SV0420 Sync Control Torque Diff fault in Fanuc drive

SV0420 Sync Control Torque Diff fault in Fanuc drive

Description

This alarm is specific to machines that have "Master/Slave" axes or "Tandem Control." For example, a large gantry router with two motors driving the X-axis (X1 and X2), or a lathe with a synchronized sub-spindle. The CNC constantly monitors the torque (current) of the Master motor and the Slave motor. In a perfect world, both motors should share the load equally (50/50). SV0420 triggers when the difference in torque between the two motors exceeds a preset parameter threshold. This means the motors are "fighting" each other.

Cause

The motors are fighting because they disagree on position or mechanical geometry.

• Mechanical Rack Alignment: On gantry machines, if the gantry is physically skewed (out of square), one motor is pushing while the other is pulling to try and straighten it.
• Backlash Discrepancy: If one gearbox has more play (slop) than the other, the control loops will become unstable, causing oscillation and torque difference.
• Reference Position Shift: If one axis lost its home position slightly (grid shift), the CNC might be commanding them to positions that are physically impossible to reach simultaneously without stretching the machine frame.
• Coupling Slip: A loose coupling on just one side of the tandem pair.

Solution

This requires mechanical squaring and parameter tuning:

1. Re-Square the Machine: You likely need to perform a "Gantry Square" procedure. This involves mechanically loosening the connection, aligning the bridge physically, and resetting the home positions for both axes so they agree.
2. Check Torque Monitoring: View the servo diagnosis screen. Watch the current % of Master vs. Slave. If they diverge significantly while moving at a constant speed, investigate the drag on the axis showing higher load.
3. Coupling Inspection: Verify both motor couplings are tight.
4. Parameter Adjustment: If the mechanics are perfect, the alarm threshold (Parameter 2031 or similar) might be too tight, or the feed-forward gains need adjustment to synchronize the servo response times.