Actuation & Instrumentation
Hysteresis
Hysteresis is the difference in valve position for the same input signal depending on the direction of approach. Ramp the signal up to 50 % and the valve sits at 48 %; ramp down to 50 % and it sits at 52 % — that spread is the hysteresis. Unlike dead band (no motion at all), hysteresis gives a different, reproducible position.
Principal sources: elastic wind-up in actuator springs and diaphragms, elastic deformation of stem/connections, friction reversals in packing and guides, and lost motion in rotary linkages. Hysteresis above roughly 1 % is usually visible in loop behaviour — slow limit cycles, offset after direction reversals, and controllers that can't be tuned consistently.
Measurement: IEC 60534-6-style staircase or ramp tests with a travel sensor produce the classic "banana" plot; smart positioners log this automatically as a valve signature. Mitigation: a good positioner (closed-loop on stem position, not pressure) largely cancels moderate hysteresis; the cure for severe hysteresis is mechanical — packing and linkage refurbishment.