Working principle The valve positioner is the main accessory of the control valve. It takes the stem displacement signal as the input feedback measurement signal and the controller output signal as the setting signal for comparison. When there is a deviation between the two, change its output to the actuator The signal causes the actuator to act, establishing a one-to-one correspondence between the valve stem displacement and the controller output signal. Therefore, the valve positioner constitutes a feedback control system that uses the displacement of the valve stem as the measurement signal and the output of the controller as the setting signal. The manipulated variable of the control system is the output signal from the valve positioner to the actuator.