[ros-users] reactive_grasp, compliant_close: not using the database

[Matei Ciocarlie]

Hi Ugo,

There are two things that the hand needs to do: move from pre-grasp to
grasp, and then close the fingers. Our nomenclature is definitely less
than ideal, they are both referred to as "grasp" in parts of the
pipeline...

Reactive grasping as defined by the ReactiveGrasp.action actually
combines both. It should be called "ReactiveApproachAndGrasp" or
something in the vicinity... The implementation we have is PR2
specific for both the approach and the finger closing part. You are
right, as a module that is (also) responsible for closing the fingers,
it should take in the grasp posture as well (not just the pose).

When not using reactive grasping, we actually do split it into two.
First, open-loop inerpolated IK is done to get the hand from pre-grasp
to grasp. Then, a call to a GraspHandPostureExecution.action is made.
This call does contain all the info about the grasp .We have a PR2
implementation that simply closes the fingers in response to that call
in the
pr2_gripper_grasp_controller package.

You could write a sh_gripper_grasp_controller package that responds to
GraspHandPostureExecution goals, either in a reactive or non-reactive
way. This would only refer to the finger closing part (not the
approach) but you could implement all the behaviors that you mentioned
in your email.

Alternatively, if you prefer a "ReactiveApproachAndGrasp" all in one
shot, we would indeed need to modify the ReactiveGrasp call to contain
the grasp posture. That also seems like a reasonable thing to do.

Best,
Matei

On Thu, Mar 3, 2011 at 4:16 AM, Ugo Cupcic <ugo at shadowrobot.com> wrote:
> Hi,
>
> In the reactive_grasp, the compliant_close function is not using the grasp
> posture from the database. I understand that it can work for the gripper,
> but for a more complex hand such as ours, I believe it would be a good start
> to have access to this grasp posture here.
>
> What I wanted to do in the reactive grasp, was (as a first approach):
>  - interpolate between current posture of the hand and "closed posture"
> (read from the database), while checking for contacts.
>  - when I have a contact on one finger, I stop moving this finger.
>  - when all fingers are touching lightly the object, close until force > X
>
> Is it not the purpose of the reactive_grasp server? Should I do that in my
> standard static grasp? Do I need to query my databases in my reactive_grasp
> node to get the list of possible grasps, or would it be possible to extend
> the content of the ReactiveGraspGoal message, so as to include this
> information?
>
> I hope my questions make sense, otherwise feel free to ask me for more info!
>
> Cheers,
>
> Ugo
>
> --
> Ugo Cupcic | Shadow Robot Company | ugo at shadowrobot.com
> Software Engineer | 251 Liverpool Road |
> need a Hand? | London N1 1LX | +44 20 7700 2487
> http://www.shadowrobot.com/hand/ @shadowrobot
>
>
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-- 
Matei Ciocarlie
Research Scientist
Willow Garage Inc.
650-475-9780