What part of a control valve controls the plug proportionally to the output of the controller?

The valve positioner is a critical component in the operation of a control valve as it receives a signal from the controller and adjusts the valve's plug position accordingly. This ensures that the flow through the valve corresponds accurately to the desired setpoint provided by the control system. The positioner interprets the control signal, often in the form of a pneumatic or electronic signal, and adjusts the actuator's movements to achieve the required valve position effectively, providing proportional control.

In contrast, a hand wheel operator is primarily used for manual adjustments and does not interact with the automatic control signals. An operating spring serves to return the valve to a fail-safe position, but it does not actively manage the valve's movement based on a control signal. The diaphragm is involved in the valve's mechanical operation but does not perform the function of controlling the valve proportionally to the controller's output. Thus, the valve positioner is specifically designed for this proportional control task, making it the correct and essential component in this context.