Added mimic joint tags

[jon-weisz]

Hey I was looking for a barrett hand urdf so I wouldn't have to roll my own, and I came upon this.

If you want to explicitly encode that the finger spread angle must always be the same in the URDF file, this works (in Hydro).

[jbohren]

Hey I was looking for a barrett hand urdf so I wouldn't have to roll my own, and I came upon this.

Ugh yeah. There are a bunch of WAM urdfs floating around out there, but none of them had everything I needed, too.

If you want to explicitly encode that the finger spread angle must always be the same in the URDF file, this works (in Hydro).

Yeah, the mimic tag is nice, where it's supported. I've been using this model with our real WAMs as well as simulated WAMs in Gazebo, and unfortunately, Gazebo doesn't support this [1] so I've just been handling them directly in the controllers.

Thanks for the contribution, I'll see if it doesn't cause any problems with my manual joint state publishing.

[1] https://bitbucket.org/osrf/gazebo/issue/1058/update-gearbox-joint-definition

[jon-weisz]

Yeah, gazebo and other simulators have to deal with some ambiguity in that it's not clear from a mimic tag whether effort, power, or flow should be preserved across the joints. Hopefully some day soon we'll start parsing URDFs in GraspIt, and that should respect the 'mimic' tag.
[sorry about closing and reopening the pull request.]

[jbohren]

Hopefully some day soon we'll start parsing URDFs in GraspIt, and that should respect the 'mimic' tag.

Now that you mention it, what's the maintenance status of GraspIt these days?

[sorry about closing and reopening the pull request.]

No worries.

[jon-weisz]

GraspIt is still under active development at Columbia University and by Matei Ciocarlie at Google, but there are some organizational details concerning releasing updates that have not been resolved. Matei is visiting soon, so look for news in a few weeks.