Section 1: rclpy Basics and Node Structure

Estimated Reading Time: 8-10 minutes

What is rclpy?

rclpy (ROS 2 Client Library for Python) is the Python interface to ROS 2. It allows you to:

• Create ROS 2 nodes in Python
• Publish and subscribe to topics
• Call and provide services
• Send and receive actions
• Use the parameter server

Basic Node Structure

Every ROS 2 Python node follows this pattern:

import rclpy
from rclpy.node import Node

class MyNode(Node):
    def __init__(self):
        super().__init__('my_node')
        self.publisher_ = self.create_publisher(Twist, '/cmd_vel', 10)
        self.timer_ = self.create_timer(0.1, self.timer_callback)

    def timer_callback(self):
        msg = Twist()
        self.publisher_.publish(msg)

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

The Control Loop

You must publish velocity commands repeatedly. Use a timer to create a control loop:

• Timer period: 0.1 seconds (10 Hz)
• Callback runs every 0.1 seconds
• Publish velocity commands in the callback
• Robot receives repeated commands and moves smoothly

Velocity Controller Example

See velocity_controller.py in code-examples for a complete example that:

1. Creates a publisher for Twist messages
2. Sets up a 10 Hz control loop
3. Publishes velocity commands
4. Moves the robot in patterns

Integration with Chapter 2

The Python control code integrates with the URDF robots:

Python Node -> Publishes to /cmd_vel -> Gazebo -> Robot moves

Key Takeaways

• rclpy is the Python interface to ROS 2
• Nodes inherit from Node class
• Publishers send messages to topics
• Timers create control loops
• Twist contains linear and angular velocity
• Control loops must run repeatedly (typically 10 Hz)

Next: Section 2: Parameters