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

[Tully Foote]

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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