Skip to main content

Week 2: ROS 2 Basics

Introduction to ROS 2​

ROS 2 (Robot Operating System 2) is the industry standard middleware for robotics. Unlike ROS 1, which relied on a custom master node, ROS 2 is built on top of DDS (Data Distribution Service), making it more real-time capable and decentralized.

Core Concepts​

ROS 2 is a graph-based system.

graph LR;
    Camera[Camera Driver Node] -->|/camera/image_raw| Vision[Object Detection Node];
    Vision -->|/targets| Planner[Path Planner Node];
    Planner -->|/joint_states| Controller[Motor Controller Node];
    style Camera fill:#e1f5fe,stroke:#01579b
    style Controller fill:#fbe9e7,stroke:#bf360c

Nodes​

A Node is a process that performs computation. A robot system is typically comprised of many nodes.

Topics (Publish/Subscribe)​

Nodes exchange data over Topics. This decouples the system. The Camera doesn't know who is listening, it just shouts data.

  • Publisher: Sends data.
  • Subscriber: Receives data.

Services​

For synchronous request/response interactions (e.g., "Take a picture now"), use Services.

Lab: Creating a Custom ROS 2 Node​

We will create a simple "Talker" node in Python.

Step 1: Create a Workspace​

mkdir -p ~/ros2_ws/src
cd ~/ros2_ws/src

Step 2: Create a Package​

ros2 pkg create --build-type ament_python my_first_package

Step 3: Write the Node​

Edit ~/ros2_ws/src/my_first_package/my_first_package/simple_publisher.py:

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, 'topic', 10)
        timer_period = 0.5  # seconds
        self.timer = self.create_timer(timer_period, self.timer_callback)
        self.i = 0

    def timer_callback(self):
        msg = String()
        msg.data = 'Hello Physical AI: %d' % self.i
        self.publisher_.publish(msg)
        self.get_logger().info('Publishing: "%s"' % msg.data)
        self.i += 1

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

if __name__ == '__main__':
    main()