Function of Valve actuator:
It is that portion of the valve that responds to the applied signal and results in the movement of the stem due to which the flow rate of fluid changes. It consists of diaphragm, stem and diaphragm returning spring
Valve positioner:
The function of valve positioner is to sense both the instrument signal and the valve position and from this measurement ensure that the valve position is directly proportional to its controller output signal. Positioner is mounted on the control valve to be able to measure the stem position. When static frictional forces are large, valve positioner is used along with actuator so as to correctly position the valve stem in response to the control signal. Valve positioner improves the speed of response and reduces the hysteresis effect.