V.2: Cascade Control

The primary disadvantage of conventional feedback control is that the corrective action for disturbances does not begin until after the controlled variable deviates from the setpoint. In other words, the disturbance must be “felt” by the process before the control system responds. Feedforward control offers large improvements over feedback control for processes that have large time constant and/or delay. However, feedforward control requires that the disturbances be measured explicitly and that a model be available to calculate the controller output. Cascade control is an alternative approach that can significantly improve the dynamic response to disturbances by employing a secondary measurement and a secondary feedback controller. The secondary measurement point is located so that it recognizes the upset condition sooner than the controlled variable, but the disturbance is not necessarily measured.

Fig V.8: Cascade Control Structure

Let us consider the following block diagram of cascade control structure.The outer loop and its controller are called master loop and master controller whereas the inner loop and its controller are called slave loop and slave controller respectively.

	V.20
Simplifying
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Similarly
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