Positioner:

For instance, as the control signal increases, a valve within the positioner admits more supply air to the actuator. Conclude, the control valve moves downward. A linkage transmits the valve position information back to the positioner. This forms a small internal feedback loop for the actuator. While the valve reaches the position which correlates to the control signal, the linkage stops supply air flow to the actuator. That will cause the actuator to stop.  Alternatively, if the control signal decreases, another valve within the positioner opens and permits the supply air pressure to decrease through venting the supply air.  Which will causes the valve to move upward and open. While the valve has opened to the proper position then the positioner stops venting air from the actuator and stops movement of the control valve.

An important safety characteristic is gives through the spring in an actuator. It could be designed to position a control valve in a safe position if a loss of supply air occurs.  With a loss of supply air, the actuator in Figure will fail open. This category of arrangement is referred to as "spring-to-open, air-to-close" or simply "fail-open." A few valves fail in the closed position. This category of actuator is referred to as "air-to-open, fail-closed "or " spring-to-close."  That "fail-safe" thought is a significant consideration in nuclear facility design.