Detailed Explanation of the 7 Major Classifications of LIDAR
Lidar is a system that combines three technologies: laser, global positioning system (GPS), and IMU (inertial measurement unit). Compared with ordinary radar, LIDAR has the advantages of high resolution, good concealment, and stronger anti-interference ability.
With the continuous development of science and technology, the application of LIDAR is becoming more and more widespread, and it can be seen in the fields of robotics, driverless, unmanned vehicles, etc. There is bound to be a market for it, and with the increasing demand for LIDAR, the types of LIDAR has become dazzling, and LIDAR can be divided into different types according to the use of function, detection mode, load platform, etc.
Lidar by function
Laser distance measurement radar
Laser distance measurement radar is used to determine the distance between the object under test and the test point by emitting a laser beam to the object under test and receiving the reflected wave of that laser beam and recording that time difference. Traditionally, LIDAR can be used in the field of security detection in industry, such as the laser wall seen in science fiction films, where the system reacts immediately with a warning when someone breaks in. In addition, laser-ranging radar is also widely used in the field of spatial mapping. However, with the rise of the artificial intelligence industry, laser ranging radar has become an indispensable core component in the robot body, used with SLAM technology, which can help the robot to conduct real-time positioning and navigation and realize autonomous walking. The rplidar series developed by Silan Technology with SLAMware module is a typical representative of autonomous positioning and navigation for service robots, which can complete laser ranging tens of thousands of times per second within a 25m ranging radius and achieve millimeter level resolution.
Laser velocimetry radar is the measurement of the movement speed of an object by performing two laser distance measurements with a specific time interval to obtain the movement speed of the object under test.
There are two main types of lidar speed measurement methods, one is based on the lidar distance measurement principle to achieve, that is, a certain time interval to continuously measure the target distance, the difference between the two target distances divided by the time interval will be able to know the speed value of the target, the direction of the speed can be determined according to the positive and negative of the distance difference. This method has a simple system structure and limited measurement accuracy, and can only be used for hard targets with strong laser reflection.
Another type of speed measurement method is the use of Doppler shift. Doppler shift is a frequency difference between the frequency of the received echo signal and the frequency of the transmitted signal when there is a relative velocity between the target and the LIDAR, and this frequency difference is the Doppler shift.
Laser imaging radar can be used to detect and track targets, obtain target orientation and speed information, etc. It can perform tasks that ordinary radar cannot, such as detecting submarines, mines, hidden military targets, and so on. It is widely used in military, aerospace, industrial and medical fields.
Atmospheric detection lidar is mainly used to detect the density of atmospheric molecules, smoke, temperature, wind speed, wind direction, and the concentration of atmospheric water vapor, in order to achieve the purpose of monitoring the atmospheric environment and the forecast of storms, dust storms and other catastrophic weather.
Tracking radar can continuously go to track a target and measure the coordinates of that target and provide the trajectory of the target's movement. It is used not only for artillery control, missile guidance, external ballistic measurement, satellite tracking, and research on surprise defense technology, but is also expanding in the fields of meteorology, transportation, and scientific research.
Classification by working medium
The high peak power of solid-state lidar, the output wavelength range matching existing optical components and devices, the output long range matching existing optical components and devices (e.g., modulators, isolators, and detectors), and atmospheric transmission characteristics, etc., and the easy implementation of the main oscillator-power amplifier (MOPA) structure, coupled with the conductors of high efficiency, small size, lightweight, high reliability, and good stability, solid-state LIDAR is preferentially used in airborne and space-based systems. In recent years, the focus of lidar development has been on diode-pumped solid-state lidars.
Gas lidar is represented by CO2 lidar, which works in the infrared band with small atmospheric transmission attenuation and long detection distance, and has played a great role in atmospheric wind field and environmental monitoring, but the large size and the use of mid-infrared HgCdTe detector must work at 77K temperature, which limits the development of gas lidar.
Semiconductor lidar can work continuously at high repetition frequency, with the advantages of long life, small size, low cost, and low damage to human eyes, and is widely used in Mie scattering measurements where the backscattering signal is relatively strong, such as detecting the height of cloud bottom. The potential applications of semiconductor lidar are measuring visibility, obtaining aerosol extinction profiles in the atmospheric boundary layer, and identifying rain and snow, etc., which can be easily made into airborne devices. Currently, the CT25K laser cloud measurement instrument developed by Vaisala, Finland, is a typical representative of semiconductor cloud measurement lidar, with a measurement range of cloud bottom height up to 7500m.
Classification by number of wires
Single-line LIDAR is mainly used for obstacle avoidance with fast scanning speed, high resolution, and high reliability. Since single-line LIDAR reflects more quickly than multi-line and 3D LIDAR in terms of angular frequency and sensitivity, it is more accurate in testing the distance and accuracy of surrounding obstacles. However, single-line radar can only scan in-plane type and cannot measure the height of objects, which has some limitations. Currently, it is mainly used in service robots, such as our common sweeping robots.
Multi-line LIDAR is mainly used in radar imaging for automobiles, which has a qualitative change in dimensional enhancement and scene restoration compared with single-line LIDAR and can identify the height information of objects. Multi-line LIDAR is 2.5D conventional and can do 3D. 4, 8, 16, 32, and 64 lines are mainly launched in the international market at present. But the price is high, most car companies will not choose.
Classification by scanning method
MEMS-based lidars can dynamically adjust their scanning patterns to focus on specific objects, picking up detailed information about farther and smaller objects and identifying them in a way that traditional mechanical lidars cannot. the complete MEMS system requires only a small reflector to direct a fixed laser beam in different directions. Since the reflector is small, its moment of inertia is not large and it can move quickly, fast enough to track to 2D scanning mode in less than a second.
Flash-type LIDAR can record the whole scene quickly, avoiding all kinds of troubles caused by the movement of the target or LIDAR during the scanning process, and it runs more like a camera. The laser beam is diffused directly in all directions so that a single flash can illuminate the entire scene. The system then uses an array of miniature sensors to capture the laser beams reflected back in different directions. flash LiDAR has its advantages, but of course, there are also certain drawbacks. When the pixels are bigger, more signals need to be processed. If a huge number of pixels are stuffed into the photodetector, it will certainly bring all kinds of interference, and the result is the degradation of accuracy.
Phased Array Lidar
Phased array lidars carry a row of emitters that can change the direction of the laser beam emission by adjusting the relative phase of the signal. Most phased-array lidars are still staying in the laboratory, while still stuck in the era of rotating or MEMS lidars.
Mechanical rotating LIDAR is the early development of LIDAR, and the technology is relatively mature, but the structure of the mechanical rotating LIDAR system is very complex, and the price of each core component is also quite expensive, which mainly includes lasers, scanners, optical components, photodetectors, receiver ICs, and position and navigation devices. Due to the high cost of hardware, which makes mass production difficult and stability needs to be improved, solid-state lidar has become the development direction of many companies.
Classification by detection method
The basic structure of a direct-detection lidar is quite similar to that of a laser rangefinder. In operation, a signal is sent by the transmitting system, reflected by the target, and collected by the receiving system, and the distance of the target is determined by measuring the time of round-trip propagation of the laser signal. As for the radial velocity of the target, it can be determined by the Doppler shift of the reflected light or the velocity can be obtained by measuring two or more distances and calculating the rate of change.
In the so-called monostable system, the transmit and receive signals share a common optical aperture and are isolated by a transmit-receive switch. In contrast, bistable systems include two optical apertures for the transmit and receive signals, and the transmit-receive switch is naturally no longer needed, while the rest of the system is identical to monostable systems.
Classification by laser emission waveform
In terms of the laser principle, a continuous laser is one that has light coming out all the time, just like turning on the switch of a flashlight, and its light will always be on (except in special cases). Continuous laser is relying on a continuous bright light to the height to be measured, for data acquisition at a certain height. Due to the working characteristics of the continuous laser, only one point of data can be collected at a certain time and moment. Because of the uncertain nature of wind data, using one point to represent the wind conditions at a certain height is obviously somewhat one-sided. Therefore, some manufacturers compromise by taking a 360-degree rotation and collecting multiple points on top of this circular edge for average evaluation, which is obviously a concept of multi-point statistics in a virtual plane.
The laser output of the pulsed laser is not continuous, but flickering. The principle of pulsed laser is to emit tens of thousands of laser particles, according to the international common Doppler principle, from the reflection of these tens of thousands of laser particles to comprehensively evaluate the wind condition at a certain height, this is a three-dimensional concept, so there is the theory of detection length. From the point of view of the characteristics of the laser, pulse laser than continuous laser measurement points dozens of times more, more accurate response to a certain height wind conditions.
Classification by load platform
Airborne LIDAR is a technology that closely integrates laser ranging equipment, GNSS equipment, INS, etc., and uses a flying platform as a carrier to obtain three-dimensional information on the ground surface by scanning the ground, recording information on the attitude, position, and reflection intensity of the target, and processing it in depth to obtain the required spatial information. It has wide potential and prospects in both military and civilian fields. The detection distance of airborne LIDAR is close, and when the laser is transmitted in the atmosphere, the energy is attenuated by atmospheric influence. The action distance of LIDAR is within 20 km, especially in bad weather conditions, such as dense fog, heavy rain and smoke, and dust, the action distance will be greatly shortened and it is difficult to work effectively. Atmospheric turbulence can also reduce the measurement accuracy of lidar to different degrees.
Vehicle-mounted LIDAR, also known as a vehicle-mounted 3D laser scanner, is a mobile 3D laser scanning system that can transmit and receive laser beams, analyze the folding time after the laser encounters the target object, calculate the relative distance between the target object and the vehicle, and use the collected 3D coordinates of a large number of dense points on the surface of the target object, reflectivity and other information to quickly reconstruct the 3D model of the target and various figure data. The 3D point cloud map is established and the environment map is drawn to achieve the purpose of environment perception. The role played by in-vehicle LIDAR in the tide of autonomous driving is becoming more and more important, such as Google, Baidu, BMW, Bosch, Delphi, and other companies, are using LIDAR in their autonomous driving systems, driving the rapid expansion of the in-vehicle LIDAR industry.
Ground-based LiDAR can obtain 3D point cloud information of the forest area and extract single wood location and tree height using point cloud information, it not only saves manpower and material resources but also improves the accuracy of extraction, which has the advantage that other remote sensing methods cannot compare. Through the analysis of the forestry application of this technology at home and abroad and the verification of the results of the later stage of the research on this invention, this technology will be used to extract various forest parameters in a larger research area in the future.
The satellite-based radar adopts a satellite platform with a high orbit and a wide observation field of view, which can reach every corner of the world. It provides a new way to obtain three-dimensional control points and digital ground models in offshore areas, which is of great significance for both national defense and scientific research. LIDAR also has the ability to observe the whole celestial body, the U.S. exploration programs such as the Moon and Mars include LIDAR on board, and the data provided can be used to produce a comprehensive three-dimensional topographic map of the celestial body. In addition, satellite-based LIDAR can play an important role in vegetation vertical distribution measurements, sea surface height measurements, cloud and aerosol vertical distribution measurements, and monitoring of special climate phenomena.
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