“For millimeter waves, everyone must have heard about it. In addition, everyone is familiar with keywords such as 5G millimeter wave and millimeter wave radar. But, can you accurately distinguish the difference between millimeter wave radar and lidar? If you don’t understand, you can find the answer in this article about millimeter wave radar.
For millimeter waves, everyone must have heard about it. In addition, everyone is familiar with keywords such as 5G millimeter wave and millimeter wave radar. But, can you accurately distinguish the difference between millimeter wave radar and lidar? If you don’t understand, you can find the answer in this article about millimeter wave radar.
1. What is Lidar
Lidar is a radar system that emits laser beams to detect the position and speed of the target and other characteristic quantities. Its working principle is to transmit a detection signal (laser beam) to the target, and then compare the received signal (target echo) from the target with the transmitted signal, and after proper processing, the relevant information of the target can be obtained, such as Target distance, azimuth, altitude, speed, attitude, and even shape and other parameters, so as to detect, track and identify aircraft, missiles and other targets. It consists of a laser transmitter, an optical receiver, a turntable, and an information processing system. The laser converts electrical pulses into light pulses and emits them. The optical receiver then restores the light pulses reflected from the target to electrical pulses and sends them to the Display.
Second, the principle of lidar
LIDAR is a system that integrates three technologies: laser, global positioning system (GPS) and inertial navigation system (INS) to obtain data and generate accurate DEM. The combination of these three technologies can locate the spot of the laser beam hitting the object with high accuracy. It is further divided into the currently maturing terrain LIDAR system for obtaining ground digital elevation model (DEM) and the hydrological LIDAR system for obtaining underwater DEM which has been matured. The common feature of these two systems is the use of lasers. Detection and measurement, this is the original English translation of the word LIDAR, namely: LIght DetecTIon And Ranging-LIDAR.
The laser itself has a very precise ranging capability, and its ranging accuracy can reach several centimeters. In addition to the laser itself, the accuracy of the LIDAR system also depends on the internal factors such as the synchronization of the laser, GPS and inertial measurement unit (IMU). . With the development of commercial GPS and IMU, it has become possible and widely used to obtain high-precision data from mobile platforms (such as on airplanes) through LIDAR.
The LIDAR system includes a single-beam narrowband laser and a receiving system. The laser generates and emits a light pulse, hits the object and reflects it back, and is finally received by the receiver. The receiver accurately measures the propagation time of the light pulse from emission to reflection. Because light pulses travel at the speed of light, the receiver always receives the reflected pulse before the next pulse. Given that the speed of light is known, travel time can be converted into a measurement of distance. Combining the height of the laser, the laser scanning angle, the position of the laser obtained from GPS and the direction of laser emission obtained from INS, the coordinates X, Y, Z of each ground spot can be accurately calculated. The frequency of laser beam emission can range from a few pulses per second to tens of thousands of pulses per second. For example, a system with a frequency of 10,000 pulses per second, the receiver will record 600,000 points in one minute. Generally speaking, the ground spot spacing of the LIDAR system ranges from 2-4m.
The working principle of lidar is very similar to that of radar. Using laser as the signal source, the pulsed laser emitted by the laser hits trees, roads, bridges and buildings on the ground, causing scattering, and some of the light waves will be reflected to the receiving of lidar. Based on the calculation of the laser ranging principle, the distance from the laser radar to the target point is obtained. The pulse laser continuously scans the target object to obtain the data of all target points on the target object. After imaging processing with this data, Accurate three-dimensional images can be obtained.
The most basic working principle of lidar is the same as that of radio radar, that is, a signal is sent by the radar transmitting system, which is reflected by the target and collected by the receiving system, and the distance of the target is determined by measuring the running time of the reflected light. As for the radial velocity of the target, it can be determined by the Doppler frequency shift of the reflected light, or it can be measured by measuring two or more distances and calculating the rate of change to obtain the velocity. This is and is also the basic principle of direct detection radars. working principle.
Third, the characteristics of lidar
High precision and strong stability.
However, the lidar detects by emitting light beams, so the detection range is narrow, and the light beam cannot be used normally after being blocked. Therefore, it cannot be turned on in severe weather such as rain, snow, haze, and sandstorms, which is greatly affected by the environment. And there is no penetrating ability, and the probe must be completely exposed to achieve the detection effect, which affects the appearance of the vehicle for the installation of the vehicle. Therefore, the lidar anti-collision device has greater limitations during use.
Fourth, what is millimeter wave radar
First of all, we must understand what millimeter waves are, millimeter waves are essentially electromagnetic waves. The frequency band of millimeter wave is quite special, its frequency is higher than radio, lower than visible light and infrared, and the approximate frequency range is 10GHz-200GHz. This is a frequency band that is very suitable for the automotive field. At present, there are three types of millimeter wave radar frequency bands that are more common in the automotive field.
1. 24-24.25GHz This is currently widely used in blind spot monitoring and lane change assistance for automobiles. The radar is installed in the rear bumper of the vehicle and is used to monitor whether there is a car in the lane on both sides of the rear of the vehicle and whether it can change lanes. This frequency band also has its shortcomings. The first is that the frequency is relatively low, and the other is that the bandwidth (Bandwidth) is relatively narrow, only 250MHz.
2. 77GHz, the frequency of this band is relatively high, and the internationally allowed bandwidth is as high as 800MHz. According to reports, the performance of the radar in this frequency band is better than that of the 24GHz radar, so it is mainly used to assemble on the front bumper of the vehicle to detect the distance to the vehicle in front and the speed of the vehicle in front, and the main realization is emergency braking and automatic car following. And other functions in the field of active safety.
3. 79GHz-81GHz, the biggest feature of this frequency band is that its bandwidth is very wide, which is more than 3 times higher than that of 77GHz, which also makes it have a very high resolution, which can reach 5cm.
Principle: The oscillator generates a signal whose frequency gradually increases with time. After encountering an obstacle, this signal will bounce back. The delay is twice the distance/speed of light. There is a frequency difference between the returned waveform and the transmitted waveform. This frequency difference and time delay are linear: the farther the object is, the later the returned wave will be received, so the frequency difference between it and the incident wave The greater the value.
By subtracting these two frequencies, the difference frequency (beat frequency) of the two frequencies can be obtained, and the distance of the obstacle can be judged by judging the height of the beat frequency.
According to the survey of domestic industry institutions, the domestic sales of automotive millimeter wave radars in 2014 were approximately 1.2 million units, and in 2015 it was approximately 1.8 million units. The main application is the short- and medium-range radar (24Ghz) for blind spot detection and rear vehicle alerting. Each vehicle requires two.
Five, millimeter wave radar characteristics
High accuracy, strong anti-interference ability, long detection distance, wide-angle detection, wide detection range, action speed can reach more than 120 yards per hour, working around the clock, rain, snow, haze, sandstorm and other bad weather, all can be turned on for normal use. Strong penetrating ability, installation can also be completely concealed, without affecting the overall appearance of the vehicle. Therefore, millimeter wave radar technology is more suitable for the field of automobile collision avoidance.
Sixth, the difference between lidar and millimeter wave radar
To put it simply, Lidar mainly uses laser beams to detect the surrounding environment. Vehicle-mounted Lidar generally uses multiple laser transmitters and receivers to establish a three-dimensional point cloud image to achieve the purpose of real-time environmental perception.
The advantage of lidar lies in its wider detection range and higher detection accuracy. However, the shortcomings of lidar are also obvious: poor performance in extreme weather such as rain, snow and fog; the amount of collected data is too large; and it is very expensive.
Technically speaking, the current traditional lidar technology is very mature, while solid-state lidar and hybrid solid-state lidar are still in their infancy. Therefore, the lidar currently used by various companies in autonomous vehicles is mostly mechanical lidar.
From the perspective of the entire lidar industry, manufacturers of high-precision automotive lidar products are mainly concentrated in foreign countries, such as Velodyne and Quanegy in the United States, and IBEO in Germany. In recent years, some companies focusing on automotive lidar have also appeared in China, and Some lidar companies that have switched careers from other fields have devoted themselves to the research and development of on-board lidar products because of the broad development prospects of autonomous vehicles. At present, the results have been remarkable.
The so-called millimeter wave radar refers to a radar whose working frequency is in the millimeter wave frequency band. The ranging principle is the same as that of a general radar, that is, sending out radio waves (radar waves), and then receiving echoes, which are measured according to the time difference between sending and receiving. The location data of the target. Millimeter wave radar means that the frequency of this radio wave is the millimeter wave frequency band.
Millimeter wave radar has been used in high-end cars since the last century, and the technology is relatively mature. The wavelength of millimeter wave is between centimeter wave and light wave. Therefore, millimeter wave has the advantages of microwave guidance and photoelectric guidance, and its guiding head has the characteristics of small size, light weight and high spatial resolution. In addition, the millimeter waveguide seeker has a strong ability to penetrate fog, smoke, and dust, which is a big advantage compared to lidar.
The shortcomings of millimeter-wave radar are also very intuitive. The detection range is directly restricted by the loss of the frequency band (if you want to detect far, you must use a high-frequency radar), it cannot perceive pedestrians, and it is impossible to accurately model all surrounding obstacles.
The above is the “millimeter wave” related content brought by this editor. Through this article, I hope everyone has a certain understanding of the difference between millimeter wave radar and lidar.
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