[ros-users] manipulation stack problems

Mon Feb 7 15:24:14 UTC 2011

Hi,

Thanks a lot for both your answers. It made things a bit clearer.

I'm using the grasping pipeline, but I had to modify some nodes because I'm
adapting it to our robot hand / arm + using the kinect as a 3d sensor.
I had to write my own move_arm_simple_action / hand_posture_execution nodes.

Modifying the hand_approach_direction in my hand_description config file
solved the problem of the "backward" approach.

I still have to get the lift action to behave, but we're getting there :)

Cheers,

Ugo

On Fri, Feb 4, 2011 at 1:07 AM, Matei Ciocarlie <matei at willowgarage.com>wrote:

> Hi Ugo,
>
> Are you using the grasping pipeline by calling the Pickup action? If so,
> the grasping pipeline will do for you all of what Kaijen explained. Here are
> a few notes:
>
> - the pre-grasp stored in the database is not used. Initially we though it
> would be, but then we realized that how far back the pre-grasp should be
> from the grasp is often application-specific.
>
> - the Pickup action goal thus specifies how far back the pre-grasp should
> be. There are two relevant fields:
>
> float32 desired_approach_distance
> float32 min_approach_distance
>
> - the grasping pipeline will attempt to find a pre-grasp that is "behind"
> the grasp by at least min_approach_distance, and ideally
> desired_approach_distance. This pre-grasp must be reachable and out of
> collision. It will do this by starting at the grasp position, then doing
> interpolated IK "backwards" as far as possible, up to
> desired_approach_distance. This explains why Interpolated IK markers show up
> the at the grasp location first, then move back.
>
> - what does "backwards" mean? That is specified on the parameter server.
> The grasping pipeline will look for a set of parameters telling it, among
> others:
>  * what is the tf name of reference frame of the hand - this is the frame
> that the grasp pose refers to
>  * what is the "approach" direction, relative to the hand frame, when
> executing a grasp. A pre-grasp will be computed by translating a grasp in
> the opposite of the approach direction.
>
> For the PR2, this is specified in
> pr2_object_manipulation_launch/config/pr2_hand_descriptions.yaml. You will
> probably need to adapt this file to your hardware.
>
> Let us know if you have more problems, we'd be glad to help. Also, if you
> could include some rviz screenshots from multiple viewpoints that would
> help, as it's hard to tell from one image what things look like in 3D.
>
> Best,
> Matei
>
>
> On Thu, Feb 3, 2011 at 1:06 PM, Kaijen Hsiao <kaijenhsiao at gmail.com>wrote:
>
>> Hi Ugo,
>> I'm not sure exactly which stack/code you are using, but those markers
>> look like they're coming from the interpolated IK motion planner.  The
>> interpolated IK motion planner can search for a consistent/feasible
>> path starting from either end--if the start_from_end parameter is
>> true, then the planner finds IK solutions for the end pose first, then
>> works its way backwards towards the start pose, using the last set of
>> found arm angles as the starting point.
>>
>> Typically when you're planning from pregrasp to grasp, you want to
>> enter pregrasp as start and grasp as end so the trajectory times are
>> correct, but you want to ask it to start searching from the grasp
>> (end) pose and work its way backwards.  That's because you typically
>> can't compromise on the final grasp pose, but you might not mind if
>> the Cartesian path backwards towards the pregrasp gets almost but not
>> all the way there.  That's what the error codes tell you--how far it
>> got.  (You can also set the steps_before_abort param to 0 if you just
>> want it to abort if it finds any infeasible/inconsistent poses at
>> all.)
>> If you get an incomplete path back, you can decide whether you're
>> willing to just motion plan to a slightly closer pregrasp pose, or if
>> you want to abort.  If the markers don't make it all the way to the
>> pregrasp, it means that the path is incomplete--the next step towards
>> the pregrasp either had no collision-free IK solution, or else had no
>> solution consistent with the last step (within consistent_angle for
>> all joint angles from the last step).
>>
>> I'm not sure what you're running to get to the pregrasp, but you
>> probably want to use move_arm or some such to move to the start of the
>> (completed portion of the) trajectory returned by the interpolated IK
>> motion planner, then use a normal joint trajectory movement to get
>> through the rest of the trajectory.  (Or you can just use a Cartesian
>> controller, assured that there actually is a feasible/consistent path
>> to actually reach the grasp pose from that set of joint angles.)
>>
>> Hope that helps.
>>
>> -Kaijen
>>
>>
>> On Thu, Feb 3, 2011 at 8:25 AM, Ugo Cupcic <ugo at shadowrobot.com> wrote:
>> > Hi all,
>> >
>> > I'm still trying to get the manipulation stack to work with our arm and
>> hand
>> > and I ran into some problem:
>> >
>> > As you can see here, A path from pregrasp to grasp is found:
>> > http://img267.imageshack.us/img267/199/rviz013.png
>> >
>> > But:
>> >  > the path seems to be the wrong way around: the first markers are at
>> the
>> > bottom, instead of at the top.
>> >  > when moving the arm, the arm moves maybe 5cm below the markers (on
>> the
>> > screenshot, it's at position 0 which should be pregrasp).
>> >
>> > The values I put in the database are:
>> > grasp_pregrasp_position =
>> > "{0,0,0.281533,0.707106781186548,0.707106781186548,0,0}"
>> > grasp_grasp_position =
>> > "{0,0,0.213018,0.707106781186548,0.707106781186548,0,0}"
>> >
>> > Any help greatly appreciated.
>> >
>> > 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
>> >
>> >
>> > _______________________________________________
>> > ros-users mailing list
>> > ros-users at code.ros.org
>> > https://code.ros.org/mailman/listinfo/ros-users
>> >
>> >
>> _______________________________________________
>> ros-users mailing list
>> ros-users at code.ros.org
>> https://code.ros.org/mailman/listinfo/ros-users
>>
>
>
>
> --
> Matei Ciocarlie
> Research Scientist
> Willow Garage Inc.
> 650-475-9780
>
> _______________________________________________
> ros-users mailing list
> ros-users at code.ros.org
> https://code.ros.org/mailman/listinfo/ros-users
>
>

-- 
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ros.org/pipermail/ros-users/attachments/20110207/1fe7990e/attachment-0003.html>