Anti-collision car system based on dual ultrasonic receiving heads

Abstract: This paper proposes a monitoring and control system for ultrasonic positioning of objects in front based on single-chip microcomputer control. Through the detection signal of the dual ultrasonic receiving heads on the left and right, the single-chip microcomputer is sent for data calculation and processing, which can accurately calculate the distance to the obstacle and determine whether the obstacle is located on the route of the vehicle. This ensures that the vehicle can accurately avoid obstacles while driving. The experimental test system shows that the effective distance of the system can reach 8m, the measurement accuracy can reach 0.05m, the car can freely avoid obstacles in front, or stop by emergency brake when encountering obstacles, but it will not stop because the obstacle is in the oblique direction of the car.

1 Introduction

Ultrasonic sensors have been widely used due to their high measurement accuracy, fast response and low price. The traditional application method is that one transmitter corresponds to one receiver, and there are also multiple transmitters corresponding to one receiver. However, we found in actual applications that if the obstacle is large (such as a wall), the ultrasonic sensor can be used to accurately measure the distance, but if it is used in the car's anti-collision system, because the obstacle is columnar and the ultrasonic transmitter has a certain scattering angle (left and right), even if the obstacle is not directly in front of the car, the ultrasonic wave can still detect the oblique front echo, which brings difficulties and misleading to the intelligent control of the vehicle. In order to solve this problem, we proposed a solution using dual receivers and gave a set of specific control strategies from a practical perspective.

2 System structure and process design

Our entire system needs to complete the functions of distance measurement, speed measurement, positioning, and control of the car's movement. The system includes the following six parts: ultrasonic transmitting circuit, ultrasonic receiving circuit, signal processor, temperature measurement, and car control circuit. The system structure block diagram is shown in Figure 1:

Figure 1: System structure diagram

A 40k square wave is generated by a single-chip microcomputer, which is amplified to drive the ultrasonic sensor transmitter, thereby emitting ultrasonic waves. After being reflected by the object in front, the ultrasonic wave is captured by the receiving end. After calculating the capture time of the two receiving heads and adding temperature compensation, the direction and distance of the final car in front are determined, and then the speed relative to the car in front is calculated by differential with the previous data. Finally, intelligent control is performed through the three data of speed, distance and position to control the car to turn or slow down, etc.

The specific hardware components are as follows: MCU uses AT89S52 single-chip microcomputer, P1.0 port outputs 40K square wave signal required by ultrasonic transducer, drives sensor after passing through inverter 7404, in order to make ultrasonic wave transmit farther, we connect three transmitters in parallel, use external interrupt 0 port to monitor the return signal output by ultrasonic receiving circuit, echo detection uses infrared detection integrated chip CX20106, display circuit uses simple 4-bit common anode LED digital tube, break code uses 74LS244, bit code uses 8550 driver. The temperature measurement part uses 18B20 to measure the current ambient temperature to determine the speed of ultrasonic wave propagation.

3 MCU algorithm control

3.1 Distance calculation and direction determination

The microcontroller can calculate the time between the emission and reception of the ultrasonic wave, find out the speed of sound at the corresponding temperature according to the actual temperature measurement of the temperature measurement system, and calculate the distance between the reflector and the two receiving ends. Theoretically, the spatial position of the object can be directly mathematically deduced from the above two data (as shown in Figure 2 and formulas 1 and 2).

Figure 2 Spatial orientation of ultrasonic sensor

Where d is the distance between R1 and R2, z1 and z2 are the distances from the object to each receiving end. If the calculation is done directly, it will be too complicated and time-consuming for ordinary single-chip microcomputers to process. Therefore, we propose a method to roughly determine the position of the object based on calculating the distance difference between the two. Generally speaking, the car only cares about the object in front of the car. We set a distance parameter l to represent the horizontal distance between the obstacle in front and the car, and then set a distance parameter h to represent the vertical distance between the obstacle in front and the car. We can deduce the relationship between h, l, d and z2-z1 from the following relationship (Formula 3-Formula 6).

By dividing the two terms in formula 6, it is not difficult to find that the first term is always greater than the second term, so z2-z1 is an increasing function of l. At the same time, as h decreases, z2-z1 will also increase. That is to say, when an obstacle approaches the car, if it deviates from the center of the car (that is, it will not hit the car), there is an obvious feature that its z2-z1 value will be larger. We can take d=5cm h=30cm, let l vary between [10cm,30cm], and the resulting curve is shown in Figure 3. The geometric relationship of each physical quantity is shown in Figure 4.

Figure 3 Relationship between z2-z1 and l

Figure 4 Geometric relationship of various physical quantities

It is not difficult to find that when the distance l is in the interval [10cm, 30cm] (h<30cm), the difference of z2-z1 will be >4cm. Based on this, we set a threshold of 4cm. When the difference is detected to be greater than 4cm, no braking control is required and the vehicle can go straight through. Through such simple calculation and judgment, we can effectively avoid the possibility of misjudgment of braking due to the small values ​​of z1 and z2 caused by obstacles that deviate from the center of the vehicle and are too close to the vehicle. When doing this project, the car model we used was not large, so the designed threshold was not very large. If it is applied to the actual car model, the size of the threshold can be changed according to the situation.

3.2 Calculation of speed

We use a simple approximate average to estimate the speed. We can calculate that the system ranging interval is about 120ms. By comparing the current ranging result with the last ranging result, we can estimate the approximate value of the current speed according to Formula 7:

3.3 System flow (see Figure 5).

Figure 5 System flow

4 Partial test results

Table 1 shows the test results of the distance measurement circuit alone: ​​(unit: cm).

Table 1 Ranging results

From this table we can see that our ranging circuit is very accurate.

Figure 6 shows the test results of our dual-receiver solution:

Figure 6 Test results of dual receiving head solution

Among these six pictures, the top three are distances calculated using the time it takes the right receiving head to receive the signal, while the bottom three are distances calculated using the time it takes the left receiving head to receive the signal. It can be seen that when the obstacle deviates from the center, the distances measured by the left and right receiving heads are obviously different, which can be used for positioning.

Finally, when we completed the debugging of the entire car system, we used it to test and found that no matter whether the obstacle was moving or stationary, the car could correctly judge and move forward or stop at any position in front of it.

5 Conclusion

In summary, this system proposes a vehicle-mounted automatic speed and distance measurement control system based on dual ultrasonic receiving heads and 3 transmitting heads, which can effectively play the role of protecting the driver and prejudging and reminding the driver. When the driver takes wrong control measures when encountering an emergency, the system can also force correction or alarm to remind the driver to check. Because the system is simple, economical and stable, it has a very large market prospect.