A sustained oscillation of constant amplitude caused by a nonlinearity rather than by tuning.
Trend showing valve stiction: the controller output ramps smoothly while the process variable moves in abrupt steps, producing a limit cycle that does not decay. A sawtooth or square-cornered cycle on the process variable against a smooth, slowly ramping output. The cycle amplitude stays constant instead of decaying. Put the loop in manual and the cycle stops — that single test separates stiction from every tuning problem.
What it means
A limit cycle is an oscillation that neither grows nor decays. It sits at a fixed amplitude indefinitely, and that is the diagnostic signature — every oscillation caused by linear tuning either decays or grows.
Limit cycles come from nonlinearities: stiction, backlash, a valve against a travel stop, an on/off element in the loop. Detuning stretches the period without touching the amplitude, which is why a loop can be detuned to the point of uselessness and still be cycling.
Trend showing valve stiction: the controller output ramps smoothly while the process variable moves in abrupt steps, producing a limit cycle that does not decay. A sawtooth or square-cornered cycle on the process variable against a smooth, slowly ramping output. The cycle amplitude stays constant instead of decaying. Put the loop in manual and the cycle stops — that single test separates stiction from every tuning problem.
Two photos, thirty seconds, no signup. The pre-flight check finds the structural
problems that no amount of tuning will fix — before you touch the controller.