Design of two-dimensional reversing radar system based on ultrasonic ranging

Abstract: Aiming at the problem that the existing one-dimensional reversing radar system cannot detect road conditions on the ground, a two-dimensional reversing radar system based on ultrasonic ranging is designed. Two rows of ultrasonic probes respectively monitor the horizontal obstacles behind the car and the ground obstacles behind the car, and give a theoretical basis for the system's ground monitoring, namely ultrasonic slope ranging. If there is an obstacle behind the car, the system will give a voice prompt. The experimental test results show that the system can achieve accurate distance measurement from 3 to 400 cm in the horizontal direction. At the same time, it has a high ability to identify obstacles and pits on the ground, and the system has high reliability.

Introduction

In recent years, with the rapid economic development, cars have entered more and more families as a convenient means of transportation. At the same time, more and more accidents caused by parking and reversing. When reversing, the mirrors inside and outside the car can expand the driver's field of vision, but obstacles behind the car and obstacles that cannot be seen through the mirror due to insufficient height may be in the blind spot of the driver's field of vision. According to relevant survey statistics, 15% of car collision accidents are caused by the lack of rear vision capabilities of the car when reversing.

Aiming at the main defects of the one-dimensional parking sensor system, this paper designs a two-dimensional parking sensor system based on ultrasonic ranging. The system can identify bumps, cliffs and other obstacles behind the car in the blind area of the driver in time. At the same time, the added intelligent voice alarm function can prompt the driver of the specific situation behind the car in time.

1 System overall design

This system is mainly composed of STC89C52 single-chip microcomputer, ultrasonic horizontal ranging module, ultrasonic ground monitoring module, stabilized power supply module, temperature compensation, intelligent voice alarm module and liquid crystal display circuit, etc. The overall design principle block diagram of the system is shown in Figure 1.


The working principle of the system is as follows: the car is in reverse gear and the system is powered on to complete the initialization. Since the speed of sound is related to temperature, a temperature sensor is used to measure the field temperature and sent back to the single-chip microcomputer for sound speed compensation. The ultrasonic horizontal ranging module emits ultrasonic waves from the rear of the car to the horizontal direction, and the ultrasonic ground monitoring module emits ultrasonic waves from the rear of the car (the installation angle is 60° with the horizontal plane). Ultrasonic waves propagate in the air and will generate echoes when they encounter obstacles. The echoes are received by the ultrasonic horizontal ranging module and the ultrasonic ground monitoring module. After the echo is pre-amplified, band-pass filtered, and voltage comparator, the single-chip microcomputer detects the time when the echo arrives, and calculates the time used by the ultrasonic wave from transmission to reception (ie transit time), thereby calculating the vehicle tail and the The distance of the obstacle. The distance is displayed on the LCD screen in real time, and a prompt voice is issued through the intelligent voice alarm module.

2 Main hardware module design

2.1 Ultrasonic horizontal ranging module design

Using the ultrasonic transceiver integrated ranging module HC-SR04 to complete the ranging task, this module includes an ultrasonic transmitting probe, a receiving probe, and a control circuit, which can achieve distance measurement from 3 to 400cm. The ultrasonic frequency emitted by the ultrasonic transmitter probe is 40 kHz, the beam angle is 30°, and the detection range is an arc-shaped area of ±30° in the axial direction.

When ultrasonic horizontal distance measurement, 3 ultrasonic modules are arranged in an equidistant manner, and the TRIG and ECHO pins of each ultrasonic module are respectively connected to the I/O port of the single-chip microcomputer. The single-chip microcomputer in turn gives the TRIG pin a high level greater than 10μs, so that the ultrasonic ranging module can give the transmitting probe 8 cycles of 40 kHz. At this time, the transmitting probe emits ultrasonic waves. After the receiving probe detects the echo, the ECHO pin outputs a high level that is the same as the time T used by the ultrasonic wave from transmission to reception. The high level time T is collected by the internal timer 0 of the single-chip microcomputer. When the single-chip microcomputer uses a 12 MHz external crystal oscillator, it takes 1 μs to execute a machine cycle, and it takes one machine cycle to add 1 to the count, so the time T (unit s) calculated by the timer is:

T=(TH0×256+TL0)×10-6 (1)

Among them, T is the time taken by the ultrasonic wave from transmission to reception, and THO and TLO respectively represent the high byte and low byte register values of the timer O of the single-chip microcomputer. The distance S between the probe and the obstacle is:


    When the system is running, the three ultrasonic ranging modules in the ultrasonic horizontal ranging circuit emit ultrasonic waves in sequence with an interval of at least greater than 60 ms to ensure that the ultrasonic echoes will not affect each other. The working diagram of the ultrasonic horizontal ranging module is shown in Figure 2.


   2.2 Design of Ultrasonic Ground Monitoring Module

    The key to ultrasonic ranging is that the probe must receive the echo. The sound wave emitted by the probe travels in a straight line, and the interface of different media will cause reflection and scattering. When the sound wave is incident on an inclined surface, the reflected wave will propagate along the direction of the reflection angle and may not be directed toward the probe. Figure 3 shows the limit of the reflected wave returning to the probe.


     In this case, H is the measurable distance, α is the inclination of the interface, and the relationship between the distances of the receiving and transmitting probes is:


    When the interface inclination α is 30° and O1O2 is 38 mm, the probe can only receive the reflected wave when H is less than 32.9 mm. The actual measurement distance of the inclined plane is much greater than this distance, so the backscattered wave becomes the echo in the actual measurement. The main ingredient. In ultrasonic ranging, for a rough inclined surface with a certain inclination angle, the scattered echo intensity when sound propagation attenuation is not considered is:


Among them, Is is the scattered echo; Wo is the transmit power of the ultrasonic probe; v is the axial concentration coefficient, which is related to the shape of the radiation surface. If it is a circular radiation surface, then v=π·d/λ; H is the probe to The vertical distance of the slope; △θ is the beam angle; β

is the inclination angle of the incident wave.

From equation (5), it can be seen that under the condition that the probe transmitting power, axial concentration factor and beam angle are fixed, the intensity of the scattered echo is related to the measurement distance and the inclination angle of the incident wave. When β=60°, the scattered echo energy is not much reduced compared with the situation when β=90°, and echo detection can still be performed.

Three ultrasonic modules HC-SR04 are fixed at the rear of the system at an angle of 60° with the ground. In the actual car reversing system, this module can be installed in a suitable place at the rear of the car. The side view and front view of the probe installation are shown in Figure 4.