Hi David!!! I've answered inline: On Fri, Aug 6, 2010 at 11:44 AM, 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. > > 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. > 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!! > > > _______________________________________________ > ros-users mailing list > ros-users@code.ros.org > https://code.ros.org/mailman/listinfo/ros-users > >