URDF Introduction - EEET2610 - Mecanum Robot Guide

Overview

URDF (Unified Robot Description Format) is an XML-based format for describing a robot’s physical structure. It defines links (rigid bodies), joints (connections), and properties like mass, inertia, and visual geometry.

Why URDF? ROS2 needs to know your robot’s physical structure for:

Coordinate frame transforms (TF)
Visualization in RViz
Simulation in Gazebo
Collision detection
Motion planning

URDF Purpose in ROS2

Visualization

RViz displays your robot’s 3D model exactly as defined in URDF, showing sensor positions and orientations.

Coordinate Transforms

TF2 system uses URDF joint definitions to compute transforms between robot parts (e.g., base_link → wheel_link).

Simulation

Gazebo uses URDF for physics simulation, including masses, inertias, and collision geometries.

Motion Planning

Nav2 and MoveIt use URDF to understand robot dimensions for collision-free path planning.

URDF Concepts

Joints

Joints connect links and define motion constraints:

Fixed: No movement (e.g., sensor rigidly mounted to base)
Revolute: Rotation around axis (e.g., steering wheel)
Continuous: Unlimited rotation (e.g., drive wheels)
Prismatic: Linear sliding (e.g., elevator)

Coordinate Frames

Each link has a coordinate frame (origin + orientation):

X-axis: Forward (red in RViz)
Y-axis: Left (green in RViz)
Z-axis: Up (blue in RViz)

ROS Convention (REP-103):

X: forward
Y: left
Z: up
Rotation: right-hand rule

Basic URDF Structure

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

  <!-- Base link (root of robot tree) -->
  <link name="base_link">
    <visual>
      <!-- How it looks -->
    </visual>
    <collision>
      <!-- Collision geometry -->
    </collision>
    <inertial>
      <!-- Mass and inertia -->
    </inertial>
  </link>

  <!-- Child link (e.g., wheel) -->
  <link name="wheel_link">
    <!-- ... -->
  </link>

  <!-- Joint connecting links -->
  <joint name="base_to_wheel" type="continuous">
    <parent link="base_link"/>
    <child link="wheel_link"/>
    <origin xyz="0.1 0.15 0" rpy="0 0 0"/>
    <axis xyz="0 1 0"/>  <!-- Rotation axis -->
  </joint>

</robot>

Minimal URDF Example

Simple Box Robot

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

  <!-- Base link: a 1x1x0.5m box -->
  <link name="base_link">
    <visual>
      <geometry>
        <box size="1.0 1.0 0.5"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>  <!-- RGB + Alpha -->
      </material>
    </visual>
  </link>

</robot>

Save as: simple_box.urdf

Visualize in RViz

# Install tools
sudo apt install ros-jazzy-urdf-tutorial ros-jazzy-joint-state-publisher-gui

# Launch visualization
ros2 launch urdf_tutorial display.launch.py model:=simple_box.urdf

Result: RViz opens showing blue box.

Link Elements

Visual (Appearance)

<visual>
  <origin xyz="0 0 0" rpy="0 0 0"/>  <!-- Position relative to link frame -->
  <geometry>
    <box size="0.5 0.3 0.2"/>  <!-- Length Width Height -->
    <!-- OR -->
    <cylinder radius="0.05" length="0.1"/>
    <!-- OR -->
    <sphere radius="0.05"/>
    <!-- OR -->
    <mesh filename="package://my_robot/meshes/base.stl" scale="0.001 0.001 0.001"/>
  </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>
    <box size="0.5 0.3 0.2"/>  <!-- Simplified geometry for collision checking -->
  </geometry>
</collision>

Simplify collision geometry: Use basic shapes (box, cylinder) even if visual is complex mesh. Faster collision checking!

Inertial (Dynamics)

<inertial>
  <origin xyz="0 0 0" rpy="0 0 0"/>  <!-- Center of mass -->
  <mass value="2.5"/>  <!-- kg -->
  <inertia
    ixx="0.01" ixy="0.0" ixz="0.0"
    iyy="0.01" iyz="0.0"
    izz="0.01"/>
</inertial>

Inertia tensor for common shapes:

Box
Cylinder
Sphere

# Box: width (w), depth (d), height (h), mass (m)
ixx = (1/12) * m * (h^2 + d^2)
iyy = (1/12) * m * (w^2 + h^2)
izz = (1/12) * m * (w^2 + d^2)
ixy = iyz = ixz = 0

Joint Types

Fixed Joint

No motion allowed:

<joint name="base_to_sensor" type="fixed">
  <parent link="base_link"/>
  <child link="lidar_link"/>
  <origin xyz="0 0 0.3" rpy="0 0 0"/>
  <!-- Sensor 0.3m above base -->
</joint>

Use for: Sensors, static mounts

Continuous Joint

Unlimited rotation:

<joint name="base_to_wheel" type="continuous">
  <parent link="base_link"/>
  <child link="wheel_link"/>
  <origin xyz="0.15 0.2 0" rpy="0 0 0"/>
  <axis xyz="0 1 0"/>  <!-- Rotates around Y-axis -->
</joint>

Use for: Drive wheels, continuously rotating parts

Revolute Joint

Limited rotation:

<joint name="base_to_arm" type="revolute">
  <parent link="base_link"/>
  <child link="arm_link"/>
  <origin xyz="0 0 0.1" rpy="0 0 0"/>
  <axis xyz="0 0 1"/>
  <limit lower="-1.57" upper="1.57" effort="10" velocity="1.0"/>
  <!-- -90° to +90° -->
</joint>

Use for: Robot arms, steering joints

Prismatic Joint

Linear motion:

<joint name="base_to_slider" type="prismatic">
  <parent link="base_link"/>
  <child link="slider_link"/>
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <axis xyz="0 0 1"/>  <!-- Slides along Z -->
  <limit lower="0" upper="0.5" effort="100" velocity="0.1"/>
</joint>

Use for: Linear actuators, elevators

Origin: Position and Orientation

Position (xyz)

<origin xyz="0.15 0.2 0" rpy="0 0 0"/>
<!--        X    Y   Z -->

X: forward/backward (m)
Y: left/right (m)
Z: up/down (m)

Orientation (rpy)

Roll-Pitch-Yaw in radians:

<origin xyz="0 0 0" rpy="0 0 1.57"/>
<!--                   R  P  Y    -->

Roll (R): Rotation around X-axis
Pitch (P): Rotation around Y-axis
Yaw (Y): Rotation around Z-axis

Common angles:

90° = 1.5708 rad (π/2)
180° = 3.1416 rad (π)
270° = 4.7124 rad (3π/2)

Python helper:

import math
angle_rad = math.radians(90)  # Convert degrees to radians

URDF Best Practices

Use Xacro for Modularity

Problem: URDF is verbose and repetitive (4 wheels = lots of copy-paste).Solution: Use Xacro (XML Macros):

Define reusable macros (e.g., wheel macro)
Use variables for dimensions
Include external files

Example:

<!-- wheel_macro.xacro -->
<xacro:macro name="wheel" params="prefix reflect">
  <link name="${prefix}_wheel">
    <!-- Wheel definition -->
  </link>
</xacro:macro>

<!-- Use macro -->
<xacro:wheel prefix="front_left" reflect="1"/>
<xacro:wheel prefix="front_right" reflect="-1"/>

We’ll use Xacro extensively for Mecanum robot!

Start Simple, Add Complexity

Step 1: Base link only (box)
Step 2: Add wheels (cylinders)
Step 3: Add sensors (simple shapes)
Step 4: Replace with detailed meshes
Step 5: Add inertia and collision

Don’t start with complex meshes - hard to debug!

Test in RViz Frequently

After each change:

ros2 launch urdf_tutorial display.launch.py model:=robot.urdf

Check:

Links appear at correct positions
Joints rotate correctly (use GUI sliders)
Coordinate frames (TF) are logical

Follow Naming Conventions

Links:

base_link (root, always)
base_footprint (projection on ground)
wheel_FL, wheel_FR, wheel_RL, wheel_RR
lidar_link, imu_link, camera_link

Joints:

base_to_wheel_FL
base_to_lidar

Consistent naming helps debugging!

Validate URDF Syntax

# Check URDF syntax
check_urdf robot.urdf

# Expected output:
# robot name is: mecanum_robot
# ---------- Successfully Parsed XML ---------------

Common errors:

Unclosed tags
Invalid joint parent/child references
Duplicate link names

URDF to Robot State Publisher

Once URDF is created, robot_state_publisher node publishes TF transforms:

# Launch robot state publisher
ros2 run robot_state_publisher robot_state_publisher --ros-args -p robot_description:="$(cat robot.urdf)"

What it does:

Reads URDF from robot_description parameter
Publishes static transforms for fixed joints
Subscribes to /joint_states for movable joints
Publishes complete TF tree

TF Tree Example:

base_link
├── wheel_FL
├── wheel_FR
├── wheel_RL
├── wheel_RR
├── lidar_link
└── imu_link

Visualization Tools

RViz

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

Features:

3D visualization
TF frame axes (RGB = XYZ)
Joint state sliders (for revolute/continuous joints)

View Frames

# Generate TF tree diagram
ros2 run tf2_tools view_frames

# Output: frames.pdf (TF tree visualization)
evince frames.pdf

URDF to Graphviz

# Generate link-joint diagram
urdf_to_graphiz robot.urdf

# Output: robot.pdf
evince robot.pdf

Common URDF Mistakes

Mistake	Symptom	Fix
Missing base_link	TF errors, no root	Always have `base_link` as root
Wrong origin	Parts floating/misaligned	Double-check xyz values
Wrong axis	Joint rotates wrong way	Verify axis direction (X/Y/Z)
Missing inertia	Gazebo crashes	Add `<inertial>` to all links
Negative mass	Physics explosion	Use positive mass values
Collision = Visual	Slow simulation	Simplify collision geometry
Large mesh scale	Giant robot in RViz	Adjust `scale` in mesh

URDF Resources

File Locations (ROS2 Package)

mecanum_description/
├── urdf/
│   ├── mecanum_robot.urdf.xacro  (Main file)
│   ├── wheel.xacro               (Wheel macro)
│   └── sensors.xacro             (Sensor definitions)
├── meshes/
│   ├── base.stl
│   ├── wheel.stl
│   └── lidar.dae
├── launch/
│   └── display.launch.py
├── rviz/
│   └── config.rviz
└── package.xml

Typical Launch File

from launch import LaunchDescription
from launch_ros.actions import Node
from launch.actions import DeclareLaunchArgument
from launch.substitutions import Command, LaunchConfiguration
import os
from ament_index_python.packages import get_package_share_directory

def generate_launch_description():
    # Get URDF file path
    pkg_share = get_package_share_directory('mecanum_description')
    urdf_file = os.path.join(pkg_share, 'urdf', 'mecanum_robot.urdf.xacro')

    # Process xacro to URDF
    robot_description = Command(['xacro ', urdf_file])

    return LaunchDescription([
        # Robot state publisher
        Node(
            package='robot_state_publisher',
            executable='robot_state_publisher',
            parameters=[{'robot_description': robot_description}]
        ),

        # Joint state publisher GUI (for testing)
        Node(
            package='joint_state_publisher_gui',
            executable='joint_state_publisher_gui'
        ),

        # RViz
        Node(
            package='rviz2',
            executable='rviz2',
            arguments=['-d', os.path.join(pkg_share, 'rviz', 'config.rviz')]
        ),
    ])

Next Steps

Mecanum URDF

Create complete URDF for Mecanum wheel robot

URDF Visualization

Visualize and test URDF in RViz

Xacro Tutorial

Learn Xacro macros for modular URDF

TF2 Transforms

Understand coordinate frame transforms

References

[1] URDF Specification: http://wiki.ros.org/urdf/XML [2] URDF Tutorials: https://docs.ros.org/en/jazzy/Tutorials/Intermediate/URDF/URDF-Main.html [3] Xacro Documentation: http://wiki.ros.org/xacro

WP1: MATLAB Simulation

WP2: Prototyping

WP3: Embedded Systems

WP4: ROS2 Setup

WP4: Robot Description

WP4: Control

WP4: Localization

WP4: Mapping

WP4: Navigation

WP4: Sensors

WP5: Integration & Presentation

​Overview

​URDF Purpose in ROS2

Visualization

Coordinate Transforms

Simulation

Motion Planning

​URDF Concepts

​Links

​Joints

​Coordinate Frames

​Basic URDF Structure

​Minimal URDF Example

​Simple Box Robot

​Visualize in RViz

​Link Elements

​Visual (Appearance)

​Collision (Physics)

​Inertial (Dynamics)

​Joint Types

​Fixed Joint

​Continuous Joint

​Revolute Joint

​Prismatic Joint

​Origin: Position and Orientation

​Position (xyz)

​Orientation (rpy)

​URDF Best Practices

​URDF to Robot State Publisher

​Visualization Tools

​RViz

​View Frames

​URDF to Graphviz

​Common URDF Mistakes

​URDF Resources

​File Locations (ROS2 Package)

​Typical Launch File

​Next Steps

Mecanum URDF

URDF Visualization

Xacro Tutorial

TF2 Transforms

​References

Overview

URDF Purpose in ROS2

URDF Concepts

Links

Joints

Coordinate Frames

Basic URDF Structure

Minimal URDF Example

Simple Box Robot

Visualize in RViz

Link Elements

Visual (Appearance)

Collision (Physics)

Inertial (Dynamics)

Joint Types

Fixed Joint

Continuous Joint

Revolute Joint

Prismatic Joint

Origin: Position and Orientation

Position (xyz)

Orientation (rpy)

URDF Best Practices

URDF to Robot State Publisher

Visualization Tools

RViz

View Frames

URDF to Graphviz

Common URDF Mistakes

URDF Resources

File Locations (ROS2 Package)

Typical Launch File

Next Steps

References