Feedforward control load response with

Feedforward control systems have gained wide acceptance in chemical engineering in the past two decades. They have demonstrated their ability to improve control, sometimes quite spectacularly. We will illustrate this improvement in this section by comparing the responses of systems with feedforward control and with conventional feedback control when load disturbances occur. 

In a feedback configuration the controlled variable (temperature) has to be upset before correction can take place. Feedforward is a mode of control that corrects for a disturbance before it can cause an upset. Figure 2.108 illustrates feedforward control of a steam heater. The feedforward portion of the loop detects the major load variables (the flow and temperature of the entering process fluid) and calculates the required steam flow (Ws) as a function of these variables. When the process flow increases, it is matched with an equal increase in the steam flow controller set point. Because instantaneous response is not possible, dynamic correction by a lead-lag element is provided. 

It might be possible to construct an extremely complete dynamic model of a process, but any compensator with more than three terms to adjust would be unreasonably difficult to cope with. Furthermore, the purpose of dynamic compensation is to minimize an error which is already transient, so perfection is not really warranted. In most cases, a simple lead-lag function will be perfectly adequate and will be able to reduce the absolute area of the response curve by tenfold or more, distributed uniformly. Figure 8.14 compares the load response of the heat exchanger under dynamically compensated feedforward control with that encountered under feedback control. 

A potential drawback of such a hierarchical system is that it is nonrobust to perturbations. Changes in ventilatory load, for example, would disrupt the ventilatory command from the feedforward signal. This is at variance with the experimental observation of a load compensation response of the controller which protects ventilation against perturbations of the mechanical plant at rest and during exercise [Poon et al, 1987a, b Poon, 1989a, b]. Furthermore, if the prime objective of the controller were indeed to meet the metabolic demand (i.e., to maintain chemical homeostasis), then the hierarchical control system seems to perform quite poorly it is well known that arterial chemical homeostasis is readily disrupted environmental changes. 

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