A control loop cycling continuously around setpoint instead of settling.
Trend of an over-tuned PID loop: the process variable hunts for several cycles after a setpoint change before settling. Four or more peaks that shrink slowly. The output mirrors the process variable and swings just as hard. The oscillation decays, which is what separates a tuning problem from a mechanical limit cycle — stiction cycles do not decay.
What it means
Hunting is the field's word for a loop that will not sit still. It is used interchangeably with oscillating, cycling and swinging, and it dominates how the symptom is described in practice — an operator reports that a valve is hunting far more often than that a loop is exhibiting sustained oscillation.
The word covers two mechanically different situations. A loop hunting because of tuning produces an oscillation that decays after a disturbance. A loop hunting because of stiction, backlash or another nonlinearity produces a limit cycle whose amplitude stays constant. The distinction determines whether anything you do at the faceplate will help.
Trend of an over-tuned PID loop: the process variable hunts for several cycles after a setpoint change before settling. Four or more peaks that shrink slowly. The output mirrors the process variable and swings just as hard. The oscillation decays, which is what separates a tuning problem from a mechanical limit cycle — stiction cycles do not decay.
Also called
On a plant you will hear this called cycling, oscillating, swinging. They mean the same thing.
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.