Servo Troubleshooting

Introduction

This guide provides some instruction on how to trouble shoot problems using MotionPlus . Problems fall into two categories:

  1. a) problem observed with an error message or error code issued

  2. b) problem observed without error codes or message.

Problems that Generate Error Codes

If MotionPlus stops due to an error, it can be from multiple possible sources.

Step 1: If you see an error code in a Polyscope popup window, please note the error code and consult the document MotionPlus URCap Errors and follow the corrective action listed in the table.

Step 2: If the corrective action does not solve your issue, then follow these steps provide information to UR Support so that we can help:

  1. Note the error code in your support request

  2. Export the logs (flight report) to a USB drive. Refer to this article for guidance: https://www.universal-robots.com/articles/ur/robot-care-maintenance/how-to-export-support-files-from-your-robot/

  3. In the MotionPlus Installation tab, export the MotionPlus log file to a USB drive. Include that in your support request.

  4. If possible, provide a photo of your work cell, including the robot, tool, and any fixtures.

  5. Indicate how long the program was running before the error appeared. For example: immediately, after a few minutes, after an hour, etc.

  6. If you can, provide a short program in UR Script that recreates the problem

  7. Include the words MotionPlus Support Request in your email subject line.

Types of Faults

Long running servo failure

After 10mins of continuous movement. This typically generates a fault and an error code and may be related to a DC (Distributed Clock) sync problem. EtherCATrelies on precise timing for transmit and receive of information packets on the bus. If the data arrives too late (after the sync), then the servo drive may generate a fault. Some things to try:

  1. Check your cable connections between the UR Control Box and the servo drive.

  2. If you are using a USB-Ethernet adapter, is it one from our approved list?

  3. You can try to swap the ethernet devices, assigning eth0 to the USB-Ethernet device and eth1 to the native ethernet port. We have a separate tech bulletin on how to do this.

  4. Try running without any other UR Caps installed. If the failure vanishes, then add the URCaps back one at a time to determine which URCap may be causing a timing failure.

Invalid number of servos on the bus

The number of ethercat axes defined and the number of servo-drives on the bus must match. No more and no less. Remove any unused servo drive if the number reported drives exceeds the expected number. If the number of detected drives is below the expected, then check cabling and power to the servo drive.

Problems that don’t generate error codes.

Some problems don’t generate stops or error codes. For these, we provide a guide to some of the more common problems:

Rough servo motion (noisy, not smooth)

If your servo is making unusual noise while running, including whining, clunking or clicking, then please do the following:

  1. Record a video with sound

  2. Does the sound only happen on startup, or when starting a motion, or does it occur continuously?

  3. If this is a continuous whining sound, can you determine the fundamental frequency of the sound? If the sound is at or near 500Hz, then the problem is likely an incorrect setting of the cycle time on the servo drive. This should be set to 2ms (500Hz). There are phone apps that will provide the fundamental frequency of the sound. For example, “Sonic Tools” on iOS and “Spectroid” on Android phones.

Servo does not turn.

Follow our MotionPlus Installation document on how to set up your servo. If the servo still fails to turn, check the following:

  • Power is connected from the servo drive to the servo motor

  • In the configuration (see installation docs) check that encoder count / revolution is set correctly. If this value is much smaller than the actual value, then the servo may be turning very slowly.

  • Check that the velocity and acceleration values for the defined axis are not near zero.

  • Check that the gear ratio is set correctly. A value set much too low may result in the servo turning very slowly.

  • Check that the feed rate is set correctly. For rotary axes, this should be either 2*PI or –2*PI.

  • If the servo motor does not have limit switches attached, then check that the servo drive limit switch inputs are deactivated. This can be done with the manufacturer’s software.

Servo turns too fast.

The potential causes are:

  1. the encoder count is too large;

  2. the feed constant is not +/- 2*PI for rotary

  3. the gear ratio is too high.

Servo turns in the wrong direction.

You can change the direction by changing the sign on the feed constant. This should be either 2*PI or –2*PI for rotary axis.

Robot does not move to correct offset from frame/axis.

If the robot does not move to the correct approach point near the axis, your calibration may be incorrect.

  1. Recalibrate and check the translation values from the robot base.

  2. Verify that Z axis is pointing in the right direction. For visualization of the orientation of your frame, you can use this on-line tool.

Servo does not move to correct position.

This can occur if the encoder setting (counts/revolution) is wrong, or the sign of the feed constant is not wrong. Also check the gear ratio. It may also be that your home position is not where you think it is. Try first moving your axis to “zero” to verify your home position.