The valve positioner is the main accessory of the regulating valve. It is used together with the pneumatic regulating valve. It uses 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, The output signal to the actuator is changed to make the actuator move, and a one-to-one correspondence between the valve stem displacement and the controller output signal is established. Therefore, the valve positioner constitutes a feedback control system that uses the valve stem displacement as the measurement signal and the controller output as the setting signal. The manipulated variable of the control system is the output signal of the valve positioner to the actuator. Valve positioner can be divided into pneumatic valve positioner, electric-pneumatic valve positioner and intelligent valve positioner according to its structure and working principle.

Principle of action of valve positioner
(1) It is used for important regulating systems with high requirements on regulating quality to improve the positioning accuracy and reliability of regulating valves.
(2) For applications where the pressure difference between the two ends of the valve is large (△ p> 1MPa). By increasing the pressure of the air source, the output force of the actuator is increased to overcome the unbalanced force generated by the liquid on the valve core and reduce the stroke error.
(3) When the medium to be adjusted is high temperature, high pressure, low temperature, toxic, flammable, and explosive, in order to prevent external leakage, the packing is often pressed tightly, so the friction between the valve stem and the packing is large. Use the positioner to overcome the time lag.
(4) When the medium to be adjusted is a viscous fluid or contains suspended solids, the positioner can overcome the resistance of the medium to the valve stem.
(5) For large-diameter (Dg> 100mm) regulating valve to increase the output thrust of the actuator.
(6) When the distance between the regulator and the actuator is more than 60m, the positioner can overcome the transmission delay of the control signal and improve the response speed of the valve action.
(7) Used to improve the flow characteristics of the regulating valve.
(8) When a regulator controls two actuators to carry out range control, two locators can be used to receive low input signal and high input signal respectively, then an actuator low range action and another elevation action constitute range control.

The valve positioner can increase the output power of the regulating valve, reduce the transmission delay of the regulating signal, speed up the movement speed of the valve stem, improve the linearity of the valve, overcome the friction of the valve stem, and eliminate the influence of unbalanced forces, thereby ensuring the adjustment Correct valve positioning.