FMCW millimeter wave collision avoidance radar system

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FMCW millimeter wave collision avoidance radar system [Copy link]

Abstract: This paper introduces the principle of active automobile anti-collision FMCW millimeter wave radar system and reports the SAE-100 millimeter wave anti-collision radar prototype developed by us.
Keywords: millimeter wave radar, automobile anti-collision

The automobile anti-collision system is very important for improving the safety of automobile driving, and the research of this system has always been highly valued. Since 1971, active automobile anti-collision systems such as ultrasonic, laser, infrared, and microwave have appeared one after another, but the above systems all have some shortcomings and have not been widely promoted and applied in automobiles. With the rapid development of highway networks in various countries, serious traffic accidents have continued to increase. In order to reduce accidents, protective measures such as driving seat belts and airbags have been adopted successively, but these technologies are passive protection and cannot fundamentally solve the problem. Millimeter wave refers to electromagnetic waves with a wavelength between 1 and 10 mm. It has a large RF bandwidth, high resolution, small antenna component size, and can adapt to harsh environments. Therefore, the millimeter wave radar system has the characteristics of light weight, small size, and all-weather. "Active automobile millimeter wave anti-collision radar system" has become a hot spot in international research and development in recent years, and some products have begun to be put on the market, with a very promising prospect.

This article introduces the principle of active automobile anti-collision millimeter wave radar and reports the SAE-100 millimeter wave anti-collision radar prototype we developed.

Principle of automobile anti-collision millimeter wave radar system

Active automobile anti-collision is based on radar ranging and speed measurement. The anti-collision radar system monitors the front of the vehicle in real time. When a dangerous target (such as a stopped or slow-moving vehicle in front of the vehicle) appears, the radar system sends an alarm to the driver in advance, allowing the driver to respond in time. At the same time, the radar output signal reaches the vehicle control system, and automatically brakes or slows down according to the situation.

There are two types of millimeter-wave anti-collision radar systems: frequency modulated continuous wave (FMCW) radar and pulse radar. For pulse radar systems, when the target is very close, the time difference between the transmitted pulse and the received pulse is very small, which requires the system to use high-speed signal processing technology. The short-range pulse radar system becomes very complex and the cost increases significantly. Therefore, the automotive millimeter-wave radar anti-collision system often uses a frequency modulated continuous wave radar system with a simple structure, low cost, and suitable for close-range detection.

Millimeter-wave FMCW radar system structure FMCW automotive radar system is shown in Figure 1, including antenna, transceiver module, signal processing module and alarm module or vehicle braking device. The RF transceiver front end is the core component of the radar system. A lot of in-depth research has been conducted on the front end at home and abroad, and great progress has been made. Various front-end structures have been developed, mainly including waveguide structure front-end, microstrip structure front-end and monolithic integration of front-end. The RF front-end developed in China is mainly a waveguide structure front-end. A typical RF front-end mainly includes three parts: linear VCO, circulator and balanced mixer, as shown in Figure 2. The intermediate frequency signal output by the front-end mixer is sent to the post-stage data processing part after intermediate frequency amplification. The basic goal of the data processing part is to eliminate unnecessary signals (such as clutter) and interference signals, and process the mixed signal after intermediate frequency amplification to extract information such as target distance and speed from the signal spectrum. Millimeter wave FMCW radar ranging and speed measurement principle The radar system transmits a series of continuously frequency-modulated millimeter waves through the antenna and receives the reflected signal of the target. The frequency of the transmitted wave changes with time according to the law of the modulation voltage. Generally, the modulation signal is a triangular wave signal, and the frequency changes of the transmitted signal and the received signal are shown in Figure 3a. The shape of the reflected wave is the same as that of the transmitted wave, except that there is a time delay (t. The relationship between (t and the target distance R can be expressed as △t=2R/c (1) where c: the frequency difference between the light-speed transmitted signal and the reflected signal at a certain moment is the intermediate frequency signal frequency (f) of the mixing output (as shown in Figure 3b). According to the triangular relationship, it can be concluded from Figure 3a that the target distance R is R=(cT/4△F)△f (2) That is, the target distance is proportional to the intermediate frequency frequency output by the front end. If the reflected signal comes from a relatively moving target, the reflected signal includes a Doppler frequency shift fd caused by the relative motion of the target (as shown in Figure 4). The intermediate frequency output at the rising and falling edges of the triangular wave can be expressed as fb + = △ ffd (3) fb- =△f+fd ( 4) where --(f: the intermediate frequency when the target is relatively stationary; fd: Doppler frequency shift, whose sign is related to the direction of the target's relative motion. According to the Doppler principle, the relative motion speed v of the target is v=c/ 4f0 (f b- -f b+ ) =λ(f b- -f b+ ) (5) Where - f0: center frequency of the transmitted wave; : wavelength of the transmitted wave. The sign of the velocity v is related to the direction of the target's relative motion. When the target is close, v is positive, otherwise v is negative. The intermediate frequency signal frequency of the rising and falling edges of the triangular wave is obtained by FFT transformation of the DSP. The target distance and the target relative motion speed can be calculated by formulas (2) and (5). Development of SAE-100 millimeter wave anti-collision radar system After nearly a year of research, Shanghai Automotive Electronics Engineering Center has developed a prototype of the SAE-100 millimeter wave anti-collision radar system. The prototype adopts the homodyne FMCW system. The system structure is shown in Figure 1. The operating frequency is 35GHz, the distance range is >100m, and the speed range is >100km/h. The system uses a small horn antenna with a gain of 26dB, a waveguide structure front end with a transmission power of 40mW, and advanced DSP data processing technology. The upper part includes the antenna, front end and intermediate frequency amplifier module, with a size of 19cm (15cm (16cm), and the output signal is an amplified intermediate frequency signal. The lower part is the data processing and display alarm module, which can display the target distance and relative movement speed. When the target distance is less than 100m, three different tones can be used to alarm according to the distance.

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