Mitsubishi MDS alarm 18 means a main side encoder initial communication error — at power-up, the drive tried to establish communication with the motor-side encoder and failed. The axis is dead until the drive can talk to its encoder. Mitsubishi's six-step investigation is a model of cheap-to-expensive discipline — here it is in bench order.
The six steps, in prose
- Check the encoder-type parameter: SV025 (ent) for servo axes, SP020 for spindles — the declared encoder type must match the installed encoder. Wrong → set it correctly (the usual arrival: restores, drive swaps, commissioning). Correct → continue.
- Check the encoder itself is the right kind: if a pulse (analog) encoder is fitted where the configuration requires a serial encoder → the hardware is wrong — fit the serial encoder. This catches mixed-generation Frankenstein builds: motor swapped for an ‘equivalent’ with the older encoder type.
- Jiggle-test the encoder connectors (drive end and encoder end): loose or disconnected → install correctly. The classic post-maintenance cause.
- Power OFF and meter the encoder cable: continuity end to end, flexing at suspect points — faulty → replace the cable (never tape a data cable).
- Swap-test: replace with another unit to determine which side the fault is on — alarm follows the drive → replace/repair the drive unit; alarm stays with the encoder → continue.
- Ambient environment: grounding and cable shield connection — noise bad enough to break the initial handshake will also corrupt running data.
Tip: Alarm 18 at first power-up after ANY work — drive swap, motor swap, cable rerouting — is caused by that work: parameter, encoder type, or connector, in that order, before instruments come out. On an untouched machine it is cable or encoder ageing — oil-rotted insulation in the loom being the usual find. The same signal-path logic as Fanuc's d2 serial sensor alarm — different brand, same physics.
Diagnosis checklist (in order)
- Verify SV025/SP020 against the installed encoder.
- Confirm the encoder is serial where serial is required.
- Reseat/jiggle-test connectors both ends.
- Meter the cable (flexing); replace if in doubt.
- Swap-test to split drive vs encoder; repair/replace the guilty side.
- Persistent intermittents → grounding and shielding.
Related guides
- Mitsubishi MDS alarm 19 (encoder error in synchronous control)
- Mitsubishi M70/M80 alarm list
- Zero point initialization after encoder work
Alarm 18 blocking an axis? JJ Automation stocks Mitsubishi encoders and cables, swap-tests drive vs encoder on site in NCR, and repairs MDS drive encoder interfaces at component level. WhatsApp us or call +91 88022 12394.