"Public transport has incomparable advantages over individual transport. Giving priority to the development of urban public transport system is the best way to solve the traffic problems of large and medium-sized cities. In recent years, the intellectualization of urban public transport system has become the main direction in the field of public transport research. Most of the existing test runs in China use GPS global positioning system for positioning and GPRS network for data transmission. The on-board GPS module can obtain navigation and positioning data such as position, direction and time in real time, and then transmit the data to the monitoring center through the on-board GPRS module, so as to realize vehicle positioning and monitoring. The monitoring center can send the real-time information or announcement information of the vehicle to the electronic station board through the GPRS module of the electronic station board to estimate the arrival time and distance, and then display it on the electronic station board. Although the existing test run intelligent public transport system has wide positioning coverage and high accuracy, it can realize the full range positioning and monitoring of vehicles. However, in the actual operation process, there are still the following deficiencies:
GPS signals will have blind spots in tunnels, viaducts and other environments;
During operation, the GPS information needs to be sent to the monitoring center through GPRS, and then the monitoring center sends the display information to the electronic station board through GPRS, so the operation cost is high;
GPRS module is expensive, there are a large number of buses, and GPRS module must be installed, and the hardware cost is high;
The communication between the bus and the stop sign cannot be realized, and the services such as station announcement in advance cannot be realized.
1. Overall system scheme
Because Xi'an has a small urban area, concentrated roads, dense bus lines, and the distance between electronic stop signs is mostly about 500m, it is not necessary for the monitoring center to monitor the buses in a real-time and full range, but only need to know the range of bus stop signs.
In order to absorb the advantages and overcome the disadvantages of the existing intelligent public transport system schemes, and combined with the characteristics of Xi'an City, this paper introduces ZigBee short-distance technology into the intelligent public transport system, and improves the data transmission mode of the schemes of GPS positioning and GPRS information transmission commonly used in the existing trial run intelligent public transport system in China, The overall architecture of the improved intelligent public transport system scheme is shown in Figure 1.
Figure 1 overall scheme of intelligent public transport system
The system is mainly composed of bus terminal, electronic stop board terminal and management monitoring center server.
The bus terminal can locate the location information of the bus in real time according to the on-board GPS module and compare it with the location information of each stop sign. When it reaches a stop sign, the bus will automatically voice report the station and display the arrival information with LCD screen.
Electronic stop sign terminal and bus terminal can communicate through ZigBee short-range wireless communication network. Bus stops can be announced in advance. When the bus arrives at a stop sign, it will package and send its vehicle information and status information to the stop sign. After receiving the information from the management center, the electronic stop board displays the location information of the bus on the electronic map of the stop board.
The management center server and the electronic station board terminal can communicate through GPRS wireless communication network. The electronic stop sign terminal processes and reseals the received bus information through the wireless networking of GPRS module, and then sends it to the wireless network. The server side is generally a PC connected to the Internet, which can receive the information on the Internet through TCP / IP protocol, and send the real-time location information and announcement information of the bus on the running line to the electronic stop sign terminal. The server can manage and query information through the database to facilitate the management and scheduling of public transport companies.
2 system hardware design
2.1 hardware composition of on-board terminal
The on-board terminal hardware in the system mainly includes power module or power access module, ARM processor, ram, flash, GPS positioning module, ZigBee RF transmission module, video monitoring module, LCD display module, serial port and debugging module, number of people in the vehicle statistics module and voice module. Fig. 2 shows the hardware composition block diagram of the on-board terminal in the system.
Fig. 2 hardware composition block diagram of vehicle terminal
ARM embedded processor is the core of the whole vehicle terminal, which can be connected with each functional module through various interfaces. The vehicle terminal adopts a 16 / 32-bit RISC embedded microprocessor S3C2410 based on ARM920T core of SamSung company in Korea. The operating frequency of S3C2410 can reach 203 MHz, which is mainly for high cost performance and low power consumption applications such as handheld devices.
In the intelligent public transportation system, the system positioning module generally adopts GPS-OEM (original equipment manufacturing) board.
In the embedded vehicle terminal system, when selecting GPS module, several factors such as positioning accuracy, price, power consumption, volume, anti-interference ability and so on should be considered. According to the above principles, this design selects the third generation SiRF star iii7855 module of GPS of Leadtek company to realize positioning. The main performance indexes of the module are as follows:
There are 20 parallel channels, which can receive 20 satellites at the same time;
Positioning time: recapture time is 0.1 s, hot start < 1 s, cold start < 42 s, and automatic search is less than 30 s;
The output differential accuracy can reach 10m and the power consumption is less than 1W;
ASCII code statements of nema-0183 protocol can be output through RS232 interface, including gpgga, gpgsa, gpgsv, GPRMC, gpvtg, gpgll, etc;
With 5 V power supply, a DB9 interface can be connected through TX and Rx pins to communicate with the serial port of embedded microprocessor.
2.2 ZigBee RF module
In the intelligent public transport system, the GPS module only completes the information collection function, and when the bus arrives, it also needs to send the information to the stop board through the ZigBee module.
After market research, it is found that Freescale mc1319x platform has low power consumption, low price, high hardware integration, convenient secondary development and high stability of RF communication system. Therefore, the xbeepro RF module compatible with ZigBee and based on Freescale mc1319x chipset is selected in this design. The design of XBee Pro module meets IEEE802.15.4 standard, the working frequency is 2.4 GHz, and its basic performance parameters are as follows:
◇ transmission power l00 MW;
◇ indoor transmission distance is 300 m and outdoor transmission distance is 1500 m;
◇ RF data transmission rate is 250 Kbps;
◇ under 3.3 V power supply, the transmitting current is 215 Ma and the receiving current is 55 ma
Figure 3 shows the pin arrangement diagram of XBee Pro module, which has 20 pins. The pins of RS232 interface circuit board can be connected to VCC, GND, dout and din pins. Where VCC is the power supply pin (2.8 ~ 3.4 V); GND grounding; Din is the signal input pin, which can be used as UART data input, and is usually connected to the UART receiving end TX of the processor; Dout is a signal output pin, which can be used as UART data output, and is usually connected to the UART receiver RX of the processor. In addition, a UART interface is integrated in the XBee / XBee Pro module, and the internal data control flow of the interface is shown in Figure 4.
Figure 3 pin arrangement of XBee Pro module
Figure 4 UART internal data control flow of XBee Pro module
When the serial data enters the XBee Pro module through the din pin, the data will be stored in the di buffer until it is sent out by the transmitter through the antenna; After the RF data is received by the antenna, the received data enters the do buffer until it is processed. Under certain conditions, the module may not be able to immediately process the data in the string bit reception buffer. If a large amount of serial data is sent to the module, CTS flow control may be needed to avoid serial receive buffer overflow. Xbeepro module can be directly connected with the UART interface of the controller through the UART interface, and the hardware interface is simple and practical.
2.3 hardware composition of electronic station board terminal
Compared with the bus terminal, the hardware composition of the electronic stop sign terminal mainly replaces the GPS positioning module on the bus with GPRS - DTU data transmission unit. GF - 2008awgprs-dtu is a GPRS wireless data communication product developed and produced by Beijing jiafuxin Technology Co., Ltd. the product has a built-in Siemens MC39i GPRS module, which has the characteristics of high accuracy, good environmental adaptability, easy installation and maintenance, and can provide users with high-speed, reliable and always online data transmission services and virtual special data communication network services, It can be widely used in remote meter reading, environmental protection data collection, traffic information release and so on. The following are the main features of gf-2008aw GPRS-DTU:
It can realize transparent wireless data transmission through serial port, and it is stable and reliable;
Highly integrated GPRS and TCP / IP technology, which can organically combine Internet and wireless network;
Support a variety of TCP / IP protocols, such as TCP, UDP, DNS, PPP, RAS, etc;
Charge by traffic, no charge if there is no traffic;
Among the standard RS232 interface products, the volume is the smallest and suitable for embedded integration;
Support peer-to-peer data transmission of point-to-point, point to multipoint and center to multipoint;
At + I instruction interface based on serial communication can save development time and cost;
Hold the always online mode, and the disconnection can be redial automatically;
It adopts 5 ~ 24 V / 1 a power supply and has energy-saving mode.
3 ZigBee communication program design
3.1 ZigBee networking scheme
Since there are usually multiple buses arriving at the stop sign at the same time, one stop sign corresponds to multiple buses. Since there are few network nodes and the network structure is relatively simple, the system adopts star model for networking.
That is, the electronic stop boards distributed on the bus lines are configured as ZigBee coordinators, and the arriving buses are configured as ZigBee terminal devices. Figure 5 shows the networking mode of buses and stop signs. When the ZigBee network coordinator on the station board selects a channel and pan ID and starts, a ZigBee personal office network (Pan) is established.
Once the coordinator starts the pan, the router and terminal equipment nodes are allowed to join the pan. When the vehicle terminal as the ZigBee terminal equipment joins the pan, the system will receive a 16 bit network address and send and receive data from the electronic station board terminal as the ZigBee coordinator. The network address of Pan coordinator is always 0. Since the network physical address of ZigBee module on the station board is unique, information can be sent to the station board through the physical address.
Figure 5 networking mode of bus and stop sign
3.2 API operation of ZigBee module
XBee Pro has five operation modes: empty mode, receiving mode, sending mode, sleep mode and command mode. For each operation mode, there are two operation modes: transparent mode and application program interface (API) mode. When working in transparent mode, the module can replace the role of serial port line and operate various information in bytes; And work
XBee Pro has five operation modes: empty mode, receiving mode, sending mode, sleep mode and command mode. For each operation mode, there are two operation modes: transparent mode and application program interface (API) mode. When working in transparent mode, the module can replace the role of serial port line and operate various information in bytes; When working in API mode, all data in and out of the module are included in the frame structure defining the operation and event of the module. This paper adopts API operation mode.
API operation requires modules to communicate through a structured interface (data is communicated interactively through a defined sequence of frames). API specifies command sending, command response, and module status information transmission and reception through serial port data frame.
( 1) ZigBee send request
After the bus arrives at the stop board, it shall send the date, time, car number, bus line, number of people in the car, driving direction and other information to the electronic stop board according to the MAC address of the stop board. The API frame structure definition of ZigBee module transmission mode of bus is shown in Figure 6. Bytes6-13 is the MAC address of the station board.
Figure 6 bus TX request API frame structure
( 2) ZigBee sending status
In order to realize reliable transmission, after the request of the bus to transmit information to the electronic stop board is completed, the confirmation information of the electronic stop board must be obtained, so the transmission status information returned by the electronic stop board to the bus must also be obtained. This message will indicate whether the packet was sent successfully or failed. If the transmission fails, the bus information must be sent again until the transmission is successful.
The electronic stop board continuously sends information to the pan according to the MAC address of the bus, and determines whether a bus joins the network by reading back the transmission status. If so, it identifies the bus according to the network address and sends the bus positioning information to the monitoring center, so as to supplement the GPS positioning mode.
Figure 7 shows the TX status frame structure of the ZigBee module of the bus
Bytes 9 is the transmission status information, and bytes 6 and 7 are the 16 bit network address of the receiving module.
( 3) ZigBee receives packets.
After receiving the status information data packet from the bus, the electronic stop board will analyze it and send it to the monitoring center through the GPRS module of the stop board. The API frame structure definition of ZigBee module receiving mode of electronic station board is shown in Figure 8. Bytes5-12 in the figure is the MAC address of the bus.
three point three GPRS network communication design
Electronic station board received
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