The design principle and the principle of achieving the index are mechanical waves with vibration frequencies higher than those of sound waves.

Design Ideas and Implementation Indicators Ultrasonic Distance Measurement Principle Ultrasound is a kind of mechanical wave whose vibration frequency is higher than that of sound wave. It is generated by vibration of the transducer wafer under the excitation of voltage. It has the characteristics of high frequency, short wavelength, small diffraction phenomenon, especially good directionality, capable of becoming ray and directed propagation. Ultrasonic distance measurement is the use of ultrasonic reflection characteristics of obstacles, a series of continuous pulses emitted by the ultrasonic generator (usually 40 KHz), sent to the ultrasonic transmitter (electroacoustic transducer), and at the same time to the measurement The logic circuit provides a short pulse. The ultrasonic receiver (acoustic transducer) also provides a short pulse to the measurement logic circuit after receiving the reflected wave reflected by the obstacle. The controller first processes the pulse data to measure the transmitted and received echoes. The time difference automatically calculates the distance between the vehicle and the obstacle. The voice alarm is issued within the warning distance and the visual warning signal is displayed by the digital tube at the same time. Its block diagram. In the distance measurement block diagram, C is the ultrasonic wave velocity, which is related to the temperature, and lists the ultrasonic wave velocity at several different temperatures.

The system of temperature system of ultrasonic wave distribution table under different temperatures consists of three parts: ultrasonic transmitter and receiver circuit, control circuit composed of single-chip microcomputer, distance display circuit and sound and light alarm circuit. Implementation Indicators The system uses the following technical indicators: two-way ultrasound positioning, digital display distance range, voice prompt distance. Implementation indicators: Two ultrasonic positioning, each detection range angle 14; three digital distance display; voice distance prompts; detection distance working temperature; measurement resolution 1 cm, error is less than 0. 5%. System hardware composition and working principle The control circuit (P87) is chosen as the controller because of its high cost performance. The display uses dynamic scanning, serial display data transmission using the serial port, the application should be connected external pull-up resistor 4. 7 K.

The power supply of the single-chip microcomputer comes from the vehicle-mounted power supply with a voltage of +12 V. The load change will cause voltage fluctuations. It will be a serious interference to the normal operation of the microcontroller. The circuit uses a reset dedicated chip, its role is to monitor the supply voltage of the microcontroller, to avoid running off the program; when the supply voltage is lower than a set threshold under harsh operating conditions (such as during engine startup), reset and power supply When the voltage recovers above the threshold value, it lasts 140 ms. This can well solve the interference caused by the monolithic microcontroller supply voltage instability on the microcontroller. Ultrasonic Transmitting and Receiving Circuits Ultrasonic transducers are customarily called ultrasonic probes. They are mainly composed of piezoelectric wafers and can emit ultrasonic waves as well as ultrasonic waves. The two transmitter and receiver circuits are identical in structure and work in rotation. The principle is as shown.

Ultrasonic Transmit and Receive Principle Figure 2. 3 Transmitting Circuit Since the microcontroller P1 port is used as an IO port, it can provide a sinking current capability of 20 mA, and the current-sinking ability is small, so an external NPN tube is connected to increase the output current capability, ensuring 40 KH. The z pulse signal has a certain power. The receiving circuit of the receiving circuit is composed of pre-amplification, band-pass filtering, echo shaping, and binarization circuit. Preamplification enables efficient amplification of small signals and improves the input impedance of the entire amplifier circuit. The circuit is designed with two-stage second-order infinite gain feedback band-pass filter amplification with a center frequency of 40 KHz; the first stage gain is A1 = -120, and the second stage gain is A1 = -320, which guarantees the micro-volt level The signal is amplified to the voltage level for shaping and binarization. The role of the shaping and binarizing circuit is to detect the echo signal first and turn it into a single-polarity signal; binary is an A/D, set the threshold level, and convert the analog echo into a level signal. Voice alarm circuit voice interface block diagram Voice alarm as a measurement instrument output is a very intuitive, easy to understand form, human-machine interface is friendly. Considering that the driver generally does not care about the meter on the car when he is reversing, attention is paid to the rear of the car. Therefore, voice alarms are used in the design.

There are many types of voice technology products on the market. This circuit uses the ISD1110 voice chip from ISD. The chip uses DAST direct analog storage technology and has a high degree of integration. Recording and playback time is 10 seconds, divided into 80 segments, and the microcomputer control can be flexibly combined. Using the parallel port of the PC, the voice is recorded to the chip and connected to the system. In this system, a total of 4 voices are recorded, which are 1. 5m area%; 1m area%; 0. 5m area%; limit alarm music alarm%. According to the size of the measured distance to play the corresponding segment. If the measurement result is 0.8 meters, report 1 meter area %. Control circuit interface As shown, A3, A4, A5 of ISD1110 select the alarm segment, and use the pulse falling edge to trigger playback. Display circuit In addition to the sound alarm, light alarm is another effective alarm mode. Three digital tubes are designed to display the current test distance (in mm), the over-range display is - - - %, and the distance from the obstacle is less than 25 mm to display SOS% flicker. The voice system plays a warning tone. The digital tube is lined with two reds, two yellows, and two greens to show the relative distance. When the measured value is greater than 1 meter, the two green tubes are on; when the measured value is 10.5 meters, the two green tubes and the two yellow tubes are lighted the first time; the measured values ​​are in meters, two green, two yellow, and two red tubes. The digital tube displays the relative value of the test distance. The closer the distance is, the more the digital tube is brightened. Display circuit schematic diagram, using RXD serial output display data, T XD output synchronization pulse, a LS164 serial data is converted to parallel data, with the output as a bit control, turns on the digital tube and digital tube row. Since there is no pull-up resistor, use it as an IO port. The software programming (flowchart) program consists of a main program, an interrupt service routine, and a display subroutine. And set a flag, determine the flag to determine the exit of the service routine, and update the flag before returning. The role of the flag is to ensure that the transmitter transmits and receives in turns. The display adopts dynamic scanning, the main program sends and receives every one way and sends it once. The display starts the serial port four times each time, and the corresponding display bit is set low. At the same time, the corresponding prompt segment is played according to the measurement result.

Anti-jamming measures hardware: As the ultrasonic receiver is a small signal in the front stage, the best connection of the sensor is to use a good quality single-core shielded cable to ensure the reliable transmission of small signals; two stages of band-pass filtering are used in the amplifier stage of the signal. The high-frequency and low-frequency interference is removed; the digital circuit and the analog circuit are preferably powered separately. The controller box is shielded with a metal shell. On the software side: Using the gross value deletion method, each measurement result is a set of 3 times. The coarse value is removed first, and the average measurement result is sent to the display. The use of 3 averages takes into account the real-time nature of the measurement. Installation requirements The two transmitter-receiver heads of the device shall be installed side by side in the rear of the vehicle, one on each side. To prevent outside clutter, use a shielded cable of about 3 m to connect to the controller box. Receiving, control and display circuits are housed in the controller box and are mounted on the driver's cab or in a position that does not obstruct the driver's vision and can be easily viewed.

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