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Following is the full code of the first version of the urdf of the robots I'm working on.

<?xml version="1.0"?>
<robot name="Saetta">
  <link name="base_link">
  </link>

  <link name="body">
    <visual>
      <geometry>
        <box size="0.195 0.145 0.055"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0.032"/>
      <material name="grey">
        <color rgba="0.5 0.5 0.5 1"/>
      </material>
    </visual>
  </link>

  <link name="cover">
    <visual>
      <geometry>
        <box size="0.195 0.145 0.02"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0.069"/>
      <material name="dark_transparent_grey">
        <color rgba="0.2 0.2 0.2 0.5"/>
      </material>
    </visual>
  </link>


  <link name="right_wheel">
    <visual>
      <geometry>
        <cylinder radius="0.032" length=".007"/>
      </geometry>
      <origin rpy="0 1.57075 1.57075" xyz="0 -0.085 0.032"/>
      <material name="wheel_metal">
        <color rgba="0.7 0.6 0.6 1"/>
      </material>
    </visual>
  </link>

  <link name="left_wheel">
    <visual>
      <geometry>
        <cylinder radius="0.032" length=".007"/>
      </geometry>
      <origin rpy="0 1.57075 1.57075" xyz="0 0.085 0.032"/>
      <material name="wheel_metal"/>
    </visual>
  </link>

  <link name="castor_support">
    <visual>
      <geometry>
        <box size="0.05 0.027 0.02"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0.1225 0 0.02"/>
      <material name="black">
        <color rgba="0 0 0 1"/>
      </material>
    </visual>
  </link>

  <link name="castor_wheel">
    <visual>
      <geometry>
        <sphere radius="0.01"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0.1225 0 0.01"/>
      <material name="black"/>
    </visual>
  </link>

  <link name="sensor_front_left">
    <visual>
      <geometry>
        <box size="0.03 0.022 0.01"/>
      </geometry>
      <origin rpy="0 1.57075 1.30915" xyz="0.082 -0.08 0.032"/>
      <material name="black"/>
    </visual>
  </link>

  <link name="sensor_front">
    <visual>
      <geometry>
        <box size="0.03 0.022 0.01"/>
      </geometry>
      <origin rpy="0 1.57075 0" xyz="0.11 0 0.05"/>
      <material name="black"/>
    </visual>
  </link>


  <link name="sensor_front_right">
    <visual>
      <geometry>
        <box size="0.03 0.022 0.01"/>
      </geometry>
      <origin rpy="0 1.57075 1.83235" xyz="0.082 0.08 0.032"/>
      <material name="black"/>
    </visual>
  </link>

  <link name="sensor_rear_right">
    <visual>
      <geometry>
        <box size="0.03 0.022 0.01"/>
      </geometry>
      <origin rpy="0 1.57075 0" xyz="-0.082 -0.08 0.032"/>
      <material name="black"/>
    </visual>
  </link>


  <link name="sensor_rear_left">
    <visual>
      <geometry>
        <box size="0.03 0.022 0.01"/>
      </geometry>
      <origin rpy="0 1.57075 0" xyz="-0.082 0.08 0.032"/>
      <material name="black"/>
    </visual>
  </link>

  <joint name="body_to_cover" type="fixed">
    <parent link="body"/>
    <child link="cover"/>
    <origin xyz="0 0 0"/>
  </joint>

  <joint name="body_to_right_wheel" type="fixed">
    <parent link="body"/>
    <child link="right_wheel"/>
    <origin xyz="0 0 0"/>
  </joint>

  <joint name="body_to_left_wheel" type="fixed">
    <parent link="body"/>
    <child link="left_wheel"/>
    <origin xyz="0 0 0"/>
  </joint>

  <joint name="body_to_castor_support" type="fixed">
    <parent link="body"/>
    <child link="castor_support"/>
    <origin xyz=" 0 0 0"/>
  </joint>

  <joint name="castor_support_to_castor_wheel" type="fixed">
    <parent link="castor_support"/>
    <child link="castor_wheel"/>
    <origin xyz=" 0 0 0"/>
  </joint>

  <joint name="base_to_body" type="fixed">
    <parent link="base_link"/>
    <child link="body"/>
    <origin xyz=" 0 0 0"/>
  </joint>

  <joint name="body_to_FL_sensor" type="fixed">
    <parent link="body"/>
    <child link="sensor_front_left"/>
    <origin xyz=" 0 0 0"/>
  </joint>

  <joint name="body_to_F_sensor" type="fixed">
    <parent link="body"/>
    <child link="sensor_front"/>
    <origin xyz=" 0 0 0"/>
  </joint>

  <joint name="body_to_FR_sensor" type="fixed">
    <parent link="body"/>
    <child link="sensor_front_right"/>
    <origin xyz=" 0 0 0"/>
  </joint>


  <joint name="body_to_RL_sensor" type="fixed">
    <parent link="body"/>
    <child link="sensor_rear_left"/>
    <origin xyz=" 0 0 0"/>
  </joint>

  <joint name="body_to_RR_sensor" type="fixed">
    <parent link="body"/>
    <child link="sensor_rear_right"/>
    <origin xyz=" 0 0 0"/>
  </joint>


</robot>

Looks complex, but it took less than an hour to write and debug. Once you have something like this (with an arbitrary number of links) you use a static_robot_publisher to publish all the static links like so (roslaunch syntax)

<!-- Robot Description -->
<param name="tf_prefix" value="YOUR_PREFIX"/>
        <param name="robot_description" command="$(find xacro)/xacro.py $(find <YOUR_PACKAGE>)/path/to/file.urdf" />

<!-- Robot State Publisher -->
<node pkg="robot_state_publisher" type="state_publisher" name="state_publisher"/>

This won't work out of the box, you have to modify the paths and names to reflect your files and setup. Anyway once you have this in place, you'll se (do a rosrun tf tf_monitor to see in realtime) that all the static links are automatically published. The only missing one, and the one RVIZ will complain about, is the link

world -> base_link

which you have to manually publish. That's because that's the "entry point" to your model: by moving the base link you can move the robot around with just one transform.

To test that you can publish a static manual transform like so

$rosrun tf static_transform_publisher x y z r p y frame1 frame2 rate

where frame1 -> world and frame2 -> base_link and the various variables can assume whatever value you want to try.

With this RVIZ should be able to visualize your robot correctly. To check your URDF tree use

$ rosrun urdf check_urdf yourfile

And finally to check that the various tf trees are correct (no unconnected branches) use

$ rosrun tf view_frames ; evince frames.pdf