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URDF Robot Modeling

Introduction​

URDF (Unified Robot Description Format) is an XML format for describing robot geometry, kinematics, dynamics, and visual properties for simulation and visualization.

Learning Objectives:

  • Understand URDF structure (links, joints, sensors)
  • Create a simple robot model from scratch
  • Visualize robots in RViz
  • Add sensors and collision geometry

What is URDF?​

URDF describes:

  • Links: Robot body parts (chassis, wheels, arms)
  • Joints: Connections between links (revolute, prismatic, fixed)
  • Visual: How the robot looks (meshes, colors)
  • Collision: Simplified geometry for physics
  • Inertial: Mass, center of mass, inertia tensor

Basic URDF Structure​

<?xml version="1.0"?>
<robot name="my_robot">
  <!-- Links -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.5 0.3 0.1"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
  </link>

  <!-- Joints -->
  <joint name="base_to_wheel" type="continuous">
    <parent link="base_link"/>
    <child link="wheel_link"/>
    <origin xyz="0 0 -0.1" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
  </joint>

  <link name="wheel_link">
    <visual>
      <geometry>
        <cylinder radius="0.05" length="0.02"/>
      </geometry>
    </visual>
  </link>
</robot>

Joint Types​

TypeDescriptionExample
fixedNo movementCamera mount
revoluteRotation with limitsRobot elbow
continuousUnlimited rotationWheel
prismaticLinear slidingElevator
floating6 DOF free movementQuadrotor

Simple Mobile Robot Example​

<?xml version="1.0"?>
<robot name="simple_robot">
  <!-- Base Link -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.6 0.4 0.2"/>
      </geometry>
      <material name="gray">
        <color rgba="0.5 0.5 0.5 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.6 0.4 0.2"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="10"/>
      <inertia ixx="0.1" ixy="0" ixz="0" iyy="0.1" iyz="0" izz="0.1"/>
    </inertial>
  </link>

  <!-- Left Wheel -->
  <joint name="left_wheel_joint" type="continuous">
    <parent link="base_link"/>
    <child link="left_wheel"/>
    <origin xyz="0 0.25 0" rpy="-1.57 0 0"/>
    <axis xyz="0 0 1"/>
  </joint>

  <link name="left_wheel">
    <visual>
      <geometry>
        <cylinder radius="0.1" length="0.05"/>
      </geometry>
      <material name="black">
        <color rgba="0 0 0 1"/>
      </material>
    </visual>
  </link>

  <!-- Right Wheel (similar structure) -->
  <!-- ... -->
</robot>

Visualizing in RViz​

# Install joint_state_publisher_gui
sudo apt install ros-humble-joint-state-publisher-gui

# Launch RViz with URDF
ros2 launch urdf_tutorial display.launch.py model:=my_robot.urdf

Adding Sensors​

Camera​

<link name="camera_link">
  <visual>
    <geometry>
      <box size="0.05 0.05 0.05"/>
    </geometry>
  </visual>
</link>

<joint name="camera_joint" type="fixed">
  <parent link="base_link"/>
  <child link="camera_link"/>
  <origin xyz="0.3 0 0.1" rpy="0 0 0"/>
</joint>

LiDAR​

<link name="lidar_link">
  <visual>
    <geometry>
      <cylinder radius="0.05" length="0.07"/>
    </geometry>
  </visual>
</link>

<joint name="lidar_joint" type="fixed">
  <parent link="base_link"/>
  <child link="lidar_link"/>
  <origin xyz="0 0 0.15" rpy="0 0 0"/>
</joint>

URDF Best Practices​

  1. Use descriptive names: left_wheel_joint not joint_1
  2. Add collision geometry: Simplified shapes for physics
  3. Include inertial properties: Required for Gazebo simulation
  4. Check with check_urdf: Validates XML structure
# Validate URDF
check_urdf my_robot.urdf

# Convert to graphical tree
urdf_to_graphiz my_robot.urdf

Xacro: Programmable URDF​

Xacro adds macros and variables to reduce repetition:

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="robot">
  <!-- Define properties -->
  <xacro:property name="wheel_radius" value="0.1"/>
  <xacro:property name="wheel_width" value="0.05"/>

  <!-- Wheel macro -->
  <xacro:macro name="wheel" params="prefix reflect">
    <link name="${prefix}_wheel">
      <visual>
        <geometry>
          <cylinder radius="${wheel_radius}" length="${wheel_width}"/>
        </geometry>
      </visual>
    </link>
    <joint name="${prefix}_wheel_joint" type="continuous">
      <parent link="base_link"/>
      <child link="${prefix}_wheel"/>
      <origin xyz="0 ${reflect * 0.25} 0" rpy="-1.57 0 0"/>
      <axis xyz="0 0 1"/>
    </joint>
  </xacro:macro>

  <!-- Use macro -->
  <xacro:wheel prefix="left" reflect="1"/>
  <xacro:wheel prefix="right" reflect="-1"/>
</robot>

Convert Xacro to URDF:

xacro robot.urdf.xacro > robot.urdf

Exercises​

  1. Create a simple robot with a base and two wheels
  2. Add a camera sensor to your robot
  3. Visualize your robot in RViz
  4. Convert your URDF to Xacro with macros for repeated parts

Summary​

URDF describes robot structure using links (parts) and joints (connections). RViz visualizes URDF models, and Xacro simplifies complex robot descriptions with macros.

Further Reading​