Fanuc alarm SV0421: EXCESS ERROR (SEMI-FULL)
αi/βi Servo (SV0xxx) · Fanuc 0i-D
What it means
The difference between semi-closed (motor pulse coder) and full-closed (separate detector) feedback exceeded the limit in parameter No. 2118. Since both should track the same motion, divergence reveals slip or measurement error somewhere in the drivetrain — typically ballscrew-to-coupling slip, or scale misreading. Backlash measurement comparing motor position vs table position will localize the loss.
Likely causes
- Coupling slippage between motor and ballscrew
- Linear scale read head gap drift or tape damage
- Accumulated mechanical backlash beyond threshold
- Parameter No. 2118 set unrealistically tight (should be ~2–3x backlash)
Need the part urgently?
Send us alarm SV0421 with your machine details — typical RFQ turnaround is 24–48h.