[ros-users] The Limitations of URDF

[Herman Bruyninckx]

On Fri, 6 Aug 2010, David Lu!! 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. 

COLLADA seems to be the future: it allows to model graphs, for example; it
is a real international standard; there are already ROS initiatives working
in this direction.

KDL is also going to adopt COLLADA, in the coming year or so.

Best regards,

Herman Bruyninckx

> 
> 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!!
> 
> 
>

--
   K.U.Leuven, Mechanical Eng., Mechatronics & Robotics Research Group
     <http://people.mech.kuleuven.be/~bruyninc> Tel: +32 16 328056
   EURON Coordinator (European Robotics Research Network) <http://www.euron.org>
   Open Realtime Control Services <http://www.orocos.org>
   Associate Editor JOSER <http://www.joser.org>, IJRR <http://www.ijrr.org>