Add 'base' coordinate system link to URDFs

[gavanderhoorn]

As discussed in ros-industrial/ros_industrial_issues#24.

[gavanderhoorn]

I've recently done this myself, and I've got values that work. For base, only a rotation over Z wrt base_link is required, as the meshes in ur_description seem to have their origins rotated when compared with the real robot.

Should the meshes perhaps be updated? Then the base coordinate frame would be coincidental with base_link and things would be much more transparent.

Same goes for tool0.

[fmessmer]

@abubeck has done this, too....umpf....
Could both of you create PullRequests?

[gavanderhoorn]

@ipa-fxm wrote:

@abubeck has done this, too [..]

I know, I spoke with him, but his branch uses even more rotations. There is only one needed, but even that one can be made unnecessary if we 'fix' the meshes (and thus the orientation of base_link).

[gavanderhoorn]

#49 (Add tool0 to URDFs) is also related.

[gavanderhoorn]

I think it would make sense to just rotate the base_link link pi rad over Z, then rotate the visual and collision geometry -pi to compensate. The base frame can then be coincident with base_link, no rotations needed anymore.

We can rotate the meshes later then.

[fmessmer]

Well, one thing that needs to be considered about your proposed solution is the following:
UR robots are also used in composition with e.g. mobile platforms (e.g. Fraunhofer IPA's rob-@-work) or within environments. They also might exist in composition with tools/grippers attached in ROS. Thus, rotating the base_link in the ur_description will change all those setups!

What we did in @abubeck's solution is add new links base0 and tool0 like this:
base_link ->TransBase1 -> base0 ->TransBase2 -> other_ur_links -> TransTool1 -> tool0 -> TransTool2 -> arm_ee_link, where TransXXX are correctly set origins within additional fixed joints....

You can find it here...but I think that file was still rather WIP than final....

However, by "squeezing"-in base0 and tool0, you keep ROS-URDF-compatibility for arms mounted to platforms/environments as well as with endeffectors mounted to the arm while still having the chance to get UR-conform transformations from base0 to tool0 using frames identical with those used within the UR controller!

[gavanderhoorn]

I forgot about bw compatibility, you're right.

I do not believe there is any reason to include base and tool0 in the kinematic chain though. tool0 will probably be the tip of the chain but doesn't have to be (to maintain bw compat again), and base can be an extra leaf frame, child of base_link. That is how it has been done in all other ros-i support pkgs.

base_link -> rest_of_chain -> wrist_3_link -> ee_link
|                             |
 -> base                       -> tool0

That should keep everything bw compat, while adding the new frames in the places where they should be with the proper orientations. We would also not have to update the MoveIt configs like this.

I do still believe that the meshes will have to be fixed though, but that is something that can then be done in K-turtle fi.

[gavanderhoorn]

See also #80.

[fmessmer]

done in #200. closing.