hector_mapping

        hector_mapping是一种无需里程计数据的SLAM方法，利用激光雷达获得二维姿态估计，虽然没有回环检测功能，但对于大多真实场景，它是较准确的。该系统已用于无人地面机器人、无人地面车辆、手持测绘设备、四旋翼无人机。

1、论文

https://www.sim.informatik.tu-darmstadt.de/publ/download/2011_SSRR_KohlbrecherMeyerStrykKlingauf_Flexible_SLAM_System.pdf

2、硬件需求

        为了使用hector，我们需要高精度的激光扫描仪（SICK、hokuyo等），扫描周围环境时，节点会使用tf变换，因此无需将雷达固定，并且不需要里程计数据。

3、ROS API

        hector_mapping是基于SLAM算法的LIDAR节点，使用时无需里程计数据。介绍ROS API时不会以所有可用于调试的选项，而是以常用选项为介绍。

3.1 订阅话题（Subscirbed Topics）

        scan（sensor_msgs/LaserScan） SLAM系统使用的激光扫描数据

Header header

float32 angle_min 

float32 angle_max

float32 angle_increment

float32 time_increment

float32 scan_time

float32 range_min

float32 range_max

float32[ ] ranges

float32[ ] intensities

        syscommand（Std_msgs/String）系统命令，如果命令是“reset”，那么地图和机器人位姿将重置为初始状态。

string命令

3.2 发布话题（Published Topics）

map_metadata(nav_msgs/MapMetaData)  从这个主题获取地图数据，锁定并定期更新

      这个消息保存了有关栅格占用特性的基本信息

      time map_load_time 加载地图时间

      float32 resolution 地图分辨率[m/cell]

      uint32 width 地图宽度cells

      uint32 height 地图高度cells

      geometry_msgs/Pose origin 原点姿势

map （nav_msgs/OccupancyGrid）从这个主题获取地图数据。。。

      Heard heard //表示2D栅格地图，其中每个单元格的值表示占用的概率

      MapMetaData info  //地图的元数据

      int8[ ] data  //  以行为主顺序从（0，0）开始的地图数据，占用概率在[0,100]，未知是-1

slam_out_pose( geometry_msgs/PoseStamped )   无协方差的机器人姿态估计

      Header header  头标题      带有参考坐标系和时间戳的位姿

      Pose pose  位姿 

poseupdate ( geometry_msgs/PoseWithCovarianceStamped )  基于不确定性高斯估计的机器人姿态估计

      表示一个带有参考坐标系和时间戳的估计位姿

      Header header

      PoseWithCovariance pose

3.3 服务（Service）

dynamic_map( nav_msgs/GetMap )  调用此服务获取地图数据  

      nav_msgs/OccupancyGrid map

3.4 参数（Parameters）

~base_frame (string, default: base_link)

• The name of the base frame of the robot. This is the frame used for localization and for transformation of laser scan data.

~map_frame (string, default: map_link)

• The name of the map frame.

~odom_frame (string, default: odom)

• The name of the odom frame.

~map_resolution (double, default: 0.025)

• The map resolution [m]. This is the length of a grid cell edge.

~map_size (int, default: 1024)

• The size [number of cells per axis] of the map. The map is square and has (map_size * map_size) grid cells.

~map_start_x (double, default: 0.5)

• Location of the origin [0.0, 1.0] of the /map frame on the x axis relative to the grid map. 0.5 is in the middle.

~map_start_y (double, default: 0.5)

• Location of the origin [0.0, 1.0] of the /map frame on the y axis relative to the grid map. 0.5 is in the middle.

~map_update_distance_thresh (double, default: 0.4)

• Threshold for performing map updates [m]. The platform has to travel this far in meters or experience an angular change as described by the map_update_angle_thresh parameter since the last update before a map update happens.

~map_update_angle_thresh (double, default: 0.9)

• Threshold for performing map updates [rad]. The platform has to experience an angular change as described by this parameter of travel as far as specified by the map_update_distance_thresh parameter since the last update before a map update happens.

~map_pub_period (double, default: 2.0)

• The map publish period [s].

~map_multi_res_levels (int, default: 3)

• The number of map multi-resolution grid levels.

~update_factor_free (double, default: 0.4)

• The map update modifier for updates of free cells in the range [0.0, 1.0]. A value of 0.5 means no change.

~update_factor_occupied (double, default: 0.9)

• The map update modifier for updates of occupied cells in the range [0.0, 1.0]. A value of 0.5 means no change.

~laser_min_dist (double, default: 0.4)

• The minimum distance [m] for laser scan endpoints to be used by the system. Scan endpoints closer than this value are ignored.

~laser_max_dist (double, default: 30.0)

• The maximum distance [m] for laser scan endpoints to be used by the system. Scan endpoints farther away than this value are ignored.

~laser_z_min_value (double, default: -1.0)

• The minimum height [m] relative to the laser scanner frame for laser scan endpoints to be used by the system. Scan endpoints lower than this value are ignored.

~laser_z_max_value (double, default: 1.0)

• The maximum height [m] relative to the laser scanner frame for laser scan endpoints to be used by the system. Scan endpoints higher than this value are ignored.

~pub_map_odom_transform (bool, default: true)

• Determine if the map->odom transform should be published by the system.

~output_timing (bool, default: false)

• Output timing information for processing of every laser scan via ROS_INFO.

~scan_subscriber_queue_size (int, default: 5)

• The queue size of the scan subscriber. This should be set to high values (for example 50) if logfiles are played back to hector_mapping at faster than realtime speeds.

~pub_map_scanmatch_transform (bool, default: true)

• Determines if the scanmatcher to map transform should be published to tf. The frame name is determined by the 'tf_map_scanmatch_transform_frame_name' parameter.

~tf_map_scanmatch_transform_frame_name (string, default: scanmatcher_frame)

• The frame name when publishing the scanmatcher to map transform as described in the preceding parameter.

3.5 需要的tf变换

扫描的每帧传入SLAM中，并换算为位姿用于计算

3.6 提供的tf变换

将换算好的位姿在地图上显示

参考：http://wiki.ros.org/hector_mapping