NexBot R-20 Axis 4 (J4) Following Error and Vibration

Description

Our NexBot R-20 Articulated Robot (SKU: NXB-ROB-R20-050-A), which is used in a palletizing cell, has stopped production due to a recurring fault. During the main traverse motion, the robot exhibits significant vibration localized around the J4 wrist joint. The teach pendant displays error E-3104, indicating an Axis 4 Following Error. We have performed a cold restart of the controller twice with no change. We also inspected the J4 assembly for any physical obstructions or loose cabling, but found nothing obvious. The robot will not complete a cycle and faults within seconds of J4 beginning its motion. The rest of the axes appear to move smoothly before the fault occurs. PROFINET communication with the master PLC appears stable and does not report any bus faults. We need assistance diagnosing whether this is a servo motor, gearbox, or drive issue.

Symptoms

• Excessive vibration on Axis 4 during motion.
• Error E-3104 (Axis 4 Following Error) on teach pendant.
• Audible grinding noise from the J4 gearbox assembly.
• Robot halts program execution.

Resolution

After remote diagnostics and analysis of the drive parameters, it was determined that the J4 servo motor's encoder was providing erratic feedback under load, coupled with high motor current, causing the following error. The audible grinding noise reported by the customer indicated a concurrent mechanical failure within the harmonic drive gearbox. The resolution was to replace the entire J4 axis motor and gearbox assembly (Part # NXB-MTR-R20-J4-ASM). After replacement, the axis was re-mastered and the motion profile was tuned. The robot passed all subsequent functional tests and was returned to production service.

Resolution Steps

1. Perform full safety lockout/tagout (LOTO) on the robot controller and cell.
2. Mechanically support the robot arm to prevent uncontrolled movement when the J4 brake is released.
3. Disconnect the J4 motor power and encoder cables from the harness.
4. Unbolt and carefully remove the failed J4 motor and gearbox assembly.
5. Install the new J4 motor/gearbox assembly, ensuring proper alignment and torque on all mounting bolts.
6. Reconnect and secure the power and encoder cables.
7. Remove LOTO and power on the system. Perform the axis mastering procedure for J4 as per the R-20 service manual.
8. Execute a low-speed motion test to verify smooth operation, then gradually increase speed to production levels.