ROS 2 Lyrical lab 06 · 75 min

Launch and parameters

Start a repeatable robot system and load reviewed limits from YAML.

Practice this lab in your browser: Validate a parameter on set

Understand before building

How this part of ROS 2 works

Launch files describe how a complete system starts. Parameters move reviewed configuration out of source code while keeping each node responsible for validation.

A loaded value is not automatically safe: the node must declare its expected type and reject values outside its operating range.

Before you start

At least two working nodes
A package with launch and config directories

Files you will touch

src/robot_bringup/launch/system.launch.py
src/robot_bringup/config/limits.yaml

Step 1

Set up

status_node:
  ros__parameters:
    max_speed: 0.25
    publish_rate: 10.0

Step 2

Create the file

from pathlib import Path
from ament_index_python.packages import get_package_share_directory
from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    config = str(Path(get_package_share_directory('robot_bringup')) / 'config' / 'limits.yaml')
    return LaunchDescription([
        Node(package='robot_basics', executable='status_node',
             parameters=[config], output='screen')
    ])

What each part does

1.The YAML nests values under the node name and ros__parameters.
2.The launch description starts the executable and supplies the reviewed configuration file.

Step 3

Build, run and inspect

ros2 launch robot_bringup system.launch.py
ros2 param get /status_node max_speed
ros2 param set /status_node max_speed 0.2

Proof that it works

The launch file starts every declared node
max_speed loads as 0.25
Runtime changes are accepted only inside the node's declared safe range

Recovery notes

Install launch and config directories from CMakeLists.txt.
Declare every parameter before reading it and validate safety-critical changes in an on-set callback.

ROS 2 launch tutorials