Chapter 3: Building with ROS 2

Duration: Weeks 4-5 Hardware Tier: Tier 2 Lessons: 4

Chapter Overview

You've learned the concepts - now it's time to build. In this chapter, you'll create complete ROS 2 packages from scratch, learn to bridge Python AI agents with robot controllers, and describe humanoid robots using URDF.

By the end, you'll have a fully functional ROS 2 package ready for simulation.

Learning Objectives

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

• Create ROS 2 packages with proper structure and dependencies
• Use rclpy to bridge Python code with ROS 2 controllers
• Write launch files to start multiple nodes with configuration
• Design URDF models that describe humanoid robot kinematics
• Build and test complete ROS 2 applications

Prerequisites

• Chapter 2: ROS 2 Architecture (completed)
• Python intermediate (classes, decorators, async basics)
• ROS 2 Humble installed (Tier 2) or cloud environment (Tier 1)

Lessons

#	Lesson	Duration	Tier	Description
3.1	Building ROS 2 Packages	75 min	2	Package structure, setup.py, dependencies
3.2	Bridging Python Agents with rclpy	75 min	2	Connecting AI/ML code to ROS 2
3.3	Launch Files and Parameters	60 min	2	Multi-node launch, configuration, namespaces
3.4	Understanding URDF for Humanoids	90 min	2	Robot description format, joints, links

Key Concepts

ROS 2 Package Structure

my_robot_pkg/
├── my_robot_pkg/
│   ├── __init__.py
│   ├── talker_node.py
│   └── listener_node.py
├── launch/
│   └── robot_launch.py
├── config/
│   └── params.yaml
├── urdf/
│   └── robot.urdf
├── package.xml
├── setup.py
└── setup.cfg

URDF Basics

<robot name="simple_humanoid">
  <link name="torso">
    <visual>
      <geometry>
        <box size="0.3 0.2 0.5"/>
      </geometry>
    </visual>
  </link>

  <joint name="left_shoulder" type="revolute">
    <parent link="torso"/>
    <child link="left_upper_arm"/>
    <axis xyz="1 0 0"/>
    <limit lower="-1.57" upper="1.57" effort="100" velocity="1.0"/>
  </joint>
</robot>

Hardware Note

Tier 2 Required

This chapter requires a local ROS 2 installation for hands-on package development. Cloud environments have limitations for package building.

Tier 1 Workaround

Tier 1 users can follow along conceptually and use pre-built packages in simulation. Full hands-on practice recommended when you upgrade to Tier 2.

What You'll Build

Module 1 Capstone: Robot Health Monitor

A complete ROS 2 package that:

1. Subscribes to simulated sensor topics (battery, temperature, joint states)
2. Processes data and detects anomalies
3. Publishes health status on a custom topic
4. Provides a service to query detailed diagnostics
5. Includes launch file for easy startup

Chapter Assessment

After completing this chapter:

• Created a ROS 2 package from scratch
• Successfully built and ran your package
• Wrote a launch file that starts multiple nodes
• Created a basic URDF for a humanoid robot
• Completed the Robot Health Monitor capstone project