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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.
- 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):
Makes it hard to do tiny adjustments to the robot coordinates.
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 Film am 30 04 20 um 13 34 - YouTube
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
- Set the robot controller speed to 2%
- Adjust any axis by pressing as short as you can (move <0.1mm)
- 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.
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.
The robot becomes bistable, doing tiny jumps between two coordinates.
Do not make tiny motions.
The robot will speed up as it tries to cross a singularity. What you’re experiencing is most likely the “WRIST ALIGNMENT SINGULARITY” in our article below:
You’ll need to take care to avoid these zones when programming.