In recent years, with the popularization of automobiles and the construction of roads, inter-city economic transactions have become more frequent, and the area of ​​activity has also become larger and larger, which has caused serious problems such as heavy traffic, increased car accidents, and increased exhaust emissions. The emergence of Intelligent Transport System (Intelligent Transport System) has effectively improved the above various transportation problems. Vehicle monitoring system is a branch of intelligent transportation system. It integrates advanced wireless positioning technology, geographic information system and modern mobile communication technology. It not only plays a major role in intelligent transportation system, but also provides anti-theft and anti-robbery alarm, emergency Medical help, entertainment and other services. All these have prompted various R & D institutions to vigorously develop vehicle monitoring systems. In this paper, the vehicle-mounted terminal modules in the entire system are selected for design and implementation.
1 Overall structure of vehicle monitoring system
The entire system consists of GPS satellites, vehicle-mounted terminals, communication networks (GPRS and Internet) and monitoring centers. During the operation of the vehicle, the GPS receiver of the vehicle terminal receives positioning data, calculates the current longitude, latitude, speed, course and other information (time, status) of the vehicle, etc., and then transmits it to the monitoring with a static IP address through the GPRS network Center, and co-existence of people center database. After receiving the information uploaded by the vehicle, the monitoring center scientifically dispatches and manages according to the current status of the vehicle, thereby improving the operation efficiency. Customers can also query the current status of a specific vehicle via the Internet or phone. The overall structure of the system is shown in Figure 1.
2 Functions of car terminal
The satellite receiving module of the vehicle-mounted GPS / GPRS terminal collects GPS satellite data, and obtains the geographic coordinate information of the vehicle through data processing. After the information is processed by the vehicle-mounted terminal, it is sent to the GPRS wireless communication network by the GPRS wireless communication module. The GPRS network establishes a data channel supporting TCP / IP between the vehicle-mounted terminal and the monitoring center connected to the Internet according to the corresponding protocol. The vehicle GPS / GPRS terminal relies on the data channel to exchange information with the monitoring center. The monitoring center can also send control commands and service information down through this channel. In addition, in addition to vehicle positioning, the system can also provide a variety of services such as anti-theft and anti-robbing, fuel and power cutoff for mobile vehicles, display of scheduling information, medical assistance, mobile phones and other services.
3 Car terminal hardware design
The vehicle terminal hardware system is based on the single-chip computer and GPS + GPRS module, which belongs to the economical functional terminal. Its structure is shown in Figure 2.
3.1 Information processing and control module
The main function of the AVR series of single-chip microcomputer system is to perform information processing and control the various components of the vehicle-mounted platform to perform response operations in accordance with the requirements of the communication protocol. The module is composed of CPU, external memory, I / O interface and control logic circuit. Among them, the CPU is implemented by ATMEGA64 (L) single-chip microcomputer. It is a low-power 8-bit CMOS microcontroller based on the enhanced AVR RISO structure. It is based on its advanced instruction set and single-clock cycle instruction execution time. ATMEGA64 (L) data The throughput rate is as high as 1 MIPS / MHz, which can alleviate the contradiction between power consumption and processing speed of the system. The MCU has two serial ports, which are used to communicate with GPS and GPRS modules.
3.2 GSM / GPRS + GPS module
Use SIM508 module of SIMCOM company. SIM508 module is a product that integrates tri-band GSM / GPRS and GPS with 20-channel capability into one module (34 mm × 55 mm × 3 mm). The design of this product fully meets the requirements of vehicle application environment (temperature, humidity, shock resistance, etc.). In particular, SIM508 with integrated additional components can save a lot of time and cost. SIM508 supports NMEA-0183, SiRF binary and RTCM SC-104 three GPS data formats, which can meet different design requirements. The vehicle-mounted terminal obtains the specific location of the vehicle through the GPS part of the module, displays it to the user after being processed by the CPU, and sends the information to the monitoring center through the GPRS part, so as to realize the real-time positioning and tracking of the vehicle, and can also achieve voice and short Message communication function.
3.3 Input and output modules
The output is realized through the 12232F liquid crystal module, which can display graphics or 7.5 × 2 (16 × 16 dot matrix) Chinese characters. The interface with the external CPU can be parallel or serial. Considering the simplicity of programming, this vehicle terminal uses serial Row interface connection.
There are two ways to input the car terminal: remote control input (main input device) and handle input. Taking into account the limitations of the handle input in an emergency, the remote control is used as the main input device here, which can be used to complete functions such as voice dialing, sending and receiving short messages, medical help, maintenance help, and opening and closing equipment. The handle device includes four function keys: confirm, return, and up and down, designed for user input.
4 Car terminal software design
This software system adopts a modular design method, each module implements a function or a protocol, and each functional module appears as a sub-function, which shortens the software development time and is easy to modify and transplant the program. At the same time, there are still Some software application interfaces facilitate software upgrades, such as adding new protocols. The functional modules of the software system are shown in Figure 3.
4.1 System workflow
The main function of the vehicle terminal software system is completed by the main program. The main program uses the system structure of the state machine, and its work flow
As shown in Figure 4
When the program works, the GPS and GPRS serial ports are initialized first, and then enter the main control loop. In the main control loop, first identify whether the GPS data is valid, that is, whether the positioning is successful. If the positioning is successful, the system goes to the next state and establishes a GPRS connection, otherwise repositioning. After the GPRS connection is established, the processed positioning data can be sent to the monitoring center. At the same time, in the process of running the main program, it can also respond to the remote controller input interrupt request in order to realize other functions.
4.2 Software system protocol stack
Borrowing from the OSI model and the standard TCP / IP protocol stack, this system uses four layers of network transmission protocols: transmission layer, network layer, data link layer and physical layer. The structure of the system protocol stack is shown in Figure 5.
Considering the situation of dozens, hundreds, or even thousands of in-vehicle terminals in the vehicle monitoring system, for such multi-point dispersion, small data volume, high real-time requirements, and a large number of terminals, the transport layer uses UDP than TCP. better. As the network layer protocol, the IP protocol mainly cuts the data stream into an appropriate size, and then selects these data packets by routing and uses different routes to transmit them to the destination IP. Above the physical layer, the PPP protocol is the only designated data link layer protocol above the physical layer of GPRS. The original physical layer connection is transformed into an error-free data link through CRC verification and confirmation. After PPP negotiation is successful, the system will successfully log in to the Internet remotely and obtain the IP assigned to it by the gateway. The physical layer channel between the terminal and the network is the GPRS connection. The specific GPRS protocol has been implemented in the GPRS modem. After setting the correct AT command on the GPRS modem through the data terminal, you can use the AT dial command to make a dial-up connection. After receiving the dial-up feedback response from the GPRS modem, a physical channel That is, the GPRS channel is established between the terminal and the network.
5 Conclusion
An implementation scheme of vehicle monitoring system terminal based on GPS / GPRS is introduced, and detailed software and hardware composition and design implementation are given. After several tests, the system is stable and the effect is good. The system can be applied to hand command monitoring system, urban rental car management department
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