The main function of the valve limiter is to improve the quality and movement speed of the regulating valve, improve the output force, and the electro pneumatic conversion. At present, most of the regulating valves are matched with computers, and the computer outputs mostly 4-20mA current signals. The electro pneumatic valve positioner can be used to meet the requirements. Valve positioner is the main accessory of control valve It takes the valve 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, it changes the output signal to the actuator to make the actuator act, and establishes a one-to-one correspondence between the valve stem displacement and the controller output signal. Therefore, the valve positioner is composed of a feedback control system with the valve stem displacement as the measuring signal and the controller output as the setting signal. The control variable of the control system is the output signal from the valve positioner to the actuator.