[ros-users] pr2 odometry tf relationship backwards?

[Lorenz Mösenlechner]

Hi,

you are right, this behavior seems somehow odd. I guess the reason is
that on the PR2, the TF tree looks like this:

 /map -> odom_combined -> base_footprint -> ...

As far as I know, Pr2Odometry is set up to not publish transforms at
all, just Odometry messages. These are read by the robot_pose_ekf node
which then publishes the odom_comined -> base_footprint transform. If
you want to use the Pr2Odometry node for doing it, I think you might
need to fork and hack it. This should not be too hard however.

Lorenz


> Hi all,
> 
> I am using the controller Pr2Odometry from the
> pr2_mechanism_controllers package to do simulated odometry for the
> KUKA YouBot in Gazebo.  I am seeing an oddity in the tf tree, and
> I'd like to find out whether it's something I'm doing.  It seems to
> me that there is a reversed parent/child relationship in the tf
> links published by Pr2Odometry.  The documentation for it at
> 
>     http://www.ros.org/wiki/pr2_mechanism_controllers/Pr2Odometry#ROS_API
> 
> says
> 
>     /tf (tf/tfMessage)
>     Two transforms are published to tf, a constant transform from
> the base_footprint frame to the base_link frame and the transform
> from the odom frame to the base footprint frame.
> 
> This conforms to my expectation, which is that /odom is the parent
> of /base_footprint.  Thus, if global localization drops out, you can
> still continue on odometry alone for as long as your dead reckoning
> will allow.  This expected behavior is also compatible with
> fake_localization, which publishes a transform from /map to /odom.
> 
> By contrast, examining tf on the running system shows me a tree in
> which /base_footprint is at the root.  /odom is a leaf and a sibling
> of /base_link, which connects to the rest of the tree.  The code at
> 
> /opt/ros/electric/stacks/pr2_controllers/pr2_mechanism_controllers/src/pr2_odometry.cpp
> 
> is clear:
> 
>       out.header.frame_id =  tf::resolve(tf_prefix_,base_footprint_frame_);
>       out.child_frame_id = tf::resolve(tf_prefix_,odom_frame_);
> 
> So /odom is the child here.  This behavior seems to contradict
> intuition and the (admittedly terse) documentation online.  However,
> since I am using this pr2 code on the YouBot, I wonder if there is
> something different about how the PR2 operates?
> 
> Thanks!
> 
>  -ross
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-- 
Lorenz Mösenlechner            | moesenle at in.tum.de
Technische Universität München | Karlstraße 45
80335 München                  | Germany
http://ias.cs.tum.edu/         | Tel: +49 (89) 289-26910