Ultimate gain
The controller gain that just drives a closed loop into a sustained oscillation of constant amplitude.
What it means
Ultimate gain is the number the closed-loop Ziegler-Nichols method exists to find: with integral and derivative off, you raise the gain until the process variable cycles at a steady amplitude, and that gain — together with the period of the cycle — is all the method's correlations need. The word ultimate is doing real work, because this is the gain at the edge of stability, a place no loop is meant to live.
Two cautions follow from the method's premise. Noise blurs the boundary, since a clean constant-amplitude cycle is exactly what a noisy process refuses to produce. And finding the number means deliberately pushing a production loop toward sustained oscillation, which is the one operating condition you would normally pay a great deal to avoid.
PIDSnap never asks for the ultimate gain, for the same reason it never drives a loop to the stability boundary: a number that has to be found by destabilising the loop is not worth the risk on anything you intend to keep running.
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Where this comes up
Last reviewed 2026-09-18.