Introduction to ROS2

Week 10 • CMPSC 304 Robotic Agents

Press Space or → to advance

The Problem: Building Robot Software Is Hard

Without ROS

Write your own sensor drivers
Write your own motor controllers
Write your own communication layer
Write your own navigation algorithms
Everything is custom, every time

With ROS

Standardized sensor interfaces
Reusable motor controllers
Built-in publish/subscribe messaging
Full navigation stack available
Write once, reuse everywhere

ROS = Robot Operating System — not actually an OS, but middleware that connects robot software components.

ROS in the Real World

Research: Most university and research lab robots run ROS
Industry: Warehouse robots, agricultural robots, inspection drones
Self-driving cars: Autoware (built on ROS2) powers autonomous vehicles
Space: NASA uses ROS for rover prototyping and ISS experiments

Why ROS2? ROS2 is the current generation. Improvements over ROS1: real-time support, better security, no single point of failure, multi-platform (Linux, macOS, Windows).

How ROS Fits Our Course

Arduino
Hardware

→

PLC
Deterministic

→

FANUC
Industrial

→

ROS2
Autonomous

You Already Know	ROS2 Equivalent
Arduino sensors	ROS topics (sensor data streams)
Arduino motor control	ROS nodes (individual processes)
PLC scan cycle	ROS publish/subscribe loop
FANUC teach pendant	ROS command-line tools
FANUC program timeline	ROS launch files

Core Concept: Nodes

A node is a single process that does one job.

Lidar
Driver

Camera
Driver

SLAM
Mapper

Path
Planner

Motor
Controller

Each node is independent — it can be started, stopped, or replaced individually
Nodes communicate with each other through topics
A robot is a graph of connected nodes

ros2 node list          # See all running nodes

Core Concept: Topics

A topic is a named channel for sending messages between nodes.

Lidar
Node

/scan

⟶

SLAM
Node

Teleop
Node

/cmd_vel

⟶

Motor
Node

Publishers send data to a topic
Subscribers receive data from a topic
Multiple nodes can publish or subscribe to the same topic

ros2 topic list         # See all active topics
ros2 topic echo /scan   # View messages on a topic

Core Concept: Messages

A message is structured data sent over a topic. Each topic has a specific message type.

Twist (velocity)

Topic: /cmd_vel

linear:
  x: 0.2    # forward m/s
  y: 0.0
  z: 0.0
angular:
  x: 0.0
  y: 0.0
  z: 0.5    # turn rad/s

LaserScan (lidar)

Topic: /scan

angle_min: -3.14
angle_max: 3.14
ranges:
  - 2.45    # distance (m)
  - 2.41
  - 1.03    # obstacle!
  - 0.52
  ...

The Publish/Subscribe Pattern

Nodes don't talk to each other directly. They communicate through topics — publishers don't know who is listening, and subscribers don't know who is sending.

Why is this useful?

Modularity: Swap a real lidar for a simulated one — subscribers don't change
Scalability: Add a new node that listens to /scan without modifying existing code
Testing: Record and replay topic data for debugging

Think about it: In your Arduino projects, the sensor code and motor code were tightly coupled in one file. ROS separates them into independent nodes connected by topics.

Simulation with Gazebo

What Gazebo Provides

Physics engine (gravity, collisions, friction)
Simulated sensors (lidar, camera, IMU)
3D environments with obstacles
Sensors publish to the same ROS topics as real hardware

Why Simulate First?

✓ No hardware damage from bugs
✓ Everyone gets their own robot
✓ Reset instantly after crashes
✓ Test algorithms before real deployment

Key insight: Code you write for a simulated TurtleBot 4 in Gazebo will also run on a real TurtleBot 4 — because both use the same ROS2 topics and messages.

TurtleBot 4: Our Robot Platform

TurtleBot 4 — built on the iRobot Create 3 base, designed for ROS2 education and research.

Hardware

iRobot Create 3 base (differential drive)
RPLIDAR A1 — 360° 2D lidar
OAK-D stereo camera (depth + RGB)
IMU, cliff sensors, bump sensors
Raspberry Pi 4

Key ROS2 Topics

/scan — lidar distance data
/odom — position estimate
/cmd_vel — velocity commands
/imu — orientation data
/oakd/rgb/preview/image_raw — camera

# Launch TurtleBot 4 in Gazebo
ros2 launch turtlebot4_ignition_bringup turtlebot4_ignition.launch.py

Essential ROS2 Commands

Command	What It Does
`ros2 node list`	List all running nodes
`ros2 topic list`	List all active topics
`ros2 topic echo /scan`	Print messages from a topic in real time
`ros2 topic echo /scan --once`	Print one message and stop
`ros2 run <pkg> <node>`	Start a single node
`ros2 launch <pkg> <file>`	Start multiple nodes at once

Tip: You only need these 6 commands for the rest of the semester. Everything else builds on them.

What's Coming Next

Week	Topic	What You'll Do
10	ROS2 Basics	Set up environment, drive TurtleBot 4, explore topics
11	SLAM & Mapping	Build a map by driving the robot around
12	Localization & Navigation	Robot finds itself on a map, plans paths to goals
13	Autonomous Navigation	Robot avoids obstacles and replans paths

Today: Set up your Docker environment and drive a simulated robot.

Key Takeaways

ROS2 is middleware that connects robot software using publish/subscribe messaging
Nodes are independent processes that each do one job
Topics are named channels that carry messages between nodes
Gazebo simulates physics and sensors — code works the same on real hardware
TurtleBot 4 is our robot platform — lidar, camera, odometry, and velocity control
You only need 6 commands to inspect and control the entire system

Now let's set up the environment! → Activity 5