2D LiDAR

Overview

2D LiDAR (also called single-line laser rangefinder or laser scanner) performs 360-degree or limited-angle distance scanning in a single plane, outputting one ring of two-dimensional distance data (LaserScan). It is the most commonly used sensor for indoor mobile robot, robot vacuum, and AGV navigation.

Working Principles

2D LiDARs typically use ToF or triangulation principles:

Triangulation

Low-cost 2D LiDARs (e.g., RPLIDAR A1) commonly use triangulation:

\[ d = \frac{f \cdot B}{\Delta p} \]

Where:

$f$ is the lens focal length
$B$ is the baseline distance between emitter and receiver
$\Delta p$ is the pixel offset

Characteristics: High accuracy at close range, decreasing accuracy at longer range, low cost.

ToF Ranging

Mid-to-high-end 2D LiDARs (e.g., Hokuyo, RPLIDAR S2) use ToF:

\[ d = \frac{c \cdot t}{2} \]

Characteristics: Good long-range accuracy, large measurement range.

Mainstream Product Comparison

RPLIDAR Series (SLAMTEC)

Model	Range	Sample Rate	Scan Frequency	Ranging Principle	Interface	Price (Ref.)
A1M8	0.15-12m	8000 pts/s	5.5Hz	Triangulation	UART	~$99
A2M12	0.15-18m	16000 pts/s	10Hz	Triangulation	UART	~$299
A3M1	0.15-25m	16000 pts/s	10Hz	Triangulation	UART	~$399
S1	0.1-40m	9200 pts/s	10Hz	ToF	UART	~$149
S2	0.05-30m	32000 pts/s	10Hz	ToF	UART	~$349
C1	0.05-12m	5000 pts/s	10Hz	Triangulation	UART	~$69

RPLIDAR A1 -- The Go-To Entry-Level Choice

RPLIDAR A1 is the most widely used entry-level LiDAR in the ROS community, with a low price, well-developed SDK, and strong community support. It is ideal for learning SLAM and mobile robot development.

YDLIDAR Series

Model	Range	Sample Rate	Scan Frequency	Ranging Principle	Interface	Price (Ref.)
X4	0.12-10m	5000 pts/s	6-12Hz	Triangulation	UART	~$69
X4PRO	0.12-10m	5000 pts/s	6-12Hz	Triangulation	UART	~$79
G4	0.26-16m	9000 pts/s	5-12Hz	Triangulation	UART	~$159
TG30	0.05-30m	20000 pts/s	10Hz	ToF	UART	~$299
TMini Pro	0.02-12m	4000 pts/s	6Hz	Triangulation	UART	~$39

Hokuyo Series

Model	Range	Sample Rate	Scan Angle	Ranging Principle	Interface	Price (Ref.)
URG-04LX	0.02-5.6m	-	240 deg	ToF	USB/UART	~$1,000
UTM-30LX	0.1-30m	-	270 deg	ToF	USB/Ethernet	~$4,500
UST-10LX	0.06-10m	-	270 deg	ToF	Ethernet	~$1,600

Hokuyo's Positioning

Hokuyo focuses on industrial-grade quality and high reliability. Prices are much higher than domestic alternatives, but they offer advantages in accuracy, stability, and lifespan. Suitable for commercially deployed AGVs and service robots.

SICK Series (Safety-Rated)

Model	Range	Scan Angle	Features	Price (Ref.)
TiM551	0.05-10m	270 deg	Industrial grade	~$1,200
TiM571	0.05-25m	270 deg	Industrial grade	~$2,000
S300 Mini	0.05-30m	270 deg	Safety certified SIL2/PLd	~$5,000+

Data Format

2D LiDARs use the sensor_msgs/msg/LaserScan message in ROS2:

# sensor_msgs/msg/LaserScan
Header header              # Timestamp and frame
float32 angle_min          # Start angle (rad)
float32 angle_max          # End angle (rad)
float32 angle_increment    # Angular increment (rad)
float32 time_increment     # Measurement time increment (s)
float32 scan_time          # Scan period (s)
float32 range_min          # Minimum valid range (m)
float32 range_max          # Maximum valid range (m)
float32[] ranges           # Range array (m)
float32[] intensities      # Reflection intensity array

Each frame contains one ring of distance measurements, from angle_min to angle_max with step size angle_increment.

ROS2 Integration

RPLIDAR ROS2 Driver

# Install rplidar_ros package
sudo apt install ros-humble-rplidar-ros

# Or build from source
cd ~/ros2_ws/src
git clone https://github.com/Slamtec/rplidar_ros.git -b ros2
cd ~/ros2_ws
colcon build --packages-select rplidar_ros

Launch node:

# RPLIDAR A1
ros2 launch rplidar_ros rplidar_a1_launch.py

# RPLIDAR A2
ros2 launch rplidar_ros rplidar_a2m12_launch.py

# View data
ros2 topic echo /scan

YDLIDAR ROS2 Driver

cd ~/ros2_ws/src
git clone https://github.com/YDLIDAR/ydlidar_ros2_driver.git
cd ~/ros2_ws
colcon build --packages-select ydlidar_ros2_driver

Launch File Example

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='rplidar_ros',
            executable='rplidar_node',
            name='rplidar_node',
            parameters=[{
                'serial_port': '/dev/ttyUSB0',
                'serial_baudrate': 115200,
                'frame_id': 'laser_frame',
                'angle_compensate': True,
                'scan_mode': 'Standard',
            }],
            output='screen',
        ),
    ])

Typical Application Scenarios

2D LiDAR is the core sensor for mobile robot SLAM. Common SLAM algorithms:

Algorithm	Type	ROS2 Package	Features
Cartographer	Graph optimization	`cartographer_ros`	Google open-source, high accuracy
SLAM Toolbox	Graph optimization	`slam_toolbox`	ROS2 recommended, supports lifelong mapping
GMapping	Particle filter	`slam_gmapping`	Classic algorithm, low resource consumption
Hector SLAM	Scan matching	`hector_slam`	No odometry required

For detailed SLAM content, see SLAM Topic

Robot vacuums are the largest consumer application for 2D LiDAR:

graph TD
    A[LiDAR Scan] --> B[SLAM Mapping]
    B --> C[Global Path Planning]
    C --> D[Area Coverage Planning]
    D --> E[Local Obstacle Avoidance]
    E --> F[Execute Motion]
    F --> A

    G[Cliff Sensor] --> E
    H[Bumper Sensor] --> E

Key characteristics:

Uses low-cost LiDAR (custom modules similar to RPLIDAR A1)
LDS (Laser Distance Sensor) mounted on top, rotating
Combined with cliff sensors and bumper sensors
Real-time mapping + zone-based cleaning planning

Safety Protection

Industrial AGVs use safety-rated LiDARs (e.g., SICK S300) for safety zone monitoring:

Protection zone: Immediate stop when obstacle detected
Warning zone: Slow down or change path
Compliant with IEC 61496 safety standard

Installation and Debugging Notes

Mounting height: Ensure the scan plane is at the desired detection height; avoid scanning the ground
Occlusion: Ensure the robot body does not block the LiDAR FOV (or configure min/max angle in URDF)
Serial port permissions: On Linux, run sudo usermod -aG dialout $USER
Coordinate frame: Ensure TF transforms are correct (base_link -> laser_frame)
Reflectivity: Dark or transparent objects may not be detected

References

SLAMTEC RPLIDAR Official Documentation: https://www.slamtec.com
YDLIDAR Development Documentation: https://www.ydlidar.com
ROS2 Navigation2 Documentation
Probabilistic Robotics - Thrun et al.