generate urdf file for a wifibot robot?

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You can try googling around to see if someone has a urdf file already or there are several tutorials on how to create your own file.

I recently just went through this process for our arm, so I'm oh too familiar. These tutorials were really helpful to me:

http://wiki.ros.org/urdf/Tutorials

Specifically, the first one in section 1 gives a great explanations of things. Then all of section 2 is really great too. I strongly recommend reading through all of them before you start writing your own file because I got to 2.4 and it starts with "if you're sick of doing all this by hand, here's how xacro can help make your life SO much easier".