Positioner is one of control valve accessory that functions as position controller.
It works to control the amount of air supply into the actuator for stroking the valve to approach desired opening as commanded by control system or local controller.
Positioner provides accurate opening of the main valve by means of its mechanical connection to a moving part of final control element or actuator.
In this arrangement, there is such a feedback to achieve desired opening according to the control system output. Besides, positioner allows faster control valve response to encounter process upset.