Traffic accident automatic positioning alarm system solution

　　1 System Principle and Structural Framework

　　One of the goals pursued by the automotive industry is to ensure the safety of drivers and passengers to the maximum extent when a car crashes unexpectedly. This automatic positioning alarm system design uses Atmega128 microcontroller as the central processor of the entire system, connected to MEMS acceleration sensor, GPS positioning module, and GSM communication module. It can automatically dial an emergency number after a car accident occurs, inform the relevant departments of the precise location and time of the accident, and enable them to send a rescue team as soon as possible, buy more time to rescue the injured, and save more lives.

　　Once the MEMS accelerometer detects a change in acceleration, Atmega128 will analyze and process the data. If it is determined that a car accident has occurred, the GSM module will send the positioning information received by the GPS to the relevant departments in the form of text messages or phone calls, so that they can quickly send out emergency teams for rescue work; otherwise, the system will automatically reset and cycle detection. The system structure diagram is shown in Figure 1.

　　2 Main Hardware Modules

　　2.1 MEMS Accelerometer Module

　　An accelerometer is an electronic device that can measure acceleration force. There are two types: angular accelerometer and linear accelerometer. The MEMS accelerometer model used in this system is ADX L202JE. This sensor has a polysilicon surface micromechanical sensor etched in the same silicon wafer and integrated with a set of sophisticated information processing circuits, as shown in Figure 2.

　　The sensor is mainly composed of a polysilicon structure processed by surface micromachining and a differential capacitor. Under the action of acceleration, the polysilicon structure will produce an offset and change the capacitance value. In the signal processing circuit, the change signal of the differential capacitor is demodulated by the modulator and sent to the duty cycle modulator through an RC filter (composed of internal RFx and external CFx, RFx). The modulator converts it into a square wave with a duty cycle proportional to the acceleration. The period of the square wave (T2) can be determined by Rs. This square wave can be directly sent to the microcontroller for processing. By using a counter to measure the square wave period T2 and the pulse width T1 (Figure 3), the acceleration value can be obtained. [page]

　　Acceleration: a=(T1/T2-Uog)/Ulg

　　Where: T2 is determined by the external resistor Rs, and the relationship is T2=Rs/125MΩ; Uog is the duty cycle corresponding to 0g, and its deformation value is 50%; Ulg represents the duty cycle change value caused by lg acceleration change. For ADXL202JE, the typical value of lg is 12.5%.

　　2.2 GPS Positioning Module

　　GPS (Global Positioning System) is the most widely used satellite navigation and positioning system. It is easy to use and low-cost. The main features of the global positioning system are: global, all-weather, continuous and real-time navigation, positioning and timing functions.

　　This module uses SKYnav GB10, and its structure and pins are shown in Figure 4. This module has the characteristics of low power consumption, strong anti-interference and anti-blocking capabilities, etc. It has 12 data parallel receiving channels, including 8 data bits, 1 start bit, 1 stop bit, no correction bit, the output level is CMOS level, and the current is 1mA. The communication method is asynchronous serial communication, and the circuit connection method is very simple. You only need to connect the TXD0 and RXD0 pins of this module to the RXD0 and TXD0 pins of the microcontroller respectively to perform data transmission. The default communication rate is 4 800 b/s, and the receiving frequency is 1 575.42±1.0MHz.

2.3 GSM Communication Module

　　TC35 is a new generation of wireless communication GSM module launched by Siemens, which can quickly, safely and reliably realize data, voice transmission, short message service (SMS) and fax in the system solution. The working voltage of the module is 3.3~5.5 V, and it can work in two frequency bands of 900 MHz and 1 800 MHz. The power consumption of the frequency bands is 2 W (900 MHz) and 1 W (1 800 MHz) respectively. In addition, the module also has phone book function, multi-party call, roaming detection function, and common working modes include power saving mode, IDLE, TALK mode, etc. Through the unique 40-pin ZIF connector, power connection, command, data, voice signal, and control signal bidirectional transmission are realized.

　　TC35i has 40 pins, 1 to 14 pins are the power supply part, of which 1 to 5 pins are the power supply voltage input terminal VBATT+, 6 to 10 pins are the power ground GND, 11 to 12 pins are the charging terminal, 13 pins are the external output voltage (for external circuit use), 24 to 29 pins are the SIM card connection terminal; 33 to 40 pins are the voice interface for connecting the phone handle. Pins 15, 30, 31 and 32 are the control part, and pin 15 is the start line IGT (Ignition). When TC35 is powered on, a low level greater than 100 mV must be given to IGT for the module to start. Pin 30 is RTC BACK UP; pin 31 is power-off control: pin 32 is SYNC, and pins 16 to 23 are data input/output terminals.

　　Here, we will focus on the SYNC pin, because this pin can well reflect the working status of TC35. The SYNC pin can be used to output a synchronization signal (synchronization sign-al), and can also control the output status of an LED lamp when applied. The SYNC terminal controls the LED through a transistor or gate circuit. The SYNC terminal is connected to the base of an NPN transistor (such as 9013) through a resistor, the emitter is directly grounded, the collector is connected to the negative end of the LED through a current limiting resistor, and the positive end of the LED is connected to VCC. The working mode of LED is exactly the same as that of synchronization signal, which shows the working status of TC35i: LED is off, indicating that TC35i is powered off, in sleep, alarm or simple charging mode; 600ms on/600ms off, indicating that the SIM card is not inserted, or the personal identity is not registered/cancelled, or the network is searching, or the management personnel identity authentication is in progress, or the network registration is in progress; 75 ms on/3 s off, indicating that the network registration is successful (the control channel and the management personnel exchange information is completed) and there is no incoming call; LED is on, depending on the call type, there are voice calls, data calls, establishing or completed states.

　　The TC35 module commonly uses the Text and PDU (Protocol Data Unit) modes to send SMS. The code for sending and receiving SMS in the Text mode is simple and easy to implement, but the biggest disadvantage is that it cannot send and receive Chinese SMS; the PDU mode not only supports Chinese SMS, but also English SMS. [page]

　　The following is an example of sending a text message:

　　1) Set up the short message center

　　at+csca="+813800731500" (SMS center);

　　2) Set the SMS sending format

　　at+cmgf=1(1-Text;0-PDU);

　　3) Send a short message (the content of the message is "Test")

　　at+cmgs=“destination number”

　　>Test^z;

　　4) Get the SMS content (Once more), assuming Index=8

　　at+cmgr=8

　　The returned information is as follows:

　　+CMGR：“RECREAD”，“+8613225 168879”，“10/03/15,22：45：25+32”

　　Ni hao!

　　OK

　　3 System Software Design

　　The central processor and each module transmit information data through UART, with a baud rate of 9600 b/s, 8 data bits, 1 stop bit, and no parity check. The software is written in C language, which has good readability and is easy to transplant and manage. The software flow chart is shown in Figure 5.

　　4 Conclusion

　　This paper designs an automatic positioning alarm system for car accidents. It realizes the functions of real-time determination of acceleration, real-time display of longitude and latitude, and automatic information transmission. With the help of the perfect data communication platform provided by GPS global positioning system and GSM communication technology, the purpose of automatic positioning alarm is achieved, which greatly reduces the death caused by untimely rescue.