Second, ITS system design based on GPS-GSM/GPRS
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The main function of the car GPS intelligent terminal software system is completed by the main program. The main program adopts the system structure of the state machine, and its overall structure is shown in Figure 4.
Among them, (a) is the overall flow chart, (b) is the GSM message processing part flow chart, and (c) is the GPS message processing part flow chart. The terminal can be in 8 states: idle, upload location information, timed upload location information, dialing, in-call, network connection, GPS cannot be located, and alarm. The switching between states is mainly controlled by the ITS center through the command message that sends the message. The program works first and then enters the main control loop. In the main control loop, first check whether there is a complete GSM data packet in the GSM data buffer, if it is, then parse it, and change the state of the terminal according to the control command in the protocol. Then it is judged whether there is a complete GPS data packet in the GPS data buffer, and if so, the current global positioning coordinates are taken out and parsed for use in uploading coordinates. Finally, the terminal is operated according to the state of the terminal, for example, if the terminal is in the state of uploading the positioning message, the GSM module is controlled to upload the message. The experiment proves that the vehicle GPS intelligent terminal can better complete the functions of terminal positioning, communication with the ITS center, alarm, and call the car, and has the characteristics of low cost, wide system coverage, low maintenance cost and reliable communication. However, since the system mainly uses short messages of the GSM system for communication, real-time communication cannot be achieved. On the basis of this system, GPRS technology can be used instead of short messages for data transmission. The "always on" feature ensures real-time data transmission.
The Intelligent Transportation System (ITS) uses information technology, computer technology, and control technology to transform the traditional traffic operation system to enhance system operation efficiency, improve system reliability and safety, and reduce energy consumption and The purpose of pollution in nature. The ITS generally consists of four parts: the traffic information collection part, the vehicle dispatch control part, the electronic toll collection system and the traffic information service. Each part of them needs the participation of the vehicle terminal: in the traffic information collection part, the vehicle terminal is required to provide the accurate positioning information of the vehicle and the vehicle running condition information; in the vehicle scheduling control part, the vehicle terminal is used as the receiving end of the control, and is responsible for receiving the ITS. The central dispatching command information; the electronic toll collection system requires the vehicle terminal and the toll station to automatically complete the payment transaction; the vehicle terminal is also the receiving platform of the traffic information service, and displays the service to the driver and the passenger of the vehicle. Therefore, the vehicle terminal is a very important combination part of the ITS system. The "vehicle GPS intelligent terminal" introduced in this paper is a specific implementation of the ITS vehicle terminal. The functions and design implementation methods of the vehicle terminal system are described in detail below.
The function of a car GPS intelligent terminal
According to the requirements of the ITS system, the car GPS intelligent terminal should have the following functions:
(1) Vehicle positioning;
(2) The terminal communicates with the ITS control center;
(3) Alarms, including active alarms and automatic alarms;
(4) Conducting in-vehicle monitoring when necessary;
(5) Control the car to stall if necessary;
(6) Display scheduling information.
In addition, the car GPS intelligent terminal also implements other functions according to user needs:
(1) can call the car phone;
(2) Limit the driving range and driving time of the vehicle, monitor the driving trajectory of the vehicle, and the like. These features of the car GPS intelligent terminal make it suitable for the management, scheduling and other fields of automobile insurance, transportation fleet or taxi team.
Figure 1 Schematic diagram of the ITS system
At present, the technology of the Global Positioning System (GPS) is relatively mature and very convenient to use. The global positioning coordinates of the in-vehicle GPS intelligent terminal can be conveniently obtained through a dedicated GPS module. Its positioning accuracy is relatively high, and the general error is less than 15m. How to send location information to the ITS center has always been one of the more difficult problems to solve. In the past, ITS systems mostly used cluster communication systems to realize communication between the vehicle terminal and the ITS center. However, such a system has the disadvantages of small coverage area, high installation and maintenance cost, and complicated technology. In the past two years, with the popularity of GSM/GPRS networks in China, the way in which vehicle terminals communicate with ITS centers via GSM/GPRS networks has become the most popular method. This is mainly due to the following reasons:
(1) The GSM/GPRS network has a wide coverage and has been spread throughout most of China, including rural areas and remote areas;
(2) No need to build network or maintain;
(3) GSM/GPRS network has high reliability and low bit error rate;
(4) Data transmission using short message function or GPRS, the cost is relatively low;
(5) Due to the wide use of GSM/GPRS technology, there are many manufacturers providing corresponding communication modules, and the price is reasonable. The ITS system designed here is based on the GPS satellite system and GSM/GPRS. Its structure is shown in Figure 1.
First, the car GPS intelligent terminal locates its global coordinates through GPS satellites; then sends the positioning information to the ITS center by means of message or GPRS data communication, and the control scheduling command of the ITS center is also sent to the car GPS intelligent terminal through the GSM/GPRS network. The communication between the terminal and the ITS center conforms to a dedicated command protocol. Finally, Internet users can also connect to the ITS center through VPN private network technology or other secure networking technologies to control and view the status of the vehicle terminal.
Design of hardware system for three-vehicle GPS intelligent terminal
The vehicle-mounted GPS intelligent terminal uses a single-chip microcomputer and a GSM module to jointly design a hardware solution that meets the requirements of economical terminal functions. Its hardware system structure is shown in Figure 2.
(1) The main control MCU
The main control MCU uses a high-performance single-chip W77E58 with two serial ports. In the system, the main control single chip is responsible for receiving the user's handle input signal and GPS input signal; calculating the GPS signal to obtain the current latitude and longitude coordinates; receiving and parsing the short message command sent by the ITS center, and uploading and positioning according to the command Coordinates, alarms, etc.; also responsible for displaying the operating status of the system and ITS information through the LCD.
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