At present, L4 and L5 level unmanned driving widely use integrated solutions of lidar, laser scanner, millimeter wave radar and vision sensor to achieve the purpose of safety redundancy. Each of the three sensing sensors has its own unique advantages. Among them, lidar has the characteristics of long detection distance, high detection accuracy, and strong reliability. Compared with millimeter wave sensors, lidar has high resolution and is not sensitive to target shapes and materials. advantage. Therefore, lidar is also considered as a necessary sensor device for unmanned driving.
L4 and L5 unmanned driving systems usually require lidar to be equipped with long-distance, large-field-of-view detection to ensure that the vehicle's environmental perception system can still maintain accurate detection even when the vehicle is running at high speed, ensuring driving safety, and also It is required to pass the TS16949 vehicle regulations and human eye safety certification, so that it has the value of being equipped in mass-produced vehicles.
Therefore, the design and development of vehicle-mounted lidar basically revolves around this goal.
Vehicle-mounted lidar usually consists of two parts: ranging module and scanning module, and there are many corresponding technical routes.
Lidar ranging technology selection:
At present, there are three mainstream ranging principles, including: triangle ranging method, PTOF ranging method and AMCW (amplitude modulated continuous wave) ranging method.
Triangulation
The triangulation method is based on the principle of triangle geometry, a light source is hit on the measured object, and the distance of the measured object is calculated by measuring the imaging position of the reflected light in the area or linear array detector. The principle diagram is as follows:
The advantages of the triangulation method are very prominent, the principle is simple, and the cost is low. Only ordinary laser transmitters and linear CCD detectors are needed to achieve distance measurement, and the detection accuracy is high at short distances. Therefore, such sensors usually have a large number of applications in sweeping robots. In addition, binocular vision and structured light ranging can be summarized as the principle of triangulation ranging.
Because of the triangulation method in long-distance detection, the detection error will increase geometrically, and in the case of direct sunlight, the reflected light spot is usually submerged in the sunlight, causing the detector to be unable to extract the reflected light spot and causing the instrument to fail. This is the fatal flaw of the triangulation method in the long-distance detection process.
PTOF ranging method
The core principle of PTOF is to strike a beam of laser light with a very short time on the detection object. By directly measuring the flight time of laser emission, hitting the detection object and returning to the detector, the distance between the detector and the object to be measured is reversed.
Due to the extremely fast flight speed of light, this solution requires a very fine clock circuit (usually ps level, 1 ps=10^-3 ns) and a very narrow pulse width laser emitting circuit (usually ns level), so The development difficulty and threshold are high, but the lidar that generally adopts the PTOF principle can usually reach a detection range of 100 meters.
AMCW (Amplitude Modulation Continuous Wave) Ranging Method
The flight time of light is extremely fast, and it is difficult to directly measure the flight time of photons. Is it possible to obtain the flight time of light through some indirect methods? The typical method is AMCW.
AMCW modulates the intensity of the light wave (such as sine wave or triangle wave, etc.), so that the light wave forms a phase difference on the light intensity waveform when it is projected onto the object and then returns to the detector. Then the phase difference can be measured indirectly. The flight time of light, and thus the flight distance is reversed.
Generally, it is easier to measure phase difference than to directly measure flight time, and it is easier to develop. Therefore, the cost of AMCW-based lidar is slightly lower than that of PTOF radar, and its unique detection method is more convenient to realize solid-state array FLASH scanning. Different from PTOF, because AMCW adopts continuous light wave modulation, it needs larger optical power during long-distance detection, especially at the detection distance of 100 meters, there are hidden dangers to human eye safety, which obviously cannot pass vehicle regulations.
The three ranging solutions have their own advantages and disadvantages. We have summarized and compared the above-mentioned three ranging methods by taking the five core capabilities required for vehicle lidar as the selection dimensions:
