Research and design of reversing radar system based on ultrasonic detection

The full name of Car Reversing Aid Systems is "reversing collision avoidance radar", also known as "parking aid system". It is a car parking safety auxiliary device that can inform the driver of the surrounding obstacles by sound or more intuitive display, eliminating the trouble caused by the driver looking around when parking and starting the vehicle, and helping the driver eliminate blind spots and blurred vision, thereby improving safety.

How the system works

The reversing radar only needs to work when the car is reversing to provide the driver with information behind the car. Since the car is moving slowly when reversing, it can be considered as stationary compared to the speed of sound, so the influence of the Doppler effect can be ignored in the system. Among many ranging methods, the pulse ranging method only needs to measure the round-trip time between the ultrasonic wave measuring point and the target, which is simple to implement, so this system adopts this method.

As shown in Figure 1, after the driver turns the handle to the reverse gear, the system starts automatically, and the ultrasonic transmitting module emits a 40kHz ultrasonic signal backward. After being reflected by the obstacle, it is collected by the ultrasonic receiving module, amplified and compared. The single-chip microcomputer AT89C2051 sends this signal to the display module and triggers the voice circuit to issue a synchronous voice prompt. When the distance to the obstacle is less than 1m, 0.5m, and 0.25m, different alarm sounds are issued to remind the driver to stop.

Figure 1 System working principle block diagram

Figure 2 Ultrasonic transmission module circuit

Hardware Design

1 Ultrasonic transmission module design

The ultrasonic transmitter consists of two parts: the ultrasonic generating circuit and the ultrasonic transmitting control circuit. The ultrasonic probe (also known as "ultrasonic transducer") uses CSB40T, which can generate ultrasonic waves by software generation method and hardware generation method. The former uses software to generate 40kHz ultrasonic signals, which are input to the driver through the output pin, and then driven by the driver to drive the probe to generate ultrasonic waves. The characteristics of this method are that it makes full use of software and has good flexibility, but it is necessary to design a driving circuit with a driving current of more than 100mA. The second method is to use an ultrasonic special generating circuit or a general generating circuit to generate ultrasonic signals, and directly drive the transducer to generate ultrasonic waves. The advantage of this method is that it does not require a driving circuit, but it lacks flexibility.

This design uses the first method to generate ultrasonic waves, and the circuit design is shown in Figure 2. The 40kHz ultrasonic wave is generated by oscillating the 555 time base circuit. The oscillation frequency is calculated as f=1.43/((R ₉ +2·R ₁₀ )·C ₅ ). The purpose of designing R ₁₀ as an adjustable resistor is to adjust the signal frequency to make it consistent with the 40kHz natural frequency of the transducer. In order to ensure that the 555 time base has sufficient driving ability, a +12V power supply is preferably used. CNT is the ultrasonic emission control signal, which is controlled by the single-chip microcomputer.

Figure 3 Ultrasonic receiving module circuit

2 Ultrasonic receiving module design

The ultrasonic receiver consists of three parts: ultrasonic receiving probe, signal amplification circuit and waveform conversion circuit. The ultrasonic probe must be of the same model as the transmitting probe. The key is that the frequency must be consistent. This design uses CSB40R, otherwise the reception effect will be affected due to the inability to generate resonance, or even no reception. Since the sinusoidal electrical signal after the probe conversion is very weak, it must be amplified by the amplifier circuit. The sinusoidal signal cannot be directly received by the microcontroller and must be converted into a waveform. According to the principles discussed above, the microcontroller only needs the moment of the first echo. The design of the receiving circuit can be implemented using a dedicated receiving circuit or a general circuit, as shown in Figure 3.

When ultrasonic waves propagate in the air, the attenuation of their energy is proportional to the distance, that is, the closer the distance, the stronger the signal, and the farther the distance, the weaker the signal, usually between 1mV and 1V. Of course, there are differences in the output signal strength of different receiving probes. Due to the large range of the input signal, two requirements are put forward for the gain of the amplifier circuit: one is that the amplification gain should be large to meet the needs of small signals; the other is that the amplification gain should be variable to meet the needs of a large signal change range. In addition, since the input signal is a sine wave, the amplifier circuit must be designed as an AC amplifier circuit. In order to reduce the use of negative power supply, the amplifier circuit is powered by a single power supply. A LM324 general-purpose operational amplifier is used for signal amplification and conversion. The first three stages are designed as amplifiers, and the last stage is designed as a comparator. LM324 can work with both dual power supplies and single power supplies, so it can meet the use requirements. In order to meet the needs of AC signals, each stage of the amplifier uses a resistor-capacitor circuit for level shifting, that is, C7, C21, C22 and C24 in Figure 3, with a capacity of 10μF, to achieve amplification of AC signals under single power supply conditions. For AC signals, the capacitor is short-circuited, so the gain of the first three amplifier circuits is 10. When the distance is close, the gain of the two-stage amplification can output a signal of sufficient strength, and the third stage may be saturated, but when the distance is far, 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, a sound is emitted to remind the driver. The voice circuit design is shown in Figure 4. M3720 is a single-sound and flashing alarm sound effect integrated circuit. An alarm sound effect is stored in the chip, which can directly drive the buzzer to sound or drive the speaker to play through an external power amplifier transistor, and can also drive an LED to flash. The functions of each pin of the chip are: 5-pin VDD; 1-pin VSS are the power input terminal and the negative terminal respectively, and the VDD voltage is 3~3.5V; 8-pin X and 1-pin Y are the external oscillation resistors of the chip respectively; 6-pin TG is the trigger control terminal, and the low level trigger is effective; 3-pin BZ and 2-pin BB are the alarm sound effect output terminals respectively, which can directly connect to an external piezoelectric ceramic buzzer. If the speaker is driven, it is led out from the 3-pin BZ terminal; 4-pin L is the flash light output terminal, which can directly drive the LED to emit light.

Software Design

AT89C2051 single-chip microcomputer and its development and application system have many advantages, such as concise language, good portability, strong expression ability, structured design, direct control of computer hardware, high quality of generated code, and easy use. The main program of the system is in key-controlled cycle working mode. When the handle is turned into reverse gear, the main program starts to call the measurement subroutine, display subroutine and voice prompt subroutine to complete the entire detection prompt process, as shown in Figure 5.

Conclusion

The reversing radar system designed in this paper is an auxiliary system to ensure the safety of car reversing. It emits ultrasonic waves through ultrasonic probes, uses high-speed single-chip microcomputers to calculate distances, and adds temperature compensation circuits to improve the accuracy of distance calculations. The LCD display installed in the system can intuitively display temperature and distance, providing convenience for the driver. When reversing, when the distance between the car and the obstacle is less than the safe distance we set, the system will issue an alarm through the voice integrated circuit to remind the driver to prevent collisions or scratches of the car, which is very practical.