SV0448

Power Down the Machine

Ensure the CNC machine is completely powered off and disconnected from the main power supply to prevent electrical hazards during inspection.

Always follow lockout-tagout (LOTO) procedures before working on electrical components.

Risk of electric shock if the machine is not properly de-energized.

Inspect Encoder Cables and Connectors

Locate the motor encoder and external scale cables. Check for visible damage, fraying, or loose connections at the encoder, scale, and control unit terminals.

Use a flashlight to check for subtle damage or corrosion in connectors.

Avoid excessive force when handling delicate encoder connectors.

Secure and Test Connections

Tighten any loose connectors and replace damaged cables if necessary. Power on the machine and monitor for the alarm after resetting the system.

Use a multimeter to check continuity in cables if damage is suspected but not visible.

Ensure replacement cables meet FANUC specifications to avoid further issues.

Access Parameter Menu

Power on the CNC machine and access the parameter menu through the control panel. Refer to the FANUC manual for the specific parameter numbers related to dual-feedback (typically in the 2000-2200 range).

Document existing parameter values before making changes for easy rollback.

Incorrect parameter changes can cause additional alarms or machine malfunction.

Verify Feedback Parameters

Check parameters for encoder type, scale resolution, and feedback mode to ensure they match the installed hardware specifications as per the machine documentation.

Cross-reference with the original machine setup sheet if available.

Do not alter unrelated parameters during this process.

Adjust and Test Settings

If discrepancies are found, adjust the parameters to the correct values, save changes, and cycle power to the machine. Test the axis movement and monitor for the alarm.

Perform a slow axis test to avoid stress on components during initial testing.

Ensure proper backup of parameters before saving changes.

Isolate Encoder for Testing

Power down the machine and disconnect the motor encoder cable from the control unit for safe testing.

Label cables before disconnection to avoid mix-ups during reassembly.

Risk of electric shock if the machine is not de-energized.

Test Encoder Output

Use a multimeter or oscilloscope to check the encoder output signals (A, B, and Z phases) while manually rotating the motor shaft. Compare readings against manufacturer specifications.

If signals are erratic or absent, the encoder is likely faulty.

Avoid excessive rotation speed to prevent damage during manual testing.

Replace Encoder if Faulty

If the encoder fails the test, replace it with a compatible unit as per FANUC specifications. Reconnect cables, power on, and test for alarm resolution.

Keep spare encoders on hand for critical machines to minimize downtime.

Ensure proper alignment during encoder installation to avoid further issues.

Inspect External Scale

Power down the machine and visually inspect the external scale for dirt, debris, or physical damage along its length and at the read head.

Use compressed air or a soft brush to clean the scale without scratching it.

Avoid using harsh chemicals that could damage the scale surface.

Test Scale Output Signals

Use a diagnostic tool or oscilloscope to measure the output signals from the scale while moving the axis manually. Compare with manufacturer specifications.

Look for signal dropouts or irregularities that indicate scale failure.

Ensure proper grounding during testing to avoid electrical interference.

Clean or Replace Scale

If contamination is found, clean the scale and read head. If signals are still incorrect or damage is evident, replace the scale. Test the system after repairs.

Schedule regular scale cleaning as part of preventive maintenance.

Improper scale installation can lead to recurring feedback issues.

Check Wiring Integrity

Power down the machine and inspect internal wiring between the encoder, scale, and control unit for signs of wear, shorts, or grounding issues.

Pay attention to areas near moving parts where wiring may rub or fray.

Risk of electric shock if the machine is not properly de-energized.

Test Signal at Control Unit

Use a diagnostic tool to check feedback signals directly at the control unit input terminals to identify signal loss or interference.

Compare signals at the source and control unit to pinpoint wiring issues.

Avoid shorting terminals during testing to prevent control damage.

Repair or Replace Components

Repair any identified wiring issues or replace faulty control unit components if necessary. Test the system after repairs to confirm alarm resolution.

Consider consulting FANUC support for control unit diagnostics if unsure.

Control unit repairs should only be performed by qualified personnel.