The full name of car reversing aid systems is “reversing anti-collision radar”, also known as “parking aid device”, which is a parking safety aid device. It can tell the driver about the obstacles around by sound or more intuitive display, and relieve the trouble caused by the driver’s front, rear, left and right visits when parking and starting the vehicle, It also helps the driver eliminate the defects of dead angle and blurred vision, and improves the safety.
System working principle
The reversing radar only needs to work when the car is reversing to provide the driver with the information behind the car. Because the vehicle’s driving speed is slow when reversing, compared with the sound speed, it can be considered that the vehicle is stationary, so the influence of Doppler effect can be ignored in the system. In many ranging methods, pulse ranging method only needs to measure the round-trip time between the measuring point and the target, which is simple to realize. Therefore, this method is adopted in this system.
As shown in Figure 1, after the driver turns the handle to the reverse gear, the system starts automatically. The ultrasonic transmitting module transmits 40KHz ultrasonic signal backward, which is reflected by the obstacle and collected by the ultrasonic receiving module for amplification and comparison. The single chip microcomputer AT89C2051 sends this signal to the display module, triggers the voice circuit and sends out synchronous voice prompt. When the distance from the obstacle is less than 1m At 0.5m and 0.25m, different alarm sounds will be sent to remind the driver to stop.
Fig. 1 system working principle block diagram
Fig. 2 circuit of ultrasonic transmitting module
1. Design of ultrasonic transmitting module
The ultrasonic transmitter includes two parts: ultrasonic generation circuit and ultrasonic emission control circuit. The ultrasonic probe (also known as “ultrasonic transducer”) adopts csb40t, which can generate ultrasonic by software generation method and hardware generation method. The former uses software to generate 40KHz ultrasonic signal, which is input to the driver through the output pin. After being driven by the driver, the probe is driven to generate ultrasonic. This method is characterized by making full use of software and good flexibility, but it is necessary to design a driving circuit with driving current above 100mA. The second method is to generate ultrasonic signal by using special ultrasonic generator circuit or general generator circuit, and directly drive the transducer to generate ultrasonic. The advantage of this method is that there is no driving circuit, but it lacks flexibility.
This design uses the first method to generate ultrasonic wave, and the circuit design is shown in Figure 2. 40KHz ultrasonic wave is generated by 555 time base circuit oscillation. The formula of oscillation frequency is f = 1.43 / ((r 9 + 2 · r 10) · C 5). The purpose of designing R10 as an adjustable resistor is to adjust the signal frequency to be consistent with the 40KHz natural frequency of the transducer. In order to ensure that 555 time base has sufficient driving capacity, + 12V power supply should be used. CNT is ultrasonic emission control signal, which is controlled by single chip microcomputer.
Fig. 3 circuit of ultrasonic receiving module
2. Design of ultrasonic receiving module
The ultrasonic receiver includes three parts: ultrasonic receiving probe, signal amplification circuit and waveform conversion circuit. The ultrasonic probe must adopt the model corresponding to the transmitting probe, and the key is that the frequency should be consistent. Csb40r is adopted in this design, otherwise the receiving effect will be affected or even cannot be received due to failure to produce resonance. Because the sine wave electrical signal transformed by the probe is very weak, it must be amplified by the amplification circuit. Sine wave signal can not be directly received by single chip microcomputer, and waveform transformation must be carried out. According to the principle discussed above, what the MCU needs is only the moment of the first echo. The receiving circuit can be designed by special receiving circuit or general circuit, as shown in Fig. 3.
When ultrasonic wave propagates in the air, its energy attenuation is directly proportional to the distance, that is, the closer the distance is, the stronger the signal is, the farther the distance is, the weaker the signal is, usually between 1mV ~ 1V. Of course, the output signal strength of different receiving probes is different. Due to the large range of input signal, two requirements are put forward for the gain of amplification circuit: first, the amplification gain should be large to meet the needs of small signal; Second, the amplification gain should be able to change to meet the needs of a wide range of signal changes. In addition, since the input signal is a sine wave, the amplification circuit must be designed as an AC amplification circuit. In order to reduce the use of negative power supply, the amplification circuit adopts single power supply, and a LM324 general-purpose operational amplifier is used for signal amplification and transformation. The first three stages are amplifier design and the last stage is comparator design. LM324 can work with dual power supply or single power supply, so it can meet the use requirements. In order to meet the needs of AC signal, the amplifier of each stage adopts resistance capacitance circuit for level offset, that is, C7, C21, C22 and C24 in Figure 3, with a capacity of 10 μ F. Realize the amplification of AC signal under the condition of single power supply. For AC signal, the capacitance is short circuited, so the gain of the first three stages of amplification circuit is 10. When the distance is close, the gain of the two-stage amplification can output a signal of sufficient strength, and the signal saturation may occur in the third stage, but when the distance is far, the three-stage amplification must be used. Reasonable adjustment of potentiometer R27 and selection of comparison reference voltage can make the measurement more accurate and stable.
Figure 4 voice circuit
3. Voice circuit design
Voice alarm means that when the distance detected by the reversing radar is less than the set safety value, it will sound to remind the driver. The voice circuit design is shown in Figure 4. M3720 is a single sound and flash alarm sound effect integrated circuit. An alarm sound effect is stored in the chip. It can directly drive the buzzer to sound or drive the speaker to play through the external power amplifier triode. At the same time, it can also drive an led to flash. The functions of each pin of the chip are: 5-pin VDD; Pin 1 VSS is the power input terminal and negative terminal respectively, and the VDD voltage is 3 ~ 3.5V; 8 pin x and 1 pin y are external oscillation resistors of the chip respectively; Pin 6 TG is the trigger control terminal, and the low-level trigger is effective; 3-pin BZ and 2-pin BB are alarm sound output terminals respectively, which can be directly externally connected with piezoelectric ceramic buzzer. If the speaker is driven, it will be led out from 3-pin BZ terminal; 4 pin L is the flashing lamp output end, which can directly drive the led to emit light.
4. Software design
AT89C2051 single chip microcomputer and its development and application system have many advantages, such as simple language, good portability, strong expression ability, structural design, direct control of computer hardware, high code quality, convenient use and so on. The main program of the system is in the keying cycle working mode. When the handle turns to reverse gear, the main program starts to call the measurement subroutine, display subroutine and voice prompt subroutine to complete the whole detection prompt process, as shown in Figure 5.
The reversing radar system designed in this paper is an auxiliary system to ensure the safety of automobile reversing. The ultrasonic wave is detected by ultrasonic wave, the distance is calculated by high-speed single chip microcomputer, and the temperature compensation circuit is added to improve the accuracy of distance calculation. The LCD installed in the system can intuitively display the temperature and distance, which provides convenience for the driver. When reversing, when the distance between the car and the obstacle is less than the safety distance we set, the system will send an alarm through the voice integrated circuit to remind the driver to prevent the car from collision or scratch, which has strong practicability.
Responsible editor: GT