positioner

A positioner is a device used in automation to place a mechanical element, such as a valve stem, damper, or actuator, at a specified location. It operates within a control loop by taking a process control signal, comparing it to the actual position, and driving the actuator to minimize the error. Positioners improve accuracy, repeatability, and stability of positioning compared with direct-drive actuators, and they help linearize valve travel and reduce hysteresis.

Positioners can be pneumatic, electric, or electro-hydraulic. Pneumatic positioners use compressed air to modulate an actuator, typically reacting to a control signal such as a 4-20 mA current or a corresponding pneumatic pressure. Electro-pneumatic and electric positioners replace or augment air pressure with electronic control, often incorporating feedback sensors and digital processing. Direct-acting and reverse-acting configurations are common, providing different responses to signals. Many positioners include fail-safe features, such as spring-return actions, to move to a safe position in power loss.

Common applications are in process control valves found in oil and gas, chemical processing, power generation, water treatment, and HVAC systems, as well as in automated equipment where precise positioning is required. Performance is evaluated by accuracy, repeatability, speed of response, and stability under temperature and supply changes. Proper selection considers actuator type, control signal, required speed, and environmental conditions.

See also: actuator, valve, damper, feedback control, PID controller.