[ros-users] Wrench and Twist: shifting and coordinate transformations

[Advait Jain]

Hi Tully,

The wiki link was very helpful.

Here's my two cents:

I believe that everything used to define a twist should be explicit in the twist
message. As a tf user, I would prefer to provide all the information rather than
follow certain conventions.

>From the wiki link:
"There is a consensus to collapse the reference point and reference frame into
a single coordinate frame."
Mathematically, this seems ok to me too. However, will this involve a
significant
overhead in the implementation, such as adding new frames to the tf tree, make
the code messy etc, if I want to find the velocity of a large number
of different
points in the frame of the gripper?

Providing a reference point (in reference frame coordinates) does not seem like
too much additional information to me.

Advait


On Tue, Jun 8, 2010 at 1:52 AM, Tully Foote <tfoote at willowgarage.com> wrote:
> Hi Advait,
>
> There has been a bit of discussion of how to deal with twists well.  However
> how to transform them has many different meanings implied by how they are
> represented.  In particular we found there were 4 different conventions
> without a clear way to distinguish between them.  There is the results of an
> API review at http://www.ros.org/wiki/tf/Reviews/2010-03-12_API_Review
>
> The biggest problem is that there is a lot of semantic information carried
> in an instance of a Twist.  Without forcing a lot of conventions on users we
> could not find a way to transform Twists without requiring a lot of extra
> information.
>
> There is an API to get the twist between two frames in the cturtle version
> of tf.
>
> And kdl provides may twist manipulation methods which are recommended for
> computing things involving twists.
>
> If anyone else has input we can come back to this.  I would like to add this
> capability, but building a consensus on representations will be required.
>
> Tully
>
> On Mon, Jun 7, 2010 at 5:00 PM, Advait Jain <advait at cc.gatech.edu> wrote:
>>
>> I just realized that the wrench and twist messages currently
>> do not explicitly store the point at which the wrench or twist
>> is expressed.
>>
>> Is there is support in TF or somewhere else for getting the
>> equivalent twist or wrench at a different point in space?
>>
>> e.g. to get the equivalent wrench at point B given a wrench
>> acting at point A, I would use:
>> Force at point A = Force at point B
>> Torque at point A  = vector from A to B X Force + Torque at point B
>>
>> It would be nice to have easy ways to be able to say, for
>> example, that given a force that the robot is applying at the
>> door handle, what the torque is at the hinge of the door.
>>
>> Another example could be if the we want a robot to perform
>> a bi-manual and need to compute the motion of two points
>> on the same rigid body.
>>
>> I think it would be really cool if TF could also shift twists,
>> wrenches correctly in addition to changing coordinate frames.
>>
>> What are other people's thoughts about this?
>>
>>
>> Advait
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>
>
>
> --
> Tully Foote
> Systems Engineer
> Willow Garage, Inc.
> tfoote at willowgarage.com
> (650) 475-2827
>
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