ROS 2 Concepts

Why this matters: ROS 2 is the lingua franca of robotics software. Understanding it is essential for any serious robotics engineer.

Introduction: The Robot Operating System

Despite its name, ROS (Robot Operating System) is not an operating system. It's a middleware framework that provides:

• Communication: Publish-subscribe messaging between processes
• Hardware abstraction: Standardized sensor/actuator interfaces
• Tools: Visualization, simulation, logging
• Ecosystem: Thousands of open-source packages

History

ROS 1 was created at Willow Garage in 2007. ROS 2 was rebuilt from scratch starting in 2014 for real-time, multi-robot, and industrial use.

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Core Concepts

Nodes

The basic unit of computation in ROS 2. Each node is a process that does one thing well.

import rclpy
from rclpy.node import Node

class MinimalNode(Node):
    def __init__(self):
        super().__init__('minimal_node')
        self.get_logger().info('Hello, ROS 2!')

def main():
    rclpy.init()
    node = MinimalNode()
    rclpy.spin(node)
    rclpy.shutdown()

Topics

Publish-subscribe channels for streaming data:

graph LR
    A[Camera Node] -->|/camera/image| B[Perception Node]
    A -->|/camera/image| C[Recording Node]
    D[LiDAR Node] -->|/lidar/points| B

Services

Request-response for one-time queries:

# Service server
@self.create_service(SetMode, 'set_mode', self.set_mode_callback)

# Service client
future = self.cli.call_async(SetMode.Request(mode='WALKING'))

Actions

Long-running tasks with feedback:

sequenceDiagram
    participant C as Client
    participant S as Action Server

    C->>S: Goal (walk to X, Y)
    S-->>C: Accepted
    loop Progress
        S-->>C: Feedback (50% complete)
    end
    S-->>C: Result (arrived)

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Data Flow

Message Types

Standard message definitions:

Package	Message	Contents
sensor_msgs	Image	RGB/Depth camera data
sensor_msgs	Imu	Accelerometer + Gyro
geometry_msgs	Twist	Linear + Angular velocity
nav_msgs	Odometry	Pose + Velocity
std_msgs	String	Text data

Custom Messages

# file: HumanoidState.msg
Header header
float64[] joint_positions
float64[] joint_velocities
geometry_msgs/Pose base_pose
bool is_stable

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Quality of Service (QoS)

ROS 2 adds QoS policies for real-time and reliability:

Policy	Options	Use Case
Reliability	RELIABLE / BEST_EFFORT	Control vs. Video
Durability	TRANSIENT_LOCAL / VOLATILE	Late-joining subscribers
History	KEEP_LAST(n) / KEEP_ALL	Buffer size
Deadline	Duration	Real-time guarantees

from rclpy.qos import QoSProfile, ReliabilityPolicy

sensor_qos = QoSProfile(
    reliability=ReliabilityPolicy.BEST_EFFORT,
    depth=1
)
control_qos = QoSProfile(
    reliability=ReliabilityPolicy.RELIABLE,
    depth=10
)

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Launch Files

Orchestrating multiple nodes:

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

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='perception',
            executable='camera_node',
            name='front_camera'
        ),
        Node(
            package='control',
            executable='walking_controller',
            parameters=[{'max_speed': 1.0}]
        ),
        Node(
            package='planning',
            executable='path_planner'
        )
    ])

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Common Tools

Command Line

# List all running nodes
ros2 node list

# See topics
ros2 topic list
ros2 topic echo /robot/state

# Call a service
ros2 service call /set_mode std_srvs/srv/SetBool "{data: true}"

# Record data
ros2 bag record -a

Visualization (RViz2)

RViz2 displays:

• Robot model (URDF)
• Sensor data (point clouds, images)
• TF frames (coordinate transforms)
• Paths and markers

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ROS 2 vs ROS 1

Feature	ROS 1	ROS 2
Communication	Custom (TCPROS)	DDS standard
Real-time	Limited	Supported
Multi-robot	Difficult	Native support
Windows	Poor	Full support
Python version	2.7	3.8+
Lifecycle	None	Managed nodes

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Key Takeaways

Summary

1. Nodes are the basic computation units
2. Topics enable streaming data (pub-sub)
3. Services are request-response
4. Actions handle long-running tasks
5. QoS provides real-time guarantees
6. ROS 2 is the modern, production-ready version

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Further Reading

• Chapter 3.2: Control Stack
• Chapter 3.3: Perception
• Chapter 1.3: Simulation Basics