Starting from measuring distance and relative speed, how to achieve effective collision avoidance?

With the development of the national economy, the number of automobiles is increasing day by day, and automobile transportation is becoming more and more busy, but at the same time, traffic accidents are also common.

Therefore, the development of automobile collision avoidance technology is of great significance to improve the level of automobile intelligence. In order to avoid a collision, the car must measure the distance of the obstacle ahead with certain equipment, and quickly feedback it to the car, so that in a critical situation, it can alarm or automatically perform a preset operation such as emergency braking to avoid the Traffic accidents caused by driver fatigue, negligence, and misjudgment. In this paper, the key points of collision avoidance technology are focused on vehicle ranging technology. From the perspective of measuring distance and relative speed, it can achieve the effect of effective collision avoidance.

1. Relevant research and setting of safety distance
1.1 Parameter determination

The so-called safe driving distance refers to the distance between the two workshops before and after driving in the same direction on the same lane (the distance between the front of the rear car and the rear of the front car), maintaining an appropriate distance that neither causes rear-end collisions nor reduces road capacity. . The formula for calculating the safety distance is:

d*=η/(vt+v2/254φ) (1)

It can be seen that to obtain the safe driving distance, the parameters that need to be determined are the vehicle speed v, the reaction time t, the adhesion coefficient φ between the tire and the road, and the system adjustment coefficient η.

1) Vehicle speed v

The real-time value can be measured using the vehicle speed sensor of the vehicle collision avoidance system.

2) Reaction time t

According to the determination of relevant experts, in general, the reaction time of most drivers is between 0.30 and 1100 s, and the total reaction time is between 1.30 and 1.98 s. s, that is, the value is 1.30 to 1.98 s.

3) Adhesion coefficient φ between tire and road

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 η according to his different requirements for the safety effect. If more conservative, choose a larger value of η. The value of η ranges from 1.05 to 1.10, usually 1.10.

5) Safety distance d0

A certain distance d0 should be maintained when the two vehicles brake and stop to ensure safety. Whether d0 is selected reasonably has a certain impact on the false alarm rate of the system. Ideally, the minimum value can be 0, but in domestic and foreign data, it is generally 2 to 5 m, and it is taken as 5 m for safety reasons.

1.2 The realization process of the system model

Given a car collision avoidance system, set the corresponding parameters according to the influencing factors and value ranges of each parameter: t=1.8 s; η=1.10; d0=5 m. In the process of driving, according to the different types of road surfaces, refer to the data in Table 1 to set the adhesion coefficient φ between the tire and the road. According to the calculation, when the speed is the same, the safe driving distance obtained by different road conditions is different, and the higher the speed, the greater the difference in the obtained safe driving distance. Therefore, it is decided to install a road condition selection switch, which is realized by a touch switch. The driver subjectively selects the adhesion coefficient according to the weather conditions, and then performs data processing to obtain the safe driving distance under the adhesion coefficient. The switch selection is shown in Figure 1.

Starting from measuring distance and relative speed, how to achieve effective collision avoidance?

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, it is necessary to use the beat method, that is, try to extract the difference fd between f0 and fr.

The classic beat method is to use coherent demodulation, multiply the received signal and the transmitted signal by a multiplier, and after a low-pass filter, the required beat frequency signal can be obtained.

2.2 Principle of coherent detection

Coherent detection, also called coherent demodulation, aims to extract the modulated signal from the received signal.Assuming that the transmitted signal and the received signal are



The general model of coherent demodulation is shown in Figure 2.

After a low pass filter, we get



3. Generation and reception of LFMCW radar signals

3.1 Generation of LFMCW Radar Signals

The basic block diagram of the LFMCW radar system is shown in Figure 3.

LFMCW is a modulated radio frequency signal, which is generated by a VCO (Voltage Controlled Oscillator) under the action of the modulated signal. The frequency of the VCO output signal varies linearly with the amplitude of the modulating signal. A part of the VCO output signal is amplified by the power amplifier and then sent out 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 during the forward process, it will be partially reflected. The reflected signal is received by the receiving antenna and mixed with the local oscillator signal, and the intermediate frequency signal is output through the band-pass filter, and the subsequent signal processing circuit can be obtained from the intermediate frequency signal. Information such as distance and speed of the target is extracted.

The waveform diagram 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, and the echo signal is consistent with the transmitted signal waveform, but there is a time delay.

3.2 Clutter processing of LFMCW radar echo signals

Coupling the transmitted signal and the echo signal to the mixer for mixing, and outputting the intermediate frequency signal. Considering the interference echo signals of various backgrounds (such as ground objects, clouds and rain, etc.), the radar target echo signals are often aliased. in the background of interference clutter. Most clutter is distributed clutter with internal motion and a wide spectrum. At this time, it is necessary to perform moving target detection (MTD) on the echo signal. The moving target detection of radar signals is to use the Doppler filter bank matched with the coherent echo pulse train to suppress various clutter, improve the power signal-to-noise ratio, and achieve coherent accumulation to enhance radar detection in the clutter background. Techniques for athletic target ability. Its essence is equivalent to coherent accumulation of different channels. The realization method is to pass the echo signal through a set of Doppler filter banks to perform signal processing to separate the moving target echo from the clutter. The simulation results of MTD processing are shown in Fig.5.

3.3 IF signal processing

For the LFMCW signal generated in Figure 4, it can be compared that only the echo signal has a certain time delay relative to the transmitted signal. Even if there is a Doppler frequency shift, the forward and backward frequency difference of the intermediate frequency signal must also exist relative to the transmission signal. In the constant difference stage with a relatively long change period, a rectangular window is used to intercept the constant stage and perform spectrum analysis on it, as shown in Figure 6(a) and (b), the forward frequency difference and the backward frequency can be obtained through processing. difference, and then obtain the distance and relative speed.

4. Ranging principle of LFMCW radar signal

The carrier frequency of FM CW radar varies linearly within the modulation period. There are also many ways to change the linearity. Combined with the analysis of test requirements, this paper adopts the LFMCW of the triangular wave modulation method to measure. From the angle of relative motion, focus on the study of the reflected signal from a relatively moving target, the reflected signal frequency will also include a Doppler frequency shift caused by the relative motion of the target, as shown in Figure 7.

Combining the solution formula of relative speed and the design idea and working principle of the collision avoidance system, it can be seen that to judge whether two vehicles are close or far away, it is only necessary to judge the size of the symbols of fb- and fb+. From the above analysis, it can be seen that obtaining the frequency difference between the upper and lower sweep frequency bands of the LFMCW and comparing the magnitude of its value is the key to achieve effective anti-collision processing.

5 Conclusion

The design system focuses on measuring the relative movement of the two workshops. As long as the distance between the two cars meets the condition of the safe distance, and the Doppler effect is used to measure the relative movement of the two workshops, it can be effectively realized. For car collision avoidance, it is not necessary to measure the relative speed value in real time. This reduces a lot of unnecessary signal processing and computation.

The Links:   SKM50GB063D LB121S03-TL04