Advait, <div>Indeed it's agreed upon, but was too close to the release of cturtle to get it implemented, reviewed, and tested before the freeze. I created a ticket to keep track of this. <a href="https://code.ros.org/trac/ros-pkg/ticket/4218">https://code.ros.org/trac/ros-pkg/ticket/4218</a></div>
<div><br></div><div>Tully<br><br><div class="gmail_quote">On Tue, Jun 8, 2010 at 7:13 AM, Advait Jain <span dir="ltr"><<a href="mailto:advait@cc.gatech.edu">advait@cc.gatech.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
Hi Tully,<br>
<br>
The wiki link was very helpful.<br>
<br>
Here's my two cents:<br>
<br>
I believe that everything used to define a twist should be explicit in the twist<br>
message. As a tf user, I would prefer to provide all the information rather than<br>
follow certain conventions.<br>
<br>
>From the wiki link:<br>
"There is a consensus to collapse the reference point and reference frame into<br>
a single coordinate frame."<br>
Mathematically, this seems ok to me too. However, will this involve a<br>
significant<br>
overhead in the implementation, such as adding new frames to the tf tree, make<br>
the code messy etc, if I want to find the velocity of a large number<br>
of different<br>
points in the frame of the gripper?<br>
<br>
Providing a reference point (in reference frame coordinates) does not seem like<br>
too much additional information to me.<br>
<font color="#888888"><br>
Advait<br>
</font><div><div></div><div class="h5"><br>
<br>
On Tue, Jun 8, 2010 at 1:52 AM, Tully Foote <<a href="mailto:tfoote@willowgarage.com">tfoote@willowgarage.com</a>> wrote:<br>
> Hi Advait,<br>
><br>
> There has been a bit of discussion of how to deal with twists well. However<br>
> how to transform them has many different meanings implied by how they are<br>
> represented. In particular we found there were 4 different conventions<br>
> without a clear way to distinguish between them. There is the results of an<br>
> API review at <a href="http://www.ros.org/wiki/tf/Reviews/2010-03-12_API_Review" target="_blank">http://www.ros.org/wiki/tf/Reviews/2010-03-12_API_Review</a><br>
><br>
> The biggest problem is that there is a lot of semantic information carried<br>
> in an instance of a Twist. Without forcing a lot of conventions on users we<br>
> could not find a way to transform Twists without requiring a lot of extra<br>
> information.<br>
><br>
> There is an API to get the twist between two frames in the cturtle version<br>
> of tf.<br>
><br>
> And kdl provides may twist manipulation methods which are recommended for<br>
> computing things involving twists.<br>
><br>
> If anyone else has input we can come back to this. I would like to add this<br>
> capability, but building a consensus on representations will be required.<br>
><br>
> Tully<br>
><br>
> On Mon, Jun 7, 2010 at 5:00 PM, Advait Jain <<a href="mailto:advait@cc.gatech.edu">advait@cc.gatech.edu</a>> wrote:<br>
>><br>
>> I just realized that the wrench and twist messages currently<br>
>> do not explicitly store the point at which the wrench or twist<br>
>> is expressed.<br>
>><br>
>> Is there is support in TF or somewhere else for getting the<br>
>> equivalent twist or wrench at a different point in space?<br>
>><br>
>> e.g. to get the equivalent wrench at point B given a wrench<br>
>> acting at point A, I would use:<br>
>> Force at point A = Force at point B<br>
>> Torque at point A = vector from A to B X Force + Torque at point B<br>
>><br>
>> It would be nice to have easy ways to be able to say, for<br>
>> example, that given a force that the robot is applying at the<br>
>> door handle, what the torque is at the hinge of the door.<br>
>><br>
>> Another example could be if the we want a robot to perform<br>
>> a bi-manual and need to compute the motion of two points<br>
>> on the same rigid body.<br>
>><br>
>> I think it would be really cool if TF could also shift twists,<br>
>> wrenches correctly in addition to changing coordinate frames.<br>
>><br>
>> What are other people's thoughts about this?<br>
>><br>
>><br>
>> Advait<br>
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><br>
><br>
><br>
> --<br>
> Tully Foote<br>
> Systems Engineer<br>
> Willow Garage, Inc.<br>
> <a href="mailto:tfoote@willowgarage.com">tfoote@willowgarage.com</a><br>
> (650) 475-2827<br>
><br>
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</div></div></blockquote></div><br><br clear="all"><br>-- <br>Tully Foote<br>Systems Engineer<br>Willow Garage, Inc.<br><a href="mailto:tfoote@willowgarage.com">tfoote@willowgarage.com</a><br>(650) 475-2827<br>
</div>