[ros-users] Weirdness in URDF tutorials

[Björn Giesler]

Hi,

well frankly, I think the documentation is very good but it makes quite a few assumptions. I've worked through the tf tutorials, and while they're great, I'm missing a step-for-step tutorial for simple platforms that don't have laser scanners. The tutorial for the navigation stack seems great, but it makes many many assumptions about system components that just aren't there if you don't have a laser scanner. Once I've worked through all of this, I may write a non-laserscanner tutorial.

I'm still confused though how the whole tf system works. Right now, I've built a simple URDF file for my robot that contains the base link and one foot, here it is:

<?xml version="1.0" ?>
<robot name="r2d2" xmlns:xacro="http://playerstage.sourceforge.net/gazebo/xmlschema/#interface">
  <link name="base_link">
    <!-- robot coord origin is between the main feet -->
    <!-- just put a tiny box there -->
    <visual>
      <geometry>
        <box size=".01 .01 .01"/> 
      </geometry>
    </visual>
  </link>
  <link name="left_foot">
    <visual>
      <geometry>
        <box size=".36 .175 .16"/>
      </geometry>
      <material name="white">
        <color rgba="1 1 1 1"/>
      </material>
      <origin rpy="0 0 0" xyz="0 0 .08"/>
    </visual>
  </link>
  <joint name="base_to_left_foot" type="fixed">
    <parent link="base_link"/>
    <child link="left_foot"/>
    <origin xyz="0 0.275 0"/>
  </joint>
</robot>

...and I've copied the transform publisher for the transform /odom->/base_link that is in the Wiki. Result: My /base_link moves around as it should, but the foot doesn't move with it. The tf tree as shown in rviz shows a tree structure "/odom -> /base_link -> /left_foot", but its status area says that there's "No transform from [/left_foot] to frame [/odom]". I thought the tf tree structure did just that?

Any help is much appreciated.

Regards,
Björn

Am 28.12.2010 um 21:53 schrieb Chad Rockey:

> I ran into a similar problem as well.  I didn't realize that joint_state_publisher wasn't included in ROS and needed to be installed.  Of course, it says right there it's needed and the warning tells you exactly what you need to do.  However, when you're focused on the URDF, these things sometimes slip by.  Maybe we should make this more obvious and explain robot_state_publisher's and joint_state_publisher's roles better or even put rosmake/rosrun tf view_frames steps in the tutorial somewhere?
> 
> - Chad
> 
> On Tue, Dec 28, 2010 at 1:59 PM, David Lu!! <davidvlu at gmail.com> wrote:
> Hey Björn,
> The tutorials were just finished last month, so they're up to date as far as I know. 
> 
> It looks like you have everything figured out, but I couldn't tell because of the last "I still need to figure out why". 
> 
> Things will appear white and at the origin when there isn't a proper transform to the link. If you're still running into that problem, make sure that rxgraph looks the way you would expect, with joint_state_publisher  connected to robot_state_publisher. 
> 
> -David!!
> 
> 

-- 
Björn Giesler
bjoern at giesler.de
Verschlüssel Deine Emails -- oder schreibst Du Deine Briefe außen auf den Umschlag?
Meinen Schlüssel findest Du auf http://giesler.biz/~bjoern/downloads/bg.asc -- schick mir Deinen!
Mehr Informationen findest Du auf http://www.gnupg.org/index.de.html