[ros-users] The Limitations of URDF

[David Lu!!]

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.

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.

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.

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.

[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.]

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,
David!!
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