Actuation & Instrumentation
Positioner
A positioner is a closed-loop controller mounted on the valve: it compares the command signal (4–20 mA → pressure, digital bus) with the measured stem position and adjusts air to the actuator until the two agree. It exists to defeat the valve's own imperfections — packing friction, process unbalance forces, hysteresis — none of which a bare actuator can overcome repeatably.
Modern digital positioners add: auto-stroke calibration, travel/PV diagnostics, partial-stroke testing for safety valves, valve signatures for dead-band and stiction trending, and HART/FF/Profibus integration.
Two rules of practice worth more than any datasheet: first, a positioner's tuning must match the actuator volume — a fast positioner on a huge diaphragm case, or a slow one on a piston, both produce hunting (boosters or volume adjustment solve this). Second, don't fit positioners where they aren't needed — on small, clean, low-friction on-off valves they add failure modes without benefit.