A valve Positioner is a device used to increase or decrease the air pressure opertaing the actuator until the valve stem reaches the posion called for by the instrument PID controller.
Positioners are generally mounted on the side or top of the actuator. They are connected mechanically to the valve stem so that stem position can be compared with the position dictated by the controller.
A positioner is a type of air relay which is used between the controller output and the valve diaphragm. The positioner acts to overcome hysteresis, packing box friction, and valve plug unbalance due to pressure drop. It assures exact positioning of the valve stem in accordance with the controller output.