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Module 1: ROS 2 Fundamentals (Weeks 1-5)

Overview​

This module introduces the Robot Operating System 2 (ROS 2), the foundational middleware for robotics applications. You'll learn the core concepts, architecture, and practical implementation of ROS 2 systems essential for humanoid robotics development.

Learning Objectives​

By the end of this module, you will be able to:

  • Explain the ROS 2 architecture and its key components
  • Create and manage ROS 2 packages, nodes, and communication patterns
  • Implement robot models using URDF (Unified Robot Description Format)
  • Work with transforms (TF) for coordinate frame management
  • Design launch files for complex robot system initialization
  • Debug and troubleshoot ROS 2 applications

Module Structure​

Week 1: ROS 2 Architecture and Setup​

  • ROS 2 vs ROS 1: Key differences and improvements
  • DDS (Data Distribution Service) and middleware
  • Workspace setup and package management
  • Basic node creation and execution

Week 2: Communication Patterns​

  • Topics and publishers/subscribers
  • Services and clients
  • Actions and clients/servers
  • Parameters and configuration

Week 3: Advanced Communication and Launch Systems​

  • Launch files and complex system management
  • Composition and node containers
  • Quality of Service (QoS) settings
  • Inter-process communication optimization

Week 4: Robot Modeling and URDF​

  • URDF (Unified Robot Description Format) basics
  • Joint types and kinematic chains
  • Visual and collision properties
  • Robot state publisher and joint state publisher

Week 5: Transform Management and Navigation​

  • TF (Transforms) and TF2 concepts
  • Static and dynamic transforms
  • Robot localization in simulation
  • Basic navigation concepts

Week 1: ROS 2 Architecture and Setup​

Introduction to ROS 2​

ROS 2 (Robot Operating System 2) is not an operating system but rather a flexible framework for writing robot software. It provides services designed for a heterogeneous computer cluster such as hardware abstraction, device drivers, libraries, visualizers, message-passing, package management, and more.

Key Improvements in ROS 2​

  1. Real-time support: ROS 2 supports real-time execution, which is crucial for safety-critical robotic applications.
  2. Multi-robot systems: Better support for multi-robot systems and distributed architectures.
  3. Security: Built-in security features including authentication, authorization, and encryption.
  4. Quality of Service (QoS): Configurable communication policies for different types of data.
  5. DDS Middleware: Pluggable middleware layer using DDS (Data Distribution Service) for communication.

Setting Up Your Workspace​

# Create a new workspace
mkdir -p ~/ros2_ws/src
cd ~/ros2_ws

# Source the ROS 2 installation
source /opt/ros/humble/setup.bash  # Replace 'humble' with your ROS 2 distribution

# Build the workspace
colcon build

# Source the workspace
source install/setup.bash

Creating Your First Package​

cd ~/ros2_ws/src
ros2 pkg create --build-type ament_python my_robot_package

Week 2: Communication Patterns​

Topics - Publisher/Subscriber Pattern​

Topics in ROS 2 implement a publish/subscribe communication model where multiple nodes can publish messages to the same topic and multiple nodes can subscribe to the same topic.

# Publisher example
import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class MinimalPublisher(Node):
    def __init__(self):
        super().__init__('minimal_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 World: %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 = MinimalPublisher()
    rclpy.spin(minimal_publisher)
    minimal_publisher.destroy_node()
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Services - Request/Response Pattern​

Services provide a request/response communication model where a client sends a request and waits for a response from a server.

Actions - Goal/Result/Feedback Pattern​

Actions are used for long-running tasks that provide feedback during execution and return a result upon completion.

Week 3: Advanced Communication and Launch Systems​

Launch Files​

Launch files allow you to start multiple nodes with a single command, configure parameters, and set up complex robot systems.

# launch/my_robot.launch.py
from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='my_robot_package',
            executable='publisher_node',
            name='publisher_node'
        ),
        Node(
            package='my_robot_package',
            executable='subscriber_node',
            name='subscriber_node'
        )
    ])

Week 4: Robot Modeling and URDF​

Understanding URDF​

URDF (Unified Robot Description Format) is an XML format for representing a robot model. It describes the kinematic and dynamic properties of a robot, as well as its visual and collision properties.

<?xml version="1.0"?>
<robot name="simple_robot">
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.5 0.5 0.2"/>
      </geometry>
    </visual>
    <collision>
      <geometry>
        <box size="0.5 0.5 0.2"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1"/>
      <inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1"/>
    </inertial>
  </link>
</robot>

Week 5: Transform Management and Navigation​

TF and TF2​

TF (Transforms) is a package that keeps track of multiple coordinate frames over time. It maintains the relationship between coordinate frames in a tree structure buffered in time, and allows you to transform points, vectors, etc. between any two coordinate frames at any desired point in time.

Troubleshooting Common Issues​

1. Package Not Found​

Issue: Package 'package_name' not found Solution: Ensure the package is in your workspace's src directory and run colcon build followed by source install/setup.bash

2. DDS Communication Issues​

Issue: Nodes in different terminals not communicating Solution: Ensure all terminals are sourcing the same ROS 2 installation and workspace

3. Permission Issues​

Issue: Cannot create or modify files Solution: Check file permissions and ensure you're working in your user workspace

Hands-on Exercise​

Create a simple ROS 2 package that includes:

  1. A publisher node that publishes a message every second
  2. A subscriber node that receives and logs the message
  3. A launch file to start both nodes
  4. Proper package configuration and dependencies

Test your package in a simulated environment and verify that the publisher and subscriber communicate correctly.