Lidar is a system that integrates three technologies: laser, global positioning system (GPS), and IMU (inertial measurement unit). Compared with ordinary radar, lidar has higher resolution, better concealment, and stronger anti-interference ability. and other advantages. With the continuous development of science and technology, the application of lidar is more and more extensive, in the fields of robots, UAV surveying and mapping, unmanned vehicles and other fields. Lidar can be divided into different types according to the use function, detection method, and load platform.
Sort By Function
Ranging lidar
Working principle of ranging lidar: The distance between the measured object and the test point is determined by emitting a laser beam to the measured object, receiving the reflected wave of the laser beam, and recording the time difference.
Ranging lidar is an indispensable core component in the robot body. When used in conjunction with SLAM technology, it can help the robot to perform real-time positioning and navigation and realize autonomous walking.
Mapping Lidar
Mapping lidar data can be used not only for traditional photogrammetry purposes, but also for forestry, agriculture, digital water conservancy, urban 3D modeling, electricity, and more.
Using a lidar surveying and mapping system requires the position and coordinate information of the laser, which can be obtained by using GPS technology, and then calculates the geodetic coordinates of each laser point.
The density of laser point cloud data is relatively high and the accuracy is very large. These data can directly examine the three-dimensional coordinate characteristics of laser points and effectively establish a digital elevation model. The laser point cloud data itself is also the most important data product of lidar mapping technology.
For example, the airborne lidar measurement system for forests can simultaneously obtain topographic information and tree height information at the bottom of the tree canopy, analyze and classify vegetation, calculate tree height, tree species and wood volume, and dynamically monitor the growth of plants and extract forests. In the water conservancy department, it is used for flood analysis, ecological assessment, shipping scheduling, water management and other applications; in the power industry, the traditional and complex power line patrol work can also be easily solved by lidar.
Speed measurement Lidar
Velocity lidar is a measurement of the moving speed of an object. By measuring the target object twice with a specific time interval, the moving speed of the object to be measured can be obtained.
There are two main types of lidar speed measurement methods. One is based on the principle of lidar ranging, that is, the target distance is continuously measured at a certain time interval, and the difference between the two target distances is divided by the time interval to know the target distance. The speed value, the direction of the speed can be determined according to the positive or negative of the distance difference. The system used to monitor the speed of vehicles on the road is the test lidar.
Another type of velocity measurement is the use of Doppler shift. Doppler frequency shift means that when there is a relative speed between the target and the lidar, there will be a frequency difference between the frequency of the received echo signal and the frequency of the transmitted signal. This frequency difference is the Doppler frequency shift.
Imaging lidar
Lidar imaging radar can be used to detect and track targets, obtain target azimuth and speed information, etc. It can accomplish 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 sounding lidar
Atmospheric detection lidar is mainly used to detect molecules in the atmosphere, the density of smoke, temperature, wind speed, wind direction and the concentration of water vapor in the atmosphere, so as to monitor the atmospheric environment and forecast severe weather such as storms and sandstorms. the goal of.
Tracking radar
Tracking radar can continuously track a target, measure the coordinates of the target, and provide the trajectory of the target. It is not only used in artillery control, missile guidance, external ballistic measurement, satellite tracking, penetration technology research, etc., but also in the fields of meteorology, transportation, and scientific research.
By working medium
Solid state lidar
Solid-state lidar has high peak power, output wavelength range and existing optical components and devices, output long range and existing optical components and devices (such as modulators, isolators and detectors) and atmospheric transmission characteristics match, etc., and It is easy to implement the master oscillator-power amplifier (MOPA) structure, coupled with conductors such as high efficiency, small size, light weight, high reliability and good stability, solid-state lidar is preferentially applied in airborne and space-based systems. In recent years, the focus of lidar development is diode-pumped solid-state lidar.
Gas lidar
Gas lidar is represented by CO2 lidar. It works in the infrared band, has small atmospheric transmission attenuation, and has a long detection distance. It has played a great role in atmospheric wind field and environmental monitoring, but it is large in size and uses mid-infrared HgCdTe detection. The device must work at a temperature of 77K, which limits the development of gas lidar.
Semiconductor Lidar
Semiconductor LiDAR can work continuously with high repetition rate and has the advantages of long life, small size, low cost and little damage to human eyes. It is widely used in Mie scattering measurements with strong backscattered signals, such as detecting cloud base heights . The potential applications of semiconductor lidar are to measure visibility, obtain aerosol extinction profiles in the atmospheric boundary layer, and identify rain and snow, etc., and are easily made into airborne equipment. At present, the CT25K laser ceilometer developed by the Finnish company Vaisala is a typical representative of semiconductor cloud measuring lidar, and its cloud base height measurement range can reach 7500m.
Sort by line
Single Line Lidar
Single-line lidar is mainly used to avoid obstacles, and its scanning speed is fast, resolution is strong, and reliability is high. Since the single-line lidar is faster in angular frequency and sensitivity than the multi-line and 3D lidar, it is more accurate in testing the distance and accuracy of surrounding obstacles. However, single-line radar can only scan in a plane and cannot measure the height of objects, which has certain limitations. Currently, it is mainly used in service robots, such as our common sweeping robots.
Multi-line LiDAR
Multi-line lidar is mainly used in automotive radar imaging. Compared with single-line lidar, it has qualitative changes in dimension improvement and scene restoration, and can identify the height information of objects. Multi-line lidar is usually 2.5D, and it can do 3D. Currently launched in the international market are mainly 4 lines, 8 lines, 16 lines, 32 lines and 64 lines. But the price is high, and most car companies will not use it.
By scanning method
MEMS lidar
MEMS lidar can dynamically adjust its scanning mode to focus on special objects, collect detailed information of farther and smaller objects and identify them, which is impossible for traditional mechanical lidars. The entire MEMS system requires only a small mirror to direct a fixed laser beam in different directions. Because the mirror is small, its moment of inertia is not large, and it can move fast enough to track a 2D scan pattern in less than a second.
Flash lidar
Flash-type lidar can quickly record the entire scene, avoiding all kinds of troubles caused by the movement of the target or lidar during the scanning process, and it operates more like a camera. The laser beam is diffused directly in all directions, so a single flash can illuminate the entire scene. The system then uses an array of tiny sensors to collect the reflected laser beams in different directions. Flash LiDAR has its advantages, but of course there are also some drawbacks. The larger the pixels, the more signals that need to be processed. If a large number of pixels are packed into the photodetector, various interferences will inevitably occur, and the result is a decrease in accuracy.
Phased Array Lidar
A row of emitters on a phased array lidar can change the direction of the laser beam by adjusting the relative phases of the signals.
Mechanical rotary lidar
Mechanical rotary lidar is a relatively early development of lidar, and the current technology is relatively mature, but the structure of mechanical rotary lidar system is very complex, and the price of each core component is quite expensive, including lasers, scanners, optical components , photodetectors, receiving ICs, and position and navigation devices. Due to the high cost of hardware, mass production is difficult, and the stability needs to be improved. At present, solid-state LiDAR has become the development direction of many companies.
By detection method
Direct detection lidar
The basic structure of the direct detection lidar is quite similar to that of the laser rangefinder. When working, a signal is sent by the transmitting system, which is collected by the receiving system after being reflected by the target, and the distance to the target is determined by measuring the round-trip propagation time of the laser signal. As for the radial velocity of the target, it can be determined by the Doppler frequency shift of the reflected light, or two or more distances can be measured and the rate of change can be calculated to obtain the velocity.
coherent detection lidar
Coherent detection type lidar is divided into monostable and bistable. In the so-called monostable system, the transmit and receive signals share an optical aperture and are isolated by the transmit-receive switch. The bistable system includes two optical apertures, which are used for transmitting and receiving signals respectively. Naturally, the transmit-receive switch is no longer required, and the rest is the same as the monostable system.
By laser emission waveform
Continuous Lidar
From the point of view of the laser principle, the continuous laser always emits light, just like turning on the switch of a flashlight, its light will always be on (except in special cases). Continuous lasers rely on continuous bright light to the height to be measured to collect data at a certain height. Due to the working characteristics of continuous lasers, only one point of data can be collected at a certain time. Because of the uncertain nature of wind data, using a point to represent wind conditions at a certain height is obviously a bit one-sided. Therefore, some manufacturers’ compromise is to rotate 360 degrees and collect multiple points on this circular edge for average evaluation. Obviously, this is the concept of multi-point statistical data in a virtual plane.
Pulsed lidar
The laser output of the pulsed laser is not continuous, but flashes and flashes. The principle of pulsed laser is to emit tens of thousands of laser particles. According to the internationally accepted Doppler principle, the wind condition at a certain height can be comprehensively evaluated from the reflection of these tens of thousands of laser particles. This is a three-dimensional concept, so There is a theory of detection length. In terms of laser characteristics, pulsed lasers measure dozens of times more points than continuous lasers, and can more accurately reflect a certain high wind condition.
By load platform
Airborne Lidar
Airborne LiDAR is a close integration of laser ranging equipment, GNSS equipment and INS and other equipment, with the flight platform as the carrier, by scanning the ground, recording the target’s attitude, position and reflection intensity and other information to obtain the three-dimensional information of the ground surface, And in-depth processing to obtain the required spatial information technology. At present, the most widely used is the vertical fixed-wing UAV equipped with lidar for terrain mapping 3D modeling.
Vehicle LiDAR
Vehicle-mounted lidar, also known as vehicle-mounted 3D laser scanner, is a mobile 3D laser scanning system. It can analyze the return time after the laser encounters the target object by emitting and receiving laser beams, and calculate the relative distance between the target object and the vehicle. And use the collected three-dimensional coordinates, reflectivity and other information of a large number of dense points on the surface of the target object to quickly reconstruct the three-dimensional model of the target and various graphic data, establish a three-dimensional point cloud map, and draw an environmental map to achieve environmental perception. Purpose. In-vehicle LiDAR is playing an increasingly important role in the tide of autonomous driving “car building”. Companies such as Google, Baidu, BMW, Bosch, Delphi and other companies have used LiDAR in their autonomous driving systems to drive in-vehicle LiDAR. The industry expanded rapidly.
Ground-based lidar
Ground-based lidar can obtain 3D point cloud information in forest areas, and use point cloud information to extract individual tree positions and tree heights. It not only saves manpower and material resources, but also improves the accuracy of extraction, and has incomparable advantages over other remote sensing methods. Through the analysis of the forestry application of this technology at home and abroad and the verification of the results of the later research of the invention, the technology will be used to extract various forest parameters in a larger research area in the future.
Spaceborne Lidar
The spaceborne radar uses a satellite platform with a high orbit and a wide observation field, which can reach every corner of the world. It provides a new way for the acquisition of 3D control points and digital ground models in overseas areas, which is of great significance for national defense or scientific research. The spaceborne lidar also has the ability to observe the entire celestial body. The spaceborne lidar is included in the moon and Mars exploration programs carried out by the United States. The data provided by it can be used to make a comprehensive three-dimensional topographic map of the celestial body. In addition, spaceborne lidar can also play an important role in the measurement of the vertical distribution of vegetation, the measurement of sea surface height, the measurement of the vertical distribution of clouds and aerosols, and the monitoring of special climatic phenomena.
Through the above introduction to the characteristics, principles, and application fields of lidar, I believe that you can also have a general understanding of the different attributes of various types of lidar. Now, in the increasingly competitive track of lidar, to create low-cost, measurable Production and production of LiDAR is a dream that many start-ups want to achieve.
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