Development of Ultrasonic Rangefinder for Parking

Development of Ultrasonic Range Finder for Parking

Abstract: This paper describes the basic principle of the ultrasonic range finder for parking which is developed by adopTIng microprocessor technology. MathemaTIcs model of measurement and calculaTIon as well as realizaTIon methods is also discussed. Software calibration method is used to increase measuring accuracy and reliability. Actual applica tion shows that the security is greatly insured when parking.

Keywords: Ultrasonic Range-finding Microprocessor

With the continuous improvement of living standards, the consumption awareness of cars entering families is increasing. China's urban car ownership has increased rapidly. As a result, traffic accidents are increasing day by day, especially in cities. The development of intelligent transportation system is an important development direction of transportation in the 21st century. Intelligent transportation system (ITS) is giving full play to the potential of existing infrastructure and improving transportation efficiency. The excellent efficiency of ensuring traffic safety, alleviating traffic congestion and improving urban environment has been widely concerned by governments at all levels. The Chinese government also attaches great importance to the research, development, promotion and application of its. The discussed in this paper focuses on reversing protection, which can effectively avoid obstacles and pedestrians that may cause harm to reversing. Effectively avoid economic losses and personal safety problems caused by reversing. Foreign high-end cars have been installed with similar systems when they leave the factory.

The system consists of three parts: (1) two air ultrasonic probes with the same transceiver to detect the obstacles at the left and right rear of the car tail. (2) The control circuit composed of single chip microcomputer and ultrasonic transmitting and receiving circuit. (3) Distance display circuit and audible and visual alarm circuit.

The system realizes the following technical indexes; Two way ultrasonic positioning, digital display distance, voice prompt distance range, row nixie tube display distance range.

The specific indicators are as follows: I. two channels of ultrasonic positioning, and the detection range angle of each channel is 14o

II three digit nixie tube distance display

III four segment voice distance range prompt, red, green and blue nixie tube distance display

working temperature: - 20-60

the measurement resolution is 1cm, and the error is less than 0.5%.

The tester has high measurement accuracy and diversified prompt methods, which can meet the objective needs of different working environments.

3 Hardware circuit composition and working principle

3.1 control chip

AT89C2051 is used as the controller in small intelligent instruments with high cost performance. The instruction system is fully compatible with 8031. It has all functional structures of 8031 except P0 and P2 ports. The measurement control system composed of it has the advantages of simple circuit, high reliability and small volume; The volume of control and transmitting and receiving circuit is, and the hardware composition block diagram is shown in Figure 1;

P1.4 and P1.5 ports of the single chip microcomputer are programmed as output ports to alternately output 40KHz square wave with a duration of 0.2ms, and send it again every 19.8ms, that is, the repetition frequency of the two channels of ultrasound is. The propagation speed of ultrasound in the air at room temperature is 340m / s, which determines the maximum detection distance of the instrument. The actual situation is that because the instruction needs time to run, and when all the transmission and reception are completed, the program will send a display. When the repetition frequency is about 50Hz, the maximum detection distance is 1.5m. The waveforms of P1.4 and P1.5 ports are shown in Figure 2. P1.6 and P1.7 are programmed as input ports to receive two ultrasonic echoes accordingly.

P3.2-p3.5 is programmed as the output port, p3.2-p3.4 controls the voice data segment of isd1110 voice chip, and p3.5 controls when to play.

The display adopts dynamic scanning, serial ports RXD and TXD are used to send display data, and p1.0-p1.3 are used to send display bits. Since P1.0 and P1.1 have no internal pull-up resistance, external pull-up resistance of 4.7K shall be connected respectively in application.

The circuit adopts max810 reset special chip. Because the power supply on the vehicle adopts DC 12V. The battery and generator are connected in parallel. Under bad working conditions (such as during engine starting), the voltage drops to about 6V. The power supply of the single chip microcomputer is taken from the 12V of the vehicle, which will be a serious interference to the normal operation of the single chip microcomputer, and the program will run. Max810 can solve this problem. Its function is to monitor the power supply voltage of single chip microcomputer; When the supply voltage is lower than a set threshold, the max810 resets and continues for 140ms when the supply voltage returns above the threshold. This can well solve the interference caused by the unstable power supply voltage of single chip microcomputer.

3.2 ultrasonic transmitting and receiving circuit

The two transmitting and receiving circuits have the same structure and work in turns. The two circuits have the same structure. The principle is shown in Figure 3;

3.2.1 transmitting circuit

Since P1 port of single chip microcomputer can provide 20mA current filling capacity when used as IO port, and the current absorption capacity is small, an NPN tube is connected externally to improve its output current capacity. Ensure that the 40KHz pulse signal has a certain power.

3.2.2 receiving circuit

The receiving circuit consists of preamplifier; Bandpass filter amplification; Echo shaping and binarization. Preamplifier can effectively amplify small signal and improve the input impedance of the whole amplification circuit. In this circuit, two-stage second-order infinite gain feedback bandpass filter amplification is designed, and the center frequency is 40KHz; The increase of the first stage is A1 = - 120 and the gain of the second stage is A1 = - 320, which ensures that the micro volt signal is amplified to the volt stage for shaping and binarization. The function of shaping and binarization circuit is to detect the echo signal into a single polarity signal; Binarization, i.e. one bit a / D, sets the threshold level, converts the analog echo into level signal and inputs it to P1.6.

3.3 voice alarm circuit

As the output of the measuring instrument, the voice alarm is a very intuitive and easy to understand form, and the man-machine interface is friendly. Considering that the driver generally has no time to estimate the instrument on the vehicle when reversing and pays attention to the rear of the vehicle, the timing adopts the voice alarm. At present, there are many kinds of voice technology products in the market. This circuit adopts isd1110 voice chip of ISD company, and the chip adopts (DAST) direct analog storage technology, which has a high degree of integration. The recording and playback time is 10 seconds and divided into 80 sections. The microcomputer control can be combined flexibly to output the required voice signals. In the application, the self-made development system shall be required, and the voice shall be recorded into the chip and connected to the system by using the parallel port of PC. A total of 4 segments were recorded, namely; "1.5m zone"; "1m zone"; "0.5m zone"; "Limit warning music alarm". Play the corresponding paragraph regularly according to the measured distance. If the measurement result is 0.8m, report "1m area". The control circuit interface is shown in Figure 4; A3, A4 and A5 of isd1110 select alarm segments, play is connected to p3.5, and the falling edge triggers playback.

3.4 display circuit

In addition to sound alarm, optical alarm is another effective alarm mode. There are two forms of optical alarm in the design. The three nixie tubes display the current test distance (unit: mm), the over range display "---", the distance from the reverse to the obstacle is less than 25mm, the display "SOS" flashes, and the voice plays warning music. The row of nixie tubes has two red, two green and two blue, showing the relative distance. When the measured value is greater than 1m, at most two green tubes are on; When the measured value is greater than 0.5m, two green tubes are on, and at most two yellow tubes are on; When the measured value is greater than 0.3m, two green and two yellow tubes are on, and at most two red tubes are on. The row nixie tube displays the relative value of the test distance. The closer the distance, the more digits the nixie tube lights.

The electrical schematic diagram is shown in Figure 5. RXD serial output is used to display data, TXD outputs synchronization pulse, and a piece of 164 is used to convert serial data into parallel data - P1.3 output as position control, turn on the nixie tube and led row. Since P1.0 and P1.1 have no pull-up resistance, a 4.7K pull-up resistance should be connected when they are used as IO ports. In order to increase the driving capacity, a mc1413 is added to the position control.

The program consists of main program (Figure 6), interrupt service program (Figure 7) and display subroutine (Figure 8). Timer T0 is used for 10ms, and 40KHz pulse is sent to P1.4 at the beginning of timing to query whether there is echo in P1.6. If there is echo, it can be obtained from t0 timing time t and sound velocity V (m); If 10ms has arrived without echo, t0 timing overflow interrupt. In t0 interrupt service subroutine, write the display "---" to the display buffer, set a flag bit, judge the flag bit to determine the exit of the service subroutine, and update the flag bit before returning. The function of flag bit is to ensure P1.4 and P1.6; P1.5 and P1.7 send and receive in turn. The display adopts dynamic scanning. Each time the main program completes the transmission and reception of one channel, it sends a display. Each display starts the serial port four times, and the corresponding display bit is set to low. At the same time, play the corresponding prompt segment according to the measurement results.

When the car is working, there is strong electromagnetic radiation due to high-voltage electric fire, and the electromagnetic environment is bad. Therefore, the anti-interference problem is considered in both hardware and software;

In terms of hardware, the ultrasonic reception is a small signal in the front stage. The sensor is connected with a high-quality single core shielded wire to ensure the reliable transmission of small signals. In the signal amplification stage, two-stage band-pass filtering is used to filter out high-frequency and low-frequency interference. The power supply of digital part and analog part is supplied separately. The controller box uses a metal shell to shield the external electromagnetic field. The software uses the gross value elimination method, and each measurement result is a group of three times. First, the gross value is eliminated, and then the average measurement result sent to the display is obtained. The use of cubic averaging takes into account the real-time performance of measurement. During reversing, I don't care much about the exact distance from the obstacle; It's whether there are obstacles and how far away from the back of the car. Practice has proved that these measures have achieved good results.

The instrument has been put into use and achieved the predetermined goal. In the process of reversing, it realizes the automatic detection of obstacles within 1.5m from the left rear and right rear of the car and pedestrians suddenly breaking into the dangerous area, and gives an alarm and prompts the driver to take measures. For new drivers, its role is more obvious. They know well in the process of reversing, which greatly improves the safety when reversing.