Emerson Exchange 365

How does PID Function Block handle bad status of feedforward variable?

We have a heat exchanger that cools the process stream with a temperature controller.  The temperature for this feedback PID control is the process outlet temperature from the heat exchanger. 

I would like to add a feedforward input to the PID function block but I'm not sure how the status will be handled.  If the feedforward variable status goes bad, how will the PID function block handle it?

I am planning to use a feed tank temperature as the feedforward variable for the downstream temperature controller.  Ambient temperature swings the feed tank in 24 hour cycles and I'd like to isolate the heat exchanger's tuning from ambient influences. 

Eric Gratz

• 14 Jul 2021 3:12 PM
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1 Reply

• James Beall

  

  • 14 Jul 2021 3:18 PM

  Eric,
  If the FFWD variable status goes bad, the PID will continue without the use of the FFWD variable and there will not be any bump in the PID/OUT. When the FFWD variable status returns to a good status, FFWD action will resume without a bump in the PID/OUT even if the value of the FFWD variable is at a different value than when it went to a bad status.
  James

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