SA011-001 Robot Arm Experiencing Position Overshoot and Vibration on Axis 4

Description

Our NexBot Robotics SA011-001 robot arm, which is part of our R-10 series cell, has started exhibiting inconsistent positioning on Axis 4 (wrist rotation). During high-speed pick-and-place cycles, the end-of-arm tooling visibly vibrates as it decelerates into its final position. This results in an overshoot of approximately 1-2mm, causing parts to be misplaced in their fixtures. The issue is intermittent but has become more frequent over the past week, leading to a higher scrap rate. We have verified that the payload data is correctly configured for our 8.5kg gripper assembly and that all mechanical fasteners on the tool flange and J4 gearbox are secure. The EtherCAT network status shows no packet loss. Occasionally, the teach pendant will flash an E-4502 alarm, which forces us to reset the cycle. We need assistance diagnosing if this is a tuning issue or a potential hardware failure in the Axis 4 servo drive.

Symptoms

• Visible vibration on the end-of-arm tooling during fast moves.
• Inconsistent final placement of parts, overshooting the target position.
• Intermittent E-4502 'Servo Lag Exceeded' alarm on the teach pendant.
• Audible whining noise from the Axis 4 motor/gearbox assembly during deceleration.

Resolution

The investigation determined that the servo tuning parameters for Axis 4 were not optimized for the specific high-inertia tooling being used, despite the payload being within the 10kg limit. The default PID (Proportional-Integral-Derivative) gain values were causing instability at high accelerations. By adjusting the proportional gain (Kp) down and the derivative gain (Kd) up for Axis 4, the servo loop was stabilized. This eliminated the position overshoot and dampened the vibration. The E-4502 error, which indicated an excessive deviation between the commanded and actual motor position, has not reoccurred since the tuning adjustments were applied.

Resolution Steps

1. Connect to the robot controller using the NexBot ServiceTool software.
2. Navigate to the 'Tuning' > 'Servo Parameters' menu.
3. Select 'Axis 4' from the axis selection dropdown.
4. Back up the current parameters by selecting 'File' > 'Export Parameters'.
5. Decrease the Proportional Gain (Kp) for Axis 4 by 15% from its current value.
6. Increase the Derivative Gain (Kd) for Axis 4 by 10% to dampen oscillations.
7. Save the new parameters to the controller and perform a cold reboot of the system.
8. Run the production cycle at 100% speed to verify the positioning accuracy and confirm that no alarms are generated.