nav2导航launch文件经过了多层套娃，真的是让初学者哭晕在厕所，今天我们就拆解一下他的launch文件，还原他最简单的状态，看看他到底启动了什么节点。

一 tb3仿真机器人启动文件：tb3_simulation_launch.py

1 文件目录结构

文件取自：Nav2 — Navigation 2 1.0.0 文档

2 文件源代码

# Copyright (c) 2018 Intel Corporation
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""This is all-in-one launch script intended for use by nav2 developers."""

import os

from ament_index_python.packages import get_package_share_directory

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, ExecuteProcess, IncludeLaunchDescription
from launch.conditions import IfCondition
from launch.launch_description_sources import PythonLaunchDescriptionSource
from launch.substitutions import LaunchConfiguration, PythonExpression
from launch_ros.actions import Node


def generate_launch_description():
    # Get the launch directory
    bringup_dir = get_package_share_directory('nav2_bringup')
    launch_dir = os.path.join(bringup_dir, 'launch')

    # Create the launch configuration variables
    slam = LaunchConfiguration('slam')
    namespace = LaunchConfiguration('namespace')
    use_namespace = LaunchConfiguration('use_namespace')
    map_yaml_file = LaunchConfiguration('map')
    use_sim_time = LaunchConfiguration('use_sim_time')
    params_file = LaunchConfiguration('params_file')
    default_bt_xml_filename = LaunchConfiguration('default_bt_xml_filename')
    autostart = LaunchConfiguration('autostart')

    # Launch configuration variables specific to simulation
    rviz_config_file = LaunchConfiguration('rviz_config_file')
    use_simulator = LaunchConfiguration('use_simulator')
    use_robot_state_pub = LaunchConfiguration('use_robot_state_pub')
    use_rviz = LaunchConfiguration('use_rviz')
    headless = LaunchConfiguration('headless')
    world = LaunchConfiguration('world')

    # Map fully qualified names to relative ones so the node's namespace can be prepended.
    # In case of the transforms (tf), currently, there doesn't seem to be a better alternative
    # https://github.com/ros/geometry2/issues/32
    # https://github.com/ros/robot_state_publisher/pull/30
    # TODO(orduno) Substitute with `PushNodeRemapping`
    #              https://github.com/ros2/launch_ros/issues/56
    remappings = [('/tf', 'tf'),
                  ('/tf_static', 'tf_static')]

    # Declare the launch arguments
    declare_namespace_cmd = DeclareLaunchArgument(
        'namespace',
        default_value='',
        description='Top-level namespace')

    declare_use_namespace_cmd = DeclareLaunchArgument(
        'use_namespace',
        default_value='false',
        description='Whether to apply a namespace to the navigation stack')

    declare_slam_cmd = DeclareLaunchArgument(
        'slam',
        default_value='False',
        description='Whether run a SLAM')

    declare_map_yaml_cmd = DeclareLaunchArgument(
        'map',
        default_value=os.path.join(bringup_dir, 'maps', 'turtlebot3_world.yaml'),
        description='Full path to map file to load')

    declare_use_sim_time_cmd = DeclareLaunchArgument(
        'use_sim_time',
        default_value='true',
        description='Use simulation (Gazebo) clock if true')

    declare_params_file_cmd = DeclareLaunchArgument(
        'params_file',
        default_value=os.path.join(bringup_dir, 'params', 'nav2_params.yaml'),
        description='Full path to the ROS2 parameters file to use for all launched nodes')

    declare_bt_xml_cmd = DeclareLaunchArgument(
        'default_bt_xml_filename',
        default_value=os.path.join(
            get_package_share_directory('nav2_bt_navigator'),
            'behavior_trees', 'navigate_w_replanning_and_recovery.xml'),
        description='Full path to the behavior tree xml file to use')

    declare_autostart_cmd = DeclareLaunchArgument(
        'autostart', default_value='true',
        description='Automatically startup the nav2 stack')

    declare_rviz_config_file_cmd = DeclareLaunchArgument(
        'rviz_config_file',
        default_value=os.path.join(bringup_dir, 'rviz', 'nav2_default_view.rviz'),
        description='Full path to the RVIZ config file to use')

    declare_use_simulator_cmd = DeclareLaunchArgument(
        'use_simulator',
        default_value='True',
        description='Whether to start the simulator')

    declare_use_robot_state_pub_cmd = DeclareLaunchArgument(
        'use_robot_state_pub',
        default_value='True',
        description='Whether to start the robot state publisher')

    declare_use_rviz_cmd = DeclareLaunchArgument(
        'use_rviz',
        default_value='True',
        description='Whether to start RVIZ')

    declare_simulator_cmd = DeclareLaunchArgument(
        'headless',
        default_value='False',
        description='Whether to execute gzclient)')

    declare_world_cmd = DeclareLaunchArgument(
        'world',
        # TODO(orduno) Switch back once ROS argument passing has been fixed upstream
        #              https://github.com/ROBOTIS-GIT/turtlebot3_simulations/issues/91
        # default_value=os.path.join(get_package_share_directory('turtlebot3_gazebo'),
        #                            'worlds/turtlebot3_worlds/waffle.model'),
        default_value=os.path.join(bringup_dir, 'worlds', 'waffle.model'),
        description='Full path to world model file to load')

    # Specify the actions
    start_gazebo_server_cmd = ExecuteProcess(
        condition=IfCondition(use_simulator),
        cmd=['gzserver', '-s', 'libgazebo_ros_init.so', world],
        cwd=[launch_dir], output='screen')

    start_gazebo_client_cmd = ExecuteProcess(
        condition=IfCondition(PythonExpression([use_simulator, ' and not ', headless])),
        cmd=['gzclient'],
        cwd=[launch_dir], output='screen')

    urdf = os.path.join(bringup_dir, 'urdf', 'turtlebot3_waffle.urdf')

    start_robot_state_publisher_cmd = Node(
        condition=IfCondition(use_robot_state_pub),
        package='robot_state_publisher',
        executable='robot_state_publisher',
        name='robot_state_publisher',
        namespace=namespace,
        output='screen',
        parameters=[{'use_sim_time': use_sim_time}],
        remappings=remappings,
        arguments=[urdf])

    rviz_cmd = IncludeLaunchDescription(
        PythonLaunchDescriptionSource(os.path.join(launch_dir, 'rviz_launch.py')),
        condition=IfCondition(use_rviz),
        launch_arguments={'namespace': '',
                          'use_namespace': 'False',
                          'rviz_config': rviz_config_file}.items())

    bringup_cmd = IncludeLaunchDescription(
        PythonLaunchDescriptionSource(os.path.join(launch_dir, 'bringup_launch.py')),
        launch_arguments={'namespace': namespace,
                          'use_namespace': use_namespace,
                          'slam': slam,
                          'map': map_yaml_file,
                          'use_sim_time': use_sim_time,
                          'params_file': params_file,
                          'default_bt_xml_filename': default_bt_xml_filename,
                          'autostart': autostart}.items())

    # Create the launch description and populate
    ld = LaunchDescription()

    # Declare the launch options
    ld.add_action(declare_namespace_cmd)
    ld.add_action(declare_use_namespace_cmd)
    ld.add_action(declare_slam_cmd)
    ld.add_action(declare_map_yaml_cmd)
    ld.add_action(declare_use_sim_time_cmd)
    ld.add_action(declare_params_file_cmd)
    ld.add_action(declare_bt_xml_cmd)
    ld.add_action(declare_autostart_cmd)

    ld.add_action(declare_rviz_config_file_cmd)
    ld.add_action(declare_use_simulator_cmd)
    ld.add_action(declare_use_robot_state_pub_cmd)
    ld.add_action(declare_use_rviz_cmd)
    ld.add_action(declare_simulator_cmd)
    ld.add_action(declare_world_cmd)

    # Add any conditioned actions
    ld.add_action(start_gazebo_server_cmd)
    ld.add_action(start_gazebo_client_cmd)

    # Add the actions to launch all of the navigation nodes
    ld.add_action(start_robot_state_publisher_cmd)
    ld.add_action(rviz_cmd)
    ld.add_action(bringup_cmd)

    return ld

二 launch文件究竟启动了什么

1 第一层套娃启动了什么

只看launch文件最后面几行。

    # Add any conditioned actions
    ld.add_action(start_gazebo_server_cmd)
    ld.add_action(start_gazebo_client_cmd)

    # Add the actions to launch all of the navigation nodes
    ld.add_action(start_robot_state_publisher_cmd)
    ld.add_action(rviz_cmd)
    ld.add_action(bringup_cmd)

    return ld

分别启动了gazebo，机器人模型文件，rviz2，和bringup_cmd，这里关于导航的只有bringup_cmd，其他的只是生成了机器人和在rviz2里面展现出来了，下面只分析最重要的—导航。

2 第二层套娃启动了什么

 bringup_cmd = IncludeLaunchDescription(
        PythonLaunchDescriptionSource(os.path.join(launch_dir, 'bringup_launch.py')),
        launch_arguments={'namespace': namespace,
                          'use_namespace': use_namespace,
                          'slam': slam,
                          'map': map_yaml_file,
                          'use_sim_time': use_sim_time,
                          'params_file': params_file,
                          'default_bt_xml_filename': default_bt_xml_filename,
                          'autostart': autostart}.items())

bringup_launch.py 文件目录结构

bringup_launch.py文件内容

# Copyright (c) 2018 Intel Corporation
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import os

from ament_index_python.packages import get_package_share_directory

from launch import LaunchDescription
from launch.actions import (DeclareLaunchArgument, GroupAction,
                            IncludeLaunchDescription, SetEnvironmentVariable)
from launch.conditions import IfCondition
from launch.launch_description_sources import PythonLaunchDescriptionSource
from launch.substitutions import LaunchConfiguration, PythonExpression
from launch_ros.actions import PushRosNamespace


def generate_launch_description():
    # Get the launch directory
    bringup_dir = get_package_share_directory('nav2_bringup')
    launch_dir = os.path.join(bringup_dir, 'launch')

    # Create the launch configuration variables
    namespace = LaunchConfiguration('namespace')
    use_namespace = LaunchConfiguration('use_namespace')
    slam = LaunchConfiguration('slam')
    map_yaml_file = LaunchConfiguration('map')
    use_sim_time = LaunchConfiguration('use_sim_time')
    params_file = LaunchConfiguration('params_file')
    default_bt_xml_filename = LaunchConfiguration('default_bt_xml_filename')
    autostart = LaunchConfiguration('autostart')

    stdout_linebuf_envvar = SetEnvironmentVariable(
        'RCUTILS_LOGGING_BUFFERED_STREAM', '1')

    declare_namespace_cmd = DeclareLaunchArgument(
        'namespace',
        default_value='',
        description='Top-level namespace')

    declare_use_namespace_cmd = DeclareLaunchArgument(
        'use_namespace',
        default_value='false',
        description='Whether to apply a namespace to the navigation stack')

    declare_slam_cmd = DeclareLaunchArgument(
        'slam',
        default_value='False',
        description='Whether run a SLAM')

    declare_map_yaml_cmd = DeclareLaunchArgument(
        'map',
        description='Full path to map yaml file to load')

    declare_use_sim_time_cmd = DeclareLaunchArgument(
        'use_sim_time',
        default_value='false',
        description='Use simulation (Gazebo) clock if true')

    declare_params_file_cmd = DeclareLaunchArgument(
        'params_file',
        default_value=os.path.join(bringup_dir, 'params', 'nav2_params.yaml'),
        description='Full path to the ROS2 parameters file to use for all launched nodes')

    declare_bt_xml_cmd = DeclareLaunchArgument(
        'default_bt_xml_filename',
        default_value=os.path.join(
            get_package_share_directory('nav2_bt_navigator'),
            'behavior_trees', 'navigate_w_replanning_and_recovery.xml'),
        description='Full path to the behavior tree xml file to use')

    declare_autostart_cmd = DeclareLaunchArgument(
        'autostart', default_value='true',
        description='Automatically startup the nav2 stack')

    # Specify the actions
    bringup_cmd_group = GroupAction([
        PushRosNamespace(
            condition=IfCondition(use_namespace),
            namespace=namespace),

        IncludeLaunchDescription(
            PythonLaunchDescriptionSource(os.path.join(launch_dir, 'slam_launch.py')),
            condition=IfCondition(slam),
            launch_arguments={'namespace': namespace,
                              'use_sim_time': use_sim_time,
                              'autostart': autostart,
                              'params_file': params_file}.items()),

        IncludeLaunchDescription(
            PythonLaunchDescriptionSource(os.path.join(launch_dir,
                                                       'localization_launch.py')),
            condition=IfCondition(PythonExpression(['not ', slam])),
            launch_arguments={'namespace': namespace,
                              'map': map_yaml_file,
                              'use_sim_time': use_sim_time,
                              'autostart': autostart,
                              'params_file': params_file,
                              'use_lifecycle_mgr': 'false'}.items()),

        IncludeLaunchDescription(
            PythonLaunchDescriptionSource(os.path.join(launch_dir, 'navigation_launch.py')),
            launch_arguments={'namespace': namespace,
                              'use_sim_time': use_sim_time,
                              'autostart': autostart,
                              'params_file': params_file,
                              'default_bt_xml_filename': default_bt_xml_filename,
                              'use_lifecycle_mgr': 'false',
                              'map_subscribe_transient_local': 'true'}.items()),
    ])

    # Create the launch description and populate
    ld = LaunchDescription()

    # Set environment variables
    ld.add_action(stdout_linebuf_envvar)

    # Declare the launch options
    ld.add_action(declare_namespace_cmd)
    ld.add_action(declare_use_namespace_cmd)
    ld.add_action(declare_slam_cmd)
    ld.add_action(declare_map_yaml_cmd)
    ld.add_action(declare_use_sim_time_cmd)
    ld.add_action(declare_params_file_cmd)
    ld.add_action(declare_autostart_cmd)
    ld.add_action(declare_bt_xml_cmd)

    # Add the actions to launch all of the navigation nodes
    ld.add_action(bringup_cmd_group)

    return ld

还是从后看，启动了一个导航组：bringup_cmd_group

 # Specify the actions
    bringup_cmd_group = GroupAction([
        PushRosNamespace(
            condition=IfCondition(use_namespace),
            namespace=namespace),

        IncludeLaunchDescription(
            PythonLaunchDescriptionSource(os.path.join(launch_dir, 'slam_launch.py')),
            condition=IfCondition(slam),
            launch_arguments={'namespace': namespace,
                              'use_sim_time': use_sim_time,
                              'autostart': autostart,
                              'params_file': params_file}.items()),

        IncludeLaunchDescription(
            PythonLaunchDescriptionSource(os.path.join(launch_dir,
                                                       'localization_launch.py')),
            condition=IfCondition(PythonExpression(['not ', slam])),
            launch_arguments={'namespace': namespace,
                              'map': map_yaml_file,
                              'use_sim_time': use_sim_time,
                              'autostart': autostart,
                              'params_file': params_file,
                              'use_lifecycle_mgr': 'false'}.items()),

        IncludeLaunchDescription(
            PythonLaunchDescriptionSource(os.path.join(launch_dir, 'navigation_launch.py')),
            launch_arguments={'namespace': namespace,
                              'use_sim_time': use_sim_time,
                              'autostart': autostart,
                              'params_file': params_file,
                              'default_bt_xml_filename': default_bt_xml_filename,
                              'use_lifecycle_mgr': 'false',
                              'map_subscribe_transient_local': 'true'}.items()),
    ])

导航最核心的部分，定位导航

很不容易懂得一个地方，condition=IfCondition(), 条件启动，弄懂这个启动文件又精简很多。

第一个条件语句块

PushRosNamespace(
            condition=IfCondition(use_namespace),
            namespace=namespace),

IfCondition条件语句，IfCondition(use_namespace) use_namespace是真这个语句启动，如果是假这个语句块想当于不存在，结合上下文 use_namespace 默认值false，所以这段语句块相当于不存在。

第二个条件语句块

IncludeLaunchDescription(
            PythonLaunchDescriptionSource(os.path.join(launch_dir, 'slam_launch.py')),
            condition=IfCondition(slam),
            launch_arguments={'namespace': namespace,
                              'use_sim_time': use_sim_time,
                              'autostart': autostart,
                              'params_file': params_file}.items()),

这段的意思是是否用slam_launch.py建图。

slam的默认取值default_value='False'，如果按默认启动这段语句也可以忽略。

如果想导航同时建图需要把slam传入True或者直接把默认值改成True。

第三个条件语句块

IncludeLaunchDescription(
            PythonLaunchDescriptionSource(os.path.join(launch_dir,
                                                       'localization_launch.py')),
            condition=IfCondition(PythonExpression(['not ', slam])),
            launch_arguments={'namespace': namespace,
                              'map': map_yaml_file,
                              'use_sim_time': use_sim_time,
                              'autostart': autostart,
                              'params_file': params_file,
                              'use_lifecycle_mgr': 'false'}.items()),

是否启动定位。

condition=IfCondition(PythonExpression(['not ', slam])),

这句很不好理解，直接的意思就是salm取False，整个语句块启动，salm取True，整个语句块不启动。

这个条件语句和上面的条件语句其实是二选一

如果slam取True，slam_launch.py建图启动，localization_launch.py 定位不启动。

如果slam取False，slam_launch.py建图不启动，localization_launch.py 定位启动。

按默认启动的话只是启动 localization_launch.py

导航语句块

 IncludeLaunchDescription(
            PythonLaunchDescriptionSource(os.path.join(launch_dir, 'navigation_launch.py')),
            launch_arguments={'namespace': namespace,
                              'use_sim_time': use_sim_time,
                              'autostart': autostart,
                              'params_file': params_file,
                              'default_bt_xml_filename': default_bt_xml_filename,
                              'use_lifecycle_mgr': 'false',
                              'map_subscribe_transient_local': 'true'}.items()),

通过前面的定位，这里才是启动导航的核心程序 navigation_launch.py

3 第三层套娃

localization_launch.py

这个文件的目录结构

文件内容

# Copyright (c) 2018 Intel Corporation
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import os

from ament_index_python.packages import get_package_share_directory

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, SetEnvironmentVariable
from launch.substitutions import LaunchConfiguration
from launch_ros.actions import Node
from nav2_common.launch import RewrittenYaml


def generate_launch_description():
    # Get the launch directory
    bringup_dir = get_package_share_directory('nav2_bringup')

    namespace = LaunchConfiguration('namespace')
    map_yaml_file = LaunchConfiguration('map')
    use_sim_time = LaunchConfiguration('use_sim_time')
    autostart = LaunchConfiguration('autostart')
    params_file = LaunchConfiguration('params_file')
    lifecycle_nodes = ['map_server', 'amcl']

    # Map fully qualified names to relative ones so the node's namespace can be prepended.
    # In case of the transforms (tf), currently, there doesn't seem to be a better alternative
    # https://github.com/ros/geometry2/issues/32
    # https://github.com/ros/robot_state_publisher/pull/30
    # TODO(orduno) Substitute with `PushNodeRemapping`
    #              https://github.com/ros2/launch_ros/issues/56
    remappings = [('/tf', 'tf'),
                  ('/tf_static', 'tf_static')]

    # Create our own temporary YAML files that include substitutions
    param_substitutions = {
        'use_sim_time': use_sim_time,
        'yaml_filename': map_yaml_file}

    configured_params = RewrittenYaml(
        source_file=params_file,
        root_key=namespace,
        param_rewrites=param_substitutions,
        convert_types=True)

    return LaunchDescription([
        # Set env var to print messages to stdout immediately
        SetEnvironmentVariable('RCUTILS_LOGGING_BUFFERED_STREAM', '1'),

        DeclareLaunchArgument(
            'namespace', default_value='',
            description='Top-level namespace'),

        DeclareLaunchArgument(
            'map',
            default_value=os.path.join(bringup_dir, 'maps', 'turtlebot3_world.yaml'),
            description='Full path to map yaml file to load'),

        DeclareLaunchArgument(
            'use_sim_time', default_value='false',
            description='Use simulation (Gazebo) clock if true'),

        DeclareLaunchArgument(
            'autostart', default_value='true',
            description='Automatically startup the nav2 stack'),

        DeclareLaunchArgument(
            'params_file',
            default_value=os.path.join(bringup_dir, 'params', 'nav2_params.yaml'),
            description='Full path to the ROS2 parameters file to use'),

        Node(
            package='nav2_map_server',
            executable='map_server',
            name='map_server',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_amcl',
            executable='amcl',
            name='amcl',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_lifecycle_manager',
            executable='lifecycle_manager',
            name='lifecycle_manager_localization',
            output='screen',
            parameters=[{'use_sim_time': use_sim_time},
                        {'autostart': autostart},
                        {'node_names': lifecycle_nodes}])
    ])

依然从后面看

       Node(
            package='nav2_map_server',
            executable='map_server',
            name='map_server',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_amcl',
            executable='amcl',
            name='amcl',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_lifecycle_manager',
            executable='lifecycle_manager',
            name='lifecycle_manager_localization',
            output='screen',
            parameters=[{'use_sim_time': use_sim_time},
                        {'autostart': autostart},
                        {'node_names': lifecycle_nodes}])
    ])

分别启动了

executable='map_server', #地图服务

executable='amcl', #amcl定位

executable='lifecycle_manager', #生命周期管理

这里注意 remappings = [('/tf', 'tf'), ('/tf_static', 'tf_static')]

机器人默认发布的是话题是'/tf' '/tf_static' 定位和导航需要接收的话题是 'tf' 'tf_static' ，需要重映射话题的节点需要加上 remappings=remappings 这也是容易产生迷惑的地方。

navigation_launch.py

文件内容

# Copyright (c) 2018 Intel Corporation
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import os

from ament_index_python.packages import get_package_share_directory

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, SetEnvironmentVariable
from launch.substitutions import LaunchConfiguration
from launch_ros.actions import Node
from nav2_common.launch import RewrittenYaml


def generate_launch_description():
    # Get the launch directory
    bringup_dir = get_package_share_directory('nav2_bringup')

    namespace = LaunchConfiguration('namespace')
    use_sim_time = LaunchConfiguration('use_sim_time')
    autostart = LaunchConfiguration('autostart')
    params_file = LaunchConfiguration('params_file')
    default_bt_xml_filename = LaunchConfiguration('default_bt_xml_filename')
    map_subscribe_transient_local = LaunchConfiguration('map_subscribe_transient_local')

    lifecycle_nodes = ['controller_server',
                       'planner_server',
                       'recoveries_server',
                       'bt_navigator',
                       'waypoint_follower']

    # Map fully qualified names to relative ones so the node's namespace can be prepended.
    # In case of the transforms (tf), currently, there doesn't seem to be a better alternative
    # https://github.com/ros/geometry2/issues/32
    # https://github.com/ros/robot_state_publisher/pull/30
    # TODO(orduno) Substitute with `PushNodeRemapping`
    #              https://github.com/ros2/launch_ros/issues/56
    remappings = [('/tf', 'tf'),
                  ('/tf_static', 'tf_static')]

    # Create our own temporary YAML files that include substitutions
    param_substitutions = {
        'use_sim_time': use_sim_time,
        'default_bt_xml_filename': default_bt_xml_filename,
        'autostart': autostart,
        'map_subscribe_transient_local': map_subscribe_transient_local}

    configured_params = RewrittenYaml(
            source_file=params_file,
            root_key=namespace,
            param_rewrites=param_substitutions,
            convert_types=True)

    return LaunchDescription([
        # Set env var to print messages to stdout immediately
        SetEnvironmentVariable('RCUTILS_LOGGING_BUFFERED_STREAM', '1'),

        DeclareLaunchArgument(
            'namespace', default_value='',
            description='Top-level namespace'),

        DeclareLaunchArgument(
            'use_sim_time', default_value='false',
            description='Use simulation (Gazebo) clock if true'),

        DeclareLaunchArgument(
            'autostart', default_value='true',
            description='Automatically startup the nav2 stack'),

        DeclareLaunchArgument(
            'params_file',
            default_value=os.path.join(bringup_dir, 'params', 'nav2_params.yaml'),
            description='Full path to the ROS2 parameters file to use'),

        DeclareLaunchArgument(
            'default_bt_xml_filename',
            default_value=os.path.join(
                get_package_share_directory('nav2_bt_navigator'),
                'behavior_trees', 'navigate_w_replanning_and_recovery.xml'),
            description='Full path to the behavior tree xml file to use'),

        DeclareLaunchArgument(
            'map_subscribe_transient_local', default_value='false',
            description='Whether to set the map subscriber QoS to transient local'),

        Node(
            package='nav2_controller',
            executable='controller_server',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_planner',
            executable='planner_server',
            name='planner_server',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_recoveries',
            executable='recoveries_server',
            name='recoveries_server',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_bt_navigator',
            executable='bt_navigator',
            name='bt_navigator',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_waypoint_follower',
            executable='waypoint_follower',
            name='waypoint_follower',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_lifecycle_manager',
            executable='lifecycle_manager',
            name='lifecycle_manager_navigation',
            output='screen',
            parameters=[{'use_sim_time': use_sim_time},
                        {'autostart': autostart},
                        {'node_names': lifecycle_nodes}]),

    ])

分别启动的节点

   Node(
            package='nav2_controller',
            executable='controller_server',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_planner',
            executable='planner_server',
            name='planner_server',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_recoveries',
            executable='recoveries_server',
            name='recoveries_server',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_bt_navigator',
            executable='bt_navigator',
            name='bt_navigator',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_waypoint_follower',
            executable='waypoint_follower',
            name='waypoint_follower',
            output='screen',
            parameters=[configured_params],
            remappings=remappings),

        Node(
            package='nav2_lifecycle_manager',
            executable='lifecycle_manager',
            name='lifecycle_manager_navigation',
            output='screen',
            parameters=[{'use_sim_time': use_sim_time},
                        {'autostart': autostart},
                        {'node_names': lifecycle_nodes}]),

executable='controller_server', #控制服务器节点

executable='planner_server', #规划服务器节点

executable='recoveries_server', #恢复服务器节点

executable='bt_navigator', #行为数节点

executable='waypoint_follower', #航点跟随节点

executable='lifecycle_manager', #声明周期管理节点

三导航启动的所有节点

1 机器人生成

gazebo仿真：gzserver， gzclient

这一步如果是实体机器人，gezabo程序不需要启动。

2 发布机器人模型并展示

executable='robot_state_publisher',

executable='rviz2',

3 定位

executable='map_server',

executable='amcl',

4 导航

ros2 launch nav2_bringup tb3_simulation_launch.py

executable='controller_server', #控制服务器节点

executable='planner_server', #规划服务器节点

executable='recoveries_server', #恢复服务器节点

executable='bt_navigator', #行为数节点

executable='waypoint_follower', #航点跟随节点

executable='lifecycle_manager', #生命周期管理节点

至此导航仿真拆解完成。

四一边建图一边导航

ros2 launch nav2_bringup tb3_simulation_launch.py slam:=True

slam:=True 通过condition=IfCondition(slam)影响了上面的第三步定位，其他节点不变

 IncludeLaunchDescription(
            PythonLaunchDescriptionSource(os.path.join(launch_dir, 'slam_launch.py')),
            condition=IfCondition(slam),
            launch_arguments={'namespace': namespace,
                              'use_sim_time': use_sim_time,
                              'autostart': autostart,
                              'params_file': params_file}.items()),

拆解slam_launch.py

目录结构

slam_launch.py

# Copyright (c) 2020 Samsung Research Russia
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import os

from ament_index_python.packages import get_package_share_directory

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, IncludeLaunchDescription
from launch.launch_description_sources import PythonLaunchDescriptionSource
from launch.substitutions import LaunchConfiguration
from launch_ros.actions import Node
from nav2_common.launch import RewrittenYaml


def generate_launch_description():
    # Input parameters declaration
    namespace = LaunchConfiguration('namespace')
    params_file = LaunchConfiguration('params_file')
    use_sim_time = LaunchConfiguration('use_sim_time')
    autostart = LaunchConfiguration('autostart')

    # Variables
    lifecycle_nodes = ['map_saver']

    # Getting directories and launch-files
    bringup_dir = get_package_share_directory('nav2_bringup')
    slam_toolbox_dir = get_package_share_directory('slam_toolbox')
    slam_launch_file = os.path.join(slam_toolbox_dir, 'launch', 'online_sync_launch.py')

    # Create our own temporary YAML files that include substitutions
    param_substitutions = {
        'use_sim_time': use_sim_time}

    configured_params = RewrittenYaml(
        source_file=params_file,
        root_key=namespace,
        param_rewrites=param_substitutions,
        convert_types=True)

    # Declare the launch arguments
    declare_namespace_cmd = DeclareLaunchArgument(
        'namespace',
        default_value='',
        description='Top-level namespace')

    declare_params_file_cmd = DeclareLaunchArgument(
        'params_file',
        default_value=os.path.join(bringup_dir, 'params', 'nav2_params.yaml'),
        description='Full path to the ROS2 parameters file to use for all launched nodes')

    declare_use_sim_time_cmd = DeclareLaunchArgument(
        'use_sim_time',
        default_value='true',
        description='Use simulation (Gazebo) clock if true')

    declare_autostart_cmd = DeclareLaunchArgument(
        'autostart', default_value='true',
        description='Automatically startup the nav2 stack')

    # Nodes launching commands
    start_slam_toolbox_cmd = IncludeLaunchDescription(
        PythonLaunchDescriptionSource(slam_launch_file),
        launch_arguments={'use_sim_time': use_sim_time}.items())

    start_map_saver_server_cmd = Node(
            package='nav2_map_server',
            executable='map_saver_server',
            output='screen',
            parameters=[configured_params])

    start_lifecycle_manager_cmd = Node(
            package='nav2_lifecycle_manager',
            executable='lifecycle_manager',
            name='lifecycle_manager_slam',
            output='screen',
            parameters=[{'use_sim_time': use_sim_time},
                        {'autostart': autostart},
                        {'node_names': lifecycle_nodes}])

    ld = LaunchDescription()

    # Declare the launch options
    ld.add_action(declare_namespace_cmd)
    ld.add_action(declare_params_file_cmd)
    ld.add_action(declare_use_sim_time_cmd)
    ld.add_action(declare_autostart_cmd)

    # Running SLAM Toolbox
    ld.add_action(start_slam_toolbox_cmd)

    # Running Map Saver Server
    ld.add_action(start_map_saver_server_cmd)
    ld.add_action(start_lifecycle_manager_cmd)

    return ld

加载主要内容

    slam_launch_file = os.path.join(slam_toolbox_dir, 'launch', 'online_sync_launch.py')
    start_slam_toolbox_cmd = IncludeLaunchDescription(
        PythonLaunchDescriptionSource(slam_launch_file),
        launch_arguments={'use_sim_time': use_sim_time}.items())

    start_map_saver_server_cmd = Node(
            package='nav2_map_server',
            executable='map_saver_server',
            output='screen',
            parameters=[configured_params])

    start_lifecycle_manager_cmd = Node(
            package='nav2_lifecycle_manager',
            executable='lifecycle_manager',
            name='lifecycle_manager_slam',
            output='screen',
            parameters=[{'use_sim_time': use_sim_time},
                        {'autostart': autostart},
                        {'node_names': lifecycle_nodes}])

executable='sync_slam_toolbox_node', #slam工具箱

executable='map_saver_server', #地图存储

executable='lifecycle_manager', #生命周期管理

五总结

tb3_simulation_launch.py 通过修改gazebo启动项可以迁移到实体机器人导航，通过设置slam:=True 或者slam:=False可以选择是否启动建图时导航，实体机器人导航需要把use_sim_time:=false（使用系统时间）, gezabo仿真导航默认的use_sim_time:=true （使用仿真时间）