search

google / brax

Massively parallel rigidbody physics simulation on accelerator hardware.
Apache License 2.0
2.38k stars 257 forks source link

Config format, Denavit-Hartenberg, UR5e #27

Closed bayerj closed 3 years ago

bayerj commented 3 years ago

Hey,

I am currently playing a bit with implementing robot arms that I have given by the Denavit-Hartenberg convention accd to Craig. My plan was to write a function that takes such a sequence of joint specifications and then puts together a config where the corresponding arm is implemented and everything nicely connected by capsules.

I got a little confused along the road, as I started out with the official UR5e DH specification (found on this page) and tried to match it to the config of the UR5e environment. Specifically, "joint2" in the UR DH parameters has an "a" value of -0.425, while the second joint in the config has a parent_offset.y = 13.8. From my view, these values should be the same, up to units.

Another potential explanation is me not understanding how Brax uses configs to build kinematic chains.

A thing that I struggle with is how a joint's axis of rotation maps to the transformation from the two connected bodies' respective frames. In my experience, these rotations are often given as the angle between the the z axes of the two consecutive frames around the x-axis of the first frame.

I am sorry that I don't have very clear questions that allow yes/no answers, but at this point I struggle quite a bit with finding how brax relates two links through a joint. I'd very much appreciate a brief explanation of this.

flobotics commented 3 years ago

hi, perhaps it would be nice to enable/disable a small plane/grid where the joint is placed. Also nice would be to place at an edge of the small plane/grid three arrows for x,y and z direction.

erikfrey commented 3 years ago

Agree the joint spec can be confusing! We're tracking this over at #112 and will be making improvements to the config documentation soon.