“Millimeter wave technology is one of the current hot technologies. Without millimeter wave, many progress made so far will be stagnant. For millimeter wave applications, 5G millimeter waves and millimeter wave communications are well known. In this article, the millimeter wave radar technology will be explained in detail to enhance everyone’s understanding of millimeter wave applications.
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Millimeter wave technology is one of the current hot technologies. Without millimeter wave, many progress made so far will be stagnant. For millimeter wave applications, 5G millimeter waves and millimeter wave communications are well known. In this article, the millimeter wave radar technology will be explained in detail to enhance 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 in pulse or phase, so as to calculate the time difference between the transmitted signal and the received signal. Then respectively correspond to the propagation speed of ultrasonic, infrared, and laser in the air, and calculate the distance and relative speed to the obstacle. Compared with optical and infrared radars, millimeter-wave radar is not affected by the shape and color of the target object, and is not affected by atmospheric turbulence compared with ultrasonic, so it has stable detection performance and good environmental adaptability. Affected by changes in the weather and the external environment, rain, snow, dust, and sunlight will not interfere with it; the Doppler frequency shift is large, and the accuracy of the relative speed measurement is improved.
The vehicle active anti-collision control system based on multi-sensor information fusion is to use 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 multiple sensors Status information, and estimate the risk of collision. 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. It enables optimal control of reaction time, distance, and speed. Reducing the driver’s burden and judgment errors will play an important role in improving traffic safety, and it is the basis for realizing auto-driving.
Among the automobile anti-collision sensors, according to the working principle and working process, they are divided into ultrasonic radar, infrared radar, lidar and millimeter wave radar. Among the first three types of radars, the echo is detected and compared with the transmitted signal to obtain the difference in pulse or phase, so as to calculate the time difference between the transmitted signal and the received signal. Then respectively correspond to the propagation speed of ultrasonic, infrared, and laser in the air, and calculate the distance and relative speed to the obstacle. Although these three types of anti-collision radars designed by the sound and light principle are simple in structure and low in price, they are susceptible to interference from severe weather conditions and cannot ensure ranging accuracy. Millimeter-wave radar shows its unique advantages. Compared with optical and infrared radars, it is not interfered by the shape and color of the target object, and compared with ultrasound, it is not affected by atmospheric turbulence, so it has stable detection performance; good environmental adaptability . Affected by changes in the weather and the external environment, rain, snow, dust, and sunlight will not interfere with it; the Doppler frequency shift is large, and the accuracy of the relative speed measurement is improved.
Below we will introduce the mainstream sensor technology step by step. At present, the most concerned sensing method is the radar system that uses millimeter waves for measurement. Today, we will introduce millimeter wave radar technology.
Radar is a device that uses radio echo to detect the direction and distance of a target. Radar is the transliteration of the English word Radar, which is abbreviated from the prefix of Radio Detection And Ranging, which means radio direction and ranging. The world became familiar with radar when the British air battle in 1940, 700 British fighters carrying radars defeated 2,000 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 aircraft and ship navigation safety, it can help pilots and airport air traffic control personnel to complete their tasks more effectively.
Millimeter wave is an electromagnetic wave that works in the millimeter wave band with a working frequency of 30-100 GHz and a wavelength of 1-10 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. Vehicle millimeter wave radars mainly include 24GHz and 77GHz millimeter wave radars.
Compared with the centimeter waveguide seeker, the millimeter waveguide seeker has the characteristics of small size, light weight and high spatial resolution. Compared with infrared, laser, TV and other optical seekers, the millimeter waveguide seeker has a strong ability to penetrate fog, smoke, and dust, and has the characteristics of all-weather (except heavy rain). Millimeter-wave radar can work around the clock, not affected by weather conditions, and the harsh climate environment is one of the main causes of traffic accidents. Compared with light waves, millimeter waves use atmospheric windows (when millimeter waves and submillimeter waves propagate in the atmosphere, some of the attenuation due to the resonance absorption of gas molecules is a minimum value). And the heat radiation source has little influence.
The main limitations of the application of millimeter wave in radar are: the attenuation of high humidity environments such as rain, fog and wet snow, and the influence of high-power devices and insertion loss will reduce the detection distance of millimeter wave radar; the penetration ability of trees is poor, compared with Microwave has low penetration into dense trees.
Like most microwave radars, millimeter waves have the concept of beams, that is, the emitted electromagnetic wave is a cone-shaped beam instead of a line like a laser. This is because the antennas in this band mainly use electromagnetic radiation instead of light particle emission as the main method. In this regard, radar and ultrasound are the same, and the way of this beam leads to its advantages and disadvantages. Advantages and reliability, because the reflective surface is large, but the disadvantage is that the resolution is not high. The millimeter-wave radar can perform detection, ranging, speed and azimuth measurement on the target.
Speaking of the principle of ranging, it is actually simple, all 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 radio wave with a very high frequency, not a sound wave. The transmitter of the radar is equivalent to the vocal cord that emits the shouting sound, and can emit pulses similar to the shouting sound. 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 function of the receiver is similar to that of the human ear to receive the echo of the electric pulse sent by the radar transmitter.
Millimeter-wave radar speed measurement is the same as ordinary radar. There are two ways. One is based on the principle of Dopler Effect. The so-called Doppler effect is that when a vibration source such as sound, light and radio waves and the observer move at a relative speed V, the frequency of the vibration 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. In other words, when the emitted electromagnetic wave and the detected target move relative to each other, 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; conversely, when the target moves away from the antenna, the reflected signal frequency will be lower than the transmitter frequency. 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 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 of the target and the radar. According to the time difference between transmitting pulse and receiving, the distance to the target can be measured. At the same time, the frequency filtering method is used to detect the Doppler frequency spectrum of the target and filter out the spectrum of the interference clutter, so that the radar can distinguish the target signal from the strong clutter. Therefore, pulse Doppler radar has stronger anti-clutter interference 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, and ranging. But this method cannot detect the tangential velocity. The second method is to obtain the velocity by tracking the position and performing differentiation.
For vehicle safety, the most important basis for judging is the relative distance and relative speed information between two vehicles. If the distance of a high-speed vehicle is too close, it is easy to cause a rear-end collision. Therefore, commonly used collision avoidance systems take the measurement of the relative distance between vehicles as the main detection task.
The millimeter-wave radar currently on the market has two specifications: 24GHz and 77GHz. Among them, the 77GHz millimeter-wave radar is mainly used in the front of the car to detect objects at medium and long distances, and the 24GHz millimeter-wave radar is generally installed on the sides 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 (ContinuousWave: CW) radar refers to the transmission of continuous wave signals, mainly used to measure the speed of the target. If you need to measure the distance of the target at the same time, you need to modulate the transmitted signal, for example, perform periodic frequency modulation on a continuous wave sine wave signal. The waveform transmitted by pulse radar is rectangular pulse, which works in a certain or staggered repetitive cycle.
Modern pulse radar technology is quite mature, but in principle, it is impossible to solve the ambiguity of distance and velocity measurement at the same time. This requires the use of multiple pulse frequency (PRF) methods to resolve the ambiguity of distance and velocity. The data transmission rate of the system is reduced, and it is not conducive to the improvement of the signal-to-noise ratio (SNR). However, continuous wave radars, such as continuous wave radars phase-modulated with pseudo codes or random codes from 0 to π, can well solve the problem of blind areas of pulse radars, and have good speed and range resolution.
At the same time, in short-range radar systems or secondary radars, continuous wave radar has unique advantages compared to pulsed radar: especially with the development of microwave solid-state devices in the world today, the use of continuous wave radar can make radar simpler. The reason is 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 beneficial to the improvement of the transmitter under the same volume and weight. In this way, continuous wave radar can achieve small size, light weight, easy implementation of the transmitter and low feeder loss.
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