SV0421 Excess Error (Semi-Full) / Dual Check fault in Fanuc drive

SV0421 Excess Error (Semi-Full) / Dual Check fault in Fanuc drive

Description

This alarm appears on machines equipped with "secondary feedback," such as linear glass scales (Heidenhain, Fagor) or rotary scales. In a standard setup (Semi-Closed Loop), the CNC trusts the motor encoder. In a "Full Closed Loop" setup, the CNC drives the motor but reads the position from the linear scale attached to the table. SV0421 indicates that the position reported by the motor encoder and the position reported by the linear scale disagree by an amount larger than the tolerance parameter. This is a "sanity check" alarm.

Cause

The disagreement usually comes from mechanical slop between the motor and the scale.

• Backlash: If the ball screw is worn, the motor turns, but the table (and scale) doesn't move immediately. The difference triggers the alarm.
• Scale Contamination: Linear scales are very sensitive. If oil gets inside the scale reader head, it may skip counts. The motor keeps moving, but the scale stops counting, causing a discrepancy.
• Coupling Slippage: If the coupling slips, the motor spins, but the ball screw (and scale) does not.
• Parameter Error: If the "Flexible Gear Ratio" (DMR/CMR) parameters are calculated incorrectly, the CNC expects a different ratio of motor turns to scale movement.

Solution

Diagnosis requires isolating which feedback device is telling the truth.

1. Clean the Scales: The most common fix is cleaning the reading head of the linear scale using air (if permitted) or manufacturer-specific solvents.
2. Check Coupling: Mark the motor shaft and coupling with a marker. Run the axis. Check if the marks are still aligned.
3. Disable Scales (Diagnostic): To run the machine temporarily, you can often disable the external scale in the parameters (typically Parameter 1815 bit 1 "OPT"). This reverts the machine to Semi-Closed loop (motor encoder only). If the machine runs fine this way, the issue is definitely the scale hardware.