Design of Ultrasonic Reversing Range Finder Based on Temperature Compensation
Ultrasonic wave has good directivity, strong penetration ability, and it is easy to obtain more concentrated acoustic energy. The detection of the effective distance of obstacles based on these characteristics of ultrasonic wave has been applied in many fields. However, the propagation speed of ultrasonic waves in the air is greatly affected by temperature. Without this factor, the accuracy of the measurement is difficult to be guaranteed.
In the development of this car reversing distance meter, the influence of the ambient temperature on the propagation speed of ultrasonic waves is fully considered, and it is corrected by the temperature compensation method. It has high measurement accuracy and can display and voice broadcast the distance of obstacles behind the car in real time. , Let the driver audio-visual combination, more humane. The whole circuit has the characteristics of simple structure, stable and reliable operation, and small measurement error.
The whole circuit of the ultrasonic reversing rangefinder. The whole circuit of this system adopts the single-chip AT89S52 as the main control unit. Other parts include the ultrasonic transmitting module, the ultrasonic receiving module, the DS18B20 temperature compensation module, the liquid crystal display module, and the voice broadcast module. The frame of the whole machine is shown in Figure 1.
Figure 1 System framework
1 Ultrasonic transmission module
The NE555 chip is used to generate a 40kHz oscillating square wave, and the microcontroller controls the transmission of ultrasonic waves through the NE555 reset terminal. Because NE555 can produce a maximum output current of 200mA, the power of the transmitted signal is limited. After the power amplifier circuit formed by NE5532AF is generated, enough power is generated to promote the transmission of ultrasonic waves, so as to meet the requirements of the transmission distance. The ultrasonic generating circuit is shown in Figure 2, and the power amplifier circuit is shown in Figure 3.
Figure 2 Ultrasonic generator
Figure 3 power amplifier circuit
2 The ultrasonic receiver module adopts LM567 phase-locked loop circuit. When the received signal is within the locked frequency range of LM567, the output of LM567 will jump from high level to low level. This signal is used as an interrupt signal of the single chip microcomputer The time difference between transmission and reception can be accurately measured. The measured distance can be obtained according to the formula S = V * T / 2. The ultrasonic detection circuit is shown in Figure 4.
Figure 4 Ultrasonic detection circuit
3 Temperature Compensation Module At present, most temperature measurement and control systems use temperature sensors to convert the temperature into electricity after the temperature is detected. After the conditioning circuit, the ADC converts it into a digital quantity for subsequent processing. The circuit structure is complicated, the debugging is cumbersome, and the accuracy is easily affected by the parameters of the components.
The temperature sensor DS18B20 module is used in this design. The sensor comes with an analog-to-digital conversion and can be used directly. Because of its simple external circuit, the temperature measurement accuracy is high.
4 The liquid crystal display module adopts liquid crystal LCD12864 to display the distance measurement information, the content is convenient and intuitive, and it can display the relevant prompt information, which is more humanized.
5 The voice broadcast module uses the digital voice chip ISD2560 as the voice recording and playback part. The recording and playback time of this chip is up to 60s, which can realize segmented recording, combined playback and loop playback of voice without using special ISD voice development equipment. The chip uses multi-level direct analog storage patent technology, eliminating ADC and DAC. Each sampled value is directly stored in a single EEPROM unit on-chip, so it can reproduce voice, music, tones and effect sounds very realistically and naturally, avoiding the quantization noise and "metallic sound" caused by quantization and compression in general solid recording circuits . In addition, ISD2560 control level is compatible with TTL level, the interface is simple and easy to use. The voice broadcast circuit is shown in Figure 5.
Figure 5 Voice broadcast circuit
Software design Software design is very important, it is the center of the system and affects the performance of the whole machine. The software flow of this ultrasonic rangefinder is shown in Figure 6.
Figure 6 Ultrasonic ranging software process
Function effect of ultrasonic reversing rangefinder
1 Temperature compensation function The speed of ultrasonic wave propagation in the air is greatly affected by temperature. In order to obtain more accurate measurement results, this system uses a temperature compensation method. The ambient temperature is measured with a high-precision temperature sensor DS18B20, and the actual propagation speed of the ultrasonic wave is obtained through the method of software table look-up, and then the precise distance is calculated by the single-chip microcomputer.
2 The size and accuracy of the distance measurement are verified by practice. The ultrasonic distance measurement reaches 8m and the accuracy reaches 0.01m. And according to the distance of obstacles behind the car is divided into three levels of "safe distance", "maintain distance", "dangerous distance" prompt.
3 LCD display function LCD 12864 is used to display text and related information in the reversing distance measuring voice system. After the system is powered on, it is displayed on the LCD in separate lines: the first line displays: the reverse ranging voice system; the second line displays: the distance from the target: × dot ×× m; the third line displays: status: ×× distance; the fourth line Display: Ambient temperature: ××. ×××× C. (× is the data measured by the system after power on, C is the symbol of temperature). This display allows users to clearly know their current location and safety level, so as to know how to proceed.
4 Voice broadcast prompt
ISD2560 is a cost-effective voice recording and playback chip whose addressable recording and playback functions are unmatched by other voice chips. The words to be read are recorded continuously. When reading the measured distance, each word is directly taken from the voice segment, and then played, and the voice broadcast function is well realized.
Conclusion This system solves the problems of low accuracy and short distance of ultrasonic ranging caused by temperature changes. In addition, the whole system is intelligent, with voice broadcast prompts and LCD display functions, which reflects humanity and is very convenient to use. This system can be installed at the rear of various vehicles to monitor the obstacles behind the vehicle in reverse. It has a good function and a significant effect.
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