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Lesson 1.1: Introduction to Nodes

Section 1: Theory​

In ROS 2, a node is the fundamental building block for creating a robotic application. Think of a node as a small, independent program that performs a specific task. For example, you might have one node for controlling the robot's wheels, another for processing camera data, and a third for planning a path.

Each node in a ROS 2 system can:

  • Be written in different programming languages (like Python, C++, etc.).
  • Run on different computers, allowing for distributed systems.
  • Communicate with other nodes using ROS 2's communication protocols (like topics, services, and actions).

By breaking down a complex robotics task into smaller, manageable nodes, you can create a system that is modular, scalable, and easy to debug.

Section 2: Code Example​

Here is a simple "Hello World" program in Python using rclpy (the ROS 2 client library for Python). This code creates a node that prints "Hello World" to the console every second.

import rclpy
from rclpy.node import Node

class HelloWorldNode(Node):
    def __init__(self):
        super().__init__('hello_world_node')
        self.timer = self.create_timer(1.0, self.timer_callback)

    def timer_callback(self):
        self.get_logger().info('Hello World')

def main(args=None):
    rclpy.init(args=args)
    node = HelloWorldNode()
    try:
        rclpy.spin(node)
    except KeyboardInterrupt:
        pass
    finally:
        node.destroy_node()
        rclpy.shutdown()

if __name__ == '__main__':
    main()

Code Breakdown:​

  • rclpy.init(): Initializes the ROS 2 client library.
  • HelloWorldNode(Node): Defines a class that inherits from rclpy.node.Node.
  • super().__init__('hello_world_node'): Initializes the node with the name hello_world_node.
  • self.create_timer(1.0, self.timer_callback): Creates a timer that calls the timer_callback function every 1.0 second.
  • self.get_logger().info('Hello World'): Logs the "Hello World" message.
  • rclpy.spin(node): Keeps the node running until it's interrupted (e.g., by pressing Ctrl+C).
  • node.destroy_node() and rclpy.shutdown(): Cleans up the node and shuts down the ROS 2 client library.

Section 3: How to Run​

  1. Save the code: Save the code above in a file named hello_world.py inside a ROS 2 package.

  2. Build the package: Navigate to your ROS 2 workspace and build the package containing hello_world.py.

    colcon build

  3. Source the workspace:

    source install/setup.bash

  4. Run the node: Use the ros2 run command to execute the node.

    ros2 run <your_package_name> hello_world

You should now see the "Hello World" message printed to your terminal every second. To stop the node, press Ctrl+C.