Which statement best describes the function of a positioner in control systems?

• A. A valve operator that automatically maintains process pressure.
• B. An instrument that records and scales position data.
• C. An actuator device that positions to a set point commanded by a controller.
• D. A sensor that detects physical position.

Answer: (C)

A positioner is the device that makes the valve move to the exact position commanded by the controller. It takes the control signal (the set-point for valve position) and drives the actuator, typically by regulating air pressure in a pneumatic actuator, to reach that position precisely. This tightens the match between the controller’s command and the actual valve position, compensating for things like friction, deadband, and variations in supply pressure. The other descriptions describe different equipment: a regulator maintains process pressure, a recorder or transducer records or scales position data, and a position sensor detects where the valve is.