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Chapter 3: Simulation Environments - The Digital Twin

Module 2: Gazebo & Unity​

Welcome to Module 2 of the Physical AI & Humanoid Robotics course! In this chapter, you'll learn how to simulate humanoid robots in physics engines before deploying them to real hardware. This is a critical skill for robotics development - simulation allows you to test designs, validate behaviors, and iterate quickly in a safe, repeatable environment.

Learning Objectives​

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

  • Set up and navigate the Gazebo simulation environment
  • Create custom simulation worlds with realistic physics properties
  • Add sensors (LIDAR, cameras, IMU) to robot models
  • Configure physics parameters (gravity, friction, collisions, inertia)
  • Visualize sensor data in RViz
  • Load and simulate humanoid robots with complete sensor suites
  • Understand when to use Gazebo vs Unity for different scenarios
  • Test and validate robot behaviors in simulation
  • Optimize simulation performance for your hardware

Why Simulation Matters​

Before deploying code to a physical robot, you need to test it. Simulation provides:

  • Rapid Prototyping: Test ideas in minutes instead of hours
  • Safe Experimentation: Try risky behaviors without damaging hardware
  • Systematic Validation: Repeatably test edge cases and failure modes
  • Parameter Optimization: Tune controllers with automated testing
  • Cost Efficiency: Develop without expensive robot hardware

However, simulation is not perfect. You'll also learn about the sim-to-real gap - differences between simulated and real-world behavior - and strategies to minimize it.

Chapter Structure​

This chapter is organized into the following sections:

  1. Gazebo Basics - Understanding the Gazebo interface and physics engine
  2. World Creation - Creating custom simulation environments with SDF
  3. Sensor Simulation - Simulating LIDAR, cameras, and IMU sensors
  4. URDF with Sensors - Adding sensor plugins to robot descriptions
  5. Physics Properties - Configuring gravity, friction, collisions, and inertia
  6. Unity Overview - When and how to use Unity for visualization
  7. Humanoid Simulation - Simulating complete humanoid robots
  8. Testing & Validation - Workflows for testing robot behaviors
  9. Exercises - Hands-on practice problems
  10. Troubleshooting - Common issues and solutions

Prerequisites​

Before starting this chapter, you should:

  • Have completed Chapter 2 (ROS 2 & URDF Basics)
  • Understand URDF robot descriptions
  • Be comfortable with ROS 2 topics and nodes
  • Have Gazebo 11 installed (installation guide in Section 1)

Tools & Technologies​

This chapter covers:

  • Gazebo 11 (Gazebo Classic) - Primary physics simulation environment
  • SDF (Simulation Description Format) - World file format
  • URDF with Gazebo plugins - Robot models with sensors
  • RViz - Visualization of sensor data
  • ROS 2 Humble - Communication framework
  • Unity (overview only) - High-fidelity visualization

Time Commitment​

Plan for 8-12 hours to complete this chapter, including:

  • Reading and understanding concepts: 3-4 hours
  • Hands-on exercises: 4-6 hours
  • Troubleshooting and experimentation: 1-2 hours

Getting Help​

If you encounter issues:

  1. Check the Troubleshooting section
  2. Verify your Gazebo installation and ROS 2 setup
  3. Review the example files in examples/chapter-3-simulation/
  4. Search the Gazebo community forums
  5. Ask on the course discussion board

What's Next?​

After completing this chapter, you'll be ready for:

  • Module 3: NVIDIA Isaac Sim for advanced perception and AI
  • Module 4: Vision-Language-Action models for embodied AI
  • Capstone Project: Simulating your humanoid robot design before deployment

Let's get started with Gazebo Basics!