With the development of national economy, the number of cars is increasing, and the automobile transportation is becoming busier, but at the same time, traffic accidents are also common. Therefore, the development of automobile anti-collision technology is of great significance to improve the level of automobile intelligence. In order to avoid collision, the vehicle must measure the distance of obstacles in front with certain equipment and quickly feed back to the vehicle, so as to avoid traffic accidents caused by driver fatigue, negligence and wrong judgment through alarm or automatic preset operation, such as emergency braking, in case of emergency. This paper focuses on the vehicle ranging technology. From the perspective of measuring distance and relative speed, effective anti-collision effect is realized.
1. Research and setting of safety distance
1.1 parameter determination
The so-called safe driving distance refers to the distance between the front and rear workshops (the distance between the front and rear of the rear vehicle) on the same lane in the same direction, so as to maintain an appropriate distance that neither rear end collision accident nor road capacity is reduced. Calculation formula of safety distance:
d*= η／ （vt+v2／254 φ） （1）
It can be seen that the required safe driving distance and the parameters to be determined include the vehicle speed V, reaction time t and the adhesion coefficient between the tire and the road φ、 System adjustment coefficient η。
1) Vehicle speed v
The real-time value can be measured by using the vehicle speed sensor of the vehicle anti-collision system.
2) Reaction time t
According to the measurement of relevant experts, generally speaking, the reaction time of most drivers is between 0.30 ~ 1100 s, plus factors such as the action time of the braking system, the total reaction time is between 1.30 ~ 1.98 s, that is, the value is 1.30 ~ 1.98 s.
3) Adhesion coefficient between tire and road φ
The adhesion coefficients of different pavements are shown in Table 1.
4) System adjustment coefficient η
For safety, the driver should set the adjustment coefficient according to his different requirements for safety effect η。 If conservative, choose η It’s worth more. η The value range is 1.05 ~ 1.10, usually 1.10.
5) Safety clearance D0
When the two vehicles stop braking, a certain distance d0 shall be maintained to ensure safety. Whether d0 is selected reasonably has a certain impact on the false alarm rate of the system. Ideally, it can be 0 at least, but it is generally 2 ~ 5m in domestic and foreign data, and 5m is taken for safety reasons.
1.2 implementation process of system model
Given an automobile anti-collision system, set the corresponding parameters according to the influencing factors and value range of each parameter: T = 1.8 s; η= 1．10； d0=5 m。 During driving, the adhesion coefficient between tire and road shall be set according to different pavement types and the data in Table 1 φ。 According to the calculation, when the speed is the same, the safe driving distance is different under different road conditions, and the greater the speed, the greater the difference of safe driving distance. Therefore, it is decided to install a road condition selection switch, which is realized by point touch switch. The driver subjectively selects the adhesion coefficient according to the weather conditions, and then carries out data processing to calculate the safe driving distance under the adhesion coefficient. The switch selection is shown in Figure 1.
2. Measurement of Doppler shift
2.1 extraction of Doppler information
If the reflected signal comes from a relatively moving target, the reflected signal includes a Doppler shift FD caused by the relative motion of the target.
To extract the Doppler frequency from the received signal, we need to use the beat method, that is, try to extract the difference FD between F0 and fr.
The classical beat method uses coherent demodulation, multiplies the received signal and the transmitted signal through a multiplier, and obtains the required difference frequency signal after passing through a low-pass filter.
2.2 coherent detection principle
Coherent detection is also called coherent demodulation. The purpose is to extract the modulated signal from the received signal. It is assumed that the transmitted signal and the received signal are
s（t）=Acos ω 0t （3）
r（t）=Bcos（ ω 0+ ω d）t （4）
The general model of coherent demodulation is shown in Fig. 2.
Obtained by low-pass filter
x（t）=Bcos ω dt／2 （6）
X (T) is the beat signal between the received signal and the transmitted signal, and the Doppler frequency shift is extracted.
3. Generation and reception of LFMCW radar signal
3.1 generation of LFMCW radar signal
The basic composition block diagram of LFMCW radar system is shown in Figure 3.
LFMCW is a modulated RF signal generated by VCO (voltage controlled oscillator) under the action of modulated signal. The frequency of the VCO output signal varies linearly with the amplitude of the modulation signal. Part of the VCO output signal is amplified by the power amplifier and transmitted wirelessly, and the other part is added to the mixer through the directional coupler as the local oscillator signal of the mixer. If the transmitted signal encounters the target in the forward process, part of it is reflected. The reflected signal is received by the receiving antenna, mixed with the LO signal, and outputs the IF signal through the band-pass filter. The subsequent signal processing circuit can extract the distance, speed and other information of the target from the if signal.
The waveform of LFMCW signal generated by MATIAB programming is shown in Figure 4. It can be seen from the figure that the transmitted signal is an LFMCW waveform whose frequency decreases linearly and then increases, while the echo signal is consistent with the transmitted signal waveform, but there is a time delay.
3.2 clutter processing of LFMCW radar echo signal
The transmitted signal and echo signal are coupled to the mixer for mixing, and the IF signal is output. Combined with the reality, considering the jamming echo signal of various backgrounds (such as ground objects, clouds and rain, etc.), the radar target echo signal is often mixed in the jamming clutter background. Most clutter belong to distributed clutter with internal motion and wide spectrum. At this time, it is necessary to carry out moving target detection (MTD) on the echo signal. Moving target detection of radar signal is a technology that uses Doppler filter banks matched with coherent echo pulse train to suppress various clutter, improve power signal-to-clutter ratio and realize coherent accumulation, so as to enhance the ability of radar to detect moving targets in clutter background. Its essence is equivalent to coherent accumulation of different channels. The realization method is to process the echo signal through a group of Doppler filter banks to separate the moving target echo from clutter. The simulation results of MTD processing are shown in Figure 5.
3.3 IF signal processing
For the LFMCW signal generated as shown in Fig. 4, only the echo signal has a certain time delay relative to the transmitted signal. Even if there is Doppler frequency shift, there must be a long constant difference stage relative to the change period for the forward and backward frequency difference of the IF signal. The rectangular window is used to intercept the constant stage and analyze its spectrum, as shown in Fig. 6 (a) (b), Through processing, the forward frequency difference and backward frequency difference can be obtained, and then the distance and relative velocity can be obtained.
4. Ranging principle of LFMCW radar signal
The carrier frequency of FM CW radar changes linearly in the modulation period. There are also many ways of linear change. Combined with the test demand analysis, LFMCW with triangular wave modulation is used for measurement in this paper. From the perspective of relative motion, focus on the study that the reflected signal comes from a relatively moving target, and the reflected signal frequency will also include a Doppler frequency shift caused by the relative motion of the target, as shown in Fig. 7.
Combined with the solution formula of relative speed and the design idea and working principle of anti-collision system, to judge whether the two vehicles are close or far away, you only need to judge the symbol size of FB – and FB +. Through the above analysis, it can be seen that obtaining the frequency difference of LFMCW up and down sweep frequency band and comparing its value become the key to realize effective anti-collision processing.
The design system focuses on measuring the relative motion of the two workshops. As long as we judge whether the distance between the two vehicles meets the conditions of safe distance, and use the Doppler effect to measure whether the relative motion of the two workshops is close or far away, we can effectively realize the vehicle anti-collision, and there is no need to measure the relative speed value in real time. This reduces a lot of unnecessary signal processing and calculation.
Responsible editor: GT