What is a valve positioner and why is it used?

A valve positioner is a controller mounted on the valve that ensures stem position accurately follows the input signal. Benefits: overcomes friction/stiction, compensates for pressure variations, provides faster response, enables split-range, and gives position feedback. Types: pneumatic (classic, uses nozzle-flapper), electro-pneumatic (input 4-20mA, output pneumatic), and smart/digital (HART/Fieldbus, diagnostics, characterization). Most control valves use positioners except for very fast on-off applications. Smart positioners provide valve diagnostics including friction, travel, and signature analysis.