Valve positioners-classified by structure into pneumatic, electro-pneumatic, and smart types-serve as key accessories for control valves. Typically paired with pneumatic control valves, they receive output signals from a controller and, in turn, generate their own output signals to actuate the pneumatic valve. Once the control valve begins to move, the displacement of the valve stem is fed back to the valve positioner via a mechanical linkage; simultaneously, the valve's position status is transmitted to a higher-level control system via an electrical signal.
The valve positioner functions as a primary accessory for control valves. It utilizes the valve stem's displacement signal as a feedback measurement input and the controller's output signal as a setpoint input. By comparing these two signals, the positioner adjusts its own output signal to the actuator whenever a deviation is detected, thereby driving the actuator into action. This process establishes a one-to-one correspondence between the valve stem's displacement and the controller's output signal. Consequently, the valve positioner forms the core of a feedback control system wherein the valve stem's displacement serves as the measured variable and the controller's output serves as the setpoint; within this system, the manipulated variable is the output signal transmitted from the valve positioner to the actuator.