Comparative merits of control strategies


Feedback control is simpler and less expensive. It relies on measured parameters and not on predictions. Most important, it does not require knowledge of the process. This is a significant advantage over feed-forward control, particularly in the case of complex processes. On the negative side, feedback control corrects the process only after the error has occurred. Feed-forward control predicts the error and reacts so as to prevent it before it occurs. It requires knowledge of the process, so that the reaction of the controlled variables to corrective actions may be calculated or estimated. It is therefore easier to apply to simple processes.

The corresponding concentration

Consider the process of producing a beverage by continuously mixing a concentrate with water. The control objective is to maintain the concentration of the mixed beverage constant within preset limits. The concentration of solutes in the concentrate may vary. In the case of feed-forward control of this process, the sensor would measure the concentration of the incoming concentrate. The controller would predict the corresponding concentration of the dilute beverage by a simple material balance and regulate the flow rate of the water or the concentrate so as to have the desired concentration in the product.

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The hardware elements always found in a loop are the controller, the measuring instrument and the actuator. The real physical loop contains additional hardware such as signal transmission lines, transducers for the conversion of one type of signal to another amplifiers used to enhance weak signals, recorders, safety switches, alarms etc.

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