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@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@willowgarage.com
(650) 475-2827