【ROS】ROS2-launch：多节点的启动和配置脚本

【ROS】郭老二博文之：ROS目录

1、简述

launch功能用于多节点的启动和配置。
在ROS1中，launch文件由XML描述文档实现
在ROS2中，launch文件由python脚本实现；

2、回顾

ROS1中launch文件是XML描述文档，它长这个样子(turtlesim.xml)：

<launch>
    <node pkg="turtlesim" type="turtlesim_node"     name="my_turtle" output="screen" />
    <node pkg="turtlesim" type="turtle_teleop_key"  name="my_key" output="screen" />
launch>

运行：

roslaunch package_name  launch_file_name

eg：

roslaunch turtlesim turtlesim.xml

3、初识ROS2-launch

先看个ROS2自带的launch脚本的例子，

/opt/ros/humble/share/turtlesim/launch$ cat multisim.launch.py

它长这个样子：

from launch import LaunchDescription           # launch文件的描述类
from launch_ros.actions import Node            # 节点启动的描述类

def generate_launch_description():             # 自动生成launch文件的函数
    return LaunchDescription([                 # 返回launch文件的描述信息
        Node(                                  # 配置一个节点的启动
            namespace= "turtlesim1",		   # 节点名字
            package='turtlesim',          	   # 节点所在的功能包
            executable='turtlesim_node',       # 节点的可执行文件
            output='screen',                   # 在屏幕显示
        ),
        Node(                                  # 配置一个节点的启动
            namespace= "turtlesim2",		   # 节点名字
            package='turtlesim',          	   # 节点所在的功能包
            executable='turtlesim_node',       # 节点的可执行文件
            output='screen',                   # 在屏幕显示
        ),
    ])

运行：

ros2 launch turtlesim multisim.launch.py

将会启动两个小海龟

4、复杂的launch脚本

下面是一个测试usb_cam的例子：

/opt/ros/humble/share/usb_cam/launch$ cat demo_launch.py

这个例子演示了，启动两个节点，并且加载了一个保存参数的文件params.yaml

import argparse
import os
import sys
from ament_index_python.packages import get_package_share_directory
from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    ld = LaunchDescription()
    parser = argparse.ArgumentParser(description='usb_cam demo')
    parser.add_argument('-n', '--node-name', dest='node_name', type=str,
                        help='name for device', default='usb_cam')
    args, unknown = parser.parse_known_args(sys.argv[4:])
    usb_cam_dir = get_package_share_directory('usb_cam')

    # get path to params file
    params_path = os.path.join(
        usb_cam_dir,
        'config',
        'params.yaml'
    )
    node_name = args.node_name
    print(params_path)
    ld.add_action(Node(
        package='usb_cam', executable='usb_cam_node_exe', output='screen',
        name=node_name,
        # namespace=ns,
        parameters=[params_path]
        ))
    ld.add_action(Node(
        package='usb_cam', executable='show_image.py', output='screen',
        # namespace=ns,
        # arguments=[image_manip_dir + "/data/mosaic.jpg"])
        # remappings=[('image_in', 'image_raw')]
        ))
    return ld

启动命令

ros2 launch usb_cam demo_launch.py