Valve Positioners: Types, Calibration, and Digital HART Communication

A valve positioner is a device mounted on a control valve actuator that ensures the valve plug or disc moves to the exact position commanded by the control signal. Without a positioner, friction, varying process pressure, and diaphragm hysteresis cause the actual valve position to deviate from the commanded position, degrading control loop performance. Positioners solve this problem by using a feedback mechanism that continuously compares the command signal to the actual valve position and corrects any error. Modern digital positioners have transformed valve diagnostics and maintenance practices across the process industries.

Types of Positioners

Positioners are classified by their input signal type and operating principle. Pneumatic positioners receive a 3 to 15 psi (0.2 to 1.0 bar) pneumatic signal from the controller and convert it to the required actuator pressure. Electro-pneumatic (I/P) positioners accept a 4 to 20 mA current signal from the DCS or controller and convert it to a pneumatic actuator pressure. Digital positioners (also called smart positioners or valve controllers) accept 4 to 20 mA signals with superimposed digital communication (HART, FOUNDATION Fieldbus, PROFIBUS PA) and offer advanced diagnostic and configuration capabilities not available with analog devices.

• Pneumatic positioners: for air-to-air signal conversion, no electronics, suitable for highest safety integrity levels

• Electro-pneumatic (I/P) positioners: convert 4-20 mA to pneumatic output, standard for most new installations

• Single-acting positioners: output pressure to one side of the actuator, spring returns the other

• Double-acting positioners: output to both sides of the actuator, no spring, for double-acting cylinders

• Digital smart positioners: HART/Fieldbus communication, full auto-calibration, diagnostic capability

Calibration Procedures

Proper calibration of a positioner ensures that the valve travels from fully closed to fully open position over the calibrated input signal range (typically 4 to 20 mA for a 0 to 100 percent stroke). Calibration involves setting the zero point (corresponding to fully closed or fully open, depending on the action) and the span (the rate of travel per unit input signal). Manual calibration of traditional positioners requires adjusting zero and span screws while observing actual valve position at defined set points. Digital positioners feature automatic auto-travel calibration routines that perform a full stroke, detect end stops, and set zero and span automatically in minutes.

HART and Digital Communication

HART (Highway Addressable Remote Transducer) protocol allows digital communication over the same two wires that carry the 4 to 20 mA analog signal. This allows plant engineers to access the positioner's configuration, calibration data, and diagnostic information from the DCS workstation or a handheld communicator without visiting the field. Key diagnostic data available through HART includes valve travel accumulation, number of direction reversals, stem friction signature, actuator pressure, supply pressure, and alerts for potential stiction or mechanical problems. FOUNDATION Fieldbus and PROFIBUS PA positioners provide fully digital, multivariable communication on fieldbus networks with even richer diagnostic integration.