Proportional Plus Integral (PI) Control System

This type of control is combination of two control modes, proportional and integral (reset). The Combining of the two modes results in gaining the advantages and compensating for the disadvantages of the two individual control modes.

The main advantage of the proportional control is that an immediate proportional output is produced as soon as an error signal exists at the controller. The proportional controller is considered a fast-acting device. This immediate output change enables the proportional controller to reposition the final control element within a relatively short period of time in response to the error.

The main disadvantage of the proportional control mode is that a residual offset error exists between the measured variable and the setpoint for all but one set of system conditions.

The main advantage of the integral control mode is that the controller output continues to reposition the final control element until the error is reduced to zero. This results in the elimination of the residual offset error allowed by the proportional control mode.

The main disadvantage of the integral control mode is that the controller output does not immediately direct the final element to a new position in response to an error signal. The controller output changes at a defined rate of change; and time is needed for the final element to be repositioned.

The combination of the two control modes is called the proportional plus integral (PI) control mode. It combines the immediate characteristics of a proportional control mode with the zero residual offset characteristics of the integral control mode.

Example of Proportional plus Integral Controller in an Industrial application

Let’s consider the heat exchanger in the figure below:

The response curves in the figure below only illustrate the demand and the measured variable which represents the hot water temperature.

Let’s assume the process undergoes a demand disturbance which reduces the flow of the hot water out of the heat exchanger. The temperature and the flow rate of the steam into the heat exchanger remain the same. As a result the temperature of the hot water out of the heat exchanger 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 in the figure (c) above. You will notice that, the residual error would still exist.

By adding the integral/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. Also, the reset/integral action acts to eliminate the offset error after a period of time.

The Disadvantage of Proportional Plus Integral (PI) Controllers

Reset windup

Proportional plus integral/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 integral/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.