Direct and Reverse Acting Controller

Controllers can be set up in either direct or reverse modes. It was stated that 99% of the positioners are direct acting, and it follows that if a balance is to be maintained in the control loop that 99% of the controllers will be reverse acting. If the control valve and its controller are not in balance, the control valve will either go to the wide-open position and stay there, or it will stay closed and act as though it is not responding. This situation can normally be corrected by reversing the action of the controller.

Direct-Acting Controller

Setpoint Increases Output Increases Increase in Setpoint  = Increase in Output Setpoint Decreases Output Decreases Decrease  in  Setpoint =  Decrease  in  Output

Reverse-Acting Controller

Setpoint Increases Output Decreases Increase in Setpoint  = Decrease in Output Setpoint Decreases Output Increases Decrease  in  Setpoint =  Increase  in  Output

Two of the more common control valve uses are for pressure control. In both instances, the controllers are reverse acting. Most pressure-reducing valves will be fail-closed and most back-pressure control valves will be fail-open. If the pressure-reducing valve were fail-open or the back-pressure valve fail-closed, then the controllers would have been direct acting.