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Robot Description with URDF

URDF (Unified Robot Description Format) is an XML-based format for describing robot geometry, joints, sensors, and visual appearance in ROS 2.

Why URDF Matters​

URDF files are the blueprint of your robot. They define:

  • Physical structure: Links (rigid bodies) and joints (connections)
  • Visual appearance: 3D models and colors for simulation
  • Collision geometry: Shapes for physics calculations
  • Sensor placement: Where cameras and LiDAR are mounted

Key Concepts​

Links represent rigid bodies (wheels, chassis, arms).

<link name="base_link">
  <visual>
    <geometry>
      <box size="0.5 0.3 0.1"/>
    </geometry>
    <material name="blue">
      <color rgba="0 0 0.8 1"/>
    </material>
  </visual>
  <collision>
    <geometry>
      <box size="0.5 0.3 0.1"/>
    </geometry>
  </collision>
</link>

2. Joints: Connections Between Parts​

Joints define how links move relative to each other.

Joint Types:

  • fixed: No movement (camera mount)
  • revolute: Rotation with limits (elbow)
  • continuous: Unlimited rotation (wheel)
  • prismatic: Linear motion (elevator)
<joint name="wheel_joint" type="continuous">
  <parent link="base_link"/>
  <child link="wheel_link"/>
  <origin xyz="0.2 0 -0.05" rpy="0 0 0"/>
  <axis xyz="0 1 0"/>
</joint>

3. Complete Example: Simple Mobile Robot​

<?xml version="1.0"?>
<robot name="simple_robot">
  <!-- Base link -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.4 0.3 0.1"/>
      </geometry>
      <material name="gray">
        <color rgba="0.5 0.5 0.5 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.4 0.3 0.1"/>
      </geometry>
    </collision>
  </link>

  <!-- Left wheel -->
  <link name="left_wheel">
    <visual>
      <geometry>
        <cylinder radius="0.05" length="0.03"/>
      </geometry>
      <material name="black">
        <color rgba="0 0 0 1"/>
      </material>
    </visual>
  </link>

  <joint name="left_wheel_joint" type="continuous">
    <parent link="base_link"/>
    <child link="left_wheel"/>
    <origin xyz="0.1 0.15 -0.05" rpy="1.5708 0 0"/>
    <axis xyz="0 0 1"/>
  </joint>

  <!-- Right wheel (mirrored) -->
  <link name="right_wheel">
    <visual>
      <geometry>
        <cylinder radius="0.05" length="0.03"/>
      </geometry>
      <material name="black">
        <color rgba="0 0 0 1"/>
      </material>
    </visual>
  </link>

  <joint name="right_wheel_joint" type="continuous">
    <parent link="base_link"/>
    <child link="right_wheel"/>
    <origin xyz="0.1 -0.15 -0.05" rpy="1.5708 0 0"/>
    <axis xyz="0 0 1"/>
  </joint>
</robot>

Viewing URDF in RViz​

To visualize your robot model:

# Launch RViz with robot state publisher
ros2 launch urdf_tutorial display.launch.py model:=my_robot.urdf

Best Practices​

  1. Start simple: Build basic shapes first, add detail later
  2. Use SI units: Meters for length, radians for angles
  3. Set collision geometry: Simplify for better performance
  4. Name consistently: Use base_link as root, descriptive joint names
  5. Test incrementally: Add one link/joint at a time

Common Pitfalls​

  • Missing parent/child links: Joints need both
  • Wrong axis definition: Check rotation direction
  • Incorrect origin: Links may overlap or disconnect
  • No collision geometry: Robot won't interact with environment

Try It Yourself​

Ask the chatbot:

  • "What's the difference between visual and collision geometry?"
  • "How do I define a revolute joint with limits?"
  • "What coordinate system does URDF use?"