omniwheel design in urdf

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As far as I know, no one has done something like this yet. However, I looked up the design of the robotino and it should be pretty easy to do! Certainly much easier than other mecanum wheels....

You'll first want to start learning the basics of URDF. I suggest that you read through David Lu!'s excellent tutorials on Learning URDF Step by Step.

Your robot has 6 rollers on each wheel, distributed 2Pi/3 away from eachother with an offset of 2Pi/6 it looks like per side. We're lucky that they are orthogonal. The only tricks I would encourage you to use would be to remember to set each roller as a continuous joint and to use the XACRO shortcuts for each roller to make changing the physics and inertias simpler if you have to. You may also want to only connect the rollers to one side. You will probably end up with something simple like three "L" shapes per side connected in the center around a hub.

Good luck! This will be a good learning experience for URDF and XACRO. I had to make an offset caster for my robot. :)