Cause : External scale error comes when Pr76 (scale error invalidation) = 0, and the driver is operated under the full-closed and hybrid control with an external encoder, the scale error input is OFF.

"alarm 28" in panasonic servo drive indicate " External scale error". This means an issue with the external scale (linear or rotary encoder) used for feedback. This error suggests that the servo drive is not receiving the expected signals from the external scale or that the signals are out of range or corrupted.

Troubleshooting Steps:

1. Check External Scale Connections:

• Wiring Integrity: Inspect all wiring between the external scale and the servo drive. Ensure that connections are secure and that there are no signs of damage, such as cuts, frays, or loose wires.
• Connector Condition: Examine the connectors for bent pins, corrosion, or any signs of wear that could affect the signal transmission.

2. Inspect the External Scale:

• Physical Condition: Check the external scale for any physical damage or signs of wear, especially if it is mounted in a harsh environment.
• Alignment and Mounting: Ensure the external scale is correctly aligned and securely mounted. Misalignment can cause inaccurate readings or signal loss.
• Signal Quality: If possible, test the output signals from the external scale with an oscilloscope to ensure they are clean and within the expected range.

3. Review Servo Drive Parameters:

• External Scale Settings: Verify the parameters in the servo drive related to the external scale, such as scale type, resolution, and input signal format. Ensure these match the specifications of the external scale being used.
• Error Detection Settings: Check the settings for error detection or tolerance related to the external scale. Adjusting these settings may help if the scale signals are borderline acceptable.

4. Check for Electrical Noise and Interference:

• Cable Routing: Ensure that the cables for the external scale are routed away from sources of electrical noise, such as power lines, high-frequency devices, or motors, which could disrupt the signals.
• Shielded Cables: Use shielded cables for the external scale if not already in place, to minimize the impact of electrical noise.

5. Test with a Different Scale:

• Alternate Scale: If available, connect a different external scale to the servo drive to determine if the issue is with the scale itself or the drive. This can help isolate the problem.

6. Inspect the Servo Drive:

• Internal Diagnostics: Run any diagnostics available on the servo drive to check for internal issues affecting how it processes signals from the external scale.
• Firmware Updates: Ensure the servo drive firmware is up to date, as updates may address known issues with external scale handling.