actual temperatures.

Good Luck,

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