Proportional Control System in Process Control

Proportional Control System in Process Control

In continuous process control, a setpoint provided by the operator is continually compared to the controlled variable output. A difference or error signal is generated which is proportional to the difference between the setpoint and the controlled variable output. This error signal is used by the controller to modify the output control module signal with an amplitude and polarity designed to reduce the amount of error.

In a proportional control system, the output signal to change the controlled variable is proportional to the error signal. If the output is greater than the set point, a negative error is input to the controller to cause the output to decrease and the error to get smaller. The opposite happens for an output less than setpoint.

It is desirable to have the system designed so that the nominal setpoint is near midrange to allow a linear response to an error condition in either direction over a wide range. If the setpoint is near the end of the linear range, very little proportional action will be possible in one direction and for this correction it will act much like an ON/OFF controller. In operation, the output of the controller will be a bias amount (to move the setpoint to mid range) plus or minus an amount which is proportional to the difference between the setpoint and the control variable (the error).

A problem with proportional control is that if a permanent disturbance occurs, this type of control will not be able to return the controlled variable to the setpoint value. The reason is that this type of control relies on an error signal to change the manipulated variable.