LiDAR, combining laser technology, Global Positioning System (GPS), and Inertial Measurement Unit (IMU), offers higher resolution, better concealment, and stronger resistance to interference compared to conventional radar. With continuous technological advancements, Lidar finds increasingly widespread applications in fields such as robotics, autonomous driving, and unmanned vehicles. Where there's demand, there's inevitably a market. With the growing demand for Lidar, the variety of Lidar types has become diverse. Lidars can be categorized into different types based on their functional use, detection method, and payload platform.
It is a device that emits a laser beam toward an object, receives the reflected wave, and records the time difference to determine the distance between the object and the test point. Traditionally, laser radar has been used in industrial safety inspection, such as the laser walls seen in science fiction movies, where the system immediately reacts and issues warnings when someone intrudes. Additionally, laser ranging radar is widely used in spatial mapping. With the rise of the artificial intelligence industry, laser ranging radar has become an indispensable core component inside robots. When used in conjunction with SLAM (Simultaneous Localization and Mapping) technology, it helps robots achieve real-time positioning and navigation, enabling autonomous movement. The RPLIDAR series developed by Slamtec, used in conjunction with the Slamware module, is a typical representative currently serving robots for autonomous positioning and navigation. Within a range of 25 meters, it can perform tens of thousands of laser ranging measurements per second with millimeter-level resolution.
Laser speed LiDAR is a device used to measure the speed of objects. It works by performing two laser ranging measurements on the target object with a specific time interval between them, thereby obtaining the speed of the target object. The method of speed measurement using laser radar can be broadly categorized into two main types. One type is based on the principle of laser ranging. It involves continuously measuring the distance to the target at regular time intervals. By calculating the difference in distance between two consecutive measurements and dividing it by the time interval, the speed of the target can be determined. The direction of the speed can be inferred based on whether the distance difference is positive or negative. This method has a simple system structure but limited measurement accuracy. It is suitable for use with hard targets with strong laser reflection. The other method of speed measurement utilizes the Doppler shift. Doppler shift refers to the frequency difference between the received echo signal and the transmitted signal when there is relative velocity between the target and the laser radar. This frequency difference is known as the Doppler shift.
Laser Imaging LiDAR can be used for detecting and tracking targets, as well as obtaining information about their direction and speed. It can accomplish tasks that conventional radar cannot, such as detecting submarines, mines, and hidden military targets. It finds widespread applications in military, aerospace, industrial, and medical fields.
Atmospheric sensing LiDAR is primarily used to detect the density, temperature, wind speed, wind direction, and concentration of water vapor in the atmosphere. Its purpose is to monitor atmospheric conditions and forecast hazardous weather events such as storms and dust storms.
Tracking LiDAR continuously tracks a target and measures its coordinates, providing the target's trajectory. It is used not only for artillery control, missile guidance, outer ballistic measurement, satellite tracking, breakthrough technology research, etc., but also increasingly in meteorology, transportation, scientific research, and other fields.
Single-line LiDAR is primarily used for obstacle avoidance. It has fast scanning speed, high resolution, and reliability. Due to its faster angular frequency and sensitivity compared to multi-line and 3D lidar, single-line lidar provides more precise distance and accuracy measurements of surrounding obstacles. However, single-line radar can only perform planar scanning and cannot measure object heights, which limits its applications. Currently, it is mainly used in service robots, such as the common robotic vacuum cleaners.
Multi-line LiDAR is primarily used in automotive radar imaging. Compared to single-line lidar, it has made a qualitative change in dimensionality enhancement and scene reconstruction, allowing for the recognition of object height information. Multi-line lidar is typically 2.5D and can also achieve 3D imaging. Currently, there are models available in the international market with 4, 8, 16, 32, and 64 lines. However, they are expensive, so most car manufacturers do not choose to use them.
MEMS-based LiDAR can dynamically adjust its scanning mode to focus on specific objects, collecting detailed information on smaller and more distant objects for identification, which traditional mechanical lidar cannot achieve. The MEMS system only requires a small mirror to guide a fixed laser beam in different directions. Due to the small size of the mirror, its inertial torque is not large, allowing for rapid movement. The speed is fast enough to track 2D scanning mode in less than a second.
Flash lidar can quickly capture the entire scene, avoiding various issues caused by target movement or lidar motion during the scanning process. It operates more like a camera. The laser beam diffuses directly in all directions, so a single flash can illuminate the entire scene. Subsequently, the system uses a micro-sensor array to collect the laser beams reflected back from different directions. Flash lidar has its advantages, but it also has some drawbacks. As the pixel size increases, the number of signals to be processed also increases. If a large number of pixels are packed into the photodetector, it will inevitably bring various interferences, resulting in a decrease in accuracy.
A row of emitters carried by phased array lidar can change the direction of the laser beam by adjusting the relative phase of the signals. Currently, most phased array lidar systems are still in the laboratory, while the technology is still in the era of rotating or MEMS lidar.
Mechanical rotating lidar is an early development in lidar technology, which is currently mature. However, the system structure of mechanical rotating lidar is very complex, and the core components are also quite expensive. These components include lasers, scanners, optical components, photodetectors, receiver ICs, position and navigation devices, and so on. Due to the high hardware costs, mass production is difficult, and stability needs to be improved. Currently, solid-state lidar has become the development direction for many companies.
The basic structure of a direct detection lidar is quite similar to that of a laser rangefinder. During operation, the transmitter system sends out a signal, which is then collected by the receiver system after being reflected by the target. The distance to the target is determined by measuring the time it takes for the laser signal to travel back and forth. As for the radial velocity of the target, it can be determined by the Doppler frequency shift of the reflected light or by measuring the changes in distance between two or more measurements and calculating their rates of change.
In coherent detection lidar, there are two types: single-stable and double-stable. In a single-stable system, the transmission and reception signals share the same optical aperture and are isolated by a transmit-receive switch. In contrast, a double-stable system consists of two separate optical apertures, each dedicated to transmission and reception signals. In this case, a transmit-receive switch is not required, and the rest of the components are similar to those of the single-stable system.
From the perspective of laser principles, continuous-wave laser means there is always light emitted, similar to turning on the switch of a flashlight, where the light remains continuously on (except in special circumstances). Continuous-wave lasers rely on continuous light emission to gather data at a certain height. Due to the continuous emission characteristic, data can only be collected for one point at a time. Because of the uncertain nature of wind data, representing wind conditions at a certain height with a single point is somewhat limited. Therefore, some manufacturers adopt a compromise by rotating 360 degrees and collecting multiple points along this circle for averaging evaluation, which represents a concept of statistical data from multiple points on a virtual plane.
Pulsed laser output is not continuous, but rather intermittent, flashing on and off. The principle of pulsed laser is to emit thousands of laser particles. Based on the internationally recognized Doppler principle, the reflection of these thousands of laser particles is used to comprehensively evaluate the wind conditions at a certain altitude. This is a three-dimensional concept, which is why there is a theoretical detection length. In terms of laser characteristics, pulsed laser measurement can capture tens of times more points than continuous laser, providing a more accurate reflection of wind conditions at a certain altitude.
According to the payload platform, LiDAR can be classified as follows:
Installed on the ground, typically used for geological exploration, urban planning, and construction measurement.
Installed on vehicles, commonly used for mapmaking, autonomous driving, and traffic monitoring.
Mounted on aircraft or other aerial platforms, used for terrain mapping, forest resource management, and geological exploration.
Installed on ships, used for marine geological exploration, coastline mapping, etc.
Carried by individuals, used for indoor navigation, virtual reality, etc.
Mounted on satellites or spacecraft, used for atmospheric detection, Earth observation, etc.
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