Mechanism: a positioner-driven actuator converts a control signal into a metered flow area
"The opening or closing of control valves is usually done automatically by electrical, hydraulic or pneumatic actuators," with the valve itself manipulating "a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance." "The actuator is the device connected to the valve through the valve stem that provides the force required to move the valve," and on the most common design, "as the supply air pressure is increased, the rubber diaphragm pushes against the spring and moves the valve stem down into the valve body." The positioner is what makes this accurate rather than approximate: it does not drive the actuator directly but "relates the input signal and the valve position, and will provide any output pressure to the actuator to satisfy this relationship," continuously comparing actual stem position — sensed through a mechanical feedback arm linked to the spindle — against the position the controller has demanded, so that "there is a linear relationship between the signal input pressure...and the position of the control valve." Most process installations run this loop on a "4 to 20 mA DC signal," with building-services installations more commonly using "0–10 V."