3. ROS 2 primer

If ROS 2 is new to you, start here. If you sat through the workshop, skip to chapter 4.

You need surprisingly little ROS to win this. A node, a subscriber, a publisher, and parameters — that is the whole surface area of a competitive entry.

3.1 The five ideas#

Node — one process doing one job. Your driver is a node. The simulator is a node. The referee is a node.

Topic — a named channel carrying one message type. /scan carries LiDAR; /drive carries steering and speed. Publishers and subscribers find each other automatically; neither knows the other exists.

Message — a typed struct. sensor_msgs/LaserScan has ranges, angle_min, angle_increment. Inspect any type with ros2 interface show <type>.

Parameter — a named value you can set at launch without editing code. This is how you tune without rebuilding, and it is worth using from day one.

Workspace — a folder of packages that colcon build compiles and source install/local_setup.bash puts on your path. Yours is /hackathon/race_ws.

3.2 The whole pattern#

import rclpy
from rclpy.node import Node
from sensor_msgs.msg import LaserScan
from ackermann_msgs.msg import AckermannDriveStamped


class Driver(Node):
    def __init__(self):
        super().__init__('driver')
        self.declare_parameter('speed', 2.0)
        self.speed = self.get_parameter('speed').value

        self.pub = self.create_publisher(AckermannDriveStamped, '/drive', 10)
        self.create_subscription(LaserScan, '/scan', self.scan_callback, 10)

    def scan_callback(self, scan):
        msg = AckermannDriveStamped()
        msg.drive.speed = self.speed
        msg.drive.steering_angle = 0.0
        self.pub.publish(msg)


def main():
    rclpy.init()
    rclpy.spin(Driver())

Everything else is the driving algorithm.

3.3 Commands worth knowing#

ros2 topic list                       # what exists
ros2 topic echo /drive                # watch messages go by
ros2 topic hz /scan                   # publishing rate — is anything alive?
ros2 topic info /scan --verbose       # who publishes and subscribes
ros2 interface show sensor_msgs/msg/LaserScan
ros2 node list
ros2 param list /driver
ros2 param set /driver speed 3.0      # retune a running node
ros2 run <package> <executable>
ros2 launch <package> <file.launch.py>

When something does not work, ros2 topic hz is almost always the fastest way to find out which link in the chain is dead.

3.4 Building#

cd /hackathon/race_ws
colcon build --symlink-install
source install/local_setup.bash

--symlink-install makes Python edits take effect immediately. You only need to rebuild after adding a new file or editing setup.py.

Build one package only:

colcon build --symlink-install --packages-select team_driver

3.5 Workshop material#

The full workshop content is in workshop/:

The workshop workspace is separate from race_ws on purpose — it is reference material, and it is not built during evaluation.

3.6 Further reading#

• ROS 2 Jazzy tutorials — the official ones are good; the beginner CLI and client library sections are enough.
• RoboRacer (F1TENTH) course materials — lectures on reactive methods, pure pursuit and planning, from the people who built this simulator.
• f1tenth_gym — the physics, if you want to know exactly what the car will do.

Next: 4. Algorithms to start from