Positioning Inaccuracy and Jitter on R-20 Joint 4 with INC142-001 Encoder

Description

We are experiencing a significant positioning issue with one of our R-20 robots on the primary assembly line. Joint 4 (J4) is exhibiting erratic behavior, including noticeable jitter during slow movements and overshooting its programmed points by 2-3mm. This is causing inconsistent part placement and subsequent failures at the vision inspection station. The controller is intermittently flagging an E-4511 'Axis Position Tracking Error' for J4. We have already tried re-mastering the robot and re-calibrating the tool center point, but the problem persists. We also swapped the suspected NXB-SNS-INC142-001 encoder with a new one from stock, but the exact same symptoms appeared, suggesting the issue might not be the encoder itself but something related to its installation or communication. The robot was operating flawlessly for 18 months prior to this issue. The cabling appears visually intact with no signs of damage. We need assistance diagnosing the root cause as this has halted production.

Symptoms

• Robot joint overshooting target position
• Visible jitter and vibration on a single axis
• Intermittent position tracking alarms on the teach pendant
• Inconsistent pick-and-place operations

Resolution

The root cause was identified as electrical noise interfering with the encoder signal due to improper cable shield grounding. Although the NXB-SNS-INC142-001 encoder is robust, its high-resolution signal is sensitive to EMI/RFI when not shielded correctly. An inspection revealed that the drain wire for the encoder cable shield was not properly terminated to the chassis ground lug inside the main controller cabinet. The connection had likely vibrated loose over time. Re-terminating the shield drain wire to the designated ground point resolved the signal integrity issue, eliminating the jitter and position tracking errors.

Resolution Steps

1. 1. Safely power down the robot controller and apply Lockout/Tagout (LOTO) procedures.
2. 2. Open the main controller cabinet and locate the terminal block for the J4 motor/encoder cable.
3. 3. Inspect the cable shield termination point for the PROFINET connection.
4. 4. Discovered the shield's drain wire was loose and making intermittent contact with the ground lug.
5. 5. Securely re-terminate the drain wire into the ground lug, ensuring a solid mechanical and electrical connection.
6. 6. Close the controller cabinet, remove LOTO, and power on the system.
7. 7. Clear the E-4511 fault code from the alarm history.
8. 8. Re-master the robot's J4 axis and execute a test program to confirm smooth, accurate motion without jitter or alarms.