What is the role of a positioner in a pneumatic valve control loop?

• A. To move the valve stem directly in response to a current signal.
• B. To provide feedback and ensure the valve reaches the commanded position by converting the control signal to proportional pneumatic pressure.
• C. To supply power to the actuator.
• D. To measure temperature.

Answer: (B)

A positioner is used to ensure the valve reaches the exact position commanded by the control signal by converting that signal into proportional pneumatic pressure for the actuator and by providing position feedback. The control signal (often 4–20 mA) represents the desired valve position. The positioner translates that signal into air pressure that moves the actuator accordingly. It also compares the actual valve position to the commanded position and makes continuous adjustments, forming a closed loop that corrects any error. This improves accuracy, stability, and speed, and helps compensate for changes in supply pressure, friction, or load. It does not supply electrical power to the actuator, and it does not measure temperature.