URDF and SDF Modeling

Robot modeling is fundamental to simulation and real-world deployment. This guide covers creating accurate robot descriptions using URDF (Unified Robot Description Format) and SDF (Simulation Description Format).

URDF Overview

URDF is an XML format for describing robot kinematics, dynamics, and visual properties. It's the standard for ROS robot descriptions.

URDF Structure

Basic URDF Syntax

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

  <!-- Base link -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.5 0.3 0.2"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 0.8 1"/>
      </material>
    </visual>

    <collision>
      <geometry>
        <box size="0.5 0.3 0.2"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="10.0"/>
      <inertia ixx="0.0875" ixy="0.0" ixz="0.0"
               iyy="0.2292" iyz="0.0"
               izz="0.2917"/>
    </inertial>
  </link>

  <!-- Wheel link -->
  <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>

    <collision>
      <geometry>
        <cylinder radius="0.1" length="0.05"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="1.0"/>
      <inertia ixx="0.0013" ixy="0.0" ixz="0.0"
               iyy="0.0013" iyz="0.0"
               izz="0.0025"/>
    </inertial>
  </link>

  <!-- Joint connecting base to wheel -->
  <joint name="left_wheel_joint" type="continuous">
    <parent link="base_link"/>
    <child link="left_wheel"/>
    <origin xyz="0 0.2 0" rpy="-1.5707 0 0"/>
    <axis xyz="0 0 1"/>
  </joint>

</robot>

Link Elements

Visual: Appearance in simulation/visualization

<visual>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <geometry>
    <mesh filename="package://my_robot/meshes/body.dae" scale="1 1 1"/>
  </geometry>
  <material name="gray">
    <color rgba="0.5 0.5 0.5 1"/>
  </material>
</visual>

Collision: Simplified geometry for collision detection

<collision>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <geometry>
    <box size="0.5 0.3 0.2"/>
  </geometry>
</collision>

Inertial: Mass and inertia properties

<inertial>
  <origin xyz="0 0 0.05" rpy="0 0 0"/>
  <mass value="5.0"/>
  <inertia ixx="0.0347" ixy="0" ixz="0"
           iyy="0.0505" iyz="0"
           izz="0.0347"/>
</inertial>

Joint Types

Type	Description	Use Case
revolute	Rotates around axis with limits	Robot arm joints
continuous	Rotates around axis without limits	Wheels
prismatic	Slides along axis with limits	Linear actuators
fixed	No movement	Mounting sensors
floating	6 DOF	Free-flying robots
planar	Moves in plane	Mobile bases

SDF Format

SDF is Gazebo's native format, offering more features than URDF.

URDF vs SDF

Feature	URDF	SDF
Multiple models	No	Yes
Closed kinematic chains	No	Yes
Joint friction	Limited	Comprehensive
Plugin system	Basic	Advanced
World description	No	Yes
Physics engines	One	Multiple

SDF Model Example

<?xml version="1.0" ?>
<sdf version="1.8">
  <model name="differential_drive_robot">
    <pose>0 0 0.15 0 0 0</pose>

    <!-- Base link -->
    <link name="chassis">
      <inertial>
        <mass>15.0</mass>
        <inertia>
          <ixx>0.1875</ixx>
          <iyy>0.4583</iyy>
          <izz>0.5208</izz>
        </inertia>
      </inertial>

      <collision name="collision">
        <geometry>
          <box>
            <size>0.6 0.4 0.2</size>
          </box>
        </geometry>
      </collision>

      <visual name="visual">
        <geometry>
          <box>
            <size>0.6 0.4 0.2</size>
          </box>
        </geometry>
        <material>
          <ambient>0.2 0.2 0.8 1</ambient>
          <diffuse>0.2 0.2 0.8 1</diffuse>
          <specular>0.1 0.1 0.1 1</specular>
        </material>
      </visual>
    </link>

    <!-- Left wheel -->
    <link name="left_wheel">
      <pose relative_to="chassis">0 0.25 0 -1.5707 0 0</pose>
      <inertial>
        <mass>1.0</mass>
        <inertia>
          <ixx>0.0013</ixx>
          <iyy>0.0025</iyy>
          <izz>0.0013</izz>
        </inertia>
      </inertial>

      <collision name="collision">
        <geometry>
          <cylinder>
            <radius>0.15</radius>
            <length>0.05</length>
          </cylinder>
        </geometry>
        <surface>
          <friction>
            <ode>
              <mu>1.0</mu>
              <mu2>1.0</mu2>
            </ode>
          </friction>
        </surface>
      </collision>

      <visual name="visual">
        <geometry>
          <cylinder>
            <radius>0.15</radius>
            <length>0.05</length>
          </cylinder>
        </geometry>
        <material>
          <ambient>0.1 0.1 0.1 1</ambient>
          <diffuse>0.1 0.1 0.1 1</diffuse>
        </material>
      </visual>
    </link>

    <!-- Joint -->
    <joint name="left_wheel_joint" type="revolute">
      <pose relative_to="left_wheel"/>
      <parent>chassis</parent>
      <child>left_wheel</child>
      <axis>
        <xyz>0 0 1</xyz>
        <limit>
          <lower>-1.79769e+308</lower>
          <upper>1.79769e+308</upper>
        </limit>
      </axis>
    </joint>

    <!-- Repeat for right wheel -->
    <link name="right_wheel">
      <pose relative_to="chassis">0 -0.25 0 -1.5707 0 0</pose>
      <inertial>
        <mass>1.0</mass>
        <inertia>
          <ixx>0.0013</ixx>
          <iyy>0.0025</iyy>
          <izz>0.0013</izz>
        </inertia>
      </inertial>

      <collision name="collision">
        <geometry>
          <cylinder>
            <radius>0.15</radius>
            <length>0.05</length>
          </cylinder>
        </geometry>
        <surface>
          <friction>
            <ode>
              <mu>1.0</mu>
              <mu2>1.0</mu2>
            </ode>
          </friction>
        </surface>
      </collision>

      <visual name="visual">
        <geometry>
          <cylinder>
            <radius>0.15</radius>
            <length>0.05</length>
          </cylinder>
        </geometry>
        <material>
          <ambient>0.1 0.1 0.1 1</ambient>
          <diffuse>0.1 0.1 0.1 1</diffuse>
        </material>
      </visual>
    </link>

    <joint name="right_wheel_joint" type="revolute">
      <pose relative_to="right_wheel"/>
      <parent>chassis</parent>
      <child>right_wheel</child>
      <axis>
        <xyz>0 0 1</xyz>
        <limit>
          <lower>-1.79769e+308</lower>
          <upper>1.79769e+308</upper>
        </limit>
      </axis>
    </joint>

    <!-- Caster wheel for stability -->
    <link name="caster">
      <pose relative_to="chassis">-0.25 0 -0.1 0 0 0</pose>
      <inertial>
        <mass>0.5</mass>
        <inertia>
          <ixx>0.0001</ixx>
          <iyy>0.0001</iyy>
          <izz>0.0001</izz>
        </inertia>
      </inertial>

      <collision name="collision">
        <geometry>
          <sphere>
            <radius>0.05</radius>
          </sphere>
        </geometry>
        <surface>
          <friction>
            <ode>
              <mu>0.1</mu>
              <mu2>0.1</mu2>
            </ode>
          </friction>
        </surface>
      </collision>

      <visual name="visual">
        <geometry>
          <sphere>
            <radius>0.05</radius>
          </sphere>
        </geometry>
      </visual>
    </link>

    <joint name="caster_joint" type="ball">
      <parent>chassis</parent>
      <child>caster</child>
    </joint>

    <!-- Differential drive plugin -->
    <plugin filename="gz-sim-diff-drive-system"
            name="gz::sim::systems::DiffDrive">
      <left_joint>left_wheel_joint</left_joint>
      <right_joint>right_wheel_joint</right_joint>
      <wheel_separation>0.5</wheel_separation>
      <wheel_radius>0.15</wheel_radius>
      <odom_publish_frequency>50</odom_publish_frequency>
      <topic>cmd_vel</topic>
    </plugin>
  </model>
</sdf>

Adding Sensors

Camera Sensor

<!-- Add to a link in URDF -->
<link name="camera_link">
  <visual>
    <geometry>
      <box size="0.05 0.05 0.05"/>
    </geometry>
  </visual>

  <collision>
    <geometry>
      <box size="0.05 0.05 0.05"/>
    </geometry>
  </collision>

  <inertial>
    <mass value="0.1"/>
    <inertia ixx="0.00002" ixy="0" ixz="0"
             iyy="0.00002" iyz="0"
             izz="0.00002"/>
  </inertial>
</link>

<!-- Gazebo-specific camera configuration -->
<gazebo reference="camera_link">
  <sensor name="camera" type="camera">
    <camera>
      <horizontal_fov>1.047</horizontal_fov>
      <image>
        <width>640</width>
        <height>480</height>
      </image>
      <clip>
        <near>0.1</near>
        <far>100</far>
      </clip>
    </camera>
    <always_on>1</always_on>
    <update_rate>30</update_rate>
    <visualize>true</visualize>
    <topic>camera/image_raw</topic>
  </sensor>
</gazebo>

LiDAR Sensor (SDF)

<link name="lidar_link">
  <pose relative_to="chassis">0.2 0 0.15 0 0 0</pose>

  <inertial>
    <mass>0.5</mass>
    <inertia>
      <ixx>0.0001</ixx>
      <iyy>0.0001</iyy>
      <izz>0.0001</izz>
    </inertia>
  </inertial>

  <collision name="collision">
    <geometry>
      <cylinder>
        <radius>0.04</radius>
        <length>0.06</length>
      </cylinder>
    </geometry>
  </collision>

  <visual name="visual">
    <geometry>
      <cylinder>
        <radius>0.04</radius>
        <length>0.06</length>
      </cylinder>
    </geometry>
    <material>
      <ambient>0.1 0.1 0.1 1</ambient>
      <diffuse>0.1 0.1 0.1 1</diffuse>
    </material>
  </visual>

  <sensor name="lidar" type="gpu_lidar">
    <topic>scan</topic>
    <update_rate>10</update_rate>
    <lidar>
      <scan>
        <horizontal>
          <samples>360</samples>
          <resolution>1</resolution>
          <min_angle>-3.14159</min_angle>
          <max_angle>3.14159</max_angle>
        </horizontal>
      </scan>
      <range>
        <min>0.1</min>
        <max>30.0</max>
        <resolution>0.01</resolution>
      </range>
      <noise>
        <type>gaussian</type>
        <mean>0.0</mean>
        <stddev>0.01</stddev>
      </noise>
    </lidar>
    <visualize>true</visualize>
  </sensor>
</link>

<joint name="lidar_joint" type="fixed">
  <parent>chassis</parent>
  <child>lidar_link</child>
</joint>

IMU Sensor

<link name="imu_link">
  <pose relative_to="chassis">0 0 0.05 0 0 0</pose>

  <inertial>
    <mass>0.01</mass>
    <inertia>
      <ixx>0.000001</ixx>
      <iyy>0.000001</iyy>
      <izz>0.000001</izz>
    </inertia>
  </inertial>

  <sensor name="imu" type="imu">
    <topic>imu</topic>
    <update_rate>100</update_rate>
    <imu>
      <angular_velocity>
        <x>
          <noise type="gaussian">
            <mean>0.0</mean>
            <stddev>0.009</stddev>
          </noise>
        </x>
        <y>
          <noise type="gaussian">
            <mean>0.0</mean>
            <stddev>0.009</stddev>
          </noise>
        </y>
        <z>
          <noise type="gaussian">
            <mean>0.0</mean>
            <stddev>0.009</stddev>
          </noise>
        </z>
      </angular_velocity>
      <linear_acceleration>
        <x>
          <noise type="gaussian">
            <mean>0.0</mean>
            <stddev>0.017</stddev>
          </noise>
        </x>
        <y>
          <noise type="gaussian">
            <mean>0.0</mean>
            <stddev>0.017</stddev>
          </noise>
        </y>
        <z>
          <noise type="gaussian">
            <mean>0.0</mean>
            <stddev>0.017</stddev>
          </noise>
        </z>
      </linear_acceleration>
    </imu>
  </sensor>
</link>

<joint name="imu_joint" type="fixed">
  <parent>chassis</parent>
  <child>imu_link</child>
</joint>

Depth Camera

<link name="depth_camera_link">
  <pose relative_to="chassis">0.25 0 0.1 0 0 0</pose>

  <inertial>
    <mass>0.2</mass>
    <inertia>
      <ixx>0.00004</ixx>
      <iyy>0.00004</iyy>
      <izz>0.00004</izz>
    </inertia>
  </inertial>

  <visual name="visual">
    <geometry>
      <box>
        <size>0.02 0.09 0.025</size>
      </box>
    </geometry>
  </visual>

  <sensor name="depth_camera" type="depth_camera">
    <topic>depth_camera</topic>
    <update_rate>30</update_rate>
    <camera>
      <horizontal_fov>1.57</horizontal_fov>
      <image>
        <width>640</width>
        <height>480</height>
        <format>R_FLOAT32</format>
      </image>
      <clip>
        <near>0.1</near>
        <far>10.0</far>
      </clip>
    </camera>
  </sensor>
</link>

<joint name="depth_camera_joint" type="fixed">
  <parent>chassis</parent>
  <child>depth_camera_link</child>
</joint>

Xacro for Modular URDF

Xacro (XML Macros) enables parameterized, reusable URDF components.

Basic Xacro Syntax

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="modular_robot">

  <!-- Parameters -->
  <xacro:property name="wheel_radius" value="0.15"/>
  <xacro:property name="wheel_width" value="0.05"/>
  <xacro:property name="wheel_mass" value="1.0"/>
  <xacro:property name="chassis_length" value="0.6"/>
  <xacro:property name="chassis_width" value="0.4"/>
  <xacro:property name="chassis_height" value="0.2"/>

  <!-- Macros -->
  <xacro:macro name="wheel" params="prefix reflect">
    <link name="${prefix}_wheel">
      <visual>
        <geometry>
          <cylinder radius="${wheel_radius}" length="${wheel_width}"/>
        </geometry>
        <material name="black">
          <color rgba="0 0 0 1"/>
        </material>
      </visual>

      <collision>
        <geometry>
          <cylinder radius="${wheel_radius}" length="${wheel_width}"/>
        </geometry>
      </collision>

      <inertial>
        <mass value="${wheel_mass}"/>
        <inertia ixx="${wheel_mass * (3 * wheel_radius * wheel_radius + wheel_width * wheel_width) / 12}"
                 iyy="${wheel_mass * wheel_radius * wheel_radius / 2}"
                 izz="${wheel_mass * (3 * wheel_radius * wheel_radius + wheel_width * wheel_width) / 12}"
                 ixy="0" ixz="0" iyz="0"/>
      </inertial>
    </link>

    <joint name="${prefix}_wheel_joint" type="continuous">
      <parent link="base_link"/>
      <child link="${prefix}_wheel"/>
      <origin xyz="0 ${reflect * chassis_width/2} 0" rpy="${-pi/2} 0 0"/>
      <axis xyz="0 0 1"/>
    </joint>
  </xacro:macro>

  <!-- Base link -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="${chassis_length} ${chassis_width} ${chassis_height}"/>
      </geometry>
      <material name="blue">
        <color rgba="0.2 0.2 0.8 1"/>
      </material>
    </visual>

    <collision>
      <geometry>
        <box size="${chassis_length} ${chassis_width} ${chassis_height}"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="15.0"/>
      <inertia ixx="${15.0 * (chassis_width * chassis_width + chassis_height * chassis_height) / 12}"
               iyy="${15.0 * (chassis_length * chassis_length + chassis_height * chassis_height) / 12}"
               izz="${15.0 * (chassis_length * chassis_length + chassis_width * chassis_width) / 12}"
               ixy="0" ixz="0" iyz="0"/>
    </inertial>
  </link>

  <!-- Instantiate wheels -->
  <xacro:wheel prefix="left" reflect="1"/>
  <xacro:wheel prefix="right" reflect="-1"/>

  <!-- Include sensor package -->
  <xacro:include filename="$(find my_robot_description)/urdf/sensors.xacro"/>

  <!-- Instantiate sensors -->
  <xacro:lidar_sensor parent="base_link"/>
  <xacro:camera_sensor parent="base_link" xyz="0.25 0 0.15"/>

</robot>

Sensor Macro Library

Create sensors.xacro:

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro">

  <!-- LiDAR macro -->
  <xacro:macro name="lidar_sensor" params="parent">
    <link name="lidar_link">
      <visual>
        <geometry>
          <cylinder radius="0.04" length="0.06"/>
        </geometry>
        <material name="black"/>
      </visual>

      <collision>
        <geometry>
          <cylinder radius="0.04" length="0.06"/>
        </geometry>
      </collision>

      <inertial>
        <mass value="0.5"/>
        <inertia ixx="0.0001" iyy="0.0001" izz="0.0001"
                 ixy="0" ixz="0" iyz="0"/>
      </inertial>
    </link>

    <joint name="lidar_joint" type="fixed">
      <parent link="${parent}"/>
      <child link="lidar_link"/>
      <origin xyz="0.2 0 0.15" rpy="0 0 0"/>
    </joint>

    <gazebo reference="lidar_link">
      <sensor name="lidar" type="gpu_lidar">
        <topic>scan</topic>
        <update_rate>10</update_rate>
        <lidar>
          <scan>
            <horizontal>
              <samples>360</samples>
              <resolution>1</resolution>
              <min_angle>-3.14159</min_angle>
              <max_angle>3.14159</max_angle>
            </horizontal>
          </scan>
          <range>
            <min>0.1</min>
            <max>30.0</max>
          </range>
        </lidar>
      </sensor>
    </gazebo>
  </xacro:macro>

  <!-- Camera macro -->
  <xacro:macro name="camera_sensor" params="parent xyz">
    <link name="camera_link">
      <visual>
        <geometry>
          <box size="0.05 0.05 0.05"/>
        </geometry>
      </visual>

      <inertial>
        <mass value="0.1"/>
        <inertia ixx="0.00002" iyy="0.00002" izz="0.00002"
                 ixy="0" ixz="0" iyz="0"/>
      </inertial>
    </link>

    <joint name="camera_joint" type="fixed">
      <parent link="${parent}"/>
      <child link="camera_link"/>
      <origin xyz="${xyz}" rpy="0 0 0"/>
    </joint>

    <gazebo reference="camera_link">
      <sensor name="camera" type="camera">
        <camera>
          <horizontal_fov>1.047</horizontal_fov>
          <image>
            <width>640</width>
            <height>480</height>
          </image>
          <clip>
            <near>0.1</near>
            <far>100</far>
          </clip>
        </camera>
        <update_rate>30</update_rate>
        <topic>camera/image_raw</topic>
      </sensor>
    </gazebo>
  </xacro:macro>

</robot>

Processing Xacro Files

# Convert xacro to URDF
xacro robot.xacro > robot.urdf

# With arguments
xacro robot.xacro use_lidar:=true wheel_radius:=0.2 > robot.urdf

# Check URDF validity
check_urdf robot.urdf

# Visualize robot
urdf_to_graphviz robot.urdf

Best Practices

1. Coordinate Frames

Follow ROS REP 105 conventions:

• X: Forward
• Y: Left
• Z: Up

2. Inertia Calculation

Use accurate inertia tensors:

# Box inertia
def box_inertia(mass, x, y, z):
    ixx = mass * (y*y + z*z) / 12
    iyy = mass * (x*x + z*z) / 12
    izz = mass * (x*x + y*y) / 12
    return ixx, iyy, izz

# Cylinder inertia (along z-axis)
def cylinder_inertia(mass, radius, length):
    ixx = mass * (3*radius*radius + length*length) / 12
    iyy = ixx
    izz = mass * radius * radius / 2
    return ixx, iyy, izz

# Sphere inertia
def sphere_inertia(mass, radius):
    i = 2 * mass * radius * radius / 5
    return i, i, i

3. Collision Geometry

• Keep collision geometry simple (boxes, cylinders, spheres)
• Use convex hulls for complex shapes
• Avoid self-collisions

4. Naming Conventions

robot_name/
  base_link
  left_wheel_link
  right_wheel_link
  sensor_link

5. File Organization

my_robot_description/
  urdf/
    robot.urdf.xacro        # Main robot file
    robot_base.xacro        # Base platform
    robot_wheels.xacro      # Wheel components
    sensors.xacro           # Sensor macros
  meshes/
    base.dae
    wheel.stl
  config/
    controllers.yaml
  launch/
    display.launch.py

Key Takeaways

• URDF is ROS-standard for robot description, SDF is Gazebo-native
• Accurate inertia and collision geometry are crucial for realistic simulation
• Xacro enables modular, maintainable robot descriptions
• Sensor integration requires both URDF links and Gazebo plugins
• Follow naming conventions and coordinate frame standards
• Use macros for repeated components (wheels, sensors)

Additional Resources

• URDF Tutorials
• SDF Specification
• Xacro Documentation
• REP 105: Coordinate Frames
• URDF Validator

Hands-On Exercises

Exercise 1: Complete Mobile Robot

Create a differential drive robot with:

• Rectangular chassis (0.6m x 0.4m x 0.2m)
• Two driven wheels (0.15m radius)
• One caster wheel
• LiDAR on top
• Front-facing camera
• IMU in center

Deliverables:

• Complete URDF/SDF file
• Launch file for visualization
• Gazebo world file
• RViz configuration

Exercise 2: Sensor Integration

Add multiple sensors to your robot:

• 2D LiDAR (360-degree, 10Hz)
• RGB camera (640x480, 30Hz)
• Depth camera (640x480, 15Hz)
• IMU (100Hz)
• GPS (1Hz)

Test each sensor in Gazebo and verify ROS topics.

Exercise 3: Xacro Modular Design

Refactor Exercise 1 using Xacro:

• Parameterize all dimensions
• Create macros for wheels
• Create macros for sensors
• Support configuration variants (2-wheel vs 4-wheel)
• Add arguments for sensor selection

Goal: Generate 3 different robot variants from same Xacro files.

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