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Understanding Basic Humanoid URDF

This chapter introduces the Unified Robot Description Format (URDF), a crucial XML format used in ROS 2 to describe all aspects of a robot model, including its visual appearance, collision properties, and physical dynamics. We'll focus on understanding the basic structure of a humanoid robot's URDF.

What is URDF?​

URDF (Unified Robot Description Format) is an XML file format that allows you to describe a robot's kinematic and dynamic properties, visual appearance, and collision models. It's essential for simulating robots in tools like Gazebo, visualizing them in RViz2, and performing motion planning.

A URDF file defines:

  • Links: These represent the rigid bodies of the robot (e.g., torso, head, upper arm).
  • Joints: These define the connections between links, specifying their type (e.g., fixed, revolute, prismatic) and degrees of freedom.

Basic Humanoid URDF Structure​

A simplified humanoid URDF typically includes a base link (e.g., base_link or torso), connected to other links (head, arms, legs) via joints.

Here’s a conceptual overview of a simple humanoid structure in URDF:

<robot name="simple_humanoid">

  <!-- Base Link -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.2 0.1 0.3"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 0.8 1"/>
      </material>
    </visual>
  </link>

  <!-- Head Link connected by a fixed joint to base_link -->
  <link name="head_link">
    <visual>
      <geometry>
        <sphere radius="0.1"/>
      </geometry>
      <material name="red">
        <color rgba="0.8 0 0 1"/>
      </material>
    </visual>
  </link>

  <joint name="base_to_head_joint" type="fixed">
    <parent link="base_link"/>
    <child link="head_link"/>
    <origin xyz="0 0 0.2" rpy="0 0 0"/>
  </joint>

  <!-- Right Arm (simplified) -->
  <link name="right_arm_link">
    <visual>
      <geometry>
        <box size="0.05 0.05 0.2"/>
      </geometry>
      <material name="green">
        <color rgba="0 0.8 0 1"/>
      </material>
    </visual>
  </link>

  <joint name="base_to_right_arm_joint" type="revolute">
    <parent link="base_link"/>
    <child link="right_arm_link"/>
    <origin xyz="0 -0.1 0.1" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-1.57" upper="1.57" effort="100" velocity="100"/>
  </joint>

  <!-- ... (similar structure for left arm, legs, etc.) ... -->

</robot>

This XML snippet defines two links (base_link and head_link) and a fixed joint connecting them. It also shows a revolute joint for a right arm. Each link has a visual element to represent its shape and color.

Visualizing URDF in RViz2​

RViz2 is a 3D visualization tool for ROS 2. It can display a robot model defined by a URDF file, along with sensor data and other debugging information.

To visualize a URDF, you typically need:

  1. A URDF file.
  2. A ROS 2 launch file that loads the URDF and starts the robot_state_publisher node (to publish the robot's joint states) and RViz2.

Assignment​

  1. Extend the simple_humanoid.urdf file to include a left arm with a similar structure to the right arm.
  2. Modify the joint type for one of the arms from revolute to prismatic and observe how the visualization changes in RViz2 (once you set up the launch file).
  3. Research and add a collision element to one of the links in your URDF.

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