The axis oscillates when it settles. Someone turns up the gains to stiffen the response. The oscillation gets worse, then the drive starts faulting on overcurrent. The machine doesn’t need more gain. It needs the right gain, and those are different numbers.
What the gains actually do
A servo loop uses proportional gain to push the motor toward the commanded position, integral gain to correct steady-state error, and derivative gain to damp overshoot. Raise proportional alone and the axis responds harder but overshoots and rings. Raise it too far and the loop goes unstable.
The mechanical system sets the ceiling. A rigid machine with stiff couplings and tight belts tolerates high gains. A compliant machine with long belts, flexible frames, or backlash will oscillate no matter how you tune the drive. You can’t tune out mechanical looseness.
Tune the mechanics first
Before touching the gains, check the physical axis. Belt tension, coupling backlash, bearing condition, and frame rigidity. An axis that clunks when you reverse by hand has backlash that the servo can’t compensate for at the tuning level. Fix the mechanics or you’ll be chasing gains forever.
Start with an auto-tune, then refine
Most drives run an auto-tuning routine that identifies the load inertia and sets baseline gains. That gets you 80 percent there. Then move the axis through the actual motion profile and watch the following error and settling. Small manual adjustments clean up what auto-tune misses.
Tune with the real load, not an empty axis. The inertia changes when the fixture and parts are on, and gains that work unloaded oscillate under load.
Filters are a tradeoff
Notch filters suppress resonance at a specific frequency, letting you run higher gains. They help when the mechanics have a known vibration mode. Add too many filters and you add delay to the loop, which hurts response. Use one or two where you’ve measured resonance; don’t stack filters to mask a floppy machine.
What good tuning looks like
The axis moves to position, settles within a few milliseconds, and doesn’t ring. Following error stays small through the move. If the machine sounds smooth but the position overshoots, or holds position but buzzes, the gains aren’t balanced. Watch the scope traces in the drive software; don’t tune by ear.
