Universal Robots+

Robot controller unstable / wiggling

Summary

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Introduction to the issue

There are “islands of instability”, where the robot jumps back and forward between two almost-equal coordinates.

Versions

  • Affected Robot Software Version(s): 5.5.1 (sept. 2019)
  • Affected Robot Hardware Version(s): ur5e
  • Robot Serial Number: if needed
  • UR+ product(s) installed:
  • URCaps Software version(s):

Impact

Makes it hard to do tiny adjustments to the robot coordinates.

Issue details

I was attempted to make very slow movements for a timelapse project. For this I was sending coordinate updates smaller then the robot precission (<0.01mm).

Use Case and Setup

What does the setup look like and what are the objectives?

I use a probe lense for filming https://www.youtube.com/watch?v=hznigEBMvMU

The objective is to create timelapse films with very slow camera motions.

Project status at point of discovered

When did you first observe the issue?
Two to three weeks ago, but I wasn’t sure about the problem. I could pinpoint it around a week ago.

Steps to Reproduce

  1. Set the robot controller speed to 2%
  2. Adjust any axis by pressing as short as you can (move <0.1mm)
  3. Repeat step 2, eventually the robot “wiggles”, ie it cannot decided on a stable position and becomes bistable

The effect is very apparent when you have a macro lense attached to the robot, i’m happy to provide a demo movie if needed.

Expected Behavior

What did you expect and why?

I expected the robot to move as close as it can to the target position, and then stop at a stable position.

Actual Behavior

The robot becomes bistable, doing tiny jumps between two coordinates.

Workaround Suggestion

Do not make tiny motions.