DH parameters of UR are different from URDF of UR in official ur_decription package

[haotianso]

Hey I got question. In this article it describes the position of joint coordinates of UR.
https://www.universal-robots.com/articles/ur/application-installation/dh-parameters-for-calculations-of-kinematics-and-dynamics/

However, in the ur_description package, the joint coordinates are different. For example, the shoulder_pan_joint is at the same position with shoulder_lift_joint, which is not the same as described in the article.

Here the upper_arm position x y z are all zero, which means the upper arm link x y z are the same with shoulder link x y z.
https://github.com/UniversalRobots/Universal_Robots_ROS2_Description/blob/rolling/config/ur10e/default_kinematics.yaml

[urfeex]

Hi,

first of all sorry that this issue slipped through my emails when I came back from vacation. They are all using the same values, the notation, however, is a bit different. Basically, the URDF uses a notation where a series of relative 3D poses (the origins of the links, defined by 6 values) are linked together by a joint that defines the rotational axis.
For DH parameters this is condensed down to 4 parameters instead. They describe links and joints in one compact notation. That has its pros and cons, but the URDF basically expands this notation into full 6D notations.

In the URDF notation, each link_i uses the d parameter its DH joint[i] for its z value and the a patamter of its DH joint[i-1] (as long as no rotation of the links is involved, such as in wrist2 and wrist3, where things are rotated around the x-axis by pi/2 making the z-axis effectively be the y-axis in that frame.

I hope, that makes sense. I'll transform this into a discussion, as I think this doesn't relate to a bug or feature request for this repo.