Control systems complementary feedback

Pure dead time cannot be generated by analog means, therefore a dead-time complementary analog controller will never be available. Dead time can be generated digitally, so the possibility exists for direct digital control systems. But in view of the problems that can be anticipated from a mismatch of process and controller dynamics, complementary feedback is of questionable value for any pure dead-time process. (A similar and more reliable method will be presented later in this chapter.)... 

Several authors have postulated a feedback control system that is modeled after the process. This kind of control action is known as complementary feedback, because the characteristics of the controller complement the dynamics of the process. A block diagram showing both process and complementary controller appears in Fig. 4.10. 

In a situation where the flow of bottoms product is much less than the flow of distillate, bottoms flow is manipulated from feed rate and com position. In this arrangement, the forward-loop calculation is complementary to that used when manipulating distillate flow. If no feedback loop is involved, accuracy of the forward loop is of paramount importance. Since the best accuracy will always be achieved by manipulating the smaller flow, the design of a control system cannot be intelligently undertaken without first writing a material balance across the tower. 

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