What is a valve positioner? What are its principles and functions?

What is a valve positioner? Valve positioners, classified by structure into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners, are major accessories for control valves. They are usually used with pneumatic control valves. They receive the output signal from the regulator and then use their output signal to control the pneumatic control valve. After the control valve moves, the displacement of the valve stem is fed back to the valve positioner through a mechanical device, and the valve position is transmitted to the upper system via an electrical signal.

The working principle is that the control valve uses the valve stem displacement signal as the input feedback measurement signal, and the controller output signal as the set signal, for comparison. When there is a deviation between the two, it changes its output signal to the actuator, causing the actuator to move, 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 with valve stem displacement as the measurement signal and controller output as the set signal. The manipulated variable of this control system is the output signal from the valve positioner to the actuator.