[ros-users] The Limitations of URDF

John Hsu johnhsu at willowgarage.com
Fri Aug 6 21:36:41 UTC 2010


Hi David!!!
I've answered inline:

On Fri, Aug 6, 2010 at 11:44 AM, David Lu!! <davidlu at wustl.edu> wrote:

> Hey ROS-community,
> I've been working with URDF extensively for awhile, and am wondering what
> the future is for its development. Specifically, whether the format will be
> extended at all to address what I perceive to be some of its limitations.
>
> I think the biggest limitation is the lack of graph support, which relates
> a link to two or more parent links (as opposed to the current tree
> structure). I believe this problem stems from KDL supporting only chains,
> and not graphs, but there are a number of situations where a graph structure
> is called for. The simplest example is a four bar linkage, which, as it
> stands, cannot be easily model led in URDF.
>
>
Thanks for the feedback.  We have started thinking about some of the
potential improvements to URDF and ticketed them on trac
https://code.ros.org/trac/ros-pkg/search?q=urdf-2.0
please feel free to contribute your comments there.  When creating
additional feature requests please put "urdf-2.0" in the "Keywords" field
for now.

One step toward fixing the problem could involve making some joints
> dependent on other joints. For the case of a parallelogram-shaped 4 bar,
> three of the joints could depend on one joint, but there is currently no
> support for that either. A similar situation involves gears/pulleys and
> other motion transferring mechanisms, i.e. two gears, each connected to a
> base with a continuous joint, and the angle of joint for the second gear is
> 4 times the angle for the first.
>

I've added your comments to this
ticket<https://code.ros.org/trac/ros-pkg/ticket/4263>.



> Having now tried to get collision detection working as well, it seems odd
> to have three different structures to specify the hierarchy of the machine.
> Its specified once in the URDF, and then separate groups are defined via
> parameters to define groups. Some of these groups coincide with whole xacro
> macros too. While I see that these distinctions may often need to be
> customized, it seems like it would be easier to do the customization via
> parameter, and not use the parameters to redefine everything again.
>

Maybe someone more familiar with the collision groups can answer this one?


>
> The last thing that concerns me is the PR2 specific extensions. I'm not
> exactly clear what they lend to the PR2, but I'm also not clear why they
> wouldn't apply to other robots.
>

can you please clarify your concern?  (e.g. package/stack names? etc.)
In general, a lot of the software packages may work their way into
non-PR2-specific status after proving its usefulness first as a PR2 specific
utility.


>
> [As an aside, does anyone know where the xml schema are for urdf? The PR2
> file links to http://playerstage.sourceforge.net/gazebo/xmlschema/, but
> there's nothing there.]
>

https://code.ros.org/trac/ros-pkg/ticket/4328


>
> What I'm wondering is whether any of these issues are currently being
> addressed, or whether I should work around them (either in my own code or in
> the ROS repository). Hopefully this will spark a discussion on any other
> hurdles people are having with URDF.
>

Thanks again for the feedbacks!
John



>
>

> Thanks,
> David!!
>
>
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