Module 2: The Digital Twin (Gazebo & Unity)

Weeks 6–7 | Physics Simulation, URDF/SDF, and High-Fidelity HRI

Learning Objectives

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

• Explain what a Digital Twin is and why it's essential for safe robot development
• Launch a Gazebo simulation of a humanoid robot from a URDF/SDF file
• Configure sensors (cameras, LiDAR) in a Gazebo world
• Use Unity's Robotics Hub for high-fidelity Human-Robot Interaction (HRI) simulation
• Understand Sim-to-Real transfer challenges and mitigation strategies

What Is a Digital Twin?

A Digital Twin is a high-fidelity virtual replica of a physical robot and its environment. It enables:

• Safe testing: Crash your robot virtually, not physically (robots are expensive!)
• Parallel development: Software teams work while hardware is being manufactured
• Dataset generation: Generate millions of training images without a physical camera
• Regression testing: Ensure software changes don't break existing behaviors

Gazebo vs Unity

Feature	Gazebo (Harmonic)	Unity (Robotics Hub)
Physics Engine	ODE/Bullet/DART	PhysX
ROS 2 Integration	Native	via ros-tcp-endpoint
Visual Fidelity	Medium	Very High
Sensor Simulation	Excellent	Good
Best For	ROS 2 development	HRI, perception datasets

URDF to SDF

Gazebo uses SDF (Simulation Description Format), which extends URDF with physics properties:

<!-- SDF world file with robot and physics - Gazebo Harmonic compatible -->
<?xml version="1.0"?>
<sdf version="1.9">
  <world name="humanoid_lab">

    <!-- Physics settings -->
    <physics name="1ms" type="ignored">
      <max_step_size>0.001</max_step_size>  <!-- 1ms timestep -->
      <real_time_factor>1.0</real_time_factor>
    </physics>

    <!-- Ground plane -->
    <model name="ground_plane">
      <static>true</static>
      <link name="link">
        <collision name="collision">
          <geometry><plane><normal>0 0 1</normal></plane></geometry>
        </collision>
        <visual name="visual">
          <geometry><plane><normal>0 0 1</normal><size>100 100</size></plane></geometry>
        </visual>
      </link>
    </model>

    <!-- Include robot from URDF/SDF -->
    <include>
      <uri>file://path/to/humanoid.sdf</uri>
      <pose>0 0 1.0 0 0 0</pose>  <!-- Spawn 1m above ground -->
    </include>

  </world>
</sdf>

Practice Exercise

1. Install Gazebo Harmonic: sudo apt install gz-harmonic
2. Convert your URDF to SDF: gz sdf -p my_humanoid.urdf > my_humanoid.sdf
3. Launch simulation: gz sim my_world.sdf
4. Connect ROS 2: ros2 launch ros_gz_sim gz_sim.launch.py

---

→ Next Module: Module 3 – The AI-Robot Brain (NVIDIA Isaac)