[ros-users] Draft REP: Coordinate Frames for Humanoids Robots

[Antonio El Khoury]

+1 for the general idea and the naming convention.

One remark:
In the frame hierarchy, torso is directly attached base_link. This may not
be always the case, e.g. for biped robots with an actuated spine. Maybe we
could just add "..." to make the hierarchy more flexible?

- base_link
   - base_footprint
   - ... - (l|r)_ankle - (l_r)_sole - (l|r)_toe
   - ... - torso    <-------------------------------
      - ... - gaze
      - ... - (l|r)_wrist - (l|r)_gripper

In any case, I think this REP is bound to evolve as new biped mechanical
structures appear.

Antonio El Khoury

On Wed, Jan 18, 2012 at 3:02 PM, Thomas Moulard <thomas.moulard at gmail.com>wrote:

> On Fri, Jan 13, 2012 at 11:17 AM, Armin Hornung
> <HornungA at informatik.uni-freiburg.de> wrote:
> > Hi Thomas!
> >
> > I edited in the description of the base_footprint with some minor
> > adjustments to keep it more general, and added a a rationale why this
> should
> > be as described (I think this makes it easier to understand). I added
> > another sentence to l/r_sole. What I don't fully understand are "It is
> > recommended to choose the origin as the projection of the body center on
> the
> > contact surface." for sole and toe. Is this the same as the sentence I
> added
> > afterwards? Then I would suggest to change "body center" into "support
> body
> > origin" or "support body link origin".
>
> (I am posting your mail back to the ml so that anyone can take part in
> the discussion)
>
> Yes this is right, I made the modification.
>
> > I also changed the frame hierarchy into a tree-format, what do you think?
>
> It is better, I fixed it to be valid rst. Is it not as nice as your
> tree, but I did not find any way
> to do a better formatting job...
>
> > You're right about the ankle, I haven't thought of non-rigid feet. I
> guess
> > I'm too fixed to our Nao's which are all rigid and have no toes. Thinking
> > about toes, I think there should be also room for an intermediate frame.
> In
> > URDF there will be a frame at the toe link which is a child of the ankle
> or
> > sole. Only then does the toe link follow (rigid or non-rigid like
> > ankle->sole, is this correct?
>
> Yes, this is correct.
>
> > Concerning Tully's comments on the mailing list I tend to agree. Torso
> also
> > sounds less medical or biological than chest (but that might only be my
> own
> > preference since I've used "Torso" more often than "Chest"). I like
> "hand"
> > more as it's more general than the "gripper" (which on the other hand is
> > very specific to the PR2) but afaik the whole grasping / arm_navigation
> code
> > in ROS is already using this convention.
>
> I switched to torso and gripper to follow PR2 naming scheme.
> I do not have any preference but we have to keep in mind that it would
> not be sufficient
> to use the PR2 arm navigation directly as PR2 frame names are
> l_gripper_* and not l_gripper directly.
> I it also a chance as it means that it is straight forward to add a
> fixed joint in the PR2 model to
> define the l_gripper / r_gripper as proposed in this REP.
>
> @Tully:
> IMHO adding a "strict" hierarchy will make the REP much more complex
> to follow when dealing with mobile robots.
> I think that humanoid robotos mechanical structure will evolve a lot
> in the future. It will be much easier to extend it when
> needed and complete it than freezing the whole thing right now.
> As far as I'm concerned, I use task space control so as long as I can
> identify the robot hands, I can generate a command which
> will move them at the appropriate position without having any
> knowledge regarding the intermediary frames and the real robot
> structure.
>
> Therefore, I would recommend not standardizing intermediary frames
> unless you find it is crucial to do so.
>
> The updated REP is here:
> - online version:
> http://laas.github.com/rep-coordinate-frames-for-biped-robots/rep-0120.html
> - git repository / sources:
> https://github.com/laas/rep-coordinate-frames-for-biped-robots
>
> Best,
> --
> Thomas Moulard
>
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