What is a valve positioner and why is it used?

A valve positioner is a controller that compares command signal with actual valve position and adjusts actuator pressure to achieve desired position. Benefits: overcomes friction and process forces, improves response speed, provides accurate positioning, enables split-ranging, and compensates for wear. Modern smart positioners add diagnostics, communication (HART/Fieldbus), auto-calibration, and valve health monitoring. Positioners are recommended for most control applications except simple on-off or when fast response and low air consumption are required (small, lightly loaded valves).