091

Check Limit Switch Status

Access the diagnostic screen on the Fanuc control to verify if the reference limit switch is active. Look for the input signal corresponding to the axis limit.

Use a multimeter to test the switch if the diagnostic screen shows inconsistent readings.

Ensure machine power is off before testing electrical components to avoid shock.

Manually Move Axis

If the limit switch is triggered, manually jog the axis away from the limit using the handwheel or jog mode on the control panel. Move at least 10-20mm away from the limit.

Move slowly to avoid overshooting and triggering other limits.

Watch for mechanical obstructions while jogging.

Attempt Homing Again

After moving away from the limit, initiate the reference point return procedure again via the control panel. Monitor for any recurrence of Alarm 091.

Record the position before and after homing to identify recurring issues.

Ensure no personnel are near moving parts during homing.

Review Homing Parameters

Access the parameter settings for the affected axis in the Fanuc control (typically under Parameter 1815 or related). Check the direction bit for reference return.

Refer to the machine’s manual for specific parameter numbers as they vary by model.

Incorrect parameter changes can cause unexpected machine behavior.

Adjust Direction Setting

If the direction is incorrect (e.g., set to positive when it should be negative), modify the parameter to the correct approach direction as per the machine setup.

Document the original setting before making changes for easy reversal if needed.

Backup parameters before editing to prevent data loss.

Test Homing Procedure

After adjusting the direction, perform a reference return operation to confirm the alarm no longer occurs. Observe the axis movement to ensure it approaches correctly.

Use a slow feed rate during testing to monitor behavior closely.

Keep clear of moving parts during the test.

Locate Deceleration Dog

Physically inspect the axis to find the deceleration dog or switch, typically mounted near the reference limit area. Refer to the machine layout diagram if needed.

Mark the current position of the dog with a marker for reference before adjustment.

Power down the machine before accessing mechanical components.

Check Dog Alignment

Verify if the dog is aligned with the axis travel path and positioned to trigger deceleration before the reference point. Compare with machine specifications.

Use a feeler gauge to ensure precise positioning if tolerances are tight.

Avoid over-tightening mounting bolts to prevent damage.

Adjust and Test

If misaligned, adjust the dog position slightly (1-2mm increments) to ensure proper triggering, then secure it. Power up and test the homing sequence.

Test multiple times to confirm consistent deceleration behavior.

Ensure no tools or hands are near the axis during testing.

Inspect Wiring

Check the wiring connected to the reference limit switch for loose connections, fraying, or breaks. Trace the cable from the switch to the control unit.

Use a cable tester to quickly identify hidden breaks in the wiring.

Disconnect power before handling electrical components.

Test Limit Switch

Use a multimeter to test the limit switch for continuity. Manually actuate the switch and confirm it changes state (open to closed or vice versa).

Listen for a click sound when actuating the switch as an initial indicator of function.

Avoid applying excessive force to the switch during testing.

Replace if Faulty

If the switch or wiring is defective, replace the faulty component with an OEM part matching the machine’s specifications. Retest the homing operation after replacement.

Keep spare limit switches on hand for quick replacements on critical machines.

Ensure proper grounding after replacement to prevent electrical issues.

Check Parameter Integrity

Compare current machine parameters related to homing (e.g., 1815, 1820) with a known good backup or the machine manual to identify discrepancies.

Use the Fanuc parameter export function to save a copy before making changes.

Incorrect parameter restoration can cause broader machine issues.

Reset Parameters if Needed

If parameters are corrupted, restore them from a backup or manually input correct values as per the machine documentation. Restart the control after changes.

Work with a second technician to double-check parameter values before saving.

Backup current settings before resetting to avoid data loss.

Test and Monitor

After parameter restoration, perform a reference return operation and monitor for Alarm 091 or other errors. Run diagnostics to confirm control stability.

Log any additional alarms during testing for future reference.

Ensure no active operations are running during parameter testing.