Chapter 2: Introduction to Gazebo Simulation

Overview

Welcome to Module 2! In this chapter, you'll learn how to create digital twins of robots using Gazebo, a powerful physics simulation environment.

What is Gazebo?

Gazebo is an open-source 3D robotics simulator that provides:

Realistic physics simulation using engines like ODE, Bullet, and Simbody
High-fidelity sensor simulation (cameras, LiDAR, IMU, force/torque)
ROS 2 integration for seamless robot control
Plugin system for custom functionality
Distributed simulation for complex scenarios

Why Simulate?

Benefits of Simulation

Cost-Effective: Test algorithms without physical hardware
Safe: Experiment with risky behaviors in a virtual environment
Rapid Iteration: Quickly test multiple scenarios
Reproducible: Reset and repeat experiments exactly
Scalable: Run multiple simulations in parallel

The Sim-to-Real Gap

While simulation is powerful, there's always a gap between simulation and reality:

Physics fidelity: Real-world physics is more complex
Sensor noise: Real sensors have noise and delays
Contact dynamics: Grasping and manipulation are challenging to simulate accurately
Environmental factors: Wind, lighting, terrain variations

Goal: Minimize this gap through careful modeling and domain randomization.

Gazebo Architecture

┌─────────────────────────────────────┐
│        User Interface (GUI)         │
└──────────────┬──────────────────────┘
               │
┌──────────────┴──────────────────────┐
│         Gazebo Server               │
│  (Physics Engine, Sensor Sim)       │
└──────────────┬──────────────────────┘
               │
┌──────────────┴──────────────────────┐
│         Plugin Layer                │
│  (ROS 2 Integration, Controllers)   │
└──────────────┬──────────────────────┘
               │
┌──────────────┴──────────────────────┐
│         World Models (SDF)          │
│  (Robots, Objects, Environments)    │
└─────────────────────────────────────┘

Key Concepts

1. World Files (.world / .sdf)

World files define the simulation environment using SDF (Simulation Description Format):

Models: Robots, objects, and structures
Lights: Illumination sources
Physics: Gravity, time step, solver parameters
Plugins: Custom behaviors and interfaces

2. Robot Models (URDF/SDF)

URDF (Unified Robot Description Format):

XML format for robot structure
Defines links (rigid bodies) and joints
Includes visual and collision geometries
Specifies inertial properties

SDF (Simulation Description Format):

More expressive than URDF
Supports complete world descriptions
Better physics specifications
Gazebo's native format

3. Physics Simulation

Gazebo simulates:

Rigid body dynamics: Forces, torques, collisions
Joint constraints: Revolute, prismatic, fixed, etc.
Contact forces: Friction, bounce, surface properties
Gravity and external forces

4. Sensor Simulation

Gazebo can simulate various sensors:

Cameras: RGB, depth, stereo
LiDAR: 2D and 3D laser scanners
IMU: Acceleration and angular velocity
Force/Torque: Contact sensing
GPS: Position sensing

Installation

Gazebo Fortress (Recommended for ROS 2 Humble)

# Install Gazebo Fortress
sudo apt-get install ros-humble-ros-gz

# Install additional packages
sudo apt-get install ros-humble-gazebo-ros-pkgs

Verify Installation

# Launch Gazebo
gz sim

# Or with ROS 2 integration
ros2 launch gazebo_ros gazebo.launch.py

Your First Simulation

Let's create a simple world with a ground plane and a sphere:

Create a World File

<?xml version="1.0" ?>
<sdf version="1.8">
  <world name="simple_world">
    
    <!-- Physics -->
    <physics name="1ms" type="ignored">
      <max_step_size>0.001</max_step_size>
      <real_time_factor>1.0</real_time_factor>
    </physics>
    
    <!-- Lighting -->
    <light type="directional" name="sun">
      <cast_shadows>true</cast_shadows>
      <pose>0 0 10 0 0 0</pose>
      <diffuse>0.8 0.8 0.8 1</diffuse>
      <specular>0.2 0.2 0.2 1</specular>
    </light>
    
    <!-- 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>
    
    <!-- Bouncing Ball -->
    <model name="ball">
      <pose>0 0 2 0 0 0</pose>
      <link name="link">
        <inertial>
          <mass>1.0</mass>
          <inertia>
            <ixx>0.04</ixx>
            <ixy>0</ixy>
            <ixz>0</ixz>
            <iyy>0.04</iyy>
            <iyz>0</iyz>
            <izz>0.04</izz>
          </inertia>
        </inertial>
        <collision name="collision">
          <geometry>
            <sphere>
              <radius>0.2</radius>
            </sphere>
          </geometry>
          <surface>
            <bounce>
              <restitution_coefficient>0.8</restitution_coefficient>
            </bounce>
          </surface>
        </collision>
        <visual name="visual">
          <geometry>
            <sphere>
              <radius>0.2</radius>
            </sphere>
          </geometry>
          <material>
            <ambient>1 0 0 1</ambient>
            <diffuse>1 0 0 1</diffuse>
          </material>
        </visual>
      </link>
    </model>
    
  </world>
</sdf>

Launch the Simulation

gz sim simple_world.sdf

Understanding URDF

Let's create a simple robot arm:

<?xml version="1.0"?>
<robot name="simple_arm">
  
  <!-- Base Link -->
  <link name="base_link">
    <visual>
      <geometry>
        <cylinder length="0.1" radius="0.2"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.1" radius="0.2"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1.0"/>
      <inertia ixx="0.01" ixy="0.0" ixz="0.0" 
               iyy="0.01" iyz="0.0" izz="0.02"/>
    </inertial>
  </link>
  
  <!-- First Arm Link -->
  <link name="arm_link">
    <visual>
      <origin xyz="0 0 0.25" rpy="0 0 0"/>
      <geometry>
        <box size="0.05 0.05 0.5"/>
      </geometry>
      <material name="red">
        <color rgba="1 0 0 1"/>
      </material>
    </visual>
    <collision>
      <origin xyz="0 0 0.25" rpy="0 0 0"/>
      <geometry>
        <box size="0.05 0.05 0.5"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="0.5"/>
      <inertia ixx="0.01" ixy="0.0" ixz="0.0" 
               iyy="0.01" iyz="0.0" izz="0.001"/>
    </inertial>
  </link>
  
  <!-- Joint connecting base to arm -->
  <joint name="base_to_arm" type="revolute">
    <parent link="base_link"/>
    <child link="arm_link"/>
    <origin xyz="0 0 0.05" rpy="0 0 0"/>
    <axis xyz="0 0 1"/>
    <limit effort="10" lower="-3.14" upper="3.14" velocity="1.0"/>
  </joint>
  
</robot>

Exercises

Exercise 1: Explore Gazebo GUI

Launch Gazebo with an empty world
Insert models from the model database
Manipulate objects using the transform tools
Adjust physics parameters (gravity, time step)

Exercise 2: Modify the Bouncing Ball

Modify the ball world file to:

Change the ball's starting height
Adjust the restitution coefficient (bounciness)
Add multiple balls at different positions
Change the ball's color and size

Exercise 3: Build a Simple Robot

Create a two-wheeled robot with:

A cylindrical body
Two cylindrical wheels
Revolute joints for the wheels
Appropriate mass and inertia values

Key Takeaways

Gazebo provides realistic physics simulation for robotics
SDF and URDF are the primary formats for describing worlds and robots
Simulation enables rapid, safe, and cost-effective development
Understanding the sim-to-real gap is crucial for deployment

Next Steps

In the next chapter, we'll cover:

Adding sensors to your robot (cameras, LiDAR)
ROS 2 integration with Gazebo
Controlling robots through ROS 2 topics
Creating custom Gazebo plugins

Additional Resources

Continue learning: Next Chapter → | Back to Module Overview

Overview​

What is Gazebo?​

Why Simulate?​

Benefits of Simulation​

The Sim-to-Real Gap​

Gazebo Architecture​

Key Concepts​

1. World Files (.world / .sdf)​

2. Robot Models (URDF/SDF)​

3. Physics Simulation​

4. Sensor Simulation​

Installation​

Gazebo Fortress (Recommended for ROS 2 Humble)​

Verify Installation​

Your First Simulation​

Create a World File​

Launch the Simulation​

Understanding URDF​

Exercises​

Exercise 1: Explore Gazebo GUI​

Exercise 2: Modify the Bouncing Ball​

Exercise 3: Build a Simple Robot​

Key Takeaways​

Next Steps​

Additional Resources​