I read the discussion of feedforward control and I began to wonder about a similar control scheme. I have a ratio controller the manipulates the flow of one stream based on the measured flow of another. Very straightfoward to setup especially with the new controllogix PIDE. What if I now want to trim the ratio based on measurement on the combined stream, lets suppose pH. The pH trim is used to adjust for small unmeasured changes in one of the streams, in this case, pH. The purpose of this trim is to make small changes in the ratio and not to change the ratio setpoint by more than say 10%. Is this setup as easy as taking the 0-100% output from the trim controller (pH control) and scaling the output to provide a small change in the ratio setpoint. For example, if the desired ratio setpoint is 1.5 then the full output of the pH controller, trim, would only adjust the setpoint from 1.4 to 1.6? For you boiler guys, this setup would be very similar to excess O2 trim on the air/fuel ratio.
Ratio/Trim Control
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