Cartographer工程化

19年到21年一直在做cartographer的延伸，谨以此篇文章纪念这几年的青春。

        此文章的重点是表述cartographer的问题、工程化应用需面对的挑战及改进思想，将从建图、定位、重定位及语义四个部分阐述，cartographer的简介以及安装不再赘述。

基本参数

外部参数

options = {
  map_builder = MAP_BUILDER,
  trajectory_builder = TRAJECTORY_BUILDER,
  map_frame = "map", --地图坐标系
  tracking_frame = "imu_link",  --跟踪坐标系，一般为IMU
  published_frame = "base_link", --发布坐标系
  odom_frame = "odom",
  provide_odom_frame = false, 
  publish_frame_projected_to_2d = false,
  use_pose_extrapolator = true,
  use_odometry = false,
  use_nav_sat = false,
  use_landmarks = false,
  num_laser_scans = 0,
  num_multi_echo_laser_scans = 1,
  num_subdivisions_per_laser_scan = 10,
  num_point_clouds = 0,
  lookup_transform_timeout_sec = 0.2,
  submap_publish_period_sec = 0.3,
  pose_publish_period_sec = 5e-3,
  trajectory_publish_period_sec = 30e-3,
  rangefinder_sampling_ratio = 1.,
  odometry_sampling_ratio = 1.,
  fixed_frame_pose_sampling_ratio = 1.,
  imu_sampling_ratio = 1.,
  landmarks_sampling_ratio = 1.,
}

前端配置（TRAJECTORY_BUILDER_2D）

TRAJECTORY_BUILDER_2D = {
  use_imu_data = true,
  min_range = 0.,
  max_range = 30.,
  min_z = -0.8,   --IMU模式下会将雷达数据转换为非水平，需谨慎调整此参数
  max_z = 2.,
  missing_data_ray_length = 5.,
  num_accumulated_range_data = 1,
  voxel_filter_size = 0.025,

  adaptive_voxel_filter = {
    max_length = 0.5,
    min_num_points = 200,
    max_range = 50.,
  },

  loop_closure_adaptive_voxel_filter = {
    max_length = 0.9,
    min_num_points = 100,
    max_range = 50.,
  },

  use_online_correlative_scan_matching = false, --若有较好的IMU和ODOM，则为false
  real_time_correlative_scan_matcher = {
    linear_search_window = 0.1,
    angular_search_window = math.rad(20.),
    translation_delta_cost_weight = 1e-1,
    rotation_delta_cost_weight = 1e-1,
  },

  ceres_scan_matcher = {
    occupied_space_weight = 1.,
    translation_weight = 10.,
    rotation_weight = 40.,
    ceres_solver_options = {
      use_nonmonotonic_steps = false,
      max_num_iterations = 20,
      num_threads = 1,
    },
  },
-- 关键帧选择条件，与计算量强相关
  motion_filter = {
    max_time_seconds = 5.,
    max_distance_meters = 0.2, --个人经验，机器人可调至0.5
    max_angle_radians = math.rad(1.), -- 可调至5
  },

  -- TODO(schwoere,wohe): Remove this constant. This is only kept for ROS.
  imu_gravity_time_constant = 10.,
  pose_extrapolator = {
    use_imu_based = false,
    constant_velocity = {
      imu_gravity_time_constant = 10.,
      pose_queue_duration = 0.001,
    },
    imu_based = {
      pose_queue_duration = 5.,
      gravity_constant = 9.806,  -- 重力加速度 9.8 or 10.0