STP113-001 Stepper Motor on S-5 Robot Losing Position on J3 Axis

Description

We have an S-5 robot on our assembly line, used for a pick-and-place operation. The J3 axis, which uses the NXB-SRV-STP113-001 stepper motor, has started exhibiting positional drift. Over the course of about 50-60 cycles, the axis drifts by approximately 1.5 degrees, causing the gripper to misalign with the target fixture. We've confirmed there is no mechanical slippage in the gearbox or couplings. When we try to increase the driver current to compensate, the motor overheats, reaching temperatures above 85°C, without solving the drift. The robot controller occasionally flags a following error, but not consistently. The issue is causing intermittent production failures and requires us to re-home the robot every hour. We suspect the motor may be failing or is no longer able to provide the required holding torque for this application.

Symptoms

• Progressive positional drift on J3 axis over several cycles.
• Audible 'clicking' or 'skipping' sound from the motor under high acceleration.
• Motor housing temperature exceeds 85°C when driver current is increased.
• Robot fails to place components accurately in the target fixture.

Resolution

Investigation determined the STP113-001 motor was not faulty. The root cause was an overly aggressive motion profile configured in the robot controller for the J3 axis. The combination of high microstepping (1/32) and a steep acceleration ramp demanded more instantaneous torque than the motor could provide at that speed, causing it to lose steps intermittently. By reducing the microstep resolution to 1/16, more torque is available per full step. Additionally, softening the acceleration ramp prevents the motor from stalling during rapid directional changes. These configuration adjustments brought the motor's operation back within its performance curve, resolving the positional drift without requiring hardware replacement.

Resolution Steps

1. Connect to the robot controller using the NexBot Service Terminal software.
2. Navigate to the Motion Configuration > Axis Parameters > J3 menu.
3. Verify the motor model selected is STP113-001.
4. Reduce the 'Microstep Resolution' setting from 1/32 to 1/16 to increase available torque per step.
5. Decrease the 'Max Acceleration' parameter for J3 from 1500 rad/s² to 1200 rad/s².
6. Save the new configuration and reboot the controller.
7. Run the 'Axis Calibration' routine for the J3 axis.
8. Execute the production cycle for 100 repetitions and verify positional accuracy is maintained.