ROS 2 Launch System Skill

Purpose: Generate accurate, educational ROS 2 Python launch files and multi-node configurations following official ROS 2 Humble patterns for the Physical AI textbook.

When to Use

• Creating lessons in Chapter 6 (Building Robot Systems)
• Writing multi-node deployment exercises
• Generating worked examples for system startup
• Teaching parameter configuration and node composition
• Chapter 7 capstone project integration

Live Documentation Access

CRITICAL: Before generating any code, fetch current ROS 2 documentation using Context7 MCP:

Tool: mcp__context7__resolve-library-id
libraryName: "ros2 launch"

Then:
Tool: mcp__context7__get-library-docs
context7CompatibleLibraryID: [resolved ID]
topic: "launch file python"

This ensures launch patterns match current ROS 2 Humble API.

Launch File Patterns

Basic Launch File (Canonical)

python
1# File: launch/robot_system.launch.py
2
3from launch import LaunchDescription
4from launch_ros.actions import Node
5
6def generate_launch_description():
7    return LaunchDescription([
8        Node(
9            package='my_robot_pkg',
10            executable='publisher_node',
11            name='sensor_publisher',
12            output='screen',
13        ),
14        Node(
15            package='my_robot_pkg',
16            executable='subscriber_node',
17            name='data_processor',
18            output='screen',
19        ),
20    ])

Launch File with Parameters

python
1from launch import LaunchDescription
2from launch_ros.actions import Node
3
4def generate_launch_description():
5    return LaunchDescription([
6        Node(
7            package='my_robot_pkg',
8            executable='configurable_node',
9            name='robot_controller',
10            output='screen',
11            parameters=[{
12                'robot_name': 'my_robot',
13                'update_rate': 10.0,
14                'max_speed': 1.5,
15                'enabled_sensors': ['lidar', 'camera', 'imu'],
16            }],
17        ),
18    ])

Launch File with Parameter File (YAML)

python
1import os
2from ament_index_python.packages import get_package_share_directory
3from launch import LaunchDescription
4from launch_ros.actions import Node
5
6def generate_launch_description():
7    config = os.path.join(
8        get_package_share_directory('my_robot_pkg'),
9        'config',
10        'robot_params.yaml'
11    )
12
13    return LaunchDescription([
14        Node(
15            package='my_robot_pkg',
16            executable='robot_node',
17            name='robot_controller',
18            output='screen',
19            parameters=[config],
20        ),
21    ])

Parameter File (YAML)

yaml
1# File: config/robot_params.yaml
2robot_controller:
3  ros__parameters:
4    robot_name: "physical_ai_bot"
5    update_rate: 20.0
6    max_linear_speed: 2.0
7    max_angular_speed: 1.57
8    sensors:
9      lidar:
10        enabled: true
11        topic: "/scan"
12      camera:
13        enabled: true
14        topic: "/image_raw"

Launch File with Arguments

python
1from launch import LaunchDescription
2from launch.actions import DeclareLaunchArgument
3from launch.substitutions import LaunchConfiguration
4from launch_ros.actions import Node
5
6def generate_launch_description():
7    # Declare launch arguments
8    robot_name_arg = DeclareLaunchArgument(
9        'robot_name',
10        default_value='default_robot',
11        description='Name of the robot'
12    )
13
14    sim_mode_arg = DeclareLaunchArgument(
15        'sim_mode',
16        default_value='true',
17        description='Run in simulation mode'
18    )
19
20    return LaunchDescription([
21        robot_name_arg,
22        sim_mode_arg,
23        Node(
24            package='my_robot_pkg',
25            executable='robot_node',
26            name='robot_controller',
27            output='screen',
28            parameters=[{
29                'robot_name': LaunchConfiguration('robot_name'),
30                'simulation': LaunchConfiguration('sim_mode'),
31            }],
32        ),
33    ])

Package Structure for Launch Files

my_robot_pkg/
├── my_robot_pkg/
│   ├── __init__.py
│   ├── publisher_node.py
│   └── subscriber_node.py
├── launch/
│   ├── robot_system.launch.py
│   └── debug_system.launch.py
├── config/
│   ├── robot_params.yaml
│   └── debug_params.yaml
├── package.xml
├── setup.py
└── setup.cfg

setup.py (Essential Parts for Launch)

python
1from setuptools import setup
2import os
3from glob import glob
4
5package_name = 'my_robot_pkg'
6
7setup(
8    name=package_name,
9    version='0.1.0',
10    packages=[package_name],
11    data_files=[
12        ('share/ament_index/resource_index/packages',
13            ['resource/' + package_name]),
14        ('share/' + package_name, ['package.xml']),
15        # Include launch files
16        (os.path.join('share', package_name, 'launch'),
17            glob('launch/*.launch.py')),
18        # Include config files
19        (os.path.join('share', package_name, 'config'),
20            glob('config/*.yaml')),
21    ],
22    install_requires=['setuptools'],
23    entry_points={
24        'console_scripts': [
25            'publisher_node = my_robot_pkg.publisher_node:main',
26            'subscriber_node = my_robot_pkg.subscriber_node:main',
27        ],
28    },
29)

Debugging Multi-Node Systems

Using ros2doctor

bash
1# Check system health
2ros2 doctor
3
4# Verbose output
5ros2 doctor --report

Using rqt_graph

bash
1# Visualize node graph
2ros2 run rqt_graph rqt_graph

Logging Levels

python
1# In node code
2self.get_logger().debug('Detailed debug info')
3self.get_logger().info('Normal operation')
4self.get_logger().warn('Warning condition')
5self.get_logger().error('Error occurred')
6self.get_logger().fatal('Fatal error')

bash
1# Set log level at runtime
2ros2 run my_pkg my_node --ros-args --log-level debug

Educational Requirements

Layer 1 (Manual Foundation)

• Explain launch file structure and purpose
• Walk through LaunchDescription components
• Demonstrate parameter passing patterns

Layer 2 (AI Collaboration)

• Generate launch files from system requirements
• Debug multi-node communication issues with AI
• Optimize node configuration (Three Roles INVISIBLE)

Layer 3 (Intelligence Design)

• System architecture patterns
• Configuration management strategies
• Reusable launch file components

Layer 4 (Spec-Driven)

• Design system from specification
• Multi-node integration testing
• Capstone project orchestration

Hardware Tier Compatibility

All launch files MUST work on:

• Tier 1: Cloud ROS 2 (TheConstruct) with turtlesim
• Tier 2+: Local ROS 2 Humble with physical robots

Launch commands:

bash
1# Build and source
2cd ~/ros2_ws
3colcon build --packages-select my_robot_pkg
4source install/setup.bash
5
6# Launch
7ros2 launch my_robot_pkg robot_system.launch.py
8
9# Launch with arguments
10ros2 launch my_robot_pkg robot_system.launch.py robot_name:=my_bot sim_mode:=false

Common Mistakes to Prevent

1. Missing data_files in setup.py: Launch files won't be installed
2. Wrong path in get_package_share_directory: Package name must match
3. Forgetting to rebuild: Changes require colcon build
4. Parameter type mismatches: YAML types must match node expectations

Integration with Other Skills

• ros2-publisher-subscriber: Nodes to launch
• ros2-service-pattern: Service nodes in system
• ros2-custom-interfaces: Interface packages as dependencies
• lesson-generator: For full lesson structure

Authoritative Source

All patterns verified against:

• https://docs.ros.org/en/humble/Tutorials/Intermediate/Launch/Creating-Launch-Files.html
• https://docs.ros.org/en/humble/Tutorials/Beginner-Client-Libraries/Using-Parameters-In-A-Class-Python.html