LA013-004 Axis 4 Overshooting Target Position, Causing Repeatability Issues

Description

Our NexBot Robotics LA013-004 arm (SN: 74B-3391A) on our engine block transfer line has developed a significant repeatability problem. Specifically, Axis 4 (the primary wrist axis) is consistently overshooting its final programmed position by 2-3mm before settling. This is causing misalignment when placing blocks into the CNC fixture, triggering faults and halting production. We have tried re-teaching the points and reducing the motion speed from 80% to 50%, but the issue persists. The problem is more pronounced when the arm is near its maximum reach. The controller is throwing an intermittent E-4502 error, but not on every cycle. We need urgent assistance as this has stopped one of our primary manufacturing lines.

Symptoms

• Axis 4 overshoots programmed setpoint.
• Inconsistent part placement and alignment.
• Visible vibration or 'settling' motion on Axis 4 after stopping.
• Poor repeatability, exceeding +/- 0.05mm specification.

Resolution

Investigation determined the root cause was a combination of a mechanically loose coupling on the Axis 4 servomotor and overly aggressive servo tuning parameters. After the customer followed Lockout/Tagout procedures, they were guided to inspect the Axis 4 drive assembly. The coupling bolts between the motor and the harmonic drive were found to be below torque specification. After torquing the bolts to the correct spec (45 Nm), we remotely guided the technician to adjust the servo gain parameters for Axis 4 via the teach pendant's maintenance screen. Reducing the proportional and derivative gains stabilized the axis, eliminating the overshoot and restoring the robot's specified repeatability.

Resolution Steps

1. Power down the robot and controller using the main disconnect (LOTO procedure).
2. Remove the access cover for the Axis 4 joint assembly.
3. Inspect the coupling between the Axis 4 servo motor and the harmonic drive input shaft. Found coupling bolts were below torque specification.
4. Torque the Axis 4 motor coupling bolts to the specified 45 Nm.
5. Reassemble the joint access cover.
6. Power on the robot and navigate to the servo tuning parameters in the teach pendant's maintenance menu.
7. Adjust the Proportional Gain (P) for Axis 4 from 120 to 105 and the Derivative Gain (D) from 85 to 75 to reduce overshoot.
8. Run the repeatability test program to confirm the issue is resolved and the robot is now operating within its +/- 0.05mm specification.