Millimeter-wave technology is one of the current hot technologies, and without millimeter-wave, many of the current progress will be stagnant. For millimeter wave applications, the well-known ones are 5G millimeter wave, millimeter wave communication, etc. In this article, the radar “target=”_blank”> millimeter wave radar technology will be explained in detail to improve everyone’s understanding of millimeter wave applications.
Ultrasonic radar, infrared radar, and lidar all detect the echo and compare it with the transmitted signal to obtain the difference between the pulse or phase, so as to calculate the time difference between the transmitted and received signals. Then, corresponding to the propagation speed of ultrasonic, infrared and laser in the air, the distance and relative speed to the obstacle are calculated. Compared with optical and infrared radar, millimeter-wave radar is not disturbed by the shape and color of the target object, and compared with ultrasonic wave, it is not affected by atmospheric turbulence, so it has stable detection performance and good environmental adaptability. It is less affected by changes in weather and external environment, and rain, snow, dust, and sunlight do not interfere with it; the Doppler frequency shift is large, and the accuracy of relative velocity measurement is improved.
The vehicle active anti-collision control system based on multi-sensor information fusion uses multi-source information fusion technology to identify the distance and speed of the vehicle in front of the vehicle based on the target information in front of the vehicle and the state information of the vehicle received by the multiple sensors. status information and perform collision risk estimation. Obviously, the vehicle active anti-collision control system based on multi-sensor information fusion is an active anti-collision and anti-lock car safety system, which can optimize the response time, distance and speed. Reducing the driver’s burden and judgment errors will play an important role in improving traffic safety and is the basis for the realization of automated driving.
Among the automotive anti-collision sensors, according to different working principles and working processes, they are divided into ultrasonic radar, infrared radar, lidar and millimeter-wave radar. Among them, the first three kinds of radars use the detection of echoes and compare them with the transmitted signal to obtain the difference of pulse or phase, so as to calculate the time difference between the transmitted and received signals. Then, corresponding to the propagation speed of ultrasonic, infrared and laser in the air, the distance and relative speed to the obstacle are calculated. Although these three kinds of automobile anti-collision radars designed using the principle of sound and light are simple in structure and low in price, they are easily interfered by bad weather conditions and cannot ensure the accuracy of ranging. Millimeter wave radar shows its unique advantages. Compared with optical and infrared radar, it is not disturbed by the shape and color of the target object. Compared with ultrasonic wave, it is not affected by atmospheric turbulence, so it has stable detection performance; good environmental adaptability . It is less affected by changes in weather and external environment, and rain, snow, dust, and sunlight do not interfere with it; the Doppler frequency shift is large, and the accuracy of relative velocity measurement is improved.
Below we will introduce the mainstream sensor technologies step by step. At present, the most concerned sensing method is the radar system that uses millimeter waves for measurement. Today, I will introduce the millimeter wave radar technology.
Radar is a device that uses radio echoes to detect the direction and distance of a target. Radar is a transliteration of the English word Radar, which is abbreviated from the prefix of the word Radio Detection And Ranging, which means radio detection and ranging. The world became familiar with radar in the 1940 Battle of Britain, when 700 British fighter jets armed with radar defeated 2,000 incoming German bombers, thus rewriting history. After World War II, radar began to have many peaceful uses. In terms of weather forecasting, it can be used to detect storms; in terms of the safety of aircraft and ships, it can help pilots and airport traffic controllers to complete their tasks more efficiently.
Millimeter wave is an electromagnetic wave operating in the millimeter wave band, with an operating frequency of 30 to 100 GHz and a wavelength of 1 to 10 mm. The wavelength of millimeter wave is between microwave and centimeter wave, so millimeter wave radar has some advantages of microwave radar and photoelectric radar. Automotive millimeter-wave radars mainly include 24GHz and 77GHz millimeter-wave radars.
Compared with the centimeter-wave seeker, the millimeter-wave seeker has the characteristics of small size, light weight and high spatial resolution. Compared with optical seekers such as infrared, laser, and TV, the millimeter-wave seeker has a strong ability to penetrate fog, smoke, and dust, and has the characteristics of all-weather (except heavy rain) all day. Millimeter-wave radars can work around the clock and are not affected by weather conditions, which are one of the main causes of traffic accidents. Compared with light waves, millimeter waves use the atmospheric window (when millimeter waves and submillimeter waves propagate in the atmosphere, some attenuation due to the resonance absorption of gas molecules is a minimum frequency), the attenuation is small when they propagate, and they are affected by natural light. and thermal radiation sources have little impact.
The main limitations of the application of millimeter waves in radar are: attenuation in high-humidity environments such as rain, fog and wet snow, as well as the impact of high-power devices and insertion loss will reduce the detection distance of millimeter-wave radar; the ability to penetrate trees is poor, compared to Microwave, low penetration to dense trees.
Like most microwave radars, millimeter waves have the concept of a beam, that is, the emitted electromagnetic wave is a cone-shaped beam, not a line like a laser. This is because the antenna in this band mainly uses electromagnetic radiation instead of light particle emission as the main method. At this point, radar and ultrasound are the same, the way of this beam, which leads to its advantages and disadvantages. The advantages are reliable, because the reflective surface is large, and the disadvantage is that the resolution is not high. Millimeter wave radar can measure the presence or absence of targets, ranging, speed and azimuth.
Speaking of the principle of ranging, it is actually simple, and it is based on the TOF (Time Of Flight) principle. The working principle of radar is very similar to the reflection of sound waves, the only difference is that the wave used is a very high frequency radio wave instead of sound wave. The transmitter of the radar is equivalent to the vocal cords that make a cry, and it can emit an electric pulse (Pulse) similar to the cry. The pointing antenna of the radar is like a shouting microphone, so that the energy of the electric pulse can be concentrated and emitted in a certain direction. The role of the receiver is similar to that of the human ear, which is used to receive the echoes of the electrical pulses sent by the radar transmitter.
The speed measurement of millimeter-wave radar is the same as that of ordinary radar. There are two ways, one is based on the principle of Doppler Effect. The so-called Doppler effect is that when a vibration source such as sound, light and radio waves moves relative to the observer at a relative speed V, the vibration frequency received by the observer is different from the frequency emitted by the vibration source. Because this phenomenon was first discovered by Austrian scientist Doppler, it is called the Doppler effect. That is to say, when the emitted electromagnetic wave and the detected target move relatively, the frequency of the echo will be different from the frequency of the emitted wave.
When the target approaches the radar antenna, the reflected signal frequency will be higher than the transmitter frequency; on the contrary, when the target moves away from the antenna, the reflected signal frequency will be lower than the transmitter probability. The frequency change formed by the Doppler effect is called the Doppler shift, which is proportional to the relative velocity V and inversely proportional to the frequency of the vibration. In this way, by detecting this frequency difference, the moving speed of the target relative to the radar can be measured, that is, the relative speed between the target and the radar. According to the time difference between the transmitted pulse and the received, the distance to the target can be measured. At the same time, the Doppler frequency spectrum line of the target is detected by the frequency filtering method, and the spectrum line of the interference clutter is filtered out, so that the radar can distinguish the target signal from the strong clutter. Therefore, the pulse Doppler radar has stronger anti-clutter ability than ordinary radar, and can detect moving targets hidden in the background. Pulse Doppler radar was successfully developed and put into use in the 1960s.
Doppler radar is a radar that uses the Doppler effect for positioning, speed measurement, ranging and other work. However, this method cannot detect the tangential velocity. The second method is to obtain the velocity by differentiating the position by tracking the position.
For vehicle safety, the most important basis for judgment is the relative distance and relative speed information between the two vehicles. Vehicles traveling at high speeds are prone to rear-end collisions if the distance is too close. Therefore, the commonly used collision avoidance systems take the measurement of the relative distance between vehicles as the main detection task.
At present, the millimeter wave radar on the market has two specifications: 24GHz and 77GHz. Among them, the 77GHz millimeter-wave radar is mainly used in front of the car to detect medium and long-distance objects, and the 24GHz millimeter-wave radar is generally installed on the side and rear of the car for blind spot detection, auxiliary parking systems, etc.
The working system of radar is mainly divided into pulse mode and continuous wave mode. Continuous wave (Continuous Wave: CW) radar refers to the emission of continuous wave signals, mainly used to measure the speed of the target. If the distance of the target needs to be measured at the same time, the transmitted signal needs to be modulated, such as periodic frequency modulation of the continuous wave sine wave signal. The waveform emitted by the pulse radar is a rectangular pulse, which works according to a certain or staggered repetition period.
Modern pulsed radar technology is quite mature, but in principle it is impossible to solve the ambiguity of distance and velocity measurement at the same time, which requires the use of multiple repetition pulse frequency (PRF) methods to solve the ambiguity of distance and velocity, so not only The data transmission rate of the system is reduced, and it is not conducive to the improvement of the signal-to-noise ratio (SNR). And continuous wave radar, such as continuous wave radar with pseudo code or random code 0 ~ π phase modulation, can solve the problem of pulse radar blind area well, and has good speed and distance resolution.
At the same time, in the short-range radar system or secondary radar, the continuous wave radar has unique advantages compared with the pulse radar: especially with the development of microwave solid-state devices in the world today, the use of continuous wave radar can make the radar simpler. The reason The transmitter of the continuous wave radar does not need very high voltage, does not produce high voltage ignition, and the modulation signal can be diversified, which is conducive to the improvement of the transmitter under the same volume and weight. In this way, the continuous wave radar can achieve small size, light weight, easy implementation of the transmitter and low feeder loss.
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