When Auto-Tuning Gives Bad Gains

… more less An auto-tune button runs a short test, often a relay test, and computes gains for you. It works well on a clean, steady, self-regulating loop. It fails in known ways. A noisy measurement makes the relay switch at the wrong times, so the period it reads is wrong. An integrating process, like a tank level, may not fit the model the tuner assumes. A sticky valve or loose linkage makes its own cycle, and the tuner measures that cycle instead of the process. Tuning at the wrong load gives gains that fit only that load. A relay step that is too small gets lost in noise and stiction. One that is too large pushes the process into its limits. A loop with long dead time next to its time constant also gets poor gains from simple formulas. So check a result before you trust it. Watch the first setpoint step and load change after tuning. Fast ringing, a slow wide cycle, or a chattering output all say the gains are wrong. Compare the gains with a quick step test and a rule you trust. Then fix the cause first: filter the measurement, repair the valve, tune at normal load, or change the relay size. Then run it again, or tune by a step test and a rule instead. After this Concept you can tell when an auto-tune result is wrong and what to do instead.