Understanding URDF

The Unified Robot Description Format (URDF) is the cornerstone of robot modeling in ROS 2. Kinetix simplifies the creation of these XML-based models through an intuitive visual interface.

Core Components

Links

Physical entities with mass, inertia, and visual geometry. Links define what your robot looks like.

Joints

Kinematic connections between links. Joints define how your robot moves (e.g., rotation, sliding).

Standard URDF Structure

A typical URDF file generated by Kinetix looks like this:

<robot name="my_robot">
  <!-- The Root Link -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.6 0.4 0.2"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 0.8 1"/>
      </material>
    </visual>
  </link>

  <!-- A Revolute Joint (e.g. Wheel) -->
  <joint name="wheel_joint" type="continuous">
    <parent link="base_link"/>
    <child link="wheel"/>
    <origin xyz="0.2 0.25 0" rpy="1.5708 0 0"/>
    <axis xyz="0 0 1"/>
  </joint>
</robot>

Coordinate Frames (TF)

Kinetix automates the calculation of coordinate transformations. When you move a child link in the builder, it updates the <origin> tag relative to the parent's frame.

Best Practices

  • Clockwise Logic: Rotations use Radians, not Degrees.

  • Naming: Use snake_case for links and joints to follow ROS 2 conventions.

  • base_footprint: Always add a virtual link at ground level for better SLAM integration.

  • Visual vs Collision: Keep visual models detailed, but collision models simple boxes/cylinders.