NexBot Robotics SA011-005 Axis 4 (J4) Position Drift and Repeatability Failure

Description

We are experiencing a critical positioning issue with our NexBot Robotics SA011-005 robot arm, serial number NB-SA11-2022-8431. The robot is used in a machine tending application, and over the last 48 hours, we've observed significant position drift on Axis 4 (the primary wrist rotation axis). After running for approximately 20-30 minutes, the J4 position drifts by up to 5mm, causing the gripper to misalign with the CNC chuck and drop parts. We have tried re-mastering the robot and re-teaching the points, but the drift returns consistently. The issue is isolated to J4; all other axes hold their position perfectly. The teach pendant intermittently flashes error code E-4114, especially after a rapid move sequence. The arm is operating well within its 10kg payload limit. Production is currently halted on this cell until we can resolve this.

Symptoms

• Inconsistent positioning on Axis 4 (J4)
• Failure to return to taught home position accurately
• Visible position drift of several millimeters after a few cycles
• Teach pendant displays error E-4114 after extended operation

Resolution

The root cause was determined to be a failing absolute encoder on the J4 servo motor. The encoder was providing inconsistent pulse counts to the controller, leading to the position drift and the E-4114 'Axis 4 Encoder Pulse Mismatch' error. Standard troubleshooting, including cable inspection and drive parameter checks, did not resolve the issue. The entire J4 servo motor assembly (Part No. NXB-SM-J4-01A) was replaced. Following the hardware replacement, a full axis mastering and calibration procedure was performed. The robot successfully passed a 4-hour continuous cycle test with position repeatability measured at ±0.015 mm, which is within spec.

Resolution Steps

1. 1. Followed standard lockout/tagout (LOTO) procedure to de-energize the robot controller.
2. 2. Manually released the brake for Axis 4 to allow for movement.
3. 3. Disconnected the power and encoder cables from the J4 servo motor.
4. 4. Unbolted and removed the faulty J4 servo motor from the robot arm housing.
5. 5. Installed the new replacement J4 servo motor, ensuring correct alignment and torquing mounting bolts to specification.
6. 6. Reconnected and secured the motor power and encoder cables.
7. 7. Powered on the controller and performed the Axis 4 mastering procedure using the service pendant.
8. 8. Loaded the calibration routine and executed a full calibration for all axes to ensure system accuracy.
9. 9. Ran the production program in a continuous test loop for 4 hours, verifying consistent positioning and confirming that error E-4114 did not reappear.