SP9020 (Alarm 20) Reference Position Loss (Spindle Differential) fault in Fanuc drive

SP9020 (Alarm 20) Reference Position Loss (Spindle Differential) fault in Fanuc drive

Description

SP9020 (Alarm 20) is specific to sophisticated spindle setups involving "Spindle Orientation," "Rigid Tapping," or "Cs Axis" contouring. To perform these tasks, the spindle must have a precise "Zero" degree mark. Alarm 20 indicates that the drive has lost the phase relationship required to maintain this zero point relative to the master encoder. This often happens on systems using two sensors (Motor sensor + External Spindle sensor).

Cause

• Belt Slip: Slippage between the motor and the spindle shaft. The drive compares the motor encoder to the external spindle encoder. If they drift apart, the "Differential" position is lost.
• Sensor Gap: The gap between the magnetic sensor and the spindle gear is inconsistent (runout).
• Cable Noise: Noise on the external sensor cable corrupting the "One Rev" pulse.

Solution

1. Check Belts: Ensure the drive belts are tight. Any slip destroys the sync between motor and spindle nose.
2. Check Proximity Switch: If the machine uses a proximity switch for orientation, ensure it is tight and the target is clean.
3. Re-Orient: Often, commanding a simple M19 (Spindle Orient) will force the drive to re-learn the position. If it fails or hunts, check the sensor gap.