URDF for Humanoids: Describing Robot Anatomy

To effectively simulate, visualize, and control humanoid robots, we need a standardized way to describe their physical characteristics. The Unified Robot Description Format (URDF) serves this purpose within the ROS ecosystem. URDF is an XML-based file format used to describe all aspects of a robot, including its kinematic and dynamic properties, visual appearance, and collision models.

What is URDF?

URDF defines a robot as a collection of links (rigid bodies) connected by joints (allowing relative motion between links). Each element within a URDF file contributes to a comprehensive model of the robot.

Key Elements of a URDF File

1. <robot> Tag: The root element, containing all other elements. It has a name attribute for the robot.

2. <link> Tag: Describes a rigid body segment of the robot.

   • <visual>: Defines the visual properties of the link, such as its geometry (e.g., box, cylinder, sphere, mesh) and material (color, texture). This is what you see in a simulator or visualization tool.
   • <collision>: Defines the collision properties of the link. This is used by physics engines to detect contact. Often a simplified version of the visual geometry to save computation.
   • <inertial>: Defines the mass, center of mass, and inertia tensor of the link. Crucial for realistic physics simulation.

3. <joint> Tag: Describes the connection between two links.

   • name: Unique identifier for the joint.
   • type: Specifies the joint's movement type (e.g., revolute for rotation, prismatic for linear motion, fixed for no motion).
   • parent / child: Specifies which link is the parent and which is the child in the kinematic chain.
   • <origin>: Defines the joint's position and orientation relative to its parent link.
   • <axis>: For revolute and prismatic joints, defines the axis of motion.
   • <limit>: For revolute and prismatic joints, defines the upper and lower position limits, velocity limits, and effort limits.
   • <calibration>, <dynamics>, <safety_controller>: Optional tags for fine-tuning joint behavior.

A Simple Humanoid Segment Example (Conceptual)

Imagine a single leg segment of a humanoid. It would have a thigh link connected to a shin link by a knee joint.

<robot name="simple_humanoid_leg">

  <link name="base_link" />

  <link name="thigh">
    <visual>
      <geometry>
        <cylinder length="0.4" radius="0.05" />
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.4" radius="0.05" />
      </geometry>
    </collision>
    <inertial>
      <mass value="2.0"/>
      <inertia ixx="0.01" ixy="0.0" ixz="0.0" iyy="0.01" iyz="0.0" izz="0.01"/>
    </inertial>
  </link>

  <link name="shin">
    <visual>
      <geometry>
        <cylinder length="0.4" radius="0.05" />
      </geometry>
      <material name="red">
        <color rgba="1 0 0 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.4" radius="0.05" />
      </geometry>
    </collision>
    <inertial>
      <mass value="1.5"/>
      <inertia ixx="0.01" ixy="0.0" ixz="0.0" iyy="0.01" iyz="0.0" izz="0.01"/>
    </inertial>
  </link>

  <joint name="hip_joint" type="revolute">
    <parent link="base_link"/>
    <child link="thigh"/>
    <origin xyz="0 0 0" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-1.57" upper="1.57" velocity="1.0" effort="100"/>
  </joint>

  <joint name="knee_joint" type="revolute">
    <parent link="thigh"/>
    <child link="shin"/>
    <origin xyz="0 0 -0.2" rpy="0 0 0"/> <!-- Position relative to thigh end -->
    <axis xyz="0 1 0"/>
    <limit lower="-1.57" upper="0" velocity="1.0" effort="100"/>
  </joint>

</robot>

Note: This is a simplified example. Real humanoid URDFs are much larger and often include meshes (for visuals) and more complex joint setups.

Xacro: Extending URDF

Writing large URDF files can be tedious and repetitive. Xacro (XML Macros) is a preprocessor for XML files (like URDF) that allows for more concise and readable descriptions. It enables:

• Macros: Define reusable blocks of XML.
• Properties: Define variables to avoid hardcoding values.
• Mathematical Expressions: Perform calculations within the file.
• Conditional Inclusion: Include or exclude parts of the description based on conditions.

Most complex humanoid robot descriptions use Xacro to generate the final URDF.

Why URDF is Essential for Humanoids

• Simulation: Physics engines (like Gazebo) use the inertial, collision, and joint properties to accurately simulate robot behavior.
• Visualization: Tools like RViz use the visual properties to render a 3D model of the robot.
• Kinematics/Dynamics: Software libraries use the link and joint descriptions to perform forward and inverse kinematics, and calculate dynamics.
• Hardware Abstraction: Provides a common interface for control algorithms, regardless of the underlying physical robot.

Understanding URDF is a foundational skill for anyone working with humanoid robots in simulation or real-world applications. In the next hands-on exercise, you will have the opportunity to control a simulated robot joint, applying some of the ROS 2 communication concepts with a robot described by URDF.