SRVO-074

Verify alarm details and affected axis

On the teach pendant, press MENU > ALARM > F1 [TYPE] > select 'Servo Alarm'. Note the specific axis number (1-6 or J1-J6) listed with SRVO-074. Record the alarm timestamp to correlate with any recent events.

Check alarm history (MENU > ALARM > F5 [HIST]) to see if this alarm occurred intermittently before becoming persistent, which suggests progressive LED failure rather than sudden damage.

Verify encoder cable connections before replacing encoder

Power off the robot controller. At the robot axis joint, inspect the encoder cable connector (typically a circular multi-pin connector). Ensure it is fully seated and the locking collar is secure. Check for corrosion, bent pins, or cable damage near the connector.

Follow LOTO (Lockout/Tagout) procedures. Ensure all power is removed and stored energy is dissipated before touching any connections.

Obtain correct replacement pulse coder

Use MENU > SYSTEM > F1 [TYPE] > 'Motors' to view the motor model for the affected axis. Cross-reference this with Fanuc parts documentation to order the exact pulse coder model. Alternatively, check the nameplate on the motor itself for the encoder part number (format: A860-xxxx-Tyyy).

The pulse coder part number is axis and motor-specific. Using an incorrect encoder model will cause mastering and calibration failures. Keep the old encoder serial number for warranty reference.

Replace the pulse coder assembly

Following Fanuc maintenance manual procedures for the specific robot model, remove the motor cover at the affected joint, disconnect the encoder cable, remove mounting bolts, and extract the failed pulse coder. Install the new pulse coder ensuring proper alignment with the motor shaft coupling. Torque mounting bolts to specification (typically 2-4 Nm depending on axis size).

Do not force the encoder onto the shaft. Misalignment can damage the new encoder immediately. Ensure the coupling shaft is clean and free of debris.

Reset encoder and perform mastering

Power on the controller. Navigate to MENU > SYSTEM > Variables. Set $MCR.$SPC_RESET to TRUE and press ENTER. Power cycle the controller completely. After reboot, perform Single Axis Mastering via MENU > SYSTEM > Master/Cal > [affected axis]. Use the mastering reference marks or Quick Master procedure per robot model.

After mastering, run MENU > SYSTEM > Master/Cal > F3 [ZERO POS] to verify the mastered position matches expected zero position values. Deviation greater than 2-3 degrees suggests mastering error.

Review alarm history for collision or crash events

Check MENU > ALARM > F5 [HIST] for any SRVO-050 (Overspeed), SRVO-043 (Overload), MOTN-023 (Collision detect), or INTP-316 (Position error) alarms that occurred just before SRVO-074. These indicate the robot experienced abnormal forces that could damage the encoder.

Alarm sequence matters: if SRVO-074 appeared immediately after collision-related alarms on the same axis, mechanical damage to the encoder is highly probable.

Inspect for visible mechanical damage

With power locked out, visually inspect the motor and encoder housing for cracks, dents, or deformation. Check if the motor shaft has play or binding. Rotate the axis manually (with brakes released via MENU > SYSTEM > F1 [TYPE] > Brake) to feel for roughness or resistance.

Before manually moving axes, ensure the robot is in a stable position and cannot fall. Use axis support fixtures or have personnel ready to support the arm weight.

Check motor and reducer condition

If mechanical shock was severe enough to damage the encoder LED internally, inspect the motor bearings and reducer for damage. Listen for grinding noises when jogging the axis slowly. Check for excessive backlash by moving the axis in small increments while observing position feedback on the teach pendant.

Parameter $SCR.$MNUTLNUM[GP,axis] shows motor utilization percentage. Values consistently over 85% before failure suggest the axis was under heavy stress, increasing likelihood of mechanical damage.

Replace pulse coder and verify mechanical integrity

Replace the pulse coder following standard replacement procedures. Before reassembly, verify the motor shaft coupling is not bent or loose. Ensure the encoder mounting flange mates flush with the motor housing without gaps or misalignment.

A bent motor shaft will cause premature failure of the replacement encoder. If shaft runout exceeds 0.05mm, the motor assembly should be replaced.

Re-master and perform accuracy verification

After encoder replacement and mastering, navigate to MENU > SYSTEM > Master/Cal > F4 [SET TOOL] to verify TCP. Run the robot through a test program with known positions. Use MENU > STATUS > Pos Reg to compare actual positions against taught positions. Deviation should be within ±0.1mm for critical applications.

After collision-related encoder replacement, reduce payload or speed parameters temporarily (reduce override or modify $SCR.$MAXSPEED) and monitor position accuracy over several hours before returning to full production.