strange behavior when moving the arm

[alonborn]

Hi,

I ran a simple test: I'm projecting a crosshair laser from the end-effector and marking the point on the table. At a height of 20 cm, the laser points to a specific spot. However, when I move the arm straight down to 13 cm—without changing the X or Y coordinates—the crosshair shifts noticeably (see attached video).

Have you encountered anything similar? It seems the arm’s positioning isn't fully accurate, and I'm trying to understand what in my setup or design might be causing this.

Any insight would be greatly appreciated.
Thanks,
Alon.

https://www.youtube.com/watch?v=Aw5JkyLV3Ks (start from 0:13)

[ycheng517]

Hi @alonborn, great experimental setup! I haven't analyzed the movement accuracy of the arm as I've been using the arm in a visual servoing type of way where the accuracy doesn't really matter, so I wouldn't be surprised if it's a little off. Perhaps the transforms I've defined in the URDF of the robot is a little off, which would affect the kinematics. Would you be able to try the same movement using the official AR4 control software and compare the accuracy? If the official software is more accurate, then I can cross-check the kinematics in this repo against it.

[alonborn]

Hey @ycheng517 , and thanks for the quick reply!

I ran the same vertical accuracy test using the original AR4 software, and the results were... well, even less encouraging :-/
I recorded a short video demonstrating the issue: video and I also posted the question on the AR4 forum.

Interestingly, with ROS and MoveIt, the laser crosshair remains fairly consistent — when I move the arm up and down, it reliably returns to the same spot (even if not the RIGHT one). With Chris’s original software, however, the crosshair quickly drifts off target.

I suspect the issue might be related to the robot’s kinematic description, but I honestly don’t know where to begin debugging it.

Any insights would be greatly appreciated — and if you have ideas for how to investigate or improve this, please share! I'm willing to look into it and I’m sure it would help others too.

[ycheng517]

@alonborn in the video you uploaded are you moving between the same 2 points? that would indicate maybe a mechanical build issue. you may want to tighten your belts.