Valve positioner is the main control valve accessories, and supporting the use of pneumatic control valve, which accepts the regulator output signal, and then use its output signal to control the pneumatic valve, when the valve action, the valve stem displacement through the machinery Device feedback to the valve positioner, valve position conditions through the electrical signal to the host system. The electrical signal is generally 4-20mA current signal, 4mA corresponds to zero range, 12mA corresponds to 50% of range, 20mA corresponds to full scale.