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To illustrate proportional plus rate control, we will use the same heat exchanger process that has been analyzed in previous chapters (see Figure 28). For this example, however, the temperature controller used is a proportional plus rate controller.
Figure 28 Heat Exchanger Process As illustrated in Figure 29, the proportional only control mode responds to the decrease in demand, but because of the inherent characteristics of proportional control, a residual offset error remains. Adding the derivative action affects the response by allowing only one small overshoot and a rapid stabilization to the new control point. Thus, derivative action provides increased stability to the system, but does not eliminate offset error.
Figure 29 Effect of Disturbance on Proportional Plus Rate Reverse Acting Controller Applications Proportional plus rate control is normally used with large capacity or slow-responding processes such as temperature control. The leading action of the controller output compensates for the lagging characteristics of large capacity, slow processes. Rate action is not usually employed with fast responding processes such as flow control or noisy processes because derivative action responds to any rate of change in the error signal, including the noise. Proportional plus rate controllers are useful with processes which are frequently started up and shut down because it is not susceptible to reset windup. Summary The proportional plus rate control mode is summarized below. Proportional Plus Rate Control Summary Derivative action is added to a controller to make it respond to the rate of change of the error signal. Derivative action cannot be used as a control mode alone. Proportional plus rate control does not eliminate offset error. Proportional plus rate control increases system stability.
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