Process Controls PROPORTIONAL PLUS RESET CONTROL SYSTEMSFigure 24 Effects of Disturbance on Reverse Acting ControllerBy adding the reset action to the proportional action the controller produces a larger output forthe given error signal and causes a greater adjustment of the control valve. This causes theprocess to come back to the setpoint more quickly. Additionally, the reset action acts toeliminate the offset error after a period of time.ResetWindupProportional plus reset controllers act to eliminate the offset error found in proportional controlby continuing to change the output after the proportional action is completed and by returningthe controlled variable to the setpoint.An inherent disadvantage to proportional plus reset controllers is the possible adverse effectscaused by large error signals. The large error can be caused by a large demand deviation orwhen initially starting up the system. This is a problem because a large sustained error signalwill 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 frequentlyshut down and started up.Rev. 0 Page 35 IC-07
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