Let's once more refer to our heat exchanger example (see Figure 23). This time we will apply a proportional plus reset controller to the process system.

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Figure 23 Heat Exchanger Process System

The response curves shown in Figure 24 illustrate only the demand and the measured variable which represents the hot water outlet temperature.

Assume the process undergoes a demand disturbance which reduces the flow of the hot water out of the heat exchanger. The temperature and flow rate of the steam into the heat exchanger remain the same. As a result, the temperature of the hot water out will begin to rise.

The proportional action of the proportional plus reset controller, if acting alone, would respond to the disturbance and reposition the control valve to a position that would return the hot water out to a new control point, as illustrated by the response curves. You'll note that a residual error would still exist.

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Figure 24 Effects of Disturbance on Reverse Acting Controller

By adding the reset action to the proportional action the controller produces a larger output for the given error signal and causes a greater adjustment of the control valve. This causes the process to come back to the setpoint more quickly. Additionally, the reset action acts to eliminate the offset error after a period of time.

Reset Windup

Proportional plus reset controllers act to eliminate the offset error found in proportional control by continuing to change the output after the proportional action is completed and by returning the controlled variable to the setpoint.

An inherent disadvantage to proportional plus reset controllers is the possible adverse effects caused by large error signals. The large error can be caused by a large demand deviation or when initially starting up the system. This is a problem because a large sustained error signal will eventually cause the controller to drive to its limit, and the result is called "reset windup."

Because of reset windup, this control mode is not well-suited for processes that are frequently shut down and started up.

Summary

The proportional plus reset control mode is summarized below.

Proportional Plus Reset Control Summary

Proportional plus reset control eliminates any offset error that would occur with proportional control only.

Reset windup is an inherent disadvantage of proportional plus reset controllers that are subject to large error signals.