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Sep
24

Feedforward Techniques – Part 4

One of the simplest and easiest techniques to evaluate and implement is set point feed forward. The maximum feedforward gain is the inverse of the process gain. You just need to make sure the process gain is converted to the same units used for the feedforward gain and only a fraction of the maximum is used as the actual feedforward gain to allow for nonlinearities, errors, and PID action. New adaptive controllers such as DeltaV Insight can find and schedule the process gains and hence the set point feedforward.

If the controller gain is large (> 1.0) and the controller structure has proportional action on error, set point feed forward has little value because there is already a sizeable step in controller output from a set point change. However, large dead times can cause low controller gains. Here, set point feedforward can get you to a set point much faster, which can be useful for loop set points driven by batch, cascade, or advanced control.