Ultimate gain
Ultimate gain is Ku: the controller gain that just holds a constant-amplitude cycle. Finding it on a running plant means driving the loop to the edge of stability. PIDSnap never asks for it.
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.
Questions that come up
What is ultimate gain?
Ultimate gain is Ku: the controller gain that just holds a constant-amplitude cycle with integral and derivative off. The Ziegler-Nichols table is built from that number and the cycle period. Finding it on a production loop means driving the loop to the edge of stability, which is why PIDSnap never asks for it.
Related
Where this comes up
Written by PIDSnap control engineering. Published 2026-08-01, last reviewed 2026-10-22.