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Gazebo Physics Simulation and URDF for Humanoids

Unified Robot Description Format (URDF) is an XML-based format used to describe robotic systems in ROS. It defines the physical and visual properties of a robot, including links, joints, and their relationships.

What is URDF?​

URDF (Unified Robot Description Format) is the standard format for representing robot models in ROS. It describes the robot's structure, kinematics, and visual appearance using XML. A URDF file contains information about:

  • Links: Rigid parts of the robot (e.g., chassis, arms, wheels)
  • Joints: Connections between links (e.g., rotational, prismatic)
  • Visual: How the robot appears in simulation
  • Collision: How the robot interacts with the environment
  • Inertial: Physical properties for dynamics simulation

Basic URDF Structure​

A basic URDF file starts with a robot tag and contains links and joints:

<?xml version="1.0"?>
<robot name="my_robot">
  <!-- Base link -->
  <link name="base_link">
    <visual>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 0.8 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="10"/>
      <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>
    </inertial>
  </link>

  <!-- Wheel links -->
  <link name="wheel_front_left">
    <visual>
      <geometry>
        <cylinder length="0.1" radius="0.1"/>
      </geometry>
      <material name="black">
        <color rgba="0 0 0 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.1" radius="0.1"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1"/>
      <inertia ixx="0.01" ixy="0.0" ixz="0.0" iyy="0.01" iyz="0.0" izz="0.01"/>
    </inertial>
  </link>

  <!-- Joint connecting wheel to base -->
  <joint name="wheel_front_left_joint" type="continuous">
    <parent link="base_link"/>
    <child link="wheel_front_left"/>
    <origin xyz="0.3 0.3 0" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
  </joint>
</robot>

Each link in a URDF has several properties:

Visual Properties​

Defines how the link appears in visualization tools:

  • geometry: Shape (box, cylinder, sphere, mesh)
  • material: Color and texture information
  • origin: Position and orientation relative to joint

Collision Properties​

Defines how the link interacts with the physics engine:

  • geometry: Shape for collision detection
  • origin: Position and orientation relative to joint

Inertial Properties​

Defines the physical properties for dynamics simulation:

  • mass: Mass of the link
  • inertia: Inertia tensor values (ixx, ixy, ixz, iyy, iyz, izz)

Joint Types​

URDF supports several joint types:

Fixed Joint​

A joint that does not move:

<joint name="fixed_joint" type="fixed">
  <parent link="base_link"/>
  <child link="sensor_link"/>
  <origin xyz="0 0 0.1" rpy="0 0 0"/>
</joint>

Continuous Joint​

A rotating joint with unlimited range:

<joint name="continuous_joint" type="continuous">
  <parent link="base_link"/>
  <child link="rotating_part"/>
  <axis xyz="0 0 1"/>
</joint>

Revolute Joint​

A rotating joint with limited range:

<joint name="revolute_joint" type="revolute">
  <parent link="base_link"/>
  <child link="arm_link"/>
  <axis xyz="0 1 0"/>
  <limit lower="-1.57" upper="1.57" effort="10" velocity="1"/>
</joint>

Prismatic Joint​

A sliding joint:

<joint name="prismatic_joint" type="prismatic">
  <parent link="base_link"/>
  <child link="slider_link"/>
  <axis xyz="1 0 0"/>
  <limit lower="0" upper="0.5" effort="10" velocity="1"/>
</joint>

Gazebo Integration​

To use URDF models in Gazebo, additional Gazebo-specific tags can be added:

<gazebo reference="link_name">
  <material>Gazebo/Blue</material>
  <mu1>0.2</mu1>
  <mu2>0.2</mu2>
</gazebo>

Robot State Publisher​

To visualize the robot in RViz, you need to publish the robot state:

# Launch the robot state publisher
ros2 run robot_state_publisher robot_state_publisher --ros-args -p robot_description:='$(xacro my_robot.urdf.xacro)'

Best Practices​

  1. Use Xacro: Xacro is a macro language that extends URDF, allowing for variables, math, and reuse
  2. Start Simple: Begin with a basic model and add complexity gradually
  3. Validate: Use check_urdf to validate your URDF file
  4. Test in Simulation: Always test your URDF in Gazebo before physical implementation
  5. Follow Naming Conventions: Use consistent and descriptive names for links and joints

Common URDF Tools​

  • check_urdf: Validates URDF syntax
  • urdf_to_graphiz: Creates a visual representation of the kinematic tree
  • xacro: Processes Xacro files to URDF

URDF is fundamental to robotics development in ROS, providing the bridge between robot design and simulation environments like Gazebo.