LiDAR Introduction and Use

1. Course Content

• Understand the LiDAR's basic parameters.

• Start the example program and view the LiDAR's visualized point cloud.

2. LiDAR Introduction

• The ROSMASTER-M1Pro package provides two LiDARs: 1. Tmini Plus, 2. Slamtec C1

2.1 T-mini Plus

2.2 Slamtec C1

3. Case Demonstration

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ros2 launch yahboomcar_bringup laser_view.launch.py

The green point cloud in the RVIZ image is the point cloud from the LiDAR scanning the contours of the surrounding environment

• In the LaserScan option in rviz's sidebar, you can view the received LiDAR data information and modify the LiDAR point cloud's color style, etc.

4. Viewing LiDAR Data

• Run the ros2 CLI command

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ros2 topic echo /scan

A single frame of data is as shown above, where:

• frame_id: represents the LiDAR's coordinate frame name.

• angle_min and angle_max: represent the minimum and maximum radians of the LiDAR scan; converted to degrees here, they are 0 degrees to 360 degrees.

• angle_increment: represents the angle increment.

• range_min and range_max: represent the minimum and maximum distance of the LiDAR scan, in meters

• ranges: represents the distance measured at each angle within the LiDAR's scan range, in meters.

5. Source Code Analysis

• car_base launch file path

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~/yahboomcar_ros2_ws/yahboomcar_ws/src/yahboomcar_bringup/launch/car_base.launch.py

• GetLidarType(): gets the current LiDAR model, and starts the corresponding driver respectively according to the return value

• When the model is c1 (Slamtec C1): starts the sllidar_node node, configuring the serial port /dev/rplidar, baud rate 460800, coordinate frame frame_id as laser, and enabling angle compensation angle_compensate

• When the model is tmini (T-mini Plus): calls ydlidar_launch.py via ExecuteProcess to start the driver

• Both are controlled by the use_lidar launch parameter; when it is False, the LiDAR is not started

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def generate_launch_description():
    #bring up LiDAR sensor
    LiDAR_Type=GetLidarType()
    if LiDAR_Type=="c1":
        bringup_LiDAR =Node(
            package='sllidar_ros2',
            executable='sllidar_node',
            name='sllidar_node',
            parameters=[{'channel_type': 'serial',
            'serial_port': '/dev/rplidar', 
            'serial_baudrate': 460800, 
            'frame_id': 'laser',
            'inverted': False, 
            'angle_compensate': True, 
            'scan_mode': 'Standard',
            'masked_sector_center_deg': 0.0,        
            'masked_sector_half_width_deg': 57.0}], 
            output='screen',
            condition=IfCondition(LaunchConfiguration('use_lidar')),
            arguments=['--ros-args', '--log-level', 'warn'])
    elif LiDAR_Type=="tmini":
        bringup_LiDAR = ExecuteProcess(
            cmd=['ros2', 'launch', 'ydlidar_ros2_driver', 'ydlidar_launch.py'],
            output='screen',
            condition=IfCondition(LaunchConfiguration('use_lidar')))

• laser_view launch file path

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~/yahboomcar_ros2_ws/yahboomcar_ws/src/yahboomcar_bringup/launch/laser_view.launch.py

• Loads the Rviz configuration file laser_view.rviz, in which the LiDAR point cloud display style is preset

• Includes car_base.launch.py via IncludeLaunchDescription, enabling only the LiDAR (use_lidar: True) and disabling the IMU, camera, magnetometer, etc.

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def generate_launch_description():

    declared_arguments = []
    declared_arguments.append(
        DeclareLaunchArgument(
            "rviz",
            default_value="true",
            description="Start Rviz2 automatically",
        )
    )

    rviz_config_file= os.path.join(get_package_share_directory('yahboomcar_bringup'), 'rviz', 'laser_view.rviz')
    bringup_laser_launch=IncludeLaunchDescription(
            PythonLaunchDescriptionSource(os.path.join(get_package_share_directory('yahboomcar_bringup'),'launch','car_base.launch.py')),
            launch_arguments={'log_level': 'info',
                              'use_imu': 'False',
                              'use_lidar': 'True',
                              'use_mag': 'False',
                              'use_camera': 'False',
                              'odom_tf_source': 'base'
                              }.items())
    rviz_node=  Node(
        package='rviz2',
        executable='rviz2',
        name='rviz2',
        arguments=['-d', rviz_config_file],
        output='screen',
        condition=IfCondition(LaunchConfiguration("rviz"))
    )