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Chapter 3: Bringing Your Robot to Life: Simulating with Gazebo

Overview: The Big Idea​

Now that we have a URDF model of our robot, it's time to bring it to life in a simulated environment. Gazebo is a powerful and popular open-source robotics simulator that is tightly integrated with ROS. In this chapter, you will learn how to launch your robot model in Gazebo, create a simulated world, and interact with your robot using ROS 2.

Key Concepts​

  • Gazebo World: A Gazebo world is a .world file that defines a simulated environment, including the ground, lighting, and any objects or obstacles.
  • Gazebo Plugins: Gazebo plugins are shared libraries that can be loaded at runtime to extend Gazebo's functionality. There are plugins for simulating sensors, actuators, and even complex robot behaviors.
  • ros_gz_bridge: The ros_gz_bridge package provides a network bridge that enables communication between ROS 2 and Gazebo. It allows you to publish messages to ROS 2 topics to control your simulated robot and subscribe to ROS 2 topics to receive sensor data from the simulation.

Real-World Use Cases​

Gazebo is used extensively in the robotics community for a wide range of simulation tasks.

  • Algorithm Testing: Gazebo provides a safe and repeatable environment for testing new algorithms for navigation, manipulation, and perception.
  • Robot Design: Different robot designs can be quickly prototyped and tested in Gazebo before any physical hardware is built.
  • CI/CD: Gazebo can be integrated into a continuous integration and continuous delivery (CI/CD) pipeline to automatically test robot software with every code change.

Technical Explanations​

Launching a Robot in Gazebo​

To launch a robot in Gazebo, you typically create a ROS 2 launch file. This launch file will do several things:

  1. Start Gazebo: It will start the Gazebo server and, optionally, the Gazebo GUI.
  2. Spawn the Robot: It will use the spawn_entity.py script to spawn your robot's URDF model into the simulation.
  3. Start the ros_gz_bridge: It will start the ros_gz_bridge to enable communication between ROS 2 and Gazebo.

You will also need to create a Gazebo world file. This is an XML file that defines the environment.

Diagram Description: A diagram showing the relationship between ROS 2, Gazebo, and the ros_gz_bridge. The diagram shows that ROS 2 and Gazebo are two separate processes. The ros_gz_bridge sits in between them, relaying messages back and forth.

Code Samples​

Here is an example of a launch file that starts Gazebo, spawns our two-wheeled robot from the previous chapter, and starts the ros_gz_bridge.

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

def generate_launch_description():
    package_dir = get_package_share_directory('your_package_name')
    urdf = os.path.join(package_dir, 'urdf', 'two_wheeled_robot.urdf')
    
    return LaunchDescription([
        ExecuteProcess(
            cmd=['gazebo', '--verbose', '-s', 'libgazebo_ros_init.so', '-s', 'libgazebo_ros_factory.so'],
            output='screen'
        ),
        Node(
            package='gazebo_ros',
            executable='spawn_entity.py',
            arguments=['-entity', 'two_wheeled_robot', '-file', urdf],
            output='screen'
        ),
        Node(
            package='ros_gz_bridge',
            executable='parameter_bridge',
            arguments=['/cmd_vel@geometry_msgs/msg/Twist@gz.msgs.Twist'],
            output='screen'
        )
    ])

This launch file first starts Gazebo. Then, it spawns the robot model into the simulation. Finally, it starts a ros_gz_bridge that bridges the /cmd_vel topic from ROS 2 to Gazebo, allowing you to control the robot's velocity.

To make the robot move, you would need to add a Gazebo plugin to your URDF file that subscribes to the Gazebo velocity command and applies forces to the wheels.

Summary​

In this chapter, we have learned how to take a URDF model of a robot and simulate it in Gazebo. We have explored the key concepts of Gazebo worlds and plugins, and we have seen how the ros_gz_bridge enables communication between ROS 2 and Gazebo.

In the next chapter, we will learn how to add sensors to our simulated robot and visualize their data in ROS 2.