This article refers to the address: http:// 600-800V Series Electric Compressor Three-phase Sealed Terminal Shenzhen Capitol Micro-Electronics Co.,LTD , https://www.capitolgtms.com
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.
one. Car GPS intelligent terminal function
According to the requirements of the ITS system, the vehicle GPS intelligent terminal should have the following functions: (1) vehicle positioning; (2) communication between the terminal and the ITS control center; (3) alarm, including active alarm and automatic alarm; (4) when necessary In-vehicle monitoring; (5) controlling the vehicle to stall when necessary; (6) displaying scheduling information. In addition, the car GPS intelligent terminal also implements other functions according to the user's needs: (1) can dial the car phone; (2) limit the driving range and driving time of the vehicle, monitor the driving track 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.
two. Design of ITS System Based on GPS-GSM/GPRS
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) GSM/GPRS network coverage is wide, and it has been spread throughout most of China, including rural areas and remote areas; (2) no network construction or maintenance is required; (3) GSM/GPRS network has high reliability. The bit error rate is low; (4) the use of short message function or GPRS for data transmission, the cost is relatively low; (5) due to the widespread 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.
three. Design of hardware system for 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.
(2) GSM module
The GSM module can be used to easily communicate using the GSM network. It is connected to the main controller in a serial port and communicates at a certain baud rate. The main controller can control the GSM module to send short messages through AT commands, use GPRS to transmit data or make voice calls. The hardware connection diagram of the GSM module is shown in Figure 3.
The standard serial port is used for communication between the GSM module and the MCU. The highest baud rate of communication can reach 115200 bit/s. Data communication between the GSM module and the SIM card is mainly through the SIMCLK and SIMDATA signal lines. In order to ensure that the time interval between the sending of the short message and the arrival of the short message is as short as possible, the selected SIM card is preferably provided by the same telecommunications carrier. When using the GPRS function, you also need to select the SIM card that supports GPRS and open the GPRS service. The GSM module also supports driving two microphones, two speakers and one buzzer. One of the microphones and speakers can be connected to the handset of the handle for car phone functionality.
(3) GPS module
The GPS module is used to receive signals from GPS satellites and calculate the current location of the vehicle terminal. The GPS module used consists of a frequency converter, a signal channel, a microprocessor and a storage unit. The GPS module sends positioning coordinates to the main controller through the serial port; the main controller can also send a setting command to the GPS module to control the status and working mode of the GPS module. The GPS module needs to be equipped with a dedicated GPS antenna to receive GPS satellite signals. Generally, in more open areas, more than three GPS satellite signals are required to be accurately located. In the car GPS intelligent terminal system, placing the antenna on the roof can have a better positioning effect.
(four) power module
Power modules are used to power other modules in the system. The terminal system requires three voltages from the power module: 3.6V, 5V, and 3.3V. Among them, the GSM module requires a relatively large current (about 2A) when transmitting and receiving data, and uses the National LM2576 power chip. It is a PWM-modulated high-power regulator chip that delivers up to 3.5A of spike current. A backup battery system is also designed in the power module to supply power to the vehicle GPS terminal when the vehicle power supply is not working or is damaged. When the vehicle power supply is working properly, the backup battery will be automatically charged.
four. Design of Car GPS Intelligent Terminal Software System
First introduce the interrupt system of the program response. Since the communication between the MCU and the module is irregular or indefinite communication, in order to ensure that there is no blocking, the system adopts the interrupt receiving mode: all the received data is placed in the corresponding circular buffer during the interruption process. The received serial data is then parsed by the main program. The microcontroller also responds to two other interrupts: one is the interrupt that is triggered when the alarm button is pressed; the other is the timed interrupt, which is triggered every 20ms to detect the timeout response of the GSM module.
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.