This repository contains the source code of the mrg_slam package for the Multi-Robot-Graph-SLAM repository. This package contains 4 ROS2 components, see apps folder:

• prefiltering_component
• scan_matching_odometry_component
• floor_detection_component
• mrg_slam_component

For running the SLAM only using LIDAR data, the prefiltering_component, scan_matching_odometry_component, and mrg_slam_component are required. The floor_detection_component is optional and can be used to improve the SLAM performance, when there is a distinct floor in the environment.

Here are some things to consider when using the mrg_slam package:

• The launch file mrg_slam.launch.py launches all the components required for the SLAM in a component container with intraprocess communication enabled. Plus additional nodes that are required for the SLAM to work.

• Command line arguments can be used in conjunction with the launch file to set certain parameters, such as the robot name and the initial pose of the robot.

  • The PARAM_MAPPING dictionary in the launch file maps the command line arguments to the parameters of the components and overwrites if they are given as command line arguments. You can remove and add parameters to the dictionary as needed.
  • All kinds of topics and services are remapped in the launch file to consider the model_namespace aka the robot name.

• The only required message for the SLAM to work is the sensor_msgs/msg/PointCloud2 message with the topic /model_namepsace/velodyne_points. The model_namespace is the name of the robot, which is used to distinguish between the different robots in the system. The frame_id of the point cloud message should be model_namespace/velodyne.

• All robot names participating in the multi-robot SLAM should be given in the multi_robot_names parameter in the used configuration file.

  • You can use the SLAM without a model_namespace by setting the model_namespace parameter to an empty string in the configuration file. This is useful when the robot uses hard-coded frames such as odom or base_link.
  • You can also insert an empty string "" into the multi_robot_names parameter in the configuration file to use the SLAM without a model_namespace in a multi-robot scenario.

• Most nodes in the mrg_slam.yaml can be enabled/disabled by setting the respective parameter to true/false for testing certain parts of the SLAM system.

• Check out the mrg_slam_velodyne_VLP16.yaml file for an example configuration file for the SLAM using live data from a Velodyne VLP-16 LIDAR sensor. use_sim_time is set to false and velodyne/ros__parameters/enable_velodyne is set to true in this configuration file.

  • We launch the velodyne driver node and the transform node ourselves in the launch file, because we want the frame_id in the point cloud message to be model_namespace/velodyne. The frame_id in the point cloud message is set to velodyne by default in the velodyne driver node and cannot be changed easily.
  • Also we don't need the laser scan message which is published by the velodyne driver standard launch file. We only need the point cloud message.

• The prefiltering_component is used to filter the point cloud data before it is used for the SLAM. The component subscribes to the /model_namespace/velodyne_points topic and publishes the filtered point cloud data on the /model_namespace/prefiltering/filtered_points topic.
• The downsample_resolution parameter for the prefiltering component can be used to downsample the point cloud data. On systems with weak computational power, it is recommended to set this parameter to a higher value to reduce the number of points in the point cloud data.

• The scan_matching_odometry_component is used to estimate the odometry of the robot using the point cloud data. The component subscribes to the /model_namespace/prefiltering/filtered_points topic and publishes the odometry data on the /model_namespace/scan_matching_odometry/odom topic.
• Odometry through scan matching is susceptible to drift over time. The enable_imu_frontend parameter is not tested in the ROS2 version of the package. This could potentially be used to reduce the drift in the odometry data by using the IMU data of the robot.

• The mrg_slam_component is used to perform the SLAM using the odometry data from the scan_matching_odometry_component. The component subscribes to the /model_namespace/scan_matching_odometry/odom and /model_namespace/prefiltering/filtered_points topics.
• Depending keyframe_delta_trans and keyframe_delta_angle parameters, the component decides when to add a new keyframe to the graph.
• The graph is updated at graph_update_interval parameter.
• When using multiple robots, the initial poses of all robots should be set w.r.t. the same global frame.
  • The initial poses of the robots needs to be set using the x, y, z (in meters) and roll, pitch, and yaw (in radians) parameters in the configuration file. Alternatively, the initial pose can be set using the command line arguments. ros2 launch mrg_slam mrg_slam.launch.py model_namespace:=robot1 x:=0.0 y:=0.0 z:=0.0 roll:=0.0 pitch:=0.0 yaw:=0.0.
  • Each robot performs SLAM in its own local frame. We enable a static transform broadcaster map2robotmap_publisher node to publish the transform between the global frame map and the local frame of the robot model_namespace/map. This way the maps of all robots can be visualized in the global frame rviz2.