Question

In a combined feedforward and feedback control strategy, the value of the feedforward signal and the Setpoint of the secondary controller are added in order to:

Group of answer choices

Initialize the operating point of the secondary controller

Initialize the operating points of the cascade controller

Determine the process variable of the primary controller

Determine the operating point of the primary controller

Answer 1

Combined feedforward and feedback control stategy involves the use of two controllers with the output of the first controller providing the set point for the second controller, the feedback loop for one controller nestling inside the other.  Such a system can give a improved response to disturbances.

Feed forward with feedback trim offers a solution for improved disturbance rejection if no practical secondary process variable can be established.

In this method, the process variable of the primary controller is determined beforehand. In traditional PID controller takes action only when the PV has been moved from set point, SP, to produce a controller error, e(t) = SP – PV. Thus, disruption to stable operation is already in progress before a feedback controller first begins to respond. From this view, a feedback strategy simply starts too late and at best can only work to minimize the upset as events unfold.

In contrast, a feed forward controller measures the disturbance,while it is still distant. The feed forward element receives the measured disturbance, uses it to predict an impact on PV, and then computes preemptive control actions, that counteract the predicted impact as the disturbance arrives. The goal is to maintain the process variable at set point (PV = SP) throughout the disturbance event.