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Chapter 2: ROS 2 Fundamentals

Overview​

This chapter covers the fundamentals of the Robot Operating System 2 (ROS 2), a key framework for robotics development.

Objectives​

  • Understand the core concepts of ROS 2 (nodes, topics, services, actions).
  • Learn how to create a simple ROS 2 package.
  • Understand the ROS 2 command-line interface.

Core Content​

ROS 2 is an open-source framework for building robotic applications. It provides a set of tools, libraries, and conventions to simplify the task of creating complex and robust robot behavior across a wide variety of robotic platforms.

Key Concepts​

  • Nodes: A node is a process that performs computation. In ROS 2, a single executable can contain one or more nodes.
  • Topics: Topics are named buses over which nodes exchange messages. Topics have anonymous publish/subscribe semantics.
  • Services: Services are a request/response type of communication. One node offers a service, and another node can call that service.
  • Actions: Actions are for long-running tasks. They provide feedback during execution and can be preempted.

Examples​

A Simple Publisher Node (Python)​

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class SimplePublisher(Node):
    def __init__(self):
        super().__init__('simple_publisher')
        self.publisher_ = self.create_publisher(String, 'chatter', 10)
        self.timer = self.create_timer(0.5, self.timer_callback)
        self.i = 0

    def timer_callback(self):
        msg = String()
        msg.data = f'Hello World: {self.i}'
        self.publisher_.publish(msg)
        self.get_logger().info(f'Publishing: "{msg.data}"')
        self.i += 1

def main(args=None):
    rclpy.init(args=args)
    simple_publisher = SimplePublisher()
    rclpy.spin(simple_publisher)
    simple_publisher.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Running the Publisher​

To run this node, you would save it as a Python file, and then run it with python your_node.py. You could then listen to the chatter topic using the ROS 2 CLI:

ros2 topic echo /chatter

Figures​

simple publisher/subscriber model

Figure 1: A simple ROS 2 system with a publisher node, a subscriber node, and a topic connecting them.

Summary​

This chapter provided a brief introduction to the fundamental concepts of ROS 2, including nodes, topics, services, and actions. We also saw a simple example of a publisher node written in Python. In the next chapters, we will build upon these concepts to create more complex robotic applications.