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Chapter 1: Gazebo Simulation Basics

Type: Theory-to-Practice
Lessons: 6
Duration: 10-12 hours

Chapter Overview​

This chapter introduces Gazebo, the physics simulation engine that enables you to develop and test robots without physical hardware. You'll learn to set up simulation environments, spawn robots, and understand physics simulation fundamentals.

By the end of this chapter, you will:

  • Understand Gazebo's role in robotics development
  • Set up Gazebo simulation environments
  • Spawn robots from URDF/SDF files
  • Understand physics engines and simulation parameters
  • Use Gazebo's visualization and debugging tools

What is Gazebo?​

Gazebo is an open-source 3D physics simulation engine designed for robotics. It provides:

  • Physics Simulation: Realistic dynamics, collisions, and contact forces
  • Sensor Simulation: Cameras, LiDAR, IMUs, and more
  • Environment Modeling: Create complex worlds with obstacles
  • Plugin System: Extend functionality with custom plugins

Gazebo vs. Real World​

Advantages of Simulation​

  • Safety: Test dangerous scenarios without risk
  • Speed: Faster iteration than physical testing
  • Cost: No hardware wear or damage
  • Reproducibility: Exact same conditions every time
  • Scalability: Test multiple robots simultaneously

Limitations​

  • Reality Gap: Simulation never perfectly matches reality
  • Computational Cost: Complex simulations are resource-intensive
  • Model Accuracy: Requires accurate robot and environment models

Setting Up Gazebo​

Installation​

For ROS 2 Humble:

sudo apt install ros-humble-gazebo-ros-pkgs

For ROS 2 Iron:

# Gazebo Garden
sudo apt install gazebo

Basic Launch​

# Launch empty Gazebo world
gazebo

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

URDF vs SDF​

URDF (Unified Robot Description Format)​

  • XML format for robot structure
  • Used by ROS 2 and Gazebo Classic
  • Simpler, less feature-rich

SDF (Simulation Description Format)​

  • More powerful than URDF
  • Supports environments, multiple robots
  • Used by Gazebo Garden
  • Can include physics, lighting, materials

Converting URDF to SDF​

# Convert URDF to SDF
gz sdf -k my_robot.urdf > my_robot.sdf

Spawning Robots in Gazebo​

Using ROS 2​

# Spawn robot from URDF
ros2 run gazebo_ros spawn_entity.py \
  -entity my_robot \
  -topic robot_description \
  -x 0 -y 0 -z 0.5

Using Launch Files​

from launch import LaunchDescription
from launch_ros.actions import Node
from ament_index_python.packages import get_package_share_directory
import os

def generate_launch_description():
    pkg_path = get_package_share_directory('my_robot_pkg')
    urdf_file = os.path.join(pkg_path, 'urdf', 'my_robot.urdf')
    
    return LaunchDescription([
        # Robot State Publisher
        Node(
            package='robot_state_publisher',
            executable='robot_state_publisher',
            parameters=[{'robot_description': open(urdf_file).read()}]
        ),
        # Spawn in Gazebo
        Node(
            package='gazebo_ros',
            executable='spawn_entity.py',
            arguments=['-entity', 'my_robot', '-topic', 'robot_description']
        ),
    ])

Physics Engines​

Gazebo supports multiple physics engines:

ODE (Open Dynamics Engine)​

  • Default for Gazebo Classic
  • Good general-purpose physics
  • Fast and stable

Bullet​

  • Better for complex collisions
  • More accurate contact forces

DART (Dynamic Animation and Robotics Toolkit)​

  • Excellent for articulated bodies
  • Good for humanoid robots

Simbody​

  • High accuracy
  • Slower but more precise

Configuring Physics​

<physics name="default_physics" default="0" type="ode">
  <max_step_size>0.001</max_step_size>
  <real_time_factor>1.0</real_time_factor>
  <real_time_update_rate>1000</real_time_update_rate>
</physics>

Sensor Simulation​

Camera​

<gazebo reference="camera_link">
  <sensor type="camera" name="camera1">
    <update_rate>30</update_rate>
    <camera>
      <horizontal_fov>1.047</horizontal_fov>
      <image>
        <width>640</width>
        <height>480</height>
      </image>
    </camera>
    <plugin name="camera_plugin" filename="libgazebo_ros_camera.so">
      <ros>
        <namespace>/camera</namespace>
      </ros>
    </plugin>
  </sensor>
</gazebo>

LiDAR​

<gazebo reference="lidar_link">
  <sensor type="ray" name="lidar">
    <ray>
      <scan>
        <horizontal>
          <samples>720</samples>
          <resolution>1</resolution>
          <min_angle>-3.14159</min_angle>
          <max_angle>3.14159</max_angle>
        </horizontal>
      </scan>
      <range>
        <min>0.10</min>
        <max>30.0</max>
      </range>
    </ray>
    <plugin name="lidar_plugin" filename="libgazebo_ros_ray_sensor.so">
      <ros>
        <namespace>/lidar</namespace>
      </ros>
    </plugin>
  </sensor>
</gazebo>

IMU​

<gazebo reference="imu_link">
  <sensor type="imu" name="imu">
    <always_on>true</always_on>
    <update_rate>100</update_rate>
    <imu>
      <angular_velocity>
        <x>
          <noise type="gaussian">
            <mean>0.0</mean>
            <stddev>0.0002</stddev>
          </noise>
        </x>
      </angular_velocity>
    </imu>
    <plugin name="imu_plugin" filename="libgazebo_ros_imu_sensor.so">
      <ros>
        <namespace>/imu</namespace>
      </ros>
    </plugin>
  </sensor>
</gazebo>

Visualization and Debugging​

Gazebo GUI​

  • Scene Graph: View robot structure
  • Physics: Monitor simulation performance
  • Sensors: View sensor data
  • World Stats: FPS, real-time factor

ROS 2 Topics​

# View camera images
ros2 topic echo /camera/image_raw

# View LiDAR data
ros2 topic echo /lidar/scan

# View IMU data
ros2 topic echo /imu/data

RViz Integration​

# Launch RViz to visualize Gazebo data
rviz2

In RViz, add displays for:

  • RobotModel (from robot_state_publisher)
  • Camera images
  • LiDAR point clouds
  • TF transforms

Chapter Projects​

Project 1: Spawn Simple Robot​

  • Create URDF for simple mobile base
  • Spawn in Gazebo
  • Verify robot appears correctly

Project 2: Add Sensors​

  • Add camera to robot
  • Add LiDAR to robot
  • Verify sensor data on ROS 2 topics

Project 3: Physics Tuning​

  • Adjust physics parameters
  • Test different physics engines
  • Compare simulation vs. expected behavior

Chapter Summary​

Key Takeaways:

  1. Gazebo enables safe, fast robot development in simulation

  2. URDF/SDF describe robots for simulation

  3. Physics engines offer trade-offs between speed and accuracy

  4. Sensor simulation provides realistic sensor data

  5. Visualization tools help debug and understand simulation

Next Steps​