Mitsubishi MDS alarm 31 means overspeed — the motor was detected rotating (or, on linear motors, moving) faster than the allowable speed. Either the axis genuinely overshot, or — at least as often — the drive's idea of speed is wrong: parameters, feedback waveform, or noise. Mitsubishi's six-step investigation splits servo from spindle first and ends with one of the most useful repeatability tests in the whole manual.
Step 1 — Servo axis or spindle?
The parameter sets differ; the alarm tells you which branch to walk.
Step 2 — Servo: the gearing/pitch parameter set
Check SV001 (PC1), SV002 (PC2), SV018 (PIT) and SV025 (MTYP) — gear ratio numerator/denominator, ballscrew pitch, and motor/encoder type. Wrong values here make the drive compute a false speed from healthy feedback — a mis-set gear ratio can read a normal rapid as overspeed. Classic after restores, drive swaps, or retrofit work. Correct → continue to step 5.
Step 3 — Spindle: SP026 (TSP, maximum speed)
The alarm trips at 115% of SP026 — if SP026 is set below the machine's real top speed, normal operation trips it. Verify against the machine specification. Correct → continue.
Step 4 — The PLG output waveform (spindle sensor)
Check the spindle PLG (pulse generator) waveform — amplitude and shape per the adjustment procedure. A weak or distorted waveform (gap drift, contamination, ageing) produces speed miscounts that read as overspeed. Problem → adjust the PLG output; unadjustable → sensor/cable replacement.
Step 5 — Is the speed waveform overshooting?
- Genuinely overshooting (visible on the speed monitor during accel/decel) → the control is too aggressive for the load: increase the acceleration/deceleration time constant, or lower the load inertia — a heavier chuck/fixture than the tuning assumed is the usual story.
- Not overshooting → the reported speed is lying → environment check (temperature, noise, grounding — the standing discipline), then step 6.
Step 6 — The repeatability test (the elegant one)
Mitsubishi's discriminator: does the alarm occur when the motor is STOPPED — or does the speed reading on the drive monitor vary while the motor stands still?
- Yes → replace the encoder or encoder cable. A stationary motor showing changing speed is pure feedback corruption — nothing mechanical can explain it. This one observation, free on the drive monitor, condemns the feedback chain beyond argument.
- The alarm occurs at all times regardless → replace (repair) the drive unit — its speed-detection processing is faulty.
Diagnosis checklist (in order)
- Servo or spindle branch?
- Servo → verify SV001/SV002/SV018/SV025; spindle → SP026 (trip = 115%).
- Spindle → check/adjust the PLG waveform.
- Speed monitor at standstill: varying → encoder/cable.
- Overshoot on accel/decel → lengthen time constants / review inertia.
- Constant alarm → drive unit to the bench.
Related guides
- Mitsubishi MDS alarm 33 (overvoltage — the decel-side companion)
- Mitsubishi MDS alarm 2F (encoder communication)
- Mitsubishi M70/M80 alarm list