PLC Feedforward with FB Trim PID Control

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Tom Taggart

This is a heating control loop using steam as a direct heating media. I plan on using the energy balance equation to determine the steam flowrate required and use a flow control loop to control the flow. For fine tuning I would like to use the temperature outlet temperature as a feedback trim in my control loop. To do this in a SLC505 do I set up the first PID loop as a flow controller and use the output (0-100%) as the bias (or feedforward parameter) in a second PID loop which is based on temperature control? Or is it the other way around? Or is there a better method.

Thanks
 
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Bill Clemons

Plan your control in stages. Overall, you want to control temperature at the exit of your steam heater. The loop between exit temperature measurement and flow control valve is your PID loop.

Test your temperature control loop and tune it. Does the control work for the most part? Are there some upstream influences on the system for which PID does not control correctly?

If you answered No to the first question or Yes to the second, then consider an advanced algorithm, like feedforward control. In your example, you mentioned a heat balance calculation (to determine heat demand upstream of the heater exit, I presume). The upstream information can be used to "feed forward" your PID control and is a bias on your final output going to the flow control valve. If the process calls for more heat vis-a-vis the heat balance calculation, you can set the feedforward bias to add % to the PID control output on the exit temperature. Effectively, you are compensating for additional heat loading anticipated through the heater, as opposed to waiting for the controller to respond to a drooping exit temperature.
 
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