NexServo AC30 motor on R-20 Joint 4 shows intermittent position tracking errors and faults out.

Description

We are experiencing a persistent issue with one of our R-20 series robots on the main assembly line. The servo motor for Joint 4, a NexServo AC30 (SKU: NXB-SRV-AC-030-A), is causing the entire robot to fault during its homing sequence or shortly after starting a production cycle. The controller logs error E-2103, which our manual indicates is a 'Position Encoder Signal Loss' fault. Before the fault occurs, we can hear a faint, high-frequency whine from the motor housing, and the teach pendant shows the axis value for Joint 4 jittering erratically. We have tried power cycling the controller and the robot multiple times. We also checked the PROFINET connection status, and all devices appear to be communicating normally. The motor is receiving its 48VDC supply as expected. This is a critical production line, and we need to get this robot operational as soon as possible.

Symptoms

• Robot fails during homing sequence.
• Controller logs E-2103 (Position Encoder Signal Loss).
• Joint 4 position value jitters on the teach pendant.
• Audible high-pitched whine from the motor before faulting.

Resolution

The issue was traced to a faulty encoder feedback cable connecting the NexServo AC30 motor to the axis drive amplifier in the main controller. Physical inspection revealed no external damage, but a pin-to-pin continuity test showed an intermittent open circuit on the A+ signal line. This signal degradation was causing the drive to lose the motor's precise position, resulting in the jitter and eventual E-2103 fault. Replacing the encoder cable (Part # NXB-CBL-ENC-M4) with a new unit fully resolved the communication issue. The robot passed all subsequent calibration and diagnostic tests.

Resolution Steps

1. 1. Performed a safe shutdown of the R-20 robot and controller using Lockout/Tagout procedures.
2. 2. Disconnected the encoder cable from both the NexServo AC30 motor at Joint 4 and its corresponding port on the axis drive module.
3. 3. Used a multimeter to perform a continuity test on all pins of the disconnected encoder cable.
4. 4. The test identified an intermittent connection on the A+ signal pin, indicating an internal cable break.
5. 5. Replaced the faulty encoder cable with a new, certified replacement part.
6. 6. Re-established all connections and torqued connectors to specification.
7. 7. Removed LOTO and powered on the system.
8. 8. Cleared the active faults from the controller log and initiated a full axis calibration routine, which completed successfully.