Hi Miguel,

In general when describing the relative pose of two rigid bodies(links), a 6dof transform is used (as in tf). That's the same approach taken in URDF. To be more specific, the same transform is the <origin> tag in your <joint>-element. In addition, for revolute, prismatic and fixed joints, the joint axis coincides with the child link origin.

If you need more information on conversion from D-H parameters to transforms between rigid body links, this link to the joint element definition should help you get started. Please let me know if you find the diagram(s) misleading or inadequate. As usual, we always appreciate any comments or code (e.g. parser) contributions :)

For example, I find it quite confusing that the dimensions of the links are specified inside the joint element.

I am not sure what you mean by above (here's the same link to the joint element definition), can you please elaborate on the question a bit?

The dimensions of links are specified here, where you can specify the dimensions for <visual> and <collision> body dimensions.

Also, is the world frame oriented like it's shown on the schematic on the urdf tutorial page? With the y axis pointing upwards?

I think you are referring to this page? The "World" frame is drawn to show how we define relative frame transforms between a sample set of frames. You can view the "world" as a special link that's inertially fixed. There is no requirement on how you define the "World" frame. The informations captured in a robot description (URDF or DH) are simply relative transforms between frames that are attached to bodies and joints. (So for example, you can take the tutorial screen and rotate it sideways, and the individual transforms between frames should remain valid).

As a side note, we are working on a URDF->COLLADA converter (package name collada_urdf in robot_model trunk) should you be interested.

Hope I am answering your questions correctly.
Best,
John

On Wed, Mar 17, 2010 at 10:41 AM, Miguel Prada <miguel.register@gmail.com> wrote:

Hi,

I'm quite new to ROS and I'm trying to describe a simple robot manipulator using urdf format.

I'm having some trouble understanding the convention used (and the parameters involved), and I just wanted to ask why is it that D-H convention is not used. Specially with it being a standard for describing manipulators, as far as I know.

For example, I find it quite confusing that the dimensions of the links are specified inside the joint element.

Also, is the world frame oriented like it's shown on the schematic on the urdf tutorial page? With the y axis pointing upwards?

Thanks,
Miguel.
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