Design of Multifunctional Car Recorder Based on GPS and GPRS

    The car driving recorder (car black box) is a digital electronic recording device used in cars. It can store the car's driving speed, time, mileage, brake and other status information and export data through USB or serial port. The car recorder plays an important role in restraining drivers' bad driving behavior, analyzing and identifying road traffic accidents, improving the level of traffic management law enforcement and transportation management, and ensuring the safety of vehicle operation. The
    current car driving recorder technology is relatively mature. Low-end products generally use 8-bit or 16-bit single-chip microcomputers as the main processor, and mid-to-high-end products use 32-bit ARM processors. However, these products can generally only record and monitor the vehicle. The vehicle driving records need to be collected one by one, which is not convenient for large fleets and enterprises to manage. Here, a design scheme of a multifunctional car recorder based on GPS and GPRS is proposed. This design is based on the realization of the basic functions of the recorder, adding GPS and GPRS modules for positioning and remote data transmission respectively, and combining database technology and corresponding monitoring and management software to realize all-round monitoring, dispatching and management of vehicles and drivers.

1 System composition and main functions
    This system mainly includes car recorder terminals, servers and monitoring and management software. The car recorder terminal includes signal acquisition, data recording, GPS and GPRS. The server receives and saves the data sent by the recorder via GPRS. The monitoring management software collects and analyzes the data on the server to obtain information such as speeding, fatigue driving and mileage.
    The recorder terminal is the basis of the entire system, and its main functions are:
    1) The real-time positioning function can collect and record the current location data of the vehicle in real time, and send the location data to the monitoring center management platform, and display the vehicle's driving trajectory on the monitoring platform; the monitoring center (monitoring station) can also issue instructions to find the location data of the target vehicle;
    2) The operation restricted area setting function uses the monitoring platform to limit the driving area of ​​the operating vehicle. Once the vehicle exceeds the limited area, the monitoring platform will issue an alarm;
    3) Parking anti-theft function After parking, press the parking anti-theft button and the vehicle will enter the parking anti-theft state. If the vehicle moves illegally, the monitoring platform will immediately send a text message to the main control mobile phone and report the current vehicle location;
    4) Emergency help alarm When the vehicle is robbed or needs help, the driver can use the button to alarm the center and send the vehicle's longitude, latitude, driving speed, direction, time and other information to the monitoring center;
    5) Vehicle full-course driving data recording The on-board terminal collects and records all data of vehicle driving and driver operations and sends them to the data center. These include: vehicle time, speed, mileage, status, location, engine speed and other data. Using the full-course driving data recorded by the all-in-one machine, the monitoring platform provides a full-course time-speed-status curve and mileage-speed-status curve for each driving record. The driver's entire driving process can be viewed through these curves;
    6) Overspeed alarm and overspeed recording The on-board terminal can make an overspeed alarm according to the pre-set speed limit. When the vehicle's driving speed exceeds the set value, the on-board terminal will alarm in an audible/optical manner to remind the driver to slow down in time. At the same time, the vehicle speeding information is sent to the monitoring center;
    7) Driver fatigue driving alarm and recording The on-board terminal can record all fatigue driving data of the driver who has been driving continuously for more than 4 hours;
    8) Accident suspicion recording The on-board terminal records the vehicle speed value, brake signal and other vehicle status signals corresponding to the real time in the 20 seconds before the vehicle stops at an interval of 0.2 seconds as accident suspicion data. Store all accident suspicion data in the last 2 months;
    9) Vehicle oil level monitoring function The on-board terminal can monitor the oil level in the fuel tank in real time. When the oil level changes abnormally, the on-board terminal can record the change in real time and send the abnormal change data to the monitoring center;
    10) Video monitoring function can be connected to two cameras to capture photos of the inside and outside of the car and transmit them to the monitoring center;
    11) Driver identity recognition function The on-board terminal uses IC card to realize the driver identity recognition function. The basic information of the driver can be set in the IC card using the management software used in conjunction with the on-board terminal, including: "Driver name", "Driver code", "Driver's license number". Insert the set IC card into the recorder, and the recorder will automatically identify the driver;
    12) Data communication function The vehicle terminal realizes data communication with the monitoring center through the built-in GPRS communication module; the vehicle terminal is equipped with a standard USB interface, and all the data recorded by the vehicle terminal can be retrieved using a USB flash drive; the vehicle terminal can be parameterized and program loaded through the USB port of the vehicle terminal;
    13) Read SMS function Read any SMS content sent by the monitoring platform.

2 Recorder hardware design
    The overall structure block diagram of the recorder is shown in Figure 1, which mainly includes an ARM processor, a GPS module, a GPRS module, a signal acquisition circuit, a real-time clock circuit, a voice alarm circuit and a data storage circuit.

2.1 Main Processor
    In order to meet the system's requirements for real-time performance, large-scale data processing, GPS signal reception, GPRS data transmission control, etc., the 32-bit ARM processor AT91SAM9260 is selected. AT91SA-M9260 uses the ARM926EJ-S core, and the external bus interface includes many controllers for controlling SDRAM and static memories including NAND Flash and Compact Flash, 7-way US-ART, 1 two-wire interface (TWI) and 4-channel 10-bit A/D converter.
2.2 Power Supply Circuit
   At present, the battery voltage of the car is between 9 and 36 V, and due to the influence of the vehicle's own environment, the car power supply voltage is unstable and there are various interferences. Therefore, this system uses a three-level voltage conversion circuit, as shown in Figure 2. The external power supply is converted to 7, 5 and 3 V by LM2576HVT-ADJ, LM2940-5.0 and LM1117 respectively, and supplied to the corresponding modules. Among them, the first-level LM2576HVT-ADJ converts the 7-40 V voltage to 7 V, so that this system can be applied to any vehicle. The LC filter before the power input voltage conversion module can effectively filter out AC interference. The back end of each power converter is connected to the ground through 100 and 0.1μF capacitors in parallel to eliminate ripple voltage and ensure the stability of the system power supply. After testing, the circuit can stably output 5 and 3.3 V voltages.

2.3 Speed ​​acquisition circuit
    Obtaining accurate speed is the basis for the normal operation of the recorder. Most cars are equipped with speed sensors. Each time the wheel rotates one circle, a certain number of pulses will be output. Accurately judging these pulses is the key. The speed acquisition circuit used in this system is shown in Figure 3. First, the high-frequency interference is removed through the front-end RC filter, and then through a follower, and then through the comparison circuit output to control the on and off of the back-end transistor VQ1 to generate a stable pulse signal.

2.4 GPS
    GPS is the core part of this system to achieve positioning. It uses Gloabalsat's ET-318 SiRF StarⅢGPS chipset. This module has high sensitivity (tracking sensitivity: -159 dbm), fast TTFF (first fix time) under low signal, 20-channel full-view tracking, speed accuracy of 0.1 m/s, supports NMEA0183 and SiRF binary protocols, and outputs data information specified by NMEA0183 through the serial port. Communication parameters: baud rate is 4 800 b/s, data bit is 8 bits, stop bit is 1 bit. No parity check. ARM receives the RMC (recommended positioning information) and obtains information such as time, longitude and latitude, ground speed, and ground heading.
2.5 GPRS
    This system sends the current speed, longitude and latitude, direction, and time to the server via GPRS every 10 seconds. The management software reads this information to monitor the current status of the vehicle. The GPRS module uses SIM300C, which is small in size, uses DIP board-to-board connector, has low power consumption, and can transmit voice, SMS (text messages), data and fax information at high speed. The most important thing is that it has a built-in powerful T-CP/IP protocol stack and supports standard AT instruction sets. This system uses SIM300C to establish a TCP pipeline to transmit data. The establishment process is as follows:

    1 024 stipulates that the maximum transmission volume each time does not exceed 1 KB.
2.6 Voice module
    When the system is speeding or driving fatigue, the corresponding high-brightness LED flashes, and the voice alarm function is activated at the same time, issuing a warning voice of "speeding, please slow down", and the recorder can play the content of the text message sent by the monitoring platform, that is, TTS (from text to speech). The recorder uses the second-generation speech synthesizer OSYNO 6288 of Voice World. It is compatible with the synthesis of 4 internal code format texts such as GB2312, GBK, BIG5 and Unicode, and can work normally at baud rates such as 9 600, 19 200, 38 400 b/s, and adds a variety of control commands. For example, synthesis, stop synthesis, pause synthesis, continue synthesis, change baud rate, etc., can automatically identify phrases and polyphonic characters. Voice is output to the speaker through PWM (pulse width modulation). In order to be compatible with monitoring software, database and GPRS module, Unicode code with better universality is adopted. Communication parameters: baud rate is 4 800 b/s, data bit is 8 bit, stop bit is 1 bit without parity check. OSYNO 6288 has built-in power amplifier, the volume is adjusted to 11 levels by software, and an external 8 Ω/0.5 W speaker is connected. The playback sound is loud and clear, and the playback content can be clearly heard even in a noisy environment. [page]
2.7 Data storage
    The car driving recorder is an electronic device that records various states of the car during driving. Various state data storage is an important part of the car driving recorder. Most previous designs use Flash and ferroelectric memory together. However, for the real-time operating system Linux, only one Flash can meet the needs. At present, the Linux operating system can support NandFlash very well. AT91sam9260 supports NandFlash booting, and the corresponding yaffs2 file system is mature, which ensures the accuracy of data. Since this system needs to save a large amount of data: GPS positioning information, suspicious records, driving records, power failure records, fatigue driving records, oil level, etc., K9F1G08UOB type 128Mx2 K page NandFlash memory is used.
2.8 Oil level sensor
    This system is equipped with an accurate oil level sensor to record the oil level in real time and transmit it to the monitoring platform via GPRS. The vehicle operating company can grasp the oil level of the vehicle in real time and completely prevent the driver from stealing oil and selling oil and doing private work.
    The vehicle electrical environment is complex and has many interferences, so a current type sensor is used. This system uses CR-606 capacitive oil level sensor. When oil enters the container, it causes changes between the sensor housing and the sensing electrode. It is suitable for any non-conductive liquid. The oil level is highly accurate. The oil level is less than 1 mm in the acquisition, and the measurement error is less than 0.1%. The standard output is a 4-20 mA linear current signal, which is converted to a 0.6-3 V voltage signal through a 150Ω precision resistor, and converted to a digital quantity and saved through the A/D conversion module of AT91sam9260.
2.9 Real-time clock
    Although the main processor AT91sam9260 has a built-in clock, when the processor is powered off, the contents of all registers of the real-time clock will be lost. For this reason, the system requires an external real-time clock. PCF8563 meets this requirement. PCF8563 has a power-off detector. When the power supply voltage is lower than a certain value, a flag bit in the second register will be set to 1, indicating that the real-time clock may generate inaccurate clock/calendar information at this time, thereby avoiding the recorder from recording the wrong time. In addition, PCF8563 can work in a wide voltage range of 1 to 5.5 V, and has the advantages of small size, simple peripheral circuit, stable operation, high precision, low power consumption, etc. The Linux kernel supports it stably and reliably, meeting the requirements of this system.
2.10 Communication interface
    According to GB/T19056-2003, the standard recorder should be equipped with at least two standard interfaces: USB standard interface and standard RS-232 type 9-pin interface. Direct data transmission through RS-232 serial port is relatively easy to implement and has high reliability. The serial port of AT91sam9260 is converted to EIA/TIA-232-E level by level converter MAX232, thus providing a standard RS-232 interface. AT91sam
9260 supports USB master-slave mode and can also be directly connected to an external USB interface.

3. Car DVR Software Design
    The car DVR is unmanned and starts automatically when the car starts running. This system uses the Linux operating system. The operation process is as follows: the car is powered on and the DVR starts, booting Bootstrap and Uboot, calling the Linux kernel through Bootloader, loading the yaffs2 file system, and automatically starting the application.
3.1 Main Program
    The application is written in C language. The main program is responsible for the initialization of the entire system. The hardware device exchanges data with the main program through signals (soft interrupts) to complete the corresponding operations. The main program flow is shown in Figure 4.

3.2 GPRS data transmission
    The data recorded by this system design include driving records, which save the time, speed, status, GPS positioning information and angle every 3 seconds from power on to power off; suspicious point records, which save the speed and status information every 0.2 seconds within 20 seconds before each stop; power-off records, which save the time when the system is powered off; fatigue driving records, which save the starting time when the driver drives continuously for more than 4 hours. All recorded data are required to be saved for 2 months, so the amount of saved data is large, and the amount of data that needs to be transmitted is also large.
    Before GPRS data transmission, the data is compressed and transmitted when new records appear in the system. When compiling busybox, the gzip function is selected. By gzipping the data, the compression ratio can generally reach 9:1. Since GPRS transparent transmission requires that the maximum data transmission size does not exceed 1 KB each time, the data to be transmitted is divided into 1,000 bytes per packet, and each packet of data is marked with an ID number.
    The server receiving end reassembles the data according to the received data packet ID, and decompresses it through the decompression software to obtain the recorded data of the recorder.
3.3 Implementation of power-off record
    In large fleets and transport companies, drivers often illegally power off the recorder during operation to avoid being constrained by the recorder, making it unable to work normally, so as to avoid monitoring. Therefore, this system is specially designed to record power off to monitor the driver's illegal power off behavior. When the system starts, the current clock is compared with the clock at the last shutdown. The time from Linux crash to restart should be within 5 minutes. If the comparison time exceeds 5 minutes, it means that the recorder is powered off for a long time. The last shutdown time is saved as a power-off record and sent to the server.
3.4 Time and speed calibration
    ARM receives the RMC information of GPS. If the difference between GPS time and current system time reaches 30 seconds, the GPS time is used as the standard and the system time and hardware clock are calibrated. The current speed is compared with the GPS speed. If the GPS speed is greater than 0 for 30 seconds in a row, and the speed acquisition circuit receives 0 all the time, it is judged that the speed sensor is faulty, and the GPS speed is used as the current speed of the system, and an alarm is sent to the monitoring center.

4 Prototype test
    After hard work, this system has been completed and a prototype has been produced. The prototype was installed on the test vehicle to conduct various performance and function tests. The test results are shown in Table 1 and Table 2.

5 Conclusion
    The car recorder combines GPS and GPRS to achieve precise positioning and remote data transmission. Through the management software, the current vehicle status information can be obtained in real time, and the vehicle driving record can be viewed in time. It plays an important role in real-time vehicle alarm and audit, and accident handling of public security traffic police departments. It adds power-off records and multiple alarm functions, which can prevent robbery and theft, and meet the needs of large and medium-sized fleets and enterprises for vehicle management and driver operation monitoring.