PROPORTIONAL PLUS RATE CONTROL SYSTEMS
Process Controls
Example of Proportional Plus Rate Control
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
Figure 28 Heat Exchanger Process
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.
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