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Lesson 1.3: Robot Modeling with URDF

Section 1: What is URDF?​

URDF (Unified Robot Description Format) is an XML format used in ROS to describe all the physical elements of a robot. It's like a blueprint that defines the robot's structure, including its links, joints, sensors, and visual appearance.

A URDF file typically contains:

  • <robot>: The root element of the URDF file.
  • <link>: Describes a rigid body part of the robot (e.g., a wheel, a gripper, the robot's base).
  • <joint>: Defines the relationship between two links, including how they are connected and their range of motion.
  • <visual>: Specifies how the link should look in simulations and visualizations (e.g., its shape, color, and texture).
  • <collision>: Defines the collision geometry of the link, which is used for physics simulations.
  • <inertial>: Describes the inertial properties of the link (mass, center of mass, inertia tensor).

Section 2: Anatomy of a URDF​

A link represents a single rigid body of the robot. Each link has a name and can have visual, collision, and inertial properties.

Joints​

A joint connects two links and defines their relative motion. There are several types of joints:

  • revolute: A hinge joint that rotates around a single axis (e.g., a wheel axle).
  • continuous: Similar to a revolute joint, but with no rotation limits.
  • prismatic: A sliding joint that moves along a single axis (e.g., a piston).
  • fixed: A rigid connection between two links that doesn't allow any motion.
  • floating: Allows for motion in all 6 degrees of freedom.
  • planar: Allows for motion in a 2D plane.

Each joint connects a parent link to a child link and has an origin that specifies the transform (position and orientation) of the child link relative to the parent.

Section 3: XML Code (Simple Box Robot)​

Here is a simple URDF example for a robot that is just a single box.

<?xml version="1.0"?>
<robot name="simple_box">

  <!-- The base link of the robot -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.5 0.5 0.2"/>
      </geometry>
      <material name="blue">
        <color rgba="0.0 0.0 1.0 1.0"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.5 0.5 0.2"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1.0"/>
      <inertia ixx="0.04" ixy="0.0" ixz="0.0" iyy="0.04" iyz="0.0" izz="0.02"/>
    </inertial>
  </link>

</robot>

How to View this URDF:​

  1. Save the XML code above in a file named simple_box.urdf.
  2. Install the joint_state_publisher and robot_state_publisher packages: 
    sudo apt-get install ros-<distro>-joint-state-publisher ros-<distro>-robot-state-publisher
  3. Create a ROS 2 launch file to visualize the robot in RViz2.
  4. Run the launch file: 
    ros2 launch <your_package_name> view_robot.launch.py

This will open RViz2 and display the simple blue box robot.