Troubleshooting Error E-8421 (J2 Axis Position Following Error) on NexBot Vision CLR032-003 SCARA Robot

Related Products

Tools Required

• Hex key set (metric)
• Multimeter
• Laptop with NexBot Controller Software
• Safety glasses
• Lockout/Tagout kit

Article

Overview

The error code E-8421: J2 Axis Position Following Error on the NexBot Vision CLR032-003 SCARA Robot (SKU: NXB-ROB-CLR032-003) indicates a significant discrepancy between the commanded position and the actual position reported by the encoder for the second (J2) axis. This safety-critical fault will cause the robot to stop its current operation and disable motor power to prevent potential damage to the robot or its payload. This article outlines the common causes and provides a systematic approach to resolving this issue.

Symptom

When the E-8421 error occurs, you will typically observe the following:

• The robot abruptly halts all motion.
• The robot's teaching pendant or controller interface displays the error message: "E-8421: J2 Axis Position Following Error".
• The J2 axis may feel rigid if the brake has engaged, or it may drift slightly if the brake has not yet set.
• An audible hum or vibration may have been present at the J2 joint immediately before the fault.
• Attempts to jog the robot, particularly the J2 axis, will fail until the error is cleared and the underlying cause is addressed.

Cause

This error can be triggered by several mechanical or electrical issues. The most common causes are listed below in order of likelihood:

1. Mechanical Obstruction or Overload: The J2 arm is physically blocked or the load exceeds the robot's dynamic capacity for the programmed move, causing the motor to lag behind the commanded position.
2. Incorrect Payload Settings: The payload data (mass, center of gravity) configured in the controller does not accurately reflect the end-of-arm tooling (EOAT). This leads to incorrect torque calculations and can cause position errors during high-acceleration moves.
3. Loose or Damaged Cabling: An intermittent or failed connection in the J2 motor power cable or the encoder feedback cable can disrupt communication between the servo drive and the motor, leading to a loss of position tracking.
4. J2 Axis Brake Malfunction: The mechanical brake on the J2 axis is not fully disengaging, creating excessive drag on the motor.
5. Servo Drive Parameter Corruption: The tuning parameters (e.g., gain, inertia ratio) for the J2 servo drive have been incorrectly modified or have become corrupted.
6. Encoder Hardware Failure: The encoder on the J2 motor is malfunctioning and providing incorrect position data to the controller.
7. Servo Motor or Drive Failure: A hardware failure within the J2 servo motor or the associated servo amplifier in the controller.

Resolution Steps

WARNING: Always follow proper lockout/tagout (LOTO) procedures before performing any physical inspection or maintenance on the robot. Ensure all stored energy is dissipated.

Step 1: Perform a Visual Inspection

1. Power down the robot controller following standard shutdown procedures.
2. Apply LOTO devices to the main power source.
3. Carefully inspect the entire range of motion for the J2 axis. Look for any foreign objects, collided parts, or other physical obstructions that would prevent free movement.
4. Ensure all mounting bolts for the robot base and the EOAT are secure.

Step 2: Verify Payload Settings

1. Remove LOTO and power on the robot controller.
2. Navigate to the payload settings menu in the control software.
3. Verify that the configured payload number corresponds to the currently installed EOAT.
4. Confirm that the mass, center of gravity, and inertia values are correct for the payload. If in doubt, re-weigh the EOAT and re-calculate the values. Make corrections as necessary.
5. Attempt to clear the fault and cautiously jog the robot at a low speed to see if the error reoccurs.

Step 3: Inspect Robot Cabling

1. Power down the robot and apply LOTO procedures.
2. Examine the main robot harness from the controller to the base of the NXB-ROB-CLR032-003. Look for signs of abrasion, pinching, or sharp bends.
3. Unscrew and re-seat the primary motor and encoder connectors at the base of the robot to ensure they are secure. Check for any bent or contaminated pins.
4. Inspect the EtherCAT communication cable (e.g., NXB-CBL-NET522-001) for damage. A poor EtherCAT connection can cause data packet loss that leads to following errors.

Step 4: Test J2 Brake Functionality

1. Power on the controller and enable motor power from the teaching pendant (this typically disengages the brakes).
2. Listen closely to the J2 joint. You should hear a distinct 'click' as the brake disengages.
3. Disable motor power. You should hear another 'click' as the brake engages.
4. If you do not hear the brake actuating, or if it sounds weak, there may be an issue with the brake coil or the 24VDC power supply to the brake. This requires advanced diagnostics.

Step 5: Contact Technical Support

If the error persists after completing the steps above, the issue is likely related to a hardware failure (encoder, motor, drive) or corrupted servo parameters. Do not attempt to modify servo tuning parameters without guidance. Please contact NexBot Robotics Technical Support and provide the following information:

• Robot Model: NexBot Vision CLR032-003
• Robot Serial Number
• Error Code: E-8421
• A description of the robot's application and recent motion patterns.
• The steps you have already taken to troubleshoot the issue.

Prevention

• Regularly Inspect: Periodically inspect robot cabling for wear, especially in applications with extensive J1 and J2 motion.
• Maintain Accurate Payload Data: Always update payload settings in the controller immediately after changing the EOAT.
• Keep Workcell Clear: Ensure the robot's work envelope is kept free of potential obstructions.
• Follow Maintenance Schedule: Adhere to the scheduled maintenance outlined in the NexBot Vision CLR032-003 user manual.

Keywords