Servo vibration or hum at standstill
A servo that hums, buzzes or hunts while it is supposed to be holding still is almost always a tuning problem: loop gains set higher than the mechanics can support, an inertia ratio setting that does not match the real load, or a machine resonance the filters do not cover. Loose couplings, backlash and worn mechanics make all three worse.
Diagnose this with AI
Try Free Servo Diagnostic Upload a photo of the alarm screen, the fault history or a data export. InduQT compares the evidence and suggests the next useful checks.
Where it shows up
- Audible hum or buzz with the axis in servo-lock; the noise stops at servo-off
- Yaskawa Σ-7: A.520 Vibration Alarm or A.911 Vibration warning
- Mitsubishi MR-J4: AL.50.4 Overload during a stop, where hunting during servo-lock is a listed cause; AL.54 Oscillation detection
- Mitsubishi MR-J5: AL.054 Oscillation detection
Likely causes and how to tell them apart
1Loop gains too high
- Points to it
- Steady buzz, often around 200 to 400 Hz; started after a tuning change.
- Check
- Compare speed loop gain, position loop gain and integral time with defaults (Yaskawa Pn100 Kv 40.0 Hz, Pn102 Kp 40.0/s, Pn101 Ti 20.0 ms) and reduce them, or rerun autotuning.
2Inertia ratio setting wrong
- Points to it
- Load or tooling changed; hum with a heavier or lighter load than when tuned.
- Check
- Check the moment of inertia ratio setting (Yaskawa Pn103) against the real load; estimate it with the servo inertia ratio calculator and remeasure with the drive’s identification function.
3Machine resonance not filtered
- Points to it
- Narrow-band tone; torque ripple at a fixed frequency.
- Check
- Measure torque ripple (MR Configurator2 or SigmaWin+), set a notch or machine resonance suppression filter; Mitsubishi notes the resonance frequency can drift with aging.
4Mechanical looseness or backlash
- Points to it
- Hunting back and forth across a small band; worse with a worn coupling or belt.
- Check
- With power isolated, check couplings, belts, gearbox backlash and mounting bolts.
5External force on the axis (gravity, belt tension)
- Points to it
- Hunting only on vertical or belt-driven axes.
- Check
- Check counterbalance and brake, and measure position deviation while holding.
Diagnostic sequence
- Confirm it is the servoIf the noise stops at servo-off, it is the control loop, not the mechanics alone.
- Check what changedNew tooling, load, gearbox or a parameter change is the usual trigger.
- Check inertia ratioCompare the inertia ratio setting with the real load before touching gains.
- Measure the frequencyUse the drive software’s FFT or torque trace to see whether it is broadband (gain) or a single tone (resonance).
- Tune, then filterRerun autotuning or reduce gains; add a notch filter at the measured resonance.
Evidence to collect
- Recording or description of the noise and when it starts
- Current gain and inertia ratio settings
- Torque or speed trace while holding
- Recent load, tooling or mechanical changes
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