Ultimate period
The time for one complete cycle of the sustained oscillation used by the closed-loop Ziegler-Nichols method.
What it means
The ultimate period is the other half of the closed-loop Ziegler-Nichols measurement: with the gain set to the ultimate gain, you time one complete cycle of the process variable and record its period. The reset and derivative of the Ziegler-Nichols rules are then simple fractions of it.
Measured carelessly, the period drags the whole tune with it. Reset and derivative scale directly off the number, so a reading taken from a single cycle on a noisy trend inherits every error of that one cycle.
The period is also where a mechanical problem disguises itself. A limit cycle from stiction or backlash has a period of its own, but it is not the oscillation the method means — it belongs to the valve, not to the gain you set.
Related
Where this comes up
Last reviewed 2026-09-18.