Add ability to attach and detach end-effector links

[jrgnicho]

Is your feature request related to a problem? Please describe.
Many industrial application involve changing end-effectors on the fly mid process and at the moment this can not be done directly; the workaround I'm aware of is to either use an attached object as and end effector or create various tool links in the URDF beforehand and turn their collision flags on and off. Therefore it would be very convenient if one could programatically attach and detach end-effector links.

Describe the solution you'd like
I believe one of the limitations lies in the URDF library which doesn't allow modifications to the chain once it's initialized from the urdf file. Therefore some work would need to be done in exploring other scene management alternatives that would allow modifying the kinematic chain at runtime; some libraries that I'm aware of are boost Graph, KDL and Ogre3D.

[AndyZe]

This sounds important. Maybe a combined request from MoveIt and ROS-Industrial would be enough to convince the powers that be to update the URDF library.

[gavanderhoorn]

This was also discussed at the 1st MoveIt con in Macau.

The URDF format does not explicitly support dynamic removal or addition of links, but that's not really important: it's a serialisation format, which is inherently static.

The in-memory representation of the URDF DOM does not really prohibit anyone from making changes. Even after initial deserialisation There is API for it currently (or at least: if you get a Model ptr it's sort-of read-only), but there is nothing inherently difficult in adding some setters.

A bigger problem is ensuring that all consumers of the URDF will be notified of that change, in a timely manner and in a coherent way.

That does not seem to be a problem "for" URDF per se, and probably also not something solved by changing URDF.

[gavanderhoorn]

of course, if move_group (or whatever hosts the planning scene and robot models) is the only consumer, then this doesn't really matter, and it could simply be added to MoveIt. But that may not be what @jrgnicho had in mind.

[jrgnicho]

@gavanderhoorn , that's more or less what I envision for this feature
I think since most moveit planning applications work with a move_group central node that host the robot model and planning scene then any online changes to the robot model would not be much of an issue as all planning operations have to go through the move_group node anyway. For applications that don't follow that workflow we could publish to a topic that notifies that changes to the robot model have been made or they could just call the get_planning_scene service to sync their local scene with the latest one

[gavanderhoorn]

Would URDF still be a limiting factor here then?

[AndyZe]

Why can't URDF have a flag to allow dynamic modification? It should be off by default.

That would be consistent and reliable for people who don't need updates, I think.

[jrgnicho]

@gavanderhoorn it was my impression that the urdf model in the urdf library was immutable but based on your comments that isn't the case, therefore it wouldn't be a limiting factor. I don't expect that this change would require adding new tags to the urdf file, if new attributes are needed we could just add those to the srdf file.

[gavanderhoorn]

What I'm confused about is the need to change the URDF format itself (so the on-disk format, not the DOM).

What change would you see needed there?

[jrgnicho]

I don't envision any changes to the URDF format actually, sorry if in my earlier comments I seemed to imply that.