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Gazebo Simulation Fundamentals

Objective​

This chapter introduces students to the fundamentals of Gazebo simulation environment, establishing a foundation for creating and operating basic simulations for robot development and testing.

Learning Outcomes​

After completing this chapter, students will be able to:

  1. Explain the core components and architecture of the Gazebo simulation environment
  2. Create and configure basic simulation environments for robot testing
  3. Implement simple robot models in simulation and spawn them in environments

Theory​

Gazebo is a 3D dynamic simulator that provides realistic environments for robot development, testing, and validation. It offers sophisticated physics simulation, high-quality graphics, and convenient programmatic interfaces. Gazebo is widely used in ROS (Robot Operating System) environments for testing and validating robotic algorithms before deployment on physical robots.

The simulator provides:

  • Physics simulation: Accurate modeling of rigid body dynamics, collisions, and contacts using ODE, Bullet, and Simbody engines
  • Sensor simulation: Support for various sensors including cameras, LiDAR, IMU, force/torque sensors, etc.
  • 3D visualization: Realistic rendering with OGRE engine
  • Plugins: Extensible architecture supporting custom controllers and sensors
  • ROS integration: Seamless communication with ROS nodes via gazebo_ros_pkgs

Gazebo's architecture consists of three main components:

  1. Server (gzserver): Runs the physics simulation and sensor models
  2. Client (gzclient): Provides visualization and user interaction
  3. Library (libgazebo): C++ library for direct integration

The simulation environment consists of models (robots, objects, sensors) within a world (environment with lighting, physics properties, and terrain). Worlds are defined using SDF (Simulation Description Format), an XML-based format that describes the complete simulation environment.

Models in Gazebo can represent robots, objects, or sensors. Robot models typically include links (rigid bodies), joints (connections between links), and plugins (custom behaviors). The SDF format defines these elements with properties like mass, inertia, visual appearance, and collision properties.

Practical Examples​

Example 1: Creating a Simple Environment​

Creating a basic world file that includes a ground plane and basic lighting:

<?xml version="1.0" ?>
<sdf version="1.6">
  <world name="small_room">
    <!-- Physics engine -->
    <physics name="ode" type="ode">
      <gravity>0 0 -9.8</gravity>
    </physics>

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

    <!-- Lighting -->
    <include>
      <uri>model://sun</uri>
    </include>

    <!-- A simple box model -->
    <model name="box1">
      <pose>0 0 0.5 0 0 0</pose>
      <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>
        </visual>
        <inertial>
          <mass>1.0</mass>
          <inertia>
            <ixx>0.1667</ixx>
            <ixy>0</ixy>
            <ixz>0</ixz>
            <iyy>0.1667</iyy>
            <iyz>0</iyz>
            <izz>0.1667</izz>
          </inertia>
        </inertial>
      </link>
    </model>
  </world>
</sdf>

Example 2: Spawning a Robot Model​

Loading and spawning a simple robot model in Gazebo using ROS commands:

# Launch Gazebo with a custom world
roslaunch gazebo_ros empty_world.launch world_name:=/path/to/small_room.world

# Spawn a robot model from a URDF file
rosrun gazebo_ros spawn_model -file /path/to/robot.urdf -urdf -model robot_name -x 0 -y 0 -z 1

Hands-on Lab​

Prerequisites​

  • ROS installation (ROS 1 or ROS 2)
  • Gazebo installed
  • Basic understanding of ROS concepts from Module 1

Step 1: Launch Basic Gazebo Environment​

Launch Gazebo with the default empty world and familiarize yourself with the interface:

roslaunch gazebo_ros empty_world.launch

Take note of the different interface elements:

  • 3D viewport
  • Scene tree
  • Toolbar options
  • Property editor

Step 2: Create a Simple Box Obstacle​

Create a custom world file that includes a simple box obstacle at coordinates (1, 0, 0.5) with dimensions 0.5m x 0.5m x 0.5m. Save this as simple_box.world.

Step 3: Run the Custom World​

Launch Gazebo with your custom world file and verify that the box appears in the correct location.

Step 4: Add a Robot Model​

Spawn a simple robot model (e.g., PR2 or TurtleBot) into your custom world. Experiment with different coordinates to place it in the scene.

Exercises​

  1. Explain the difference between SDF and URDF formats in the context of Gazebo simulation.

  2. Describe the physics properties that can be configured in a Gazebo world file and how they affect simulation outcomes.

  3. Compare the advantages and limitations of using Gazebo versus real-world robot testing for algorithm validation.

  4. Create a simple world file that includes two different shaped objects (e.g., box and cylinder) placed at different locations.

  5. Research and explain how Gazebo's physics engine affects the realism of simulation results.

Summary​

This chapter covered the fundamentals of the Gazebo simulation environment, including its architecture, components, and basic usage. Students learned how to create simple simulation environments and spawn models within them. Understanding these fundamentals is crucial for effective robotics development and testing in simulation before deployment to physical hardware.

Further Reading​