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ROS 2 URDF Model

1. Introduction to URDF​

The modeling method in ROS is called URDF, which stands for Unified Robot Description Format. It is used to describe:

  • The robot model itself\
  • The robot's external environment

URDF model files use the XML format.

External reference:

2. Robot Components​

When modeling and describing a robot, we must first understand its components and parameters. A robot is generally composed of four major systems:

  • Hardware structure
  • Drive system
  • Sensor system
  • Control system

This applies to both mobile robots and robotic arms.

Component Breakdown​

  • Hardware Structure --- chassis, housing, motors\
  • Drive System --- motor drivers, power electronics\
  • Sensor System --- IMU, cameras, LiDAR, encoders\
  • Control System --- embedded computer, OS, middleware

The robot modeling process follows the same idea: each component is described using a modeling language and then assembled into a complete system.

3. URDF Syntax​

The <link> tag describes a robot's rigid body:

  • Appearance (visual)
  • Physical properties (inertia, mass)
  • Collision model

The name attribute defines the link name.

Visual Section​

  • <geometry> --- Defines shape\
  • <mesh> --- Loads STL model\
  • <origin> --- Position and orientation offset

Collision Section​

  • Used for physical interaction\
  • Often simplified to reduce computation

Difference:

  • <visual> → Appearance only\
  • <collision> → Physics calculations

3.2 Joint Description​

Rigid bodies are connected using joints.

Supported joint types:

  1. Continuous --- Infinite rotation (e.g., wheels)\
  2. Revolute --- Limited rotation (robot joints)\
  3. Prismatic --- Linear sliding\
  4. Fixed --- No movement\
  5. Floating --- Free 6-DOF motion\
  6. Planar --- Planar motion

Each joint includes:

  • <parent>
  • <child>
  • <origin>
  • <axis>
  • <limit>

4. Complete Robot Model Structure​

All <link> and <joint> elements are placed inside a <robot> root tag.

  1. Identify all links\
  2. Identify all joints\
  3. Understand full structure\
  4. Analyze parameter values

5. Creating a Robot Model​

Using a mobile robot model as an example, copy the robot description package into your workspace src directory.

Directory Structure​

  • urdf --- URDF/XACRO files\
  • meshes --- STL mesh files\
  • launch --- Launch files\
  • rviz --- RViz display configs

6. Compile the Description Package​

colcon build --packages-select hemihexcar_description

7. Model Visualization in RViz​

Refresh environment and launch:

ros2 launch hemihexcar_description display.launch.py

RViz will automatically start and display the robot model.

8. Development Workflow​

Typical URDF workflow:

  1. Define links\
  2. Define joints\
  3. Load meshes\
  4. Verify collisions\
  5. Visualize in RViz\
  6. Adjust TF tree alignment

This documentation is maintained by HemiHex for ROS 2 robot modeling and URDF development workflows.