The second in command of intelligent bus monitoring: GPRS and ZigBee

introduction

In today's modern life, buses are the most important part of urban transportation. Their operating efficiency and service quality greatly affect urban traffic conditions and citizens' travel conditions. The important criteria for measuring operating efficiency and service quality are whether buses can arrive at each station on time and whether people can know the operation status of the buses they are waiting for.

At present, except for the departure station and the terminal station, many intermediate stations cannot guarantee the punctuality of buses; relying on the driver to press buttons to announce the stations, it is inevitable that errors will occur and mislead passengers; and the waiting people do not know the operation status of the waiting buses. Therefore, this paper develops a bus operation monitoring system based on GPRS and ZigBee, in order to better solve these problems.

1 Overall system design

The system consists of a bus monitoring center, a platform monitor at the bus stop, and an intelligent wireless terminal on the bus (hereinafter referred to as the monitoring center, monitor, and wireless terminal), as shown in Figure 1. The wireless terminal reports the arrival and departure time of the bus to the monitor through ZigBee technology. The monitor receives the signal sent by the wireless terminal, checks the "identification number" of the vehicle, identifies the incoming vehicle, and transmits the arrival time, vehicle number, and other information of the vehicle to the monitoring center through the GPRS network.

The bus identifies the platform name based on the platform identifier sent by the detector and announces the station through voice and LED screen. After that, the monitor continuously detects the signal strength sent by the wireless terminal. When it weakens to a certain level, it is considered that the bus has left the station and then sends relevant information to the monitoring center. The monitoring center stores the information sent by the monitor, determines the bus driving section based on the received information, and sends the information to the monitor, which displays it to the waiting passengers through the running status indicator light.

2 Hardware Design

2.1 Monitor

2.1.1 Overall design

The monitor is shown in Figure 2. This part consists of a CPU, a wireless GPRS communication module, a wireless ZigBee communication module, a bus running status indicator light, and other peripheral circuits. The CPU is Samsung's S3C44B0X, which has the advantages of low power consumption, high performance, and high cost performance. It also has a wealth of built-in components, which greatly reduces the configuration of components other than the processor in the system circuit, reduces costs and reduces the complexity of the system. At the same time, it has a large number of I/O ports, which can realize the control of a large number of status indicators. GPRS can support both intermittent burst data transmission and occasional large amounts of data transmission. The data transmission speed is fast and is charged according to traffic. Therefore, GPRS is suitable for this kind of monitoring system with frequent communication, large data volume, and high real-time requirements. This design selects GPRS as the wireless connection method between the monitor and the monitoring center, and the monitor and the bus terminal communicate using ZigBee wireless communication. ZigBee is a short-range, low-complexity, low-power, low-rate, and low-cost wireless network technology. The ZigBee communication module in the design uses Freescale's MC13192, which has an operating frequency of 21405~21480GHz and adopts direct sequence spread spectrum communication technology. The data transmission rate is 250kb/s, which meets the design requirements.

2.1.2 GPRS communication module

The GPRS module is Q2403 produced by French WAVECOM. This module complies with ETSI standards GSM0707 and GSM0705, with a download speed of 5316kb/s and an upload speed of 2618kb/s. The module provides an asynchronous serial communication interface that complies with the V24 protocol, supports encryption algorithms, integrates RF circuits and baseband, has stable performance, and can transmit quickly and reliably. Q2403 and S3C44B0X are connected through a serial interface, as shown in Figure 3.

2.2 Wireless Terminal

The wireless terminal is mainly composed of an audio playback module, a key response circuit, a wireless ZigBee communication module and an LED screen display module, as shown in Figure 4. The audio playback module is responsible for recording and playing the voice announcement information.

The key response circuit is responsible for responding to the bus driver's key operation.

2.2.1 ZigBee wireless communication module

Since the RF signal of MC13192 adopts differential mode and the inverted F-type antenna is a single-ended antenna, a balanced/unbalanced impedance conversion circuit is required between the chip and the antenna to achieve the best transmission and reception effect.

The circuit uses UPG2012TK and the balun circuit dedicated chip LDB212G4020C. UPG2012TK is a gallium arsenide single-pole double-throw (SPDT) RF switch manufactured by NEC for mobile phones and other L-band applications. Its operating frequency is 0.15~2.15GHz, with very low insertion loss and high isolation performance. The connection between MC13192 and S3C44B0X is shown in Figure 5.

2.2.2 LED screen display module

The LED dot matrix screen in the design is composed of 4 LED dot matrix modules. The modules need to be controlled by the anode and cathode. The row is the anode and the column is the cathode. Therefore, the LED dot matrix screen drive circuit is divided into two parts: the row drive circuit and the column drive circuit, as shown in Figure 6. The row drive circuit uses 16 8050D NPN transistors and 16 pull-up resistors to complete the drive. The column drive circuit is driven by 16 S8550D PNP transistors and 16 pull-up resistors.

Therefore, the distortion is small, it is easy to use, does not require special voice development tools, and is low cost. The keyboard uses an independent keyboard, and the driver chip uses ZLG7290. The RS232 communication part is completed by MAX233A. The reset part is implemented by a professional reset circuit chip IMP811.

3 Software Design

3.1 ZigBee Network Address Allocation

The design uses a distributed address allocation scheme to allocate ZigBee network addresses, and adopts a peer-to-peer network structure to build the network, with the monitor as the parent device and the wireless terminal as the child device. The parent device of the terminal station acts as a network coordinator to start the establishment of the network, select a channel, determine a unique PAN address and broadcast the network establishment information. After the parent device establishes the network, it sets its own address to 0X0000, and other monitors act as routers and wireless terminals as terminal nodes to join the network. The allocation of network addresses is related to three parameters, namely the maximum number of child nodes allowed Cm, the maximum number of routing nodes allowed Rm and the maximum network depth allowed Lm. Based on these three parameters, the address interval Is (d) between adjacent nodes at each level can be calculated from the bottom up:

Where d is the router level, and the address of the nth level parent device Ap is

The address of the wireless terminal device is determined according to the order of access to the network. For example, the address An of the wireless terminal device that accesses the network for the nth time is

Among them, An is the node with sequence n in the same level depth nodes, 1≤n≤Cm-Rm, and Ap is the address of its parent node at the previous level.

3.2 Software Process

The system software design includes three parts: wireless terminal, monitor and monitoring center software design. This article only introduces the wireless terminal and monitoring station software design. For the monitoring center software design, please refer to other materials.

After the monitor is powered on, it initializes Q2403 and ZigBee and prepares for ZigBee communication, waiting for the connection request of the ZigBee device. When receiving the connection request of a device, it confirms whether it is a legitimate user. If so, it issues a command to allow the connection to realize the wireless connection between the wireless terminal and the monitor. After the connection is established, the monitor obtains the unique identification number of the bus, registers the bus, and sends the bus number and time information to the monitoring center through the GPRS network. When the bus leaves the platform, the signal strength drops to a certain level, the bus disconnects from the monitor, and it is considered that the bus has left the station. The monitoring station also receives the bus operation status information sent by the monitoring center at all times, and displays it to the waiting passengers through the operation status indicator. The workflow is shown in Figure 7.

After the wireless terminal is powered on, it performs ZigBee initialization and searches for the monitor. When it detects that the signal strength of the monitor is greater than a certain value, it sends a request to establish a connection to the monitor, obtains the identifier of the monitor, and thus knows which station it is, and uses voice and LED screen to realize automatic station announcement. When leaving the platform monitor, it detects that the signal strength of the monitor is weak to a certain extent, and then sends a disconnection request to the monitor. Its workflow is shown in Figure 8.

4 Conclusion

Applying GPRS and ZigBee technology to the intelligent bus monitoring system has solved many problems that have plagued the bus monitoring system for many years, making it more prominent, improving the service quality and operating efficiency of the bus, and has high practical value. In this system, the GPRS technology used in long-distance wireless communication and the ZigBee technology used in short-distance wireless communication complement each other, while expanding the monitoring range, it also improves the intelligence level of the monitoring system. This monitoring network model has a certain degree of versatility and can be promoted and applied to industrial sites with a wide range of working areas such as oil and coal production.