Example of Proportional Plus Rate Control:
To describe proportional plus rate control, we will use the similar heat exchanger process which has been analyzed in previous chapters. For this instance, therefore, the temperature controller used is a proportional plus rate controller.
Figure: Heat Exchanger Process
As described in Figure, the proportional only control mode responds to the decrease in demand, but since of the inherent characteristics of proportional control then a residual offset error remains. By Adding the derivative action affects the response through permitting only one small overshoot and a rapid stabilization to the new control point. Therefore, derivative action gives increased stability to the system, but does not eliminate offset error.
Figure: Effect of Disturbance on Proportional plus Rate Reverse Acting Controller