[ros-users] The Limitations of URDF

[Rosen Diankov]

This is a great point Herman. Although there is a urdf2collada tool,
most of the tools developed by Willow Garage in ROS directly use URDF.
We're hoping to eventually have a collada2urdf tool to resolve these
issues so that a robot's data can be natively stored in collada.
rosen,

2010/8/7 Herman Bruyninckx <Herman.Bruyninckx at mech.kuleuven.be>:
> 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>
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