Lidar Introduction and Use

1. Course Content

• Understand the basic parameters of Lidar.
• Start the sample program to view the visualized point cloud of the Lidar.

1. Lidar Introduction

1.1 T-mini Plus Lidar

The ROSMASTER-M3 uses the T-mini Plus Lidar, which is a 360° 2D Lidar. The T-mini Plus is based on the pulsed Time-of-Flight (ToF) ranging principle, combined with relevant optical, electrical, and algorithmic designs to achieve high-frequency and high-precision distance measurement. At the same time, its mechanical structure rotates 360 degrees to continuously acquire angle information, thus achieving 360-degree scanning and ranging, and outputting point cloud data of the scanned environment.

1.2 Lidar Features

• 360-degree omnidirectional scanning, 6-12Hz adjustable scanning frequency
• High-speed ranging, with a ranging frequency of 4000Hz
• Small ranging error and good ranging stability
• Strong resistance to ambient light interference
• Class I eye safety standard

1.3 Performance Parameters

2. Start the Agent

Note: If it is already running, you do not need to start it again.

Enter the command in the vehicle's terminal:

start_agent

The terminal will print the following information, indicating a successful connection:

3. View Lidar Data

• The ROSMASTER-M3 uses a self-developed ROS control board by Yahboom to drive the Lidar.

• After starting the micro-ROS agent, the data communication node with the chassis will be launched.

• This node forwards the chassis's data topics. For the Lidar topics:

  • Single Lidar version: The published Lidar topics are /scan2 and /scan3. Here, the complete data of a single Lidar is split into left-half and right-half circles and published as two topics.
  • Dual Lidar version: The Lidar at the left rear of the car is /scan0, and the Lidar at the right front is /scan1.

• Here, we take the dual Lidar version as an example to view the data of the /scan0 topic:

For Raspberry Pi 5 users, if the ROS container is not started, you need to start it first,

xxxxxxxxxx
bringup_m3

Open a terminal inside the container,

xxxxxxxxxx
exec_m3

Enter the command,

xxxxxxxxxx
ros2 topic echo /scan0

A frame of data is shown in the figure above, where:

• frame_id: Represents the name of the Lidar's coordinate system.
• angle_min and angle_max: Represent the minimum and maximum radians of the Lidar scan, which translates to 0 to 360 degrees.
• angle_increment: Represents the angular increment.
• range_min and range_max: Represent the minimum and maximum scanning distances of the Lidar, in meters. Here, the minimum distance is 0.05 meters, and the maximum is 12.0 meters.
• ranges: Represents the distance scanned at each angle within the Lidar's scanning range, in meters.

3. Visualize Lidar Point Cloud

• Use RVIZ to visualize the Lidar's point cloud data.

xxxxxxxxxx
ros2 launch m3_bringup laser_view.launch.py

• After starting RVIZ, as shown in the figure above, the colored point cloud is the Lidar's point cloud. By default, it displays the point cloud of the fused /scan topic.

[!NOTE]

• For the single Lidar version: Because the Lidar data is transmitted from the chassis as two topics (left-half and right-half circles), the point cloud fusion combines the left-half and right-half data into a complete circle.
• For the dual Lidar version: The point cloud fusion combines the data from the left-front and right-rear Lidars.

• If you need to view the topic of a single Lidar (dual Lidar version) or a half-circle Lidar (single Lidar version):
• Click on Topic in the LaserScan item on the left and select the corresponding topic name to view the corresponding point cloud.