自己位置推定の入出力について
自己位置推定の入出力は下記の図のようになっています。
地図作成(SLAM)の入出力と逆順になり、最終的に/tfトピック(map→odom)を出力しています。
自己位置推定のアルゴリズム自体から出力されるのは、/tfトピック(map→base_footprint)ですが、TFのルール上、親フレームが1つでなければならないので、(map→base_footprint)と(odom→base_footprint)のベクトルの引き算により、(map→odom)を算出し出力しています。
このような流れになっているのはルール上仕方なくということではなく意味があります。
(map→base_footprint)を出力としてしまうと、自己位置推定が誤ったときなどに大きなズレが生じますし、実行周期も速くはありません。その場合、後段の経路計画や経路追従制御に大きな悪影響を及ぼします。(具体例としてはDARPAグランド・チャレンジでの失敗)
なので、直近の連続性や実行周期を保つために(map→odom→base_footprint)となっています。
localization_launch.pyの内容
localization_launch.pyでは、map_serverノードとamclノードを起動しています。
# localization_launch.py
import os
from ament_index_python.packages import get_package_share_directory
from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, GroupAction, SetEnvironmentVariable
from launch.conditions import IfCondition
from launch.substitutions import LaunchConfiguration, PythonExpression
from launch_ros.actions import LoadComposableNodes
from launch_ros.actions import Node
from launch_ros.descriptions import ComposableNode
from nav2_common.launch import RewrittenYaml
def generate_launch_description():
# Get the launch directory
# bringup_dir = get_package_share_directory('nav2_bringup')
bringup_dir = get_package_share_directory('lecture_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')
use_composition = LaunchConfiguration('use_composition')
container_name = LaunchConfiguration('container_name')
use_respawn = LaunchConfiguration('use_respawn')
log_level = LaunchConfiguration('log_level')
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)
stdout_linebuf_envvar = SetEnvironmentVariable(
'RCUTILS_LOGGING_BUFFERED_STREAM', '1')
declare_namespace_cmd = DeclareLaunchArgument(
'namespace',
default_value='',
description='Top-level namespace')
declare_map_yaml_cmd = DeclareLaunchArgument(
'map',
default_value=os.path.join(bringup_dir, 'maps', 'turtlebot3_world.yaml'),
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_autostart_cmd = DeclareLaunchArgument(
'autostart', default_value='true',
description='Automatically startup the nav2 stack')
declare_use_composition_cmd = DeclareLaunchArgument(
'use_composition', default_value='False',
description='Use composed bringup if True')
declare_container_name_cmd = DeclareLaunchArgument(
'container_name', default_value='nav2_container',
description='the name of conatiner that nodes will load in if use composition')
declare_use_respawn_cmd = DeclareLaunchArgument(
'use_respawn', default_value='False',
description='Whether to respawn if a node crashes. Applied when composition is disabled.')
declare_log_level_cmd = DeclareLaunchArgument(
'log_level', default_value='info',
description='log level')
load_nodes = GroupAction(
condition=IfCondition(PythonExpression(['not ', use_composition])),
actions=[
Node(
package='nav2_map_server',
executable='map_server',
name='map_server',
output='screen',
respawn=use_respawn,
respawn_delay=2.0,
parameters=[configured_params],
arguments=['--ros-args', '--log-level', log_level],
remappings=remappings),
Node(
package='nav2_amcl',
executable='amcl',
name='amcl',
output='screen',
respawn=use_respawn,
respawn_delay=2.0,
parameters=[configured_params],
arguments=['--ros-args', '--log-level', log_level],
remappings=remappings),
Node(
package='nav2_lifecycle_manager',
executable='lifecycle_manager',
name='lifecycle_manager_localization',
output='screen',
arguments=['--ros-args', '--log-level', log_level],
parameters=[{'use_sim_time': use_sim_time},
{'autostart': autostart},
{'node_names': lifecycle_nodes}])
]
)
load_composable_nodes = LoadComposableNodes(
condition=IfCondition(use_composition),
target_container=container_name,
composable_node_descriptions=[
ComposableNode(
package='nav2_map_server',
plugin='nav2_map_server::MapServer',
name='map_server',
parameters=[configured_params],
remappings=remappings),
ComposableNode(
package='nav2_amcl',
plugin='nav2_amcl::AmclNode',
name='amcl',
parameters=[configured_params],
remappings=remappings),
ComposableNode(
package='nav2_lifecycle_manager',
plugin='nav2_lifecycle_manager::LifecycleManager',
name='lifecycle_manager_localization',
parameters=[{'use_sim_time': use_sim_time,
'autostart': autostart,
'node_names': lifecycle_nodes}]),
],
)
# 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_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_use_composition_cmd)
ld.add_action(declare_container_name_cmd)
ld.add_action(declare_use_respawn_cmd)
ld.add_action(declare_log_level_cmd)
# Add the actions to launch all of the localiztion nodes
ld.add_action(load_nodes)
ld.add_action(load_composable_nodes)
return ld