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.
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.
Proportional plus rate controllers are useful with processes which are frequently started up and shut down because it is not susceptible to reset windup.