The valve positioner is a necessary configuration of the pneumatic control valve. The valve positioner can be used to control the position of the valve at any opening degree. It uses the valve stem displacement signal as the input feedback measurement signal and the controller output signal as the seting signal. For comparison, when there is a deviation between the two,the output signal to the actuator is changed to make the actuator operate, and a one-to-one correspondence between the displacement of the valve stem and the output signal of the controller is established.