Intermittent position loss on R-50 Joint 4, requires re-mastering after power cycle

Description

We are experiencing a critical issue with one of our R-50 robots on the main production line. The robot's Joint 4, which uses a NexBot Drives ABS141-006 Absolute Encoder, is losing its absolute position data every time the main controller is shut down and restarted. This forces our maintenance team to re-master the axis at the start of every shift, causing significant downtime. The controller log shows error E-4502 (Absolute Position Mismatch) upon startup. We have visually inspected the encoder cabling and connections to the drive amplifier, and they appear secure. We also verified that the encoder is receiving a stable 24VDC supply. The robot has been in service for approximately three years without this issue. We suspect the encoder itself has developed an internal fault preventing it from retaining its position data while powered off. Please advise on further diagnostic steps or if a replacement is warranted.

Symptoms

• Robot requires re-mastering/calibration after every power cycle
• Controller displays 'Absolute Position Mismatch' alarm
• Loss of productivity due to daily recalibration
• Inaccurate initial positioning on the affected joint at startup

Resolution

The investigation confirmed that the NexBot Drives ABS141-006 Absolute Encoder was faulty. The internal energy storage component (e.g., supercapacitor or battery) used to maintain the turn count during power-off had failed. This is a non-serviceable component within the sealed IP67-rated encoder unit. As a result, the encoder could not retain its multi-turn position data across a power cycle, leading to the E-4502 error. The resolution was to replace the faulty encoder. A new unit was installed, and after a one-time mastering procedure, the robot successfully retained its absolute position across multiple power cycles, restoring normal operation and eliminating production downtime.

Resolution Steps

1. Confirm stable 24VDC power at the encoder's connector pins.
2. Inspect the BISS-C communication cable for any signs of physical damage, especially near flex points.
3. If power and cabling are confirmed to be good, diagnose the encoder as the point of failure.
4. Power down and perform a full lock-out/tag-out procedure on the robot controller.
5. Mechanically brace the robot's J4 arm to prevent unexpected movement upon brake release.
6. Disconnect the cable and unbolt the faulty encoder from the motor housing.
7. Install the new NexBot Drives ABS141-006 Absolute Encoder (SKU: NXB-SNS-ABS141-006) and reconnect the cable.
8. Power on the system, clear the initial faults, and perform the single-axis mastering procedure for Joint 4 as per the R-50 maintenance manual.