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Gazebo & Digital Twin Refresher (Chapter 2 Reference)

Use this refresher when: Lessons reference simulation concepts from Chapter 2

Quick Reference: Gazebo Simulation​

What is a Digital Twin?​

A digital twin is a virtual representation of a physical robot that:

  • Mirrors the robot's sensors and actuators
  • Provides realistic physics simulation
  • Enables safe testing before real-world deployment

Gazebo Architecture​

┌─────────────────────────────────────────────────┐
│                  Gazebo Server                   │
│  ┌──────────┐  ┌──────────┐  ┌──────────────┐  │
│  │ Physics  │  │ Sensors  │  │   Plugins    │  │
│  │  Engine  │  │Simulation│  │ (ROS 2 etc.) │  │
│  └──────────┘  └──────────┘  └──────────────┘  │
└─────────────────────────────────────────────────┘
                       │
                       ▼
┌─────────────────────────────────────────────────┐
│               ROS 2 Interface                    │
│  /cmd_vel  /scan  /camera/image  /odom  /tf     │
└─────────────────────────────────────────────────┘

Key Components​

ComponentPurpose
World file (.world)Defines environment, physics, lighting
URDF/SDFRobot model description
PluginsConnect sensors/actuators to ROS 2
ros_gz_bridgeTranslates Gazebo↔ROS 2 messages

Essential Commands​

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

# Spawn a robot model
ros2 run gazebo_ros spawn_entity.py -entity robot_name -file model.urdf

# List Gazebo topics (bridged to ROS 2)
ros2 topic list | grep -E "scan|image|odom"

Simulated Sensors​

Camera​

# Publishes to: /camera/image_raw (sensor_msgs/Image)
# Provides: RGB images at specified resolution and FPS

Depth Camera​

# Publishes to: /depth_camera/depth/image_raw (sensor_msgs/Image)
# Provides: Depth images (32-bit float, meters)

LIDAR​

# Publishes to: /scan (sensor_msgs/LaserScan)
# Provides: 2D range measurements

IMU​

# Publishes to: /imu (sensor_msgs/Imu)
# Provides: Orientation, angular velocity, linear acceleration

Spawning a Robot​

# In a launch file
from launch import LaunchDescription
from launch.actions import IncludeLaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    # Start Gazebo
    gazebo = IncludeLaunchDescription(
        'gazebo_ros', 'gazebo.launch.py',
        launch_arguments={'world': 'my_world.world'}.items()
    )

    # Spawn robot
    spawn = Node(
        package='gazebo_ros',
        executable='spawn_entity.py',
        arguments=['-entity', 'robot', '-topic', 'robot_description']
    )

    return LaunchDescription([gazebo, spawn])

Coordinate Frames in Simulation​

world (fixed)
  └── odom (drifts over time)
        └── base_link (robot body)
              ├── base_scan (LIDAR)
              ├── camera_link (camera)
              └── imu_link (IMU)

The odom frame provides the robot's estimated position based on wheel odometry, which drifts over time. SLAM corrects this drift.

Need More Detail?​

See Chapter 2: Digital Twin Simulation for complete coverage.