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Chapter 3: URDF and Robot Description

Learning Objectives

By the end of this section, you will be able to:

Understand URDF (Unified Robot Description Format)
Create robot models with links and joints
Define visual, collision, and inertial properties
Convert URDF to SDF for Gazebo
Use Xacro for modular robot descriptions

Introduction

URDF (Unified Robot Description Format) is the standard XML format for describing robot kinematics and dynamics in ROS. It defines:

Links: Rigid bodies (torso, legs, arms)
Joints: Connections between links (revolute, prismatic, fixed)
Sensors: Cameras, LiDAR, IMUs
Visual/Collision: Meshes and shapes

This section teaches you how to create URDF models for humanoid robots.

URDF Basics

Minimal Robot Example

simple_robot.urdf:

<?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="blue">
        <color rgba="0 0 0.8 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.6 0.4 0.2"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="10.0"/>
      <inertia ixx="0.4" ixy="0.0" ixz="0.0"
               iyy="0.6" iyz="0.0" izz="0.8"/>
    </inertial>
  </link>
  
  <!-- Wheel link -->
  <link name="wheel_left">
    <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="0.5"/>
      <inertia ixx="0.001" ixy="0.0" ixz="0.0"
               iyy="0.001" iyz="0.0" izz="0.001"/>
    </inertial>
  </link>
  
  <!-- Joint connecting base to wheel -->
  <joint name="base_to_wheel_left" type="continuous">
    <parent link="base_link"/>
    <child link="wheel_left"/>
    <origin xyz="0 0.25 0" rpy="1.5708 0 0"/>
    <axis xyz="0 0 1"/>
  </joint>
</robot>

Links: Robot Parts

Link Components

Every link has three optional elements:

Visual (Appearance)

<visual>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <geometry>
    <box size="1 1 1"/>
    <!-- OR -->
    <cylinder radius="0.5" length="1"/>
    <!-- OR -->
    <sphere radius="0.5"/>
    <!-- OR -->
    <mesh filename="package://my_robot/meshes/part.dae" scale="1 1 1"/>
  </geometry>
  <material name="red">
    <color rgba="1 0 0 1"/>
  </material>
</visual>

Collision (Physics)

<collision>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <geometry>
    <!-- Simplified geometry for faster collision detection -->
    <box size="1 1 1"/>
  </geometry>
</collision>

Best Practice: Use simplified collision shapes (boxes, cylinders) instead of complex meshes for performance.

Inertial (Dynamics)

<inertial>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <mass value="10.0"/>
  <inertia ixx="1.0" ixy="0.0" ixz="0.0"
           iyy="1.0" iyz="0.0" izz="1.0"/>
</inertial>

Calculating Inertia (for a box):

ixx = (1/12) * mass * (height² + depth²)
iyy = (1/12) * mass * (width² + depth²)
izz = (1/12) * mass * (width² + height²)

Joints: Connecting Links

Joint Types

Type	Description	Use Case
revolute	Rotates around axis (limited range)	Knee, elbow
continuous	Rotates around axis (unlimited)	Wheels
prismatic	Slides along axis	Telescoping arm
fixed	No movement	Sensor mounts
planar	Moves in a plane	Rarely used
floating	6-DOF (position + orientation)	Base of humanoid

Revolute Joint Example

<joint name="knee_joint" type="revolute">
  <parent link="thigh"/>
  <child link="shin"/>
  <origin xyz="0 0 -0.5" rpy="0 0 0"/>
  <axis xyz="0 1 0"/>
  <limit lower="-2.0" upper="0.0" effort="100" velocity="10"/>
  <dynamics damping="0.7" friction="0.0"/>
</joint>

Parameters:

lower/upper: Joint limits (radians)
effort: Maximum torque (N·m)
velocity: Maximum speed (rad/s)
damping: Joint damping coefficient
friction: Joint friction

Humanoid Leg Example

Simple Biped Leg

<?xml version="1.0"?>
<robot name="humanoid_leg">
  <!-- Hip link -->
  <link name="hip">
    <visual>
      <geometry>
        <box size="0.2 0.3 0.1"/>
      </geometry>
      <material name="gray">
        <color rgba="0.5 0.5 0.5 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.2 0.3 0.1"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="5.0"/>
      <inertia ixx="0.05" ixy="0" ixz="0"
               iyy="0.04" iyz="0" izz="0.04"/>
    </inertial>
  </link>
  
  <!-- Thigh link -->
  <link name="thigh">
    <visual>
      <origin xyz="0 0 -0.25" rpy="0 0 0"/>
      <geometry>
        <cylinder radius="0.05" length="0.5"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 0.8 1"/>
      </material>
    </visual>
    <collision>
      <origin xyz="0 0 -0.25" rpy="0 0 0"/>
      <geometry>
        <cylinder radius="0.05" length="0.5"/>
      </geometry>
    </collision>
    <inertial>
      <origin xyz="0 0 -0.25" rpy="0 0 0"/>
      <mass value="3.0"/>
      <inertia ixx="0.06" ixy="0" ixz="0"
               iyy="0.06" iyz="0" izz="0.001"/>
    </inertial>
  </link>
  
  <!-- Hip joint -->
  <joint name="hip_joint" type="revolute">
    <parent link="hip"/>
    <child link="thigh"/>
    <origin xyz="0 0 -0.05" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-1.57" upper="1.57" effort="150" velocity="5"/>
    <dynamics damping="1.0"/>
  </joint>
  
  <!-- Shin link -->
  <link name="shin">
    <visual>
      <origin xyz="0 0 -0.25" rpy="0 0 0"/>
      <geometry>
        <cylinder radius="0.04" length="0.5"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 0.8 1"/>
      </material>
    </visual>
    <collision>
      <origin xyz="0 0 -0.25" rpy="0 0 0"/>
      <geometry>
        <cylinder radius="0.04" length="0.5"/>
      </geometry>
    </collision>
    <inertial>
      <origin xyz="0 0 -0.25" rpy="0 0 0"/>
      <mass value="2.0"/>
      <inertia ixx="0.04" ixy="0" ixz="0"
               iyy="0.04" iyz="0" izz="0.001"/>
    </inertial>
  </link>
  
  <!-- Knee joint -->
  <joint name="knee_joint" type="revolute">
    <parent link="thigh"/>
    <child link="shin"/>
    <origin xyz="0 0 -0.5" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-2.0" upper="0.0" effort="100" velocity="5"/>
    <dynamics damping="0.7"/>
  </joint>
  
  <!-- Foot link -->
  <link name="foot">
    <visual>
      <origin xyz="0.05 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="0.2 0.1 0.05"/>
      </geometry>
      <material name="black">
        <color rgba="0 0 0 1"/>
      </material>
    </visual>
    <collision>
      <origin xyz="0.05 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="0.2 0.1 0.05"/>
      </geometry>
    </collision>
    <inertial>
      <origin xyz="0.05 0 0" rpy="0 0 0"/>
      <mass value="0.5"/>
      <inertia ixx="0.001" ixy="0" ixz="0"
               iyy="0.002" iyz="0" izz="0.002"/>
    </inertial>
  </link>
  
  <!-- Ankle joint -->
  <joint name="ankle_joint" type="revolute">
    <parent link="shin"/>
    <child link="foot"/>
    <origin xyz="0 0 -0.5" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>
    <limit lower="-0.5" upper="0.5" effort="50" velocity="5"/>
    <dynamics damping="0.5"/>
  </joint>
</robot>

Xacro: Modular URDF

Xacro (XML Macros) allows you to create reusable, parameterized robot descriptions.

Why Xacro?

✅ Reduce repetition (define a leg once, use twice)
✅ Parameterization (change dimensions easily)
✅ Modularity (include files)
✅ Math expressions (calculate inertia automatically)

Xacro Example

robot.urdf.xacro:

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="my_robot">
  <!-- Parameters -->
  <xacro:property name="leg_length" value="0.5"/>
  <xacro:property name="leg_radius" value="0.05"/>
  <xacro:property name="leg_mass" value="3.0"/>
  
  <!-- Macro for a leg -->
  <xacro:macro name="leg" params="prefix reflect">
    <link name="${prefix}_thigh">
      <visual>
        <origin xyz="0 0 -${leg_length/2}" rpy="0 0 0"/>
        <geometry>
          <cylinder radius="${leg_radius}" length="${leg_length}"/>
        </geometry>
      </visual>
      <inertial>
        <mass value="${leg_mass}"/>
        <inertia ixx="${leg_mass * leg_length * leg_length / 12}"
                 iyy="${leg_mass * leg_length * leg_length / 12}"
                 izz="${leg_mass * leg_radius * leg_radius / 2}"
                 ixy="0" ixz="0" iyz="0"/>
      </inertial>
    </link>
    
    <joint name="${prefix}_hip" type="revolute">
      <parent link="base_link"/>
      <child link="${prefix}_thigh"/>
      <origin xyz="0 ${reflect * 0.15} 0" rpy="0 0 0"/>
      <axis xyz="0 1 0"/>
      <limit lower="-1.57" upper="1.57" effort="150" velocity="5"/>
    </joint>
  </xacro:macro>
  
  <!-- Base link -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.4 0.3 0.2"/>
      </geometry>
    </visual>
  </link>
  
  <!-- Instantiate legs -->
  <xacro:leg prefix="left" reflect="1"/>
  <xacro:leg prefix="right" reflect="-1"/>
</robot>

Converting Xacro to URDF

xacro robot.urdf.xacro > robot.urdf

URDF to SDF Conversion

Gazebo uses SDF, but ROS uses URDF. Conversion happens automatically, but you can do it manually:

gz sdf -p robot.urdf > robot.sdf

Key Differences

Feature	URDF	SDF
Format	ROS-specific	Gazebo-standard
Closed loops	No	Yes
Multiple robots	No	Yes
Plugins	Limited	Extensive

Visualizing URDF

In RViz

# Chapter 3: Install joint_state_publisher_gui
sudo apt install ros-humble-joint-state-publisher-gui

# Chapter 3: Launch
ros2 launch urdf_tutorial display.launch.py model:=robot.urdf

In Gazebo

gz sim robot.urdf

Key Takeaways

✅ URDF is the standard format for robot description in ROS

✅ Links define rigid bodies with visual, collision, and inertial properties

✅ Joints connect links (revolute, continuous, prismatic, fixed)

✅ Xacro enables modular, parameterized robot descriptions

✅ URDF → SDF conversion happens automatically in Gazebo

Reflection Questions

Why use simplified collision geometries instead of detailed meshes?
How would you calculate the inertia tensor for a cylinder?
When would you use a continuous joint vs. a revolute joint?
What are the advantages of using Xacro over plain URDF?

Learning Objectives​

Introduction​

URDF Basics​

Minimal Robot Example​

Links: Robot Parts​

Link Components​

Visual (Appearance)​

Collision (Physics)​

Inertial (Dynamics)​

Joints: Connecting Links​

Joint Types​

Revolute Joint Example​

Humanoid Leg Example​

Simple Biped Leg​

Xacro: Modular URDF​

Why Xacro?​

Xacro Example​

Converting Xacro to URDF​

URDF to SDF Conversion​

Key Differences​

Visualizing URDF​

In RViz​

In Gazebo​

Key Takeaways​

Reflection Questions​

Further Reading​