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Chapter 03: ROS 2 Fundamentals

Learning Objectives​

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

  1. Understand ROS 2 architecture - nodes, topics, services, actions, parameters
  2. Create publisher/subscriber nodes in Python using rclpy
  3. Implement service clients and servers for request-response patterns
  4. Use action servers for long-running robot tasks with feedback
  5. Configure parameters and launch files for multi-node systems
  6. Debug ROS 2 applications using command-line tools and visualization
Prerequisites

Before starting this chapter, ensure you have:

  • Completed Chapter 02 (hardware setup or cloud instance)
  • Ubuntu 22.04 with ROS 2 Jazzy installed
  • Basic Python knowledge (functions, classes, imports)
  • Terminal/command line familiarity

Introduction: Why ROS 2?​

Robot Operating System 2 (ROS 2) is not an operating system like Linux or Windows. It's a middleware framework that provides:

  • Communication infrastructure: Nodes send messages to each other
  • Hardware abstraction: Same code works with different robots
  • Standard libraries: Motion planning, localization, perception, control
  • Tools ecosystem: Debugging, visualization, simulation integration

ROS 1 vs ROS 2: Why the Rewrite?​

FeatureROS 1 (Legacy)ROS 2 (Modern)
Real-time supportNoYes (DDS middleware)
Multi-robotDifficult (single master)Native support
SecurityNoneDDS Security (encryption, auth)
Platform supportLinux onlyLinux, Windows, macOS
Production readinessResearch-focusedIndustry-grade
Lifecycle managementManualManaged nodes

For Physical AI: ROS 2 is mandatory because:

  • Real-time: Humanoid balance control requires 1000 Hz control loops
  • Reliability: Production robots can't tolerate single-point failures (ROS 1 master node)
  • Security: Commercial deployments need encrypted communication

ROS 2 Architecture Overview​

Key Concepts​

  1. Node: An independent process (Python script, C++ program) that performs computation
  2. Topic: A named communication channel for streaming data (pub-sub pattern)
  3. Service: Request-response pattern (client asks, server responds once)
  4. Action: Long-running task with goal, feedback, and result (e.g., "navigate to waypoint")
  5. Parameter: Runtime configuration (e.g., robot speed, sensor calibration)

Chapter Structure​

This chapter is divided into 4 sections:

  1. Nodes & Topics - Publisher/subscriber communication
  2. Services & Actions - Request-response and goal-based tasks
  3. Parameters & Launch Files - Configuration and multi-node startup
  4. Debugging Tools - rqt, rviz2, ros2 CLI commands

Each section includes:

  • Conceptual explanation with diagrams
  • Hands-on Python code examples
  • Common pitfalls and troubleshooting tips
Estimated Time
  • Reading: 2-3 hours for all 4 sections
  • Coding exercises: 3-4 hours to complete all examples
  • Total: 5-7 hours to master ROS 2 fundamentals

Quick Start: Your First ROS 2 Node​

Let's verify your ROS 2 installation works with a simple "Hello World":

# Source ROS 2 environment (add to ~/.bashrc for persistence)
source /opt/ros/jazzy/setup.bash

# Check ROS 2 version
ros2 --version
# Expected output: ros2 cli version: 0.XX.X

# Run demo talker node (publisher)
ros2 run demo_nodes_cpp talker

Expected output:

[INFO] [talker]: Publishing: 'Hello World: 1'
[INFO] [talker]: Publishing: 'Hello World: 2'
[INFO] [talker]: Publishing: 'Hello World: 3'
...

Open a second terminal and run the listener (subscriber):

source /opt/ros/jazzy/setup.bash
ros2 run demo_nodes_py listener

Expected output:

[INFO] [listener]: I heard: [Hello World: 1]
[INFO] [listener]: I heard: [Hello World: 2]
[INFO] [listener]: I heard: [Hello World: 3]
...

What just happened?

  1. talker node publishes messages to /chatter topic
  2. listener node subscribes to /chatter topic
  3. ROS 2 middleware (DDS) delivers messages automatically
  4. Nodes can be in different languages (C++ vs Python) - they communicate transparently

Installation Verification​

Run these commands to verify your ROS 2 setup:

# Check ROS 2 environment variables
env | grep ROS
# Should show: ROS_VERSION=2, ROS_DISTRO=jazzy

# List available ROS 2 packages
ros2 pkg list | head -10

# Check DDS implementation (default: Fast DDS)
ros2 doctor --report | grep middleware
# Should show: middleware name: rmw_fastrtps_cpp

# Test topic communication
ros2 topic list
# Should show: /parameter_events, /rosout (always present)

# Check node discovery
ros2 node list
# Should show: (empty if no nodes running)

If any command fails, revisit Chapter 02 - Appendix A: Lab Build Guides for ROS 2 installation troubleshooting.

Next Steps​

Ready to dive deeper? Start with Section 1: Nodes & Topics to learn the foundation of ROS 2 communication.

Prefer a different learning path?

  • Hands-on learner: Jump to code examples in each section
  • Visual learner: Focus on Mermaid diagrams and ros2 CLI visualization
  • System designer: Read about architecture patterns in Services & Actions section
Quick Navigation
  • Next: Nodes & Topics - Publisher/subscriber pattern
  • Skip ahead: Debugging Tools - Troubleshooting ROS 2 issues
  • Related: Chapter 04 (URDF modeling) builds on ROS 2 fundamentals

Further Reading​