General:
The controlled or auxiliary variable (e.g. distance, force, speed among others) necessary for a specific result is converted into a corresponding signal that is fed back to the input on the controlled subsystem.
On continuously adjustable valves:
Here as a rule the distance moved by the valve control spool is the auxiliary controlled variable that is measured by an inductive displacement transducer and fed as the actual value to the (internal) control circuit (Figure E 17). The actual controlled variable would be the flow rate or the pressure.
Figure E 17: Block diagram for electrical feedback