What is the definition and classification of lidar sensing technology?
1. The definition of lidar
LiDAR (LiDAR) is a sensor used to accurately obtain three-dimensional position information. Like the human eye, it can determine the position, size, external appearance and even material of an object. It is a collective term for active sensors that detect environmental information through laser ranging technology. It uses laser beams to detect targets, obtain data and generate accurate digital engineering models.
Lidar is composed of three parts: transmitting system, receiving system, and information processing. The working principle of lidar is to use visible and near-infrared light waves (mostly infrared light near the 950nm band) to emit, reflect and receive to detect objects.
2. Classification of lidar
According to the structure, lidar is divided into mechanical lidar, solid-state lidar and hybrid solid-state lidar.
1, mechanical lidar
Mechanical lidar refers to the macroscopic rotation of its transmitting system and receiving system, that is, by continuously rotating the transmitting head, the laser with faster speed and more accurate transmission is changed from "line" to "surface", and it is vertical Arrange multiple laser beams in the straight direction to form multiple surfaces to achieve the purpose of dynamic scanning and dynamic receiving of information.
Because it has a mechanical rotating mechanism, the biggest feature of the mechanical lidar is that it can turn by itself and is relatively large.
Nowadays, mechanical lidar technology is relatively mature, but the price is expensive, and the possibility of mass production for the main engine factory is low. At the same time, there are optical debugging and complex assembly, long production cycle, and the reliability of mechanical rotating parts in the driving environment is not high, and it is difficult Meet the stringent requirements of car regulations and other deficiencies.
2, hybrid solid-state lidar
When the mechanical lidar is working, the transmitting system and the receiving system will always rotate 360 degrees, while when the hybrid solid-state lidar is working, the rotation is not visible from the appearance. The ingenious thing is that the mechanical rotating parts are made more compact And deeply hidden in the shell.
It is generally believed in the industry that hybrid solid-state lidar refers to the use of semiconductor "micro-moving" devices (such as MEMS scanning mirrors) to replace macro-mechanical scanners to achieve laser scanning at the radar transmitting end on a micro scale. The MEMS scanning mirror is a silicon-based semiconductor component, which is a solid-state electronic component; but the MEMS scanning mirror is not "secure", it integrates a "movable" micro-mirror; it can be seen that MEMS scanning mirrors are both "solid" And the two attributes of "movement", so called "mixed solid state."
For lidar, the greatest value of MEMS is: originally in order to realize scanning by mechanical lidar, the laser transmitter must be rotated. The MEMS micro-electromechanical system can directly integrate a very compact micro galvanometer on the silicon-based chip, and the rotating micro galvanometer reflects the light of the laser to realize scanning.
In this way, the lidar itself does not need to be rotated substantially, which can effectively reduce the probability of problems in the driving environment of the entire system. In addition, after the main components are produced using chip technology, the mass production capacity has also been greatly improved, which is conducive to reducing the cost of lidar, which can be reduced from thousands to tens of thousands of dollars to hundreds of dollars.
3, solid-state lidar
Compared with mechanical lidar, the biggest feature of solid-state lidar structure is that it has no rotating parts and is relatively small.
The advantages of solid-state lidar include: fast data acquisition speed, high resolution, and strong adaptability to temperature and vibration; through beam control, the detection points (point clouds) can be arbitrarily distributed, for example, in the main scan of the highway far ahead, For the sparse scan of the side but not completely ignored, strengthen the side scan at the intersection. However, a mechanical lidar that can only rotate at a constant speed cannot perform this fine operation.
From the technology used, solid-state lidar is divided into OPA solid-state lidar and Flash solid-state lidar.
(1) OPA solid-state lidar
OPA (opticalphasedarray) optical phased array technology. Readers who know something about military affairs should know the phased array radar, the honeycomb "board" on the US Navy Aegis ship.
The optical phased array uses the same technology. OPA uses the principle of coherence (similarly, after two circles of water waves are superimposed on each other, some directions will cancel each other, some will enhance each other), using multiple light sources to form an array, by controlling the light-emitting time difference of each light source, synthesize the main beam with a specific direction . Then control it, and the main beam can scan in different directions.
Compared with MEMS, this technology is more electronic, it completely eliminates the mechanical structure, and changes the laser emission angle by adjusting the phase difference of each emitting unit in the emitting array.
Because there is no mechanical structure, naturally there is no rotation. Therefore, compared with traditional mechanical radar, OPA solid-state lidar has the advantages of fast scanning speed, high precision, good controllability, and small size. However, side lobes are also easy to form, which affects the distance and angular resolution of the beam, and the production is difficult.
(2) Flash solid-state lidar
Flash originally means flash. The principle of Flash Lidar is also fast flashing. Unlike the MEMS or OPA solution, which scans, it directly emits a large area of laser light covering the detection area in a short time, and then uses a highly sensitive receiver to complete the surrounding environment. The drawing of the image.
Therefore, the Flash solid-state lidar is a non-scanning radar that emits area array light and is a lidar that focuses on 2D or 3D images. In a sense, it is somewhat similar to a camera in the dark, where the light source is emitted by itself.
One big advantage of Flash solid-state radar is that it can quickly record the entire scene, avoiding various troubles caused by the movement of the target or lidar during the scanning process. However, this method also has its own shortcomings, such as a shorter detection range.
This means that Flash solid-state lidar does not have "farsightedness" and is not suitable for remote detection in actual use. Industry experts firmly believe that the lidar on fully autonomous vehicles must see objects 200 to 300 meters away at least at a glance.
In fact, the cost of Flash solid-state lidar is relatively low, but solid-state lidar based on 3DFlash technology still has problems in terms of technical reliability.
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