Lesson 1: Gazebo Basics and World Building

Learning Objectives

By the end of this lesson, students will be able to:

• Install and configure Gazebo Harmonic
• Create custom simulation worlds
• Understand Gazebo's physics engine integration
• Work with basic Gazebo models and environments

Introduction to Gazebo

Gazebo is a 3D simulation environment for robotics that provides realistic physics simulation, high-quality rendering, and sensor simulation. It's an essential tool for testing and validating robotic systems before deployment on real hardware.

Key Features of Gazebo

1. Physics Simulation: Accurate simulation of rigid body dynamics using ODE, Bullet, or DART
2. Sensor Simulation: Realistic simulation of cameras, LiDAR, IMUs, GPS, and other sensors
3. Rendering: High-quality 3D visualization with support for multiple rendering engines
4. ROS Integration: Seamless integration with ROS/ROS 2 for robotic simulation
5. Model Database: Access to a large database of pre-built robot and environment models

Installing Gazebo Harmonic

For this course, we'll use Gazebo Harmonic (the latest stable version):

# On Ubuntu 22.04
sudo apt update
sudo apt install gazebo libgazebo-dev

Basic Gazebo World Structure

Gazebo worlds are defined using SDF (Simulation Description Format), an XML-based format:

<?xml version="1.0" ?>
<sdf version="1.7">
  <world name="default">
    <!-- Include the default Gazebo world -->
    <include>
      <uri>model://ground_plane</uri>
    </include>

    <include>
      <uri>model://sun</uri>
    </include>

    <!-- Add your custom models here -->
    <model name="my_robot">
      <!-- Model definition -->
    </model>
  </world>
</sdf>

World Elements

• <physics>: Physics engine configuration (ODE, Bullet, DART)
• <scene>: Visual properties (ambient lighting, shadows)
• <light>: Light sources in the environment
• <model>: Robot and object definitions
• <include>: References to models in the Gazebo model database

Creating a Simple World

Let's create a custom world file called simple_room.world:

<?xml version="1.0" ?>
<sdf version="1.7">
  <world name="simple_room">
    <!-- Physics engine -->
    <physics type="ode">
      <gravity>0 0 -9.8</gravity>
      <max_step_size>0.001</max_step_size>
      <real_time_factor>1</real_time_factor>
      <real_time_update_rate>1000</real_time_update_rate>
    </physics>

    <!-- Scene settings -->
    <scene>
      <ambient>0.4 0.4 0.4</ambient>
      <background>0.7 0.7 0.7</background>
      <shadows>true</shadows>
    </scene>

    <!-- Sun light -->
    <light name="sun" type="directional">
      <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>
      <attenuation>
        <range>1000</range>
        <constant>0.9</constant>
        <linear>0.01</linear>
        <quadratic>0.001</quadratic>
      </attenuation>
      <direction>-0.6 0.4 -0.8</direction>
    </light>

    <!-- Ground plane -->
    <include>
      <uri>model://ground_plane</uri>
    </include>

    <!-- Cube obstacle -->
    <model name="cube_obstacle">
      <pose>-1 0 0.5 0 0 0</pose>
      <static>true</static>
      <link name="link">
        <collision name="collision">
          <geometry>
            <box>
              <size>1 1 1</size>
            </box>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <box>
              <size>1 1 1</size>
            </box>
          </geometry>
          <material>
            <ambient>0.8 0.2 0.2 1</ambient>
            <diffuse>0.8 0.2 0.2 1</diffuse>
          </material>
        </visual>
      </link>
    </model>

    <!-- Wall -->
    <model name="wall">
      <pose>0 2 1 0 0 0</pose>
      <static>true</static>
      <link name="link">
        <collision name="collision">
          <geometry>
            <box>
              <size>5 0.2 2</size>
            </box>
          </geometry>
        </collision>
        <visual name="visual">
          <geometry>
            <box>
              <size>5 0.2 2</size>
            </box>
          </geometry>
          <material>
            <ambient>0.5 0.5 0.5 1</ambient>
            <diffuse>0.5 0.5 0.5 1</diffuse>
          </material>
        </visual>
      </link>
    </model>
  </world>
</sdf>

Running Gazebo with Custom Worlds

To run Gazebo with your custom world:

# Run with a specific world file
gazebo simple_room.world

# Or run Gazebo and load the world later
gazebo --verbose

Gazebo GUI Overview

When Gazebo starts, you'll see several key components:

1. 3D View: The main simulation environment
2. World Control: Play, pause, step through simulation
3. Model Database: Pre-built models to insert into the world
4. Scene Graph: Tree view of all objects in the simulation
5. Tools: Additional utilities like the model editor

Physics Configuration

The physics engine is crucial for realistic simulation. Key parameters include:

• max_step_size: Time step for physics updates (smaller = more accurate but slower)
• real_time_factor: Target simulation speed (1.0 = real-time)
• real_time_update_rate: Update rate in Hz

Physics Engine Comparison

• ODE (Open Dynamics Engine): Stable and widely used, good for most applications
• Bullet: Good for complex collision detection, handles soft bodies well
• DART: Advanced dynamics, good for articulated robots and complex contacts

Practical Exercise: Create Your Own World

Create a simple world file with the following elements:

1. A ground plane
2. A light source
3. At least 3 different obstacles (cubes, spheres, or cylinders)
4. A simple maze or path for navigation

Steps:

1. Create a new directory for your world files: mkdir ~/gazebo_worlds
2. Create your world file: nano ~/gazebo_worlds/my_world.world
3. Copy the structure from the example above
4. Add your custom elements
5. Run with: gazebo ~/gazebo_worlds/my_world.world

Gazebo Commands and Tools

Command Line Options

• gazebo --verbose: Show detailed output
• gazebo --physics=bullet: Use a specific physics engine
• gazebo --play: Start simulation automatically

Model Database

Gazebo comes with a rich database of models:

• Robots (PR2, TurtleBot, etc.)
• Objects (cups, books, furniture)
• Environments (rooms, outdoor scenes)

Troubleshooting Common Issues

Simulation Runs Too Slow

• Reduce max_step_size or increase real_time_update_rate
• Simplify collision geometries
• Reduce the number of objects in the scene

Physics Behaviors Are Unrealistic

• Check mass and inertia values
• Verify friction and damping coefficients
• Adjust solver parameters

Rendering Issues

• Check graphics drivers
• Try running with --verbose to see error messages
• Consider using software rendering if hardware acceleration fails

Summary

This lesson introduced Gazebo simulation environment and how to create custom worlds. Understanding world building is fundamental for creating realistic testing environments for your robots.

Next Steps

In the next lesson, we'll explore sensor simulation in Gazebo, including cameras, LiDAR, and IMU sensors.