When the disturbance is measurable, feedforward control is an effective means for cancelling the effects of disturbance on the system output. This is advantageous, since in a simple feedback system, the corrective action starts after the effect of disturbance is reflected at the output. On the other hand, in feedforward control the change in disturbance signal is measured and the corrective action takes place immediately. As a result, the speed and performance of the overall system improves, if feedforward control, together with feedback action is employed.
In order to illustrate the effect of feedforward control, let us consider the heat exchange process shown in Fig.1. The cold water comes from a tank and flows to the heat exchanger. The flow rate of cold water can be considered as a disturbance. The change in input flow line may occur due to the change in water level in the tank. Suppose, the feedforward line is not connected, and the controller acts as a feedback control only. If the water inlet flow rate increases, the temperature of the outlet hot water flow will decrease. This will be sensed by the temperature sensor that will compare with the set point temperature and the temperature controller will send signal to open the control valve to allow more steam at the steam inlet. The whole operation is a time consuming and as a result the response of the controller due to the disturbance (inlet water flow rate) is normally slow. But if we measure the change in inlet flow rate by a flowmeter and feed this information to the controller, the controller can immediately take the correcting action anticipating the change in outlet temperature. This will improve the speed of response. Thus feedforward action, in addition to the feedback control improves the performance of the system, but provided, the disturbance is measurable.
Let us now draw the block diagram of the overall control operation of the system shown in Fig. The block diagram representation is shown in below fig.