PROPORTIONAL PLUS RESET CONTROL SYSTEMS
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.
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.