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Research on the application of wireless sensor network in parking space control

1 Introduction

wireless sensor network is a new information acquisition and processing technology, which has been more and more widely used in real life. With the development of communication technology, embedded technology and sensor technology, sensors are gradually developing towards intelligence, miniaturization and wireless networking [1]. At present, domestic and foreign researchers mainly study the low-power hardware platform design of wireless sensor network nodes, topology control and network protocol, positioning technology, etc. This design takes the sensor detecting ultrasonic intensity as an example, and realizes a wireless sensor network. According to the ultrasonic intensity detected by the sensor, it decides to turn on or off the parking space indicator light, so as to judge whether there is a vehicle entering the detection area. This kind of sensor network integrates embedded technology, sensor technology and short-range wireless communication technology, and has a wide range of applications. The system does not need to change the site structure, does not need any support of the original fixed network, can be quickly arranged, easy to adjust, and has good maintainability and expansibility

2 IEEE 802.15.4 standard

ieee 802.15.4 standard [2] is applicable to wireless personal domain (WPAN) with low rate, low power consumption, low complexity and short distance data transmission. In the process of wireless transmission in the network, the carrier sense multiple access mechanism (CSMA/CA) with collision avoidance is adopted, and the superframe structure and time slot guarantee mechanism (GTS) are supported. The network topology can be star shaped or peer-to-peer. The standard defines three data transmission frequencies, namely 868mhz, 915MHz and 2.4GHz. The first two transmission frequencies adopt BPSK modulation mode, and the latter adopts PSK modulation mode. Various frequencies support wireless data transmission rates of 20 kbit/s, 40kbit/s and 250 kbit/s respectively, and the transmission distance is between 0m and 70m. The wireless transmission module with frequency of 2.4GHz is used in this paper

3 realization of wireless sensor network

3.1 network platform formation

wireless sensor network platform is composed of three parts: ultrasonic sensor module, microprocessor module and wireless transmission module [3], as shown in Figure 1. The microprocessor module and the wireless transmission module are integrated on a board, and the ultrasonic sensor module is connected with the microprocessor through the interface, so it can be used in various occasions by replacing different sensor modules

3.1.1 ultrasonic sensor module

Figure 1 wireless sensor network node structure

due to the strong directivity of ultrasonic wave, it spreads far in the medium, so ultrasonic wave is often used for distance measurement, such as distance measuring instrument and level measuring instrument, which can be realized by ultrasonic wave [4]. Ultrasonic testing is often relatively fast, convenient, simple calculation, easy to achieve real-time control, and can meet the requirements of industrial practice in terms of measurement accuracy. In order to enable the vehicle to automatically avoid obstacles and walk, it must be equipped with a ranging system to obtain the distance information (distance and direction) from the obstacles in time. The three-way (front, left and right) ultrasonic ranging system introduced in this paper is to provide a moving distance information for the backstage staff to understand the environment in front, left and right [5]

Figure 2 wireless sensor network node communication topology

sl-srf-25 ultrasonic sensor, connected to the power supply, can be used alone as ultrasonic ranging. The obstacle distance is displayed by the 3-bit LED digital tube. The 3-bit LED digital tube adopts the building block plug-in method, which is convenient for debugging, inspection and use in different occasions. The measurement range is 10cm-250cm. When the ranging is less than 100cm, the error is 1 ~ 2cm, When it is greater than 100cm, the error is 4 ~ 5cm. Sl-srf-25 ultrasonic sensor can also specify to output segmented distance detection signal from the i/o port of single chip microcomputer

3.1.2 microprocessor module

MICa2 model node of University of California Berkeley is selected as the processor module. The node board provides the following functions: 433MHz central frequency wireless communication interface, which can customize various functions through programming: it can provide -20db~10db various communication power; It can provide multiple transmission rates from 0.3kbps to 38.4kbps in Manchester coding mode; A variety of communication frequencies can be set near 433M with a frequency interval of 76k. Its high-speed and large capacity ram characteristics provide convenience for processing data packets

3.1.3 wireless transmission module

the wireless transmission module adopts srwf-501 micro power wireless module RF transceiver of sunray electronic technology company. The chip requires only a few external components, with stable performance and extremely low power consumption. The transceiver provides three serial ports and three interface modes. COM1 is the TTL level UART interface, com2 is the standard RS-232 interface, and the standard RS-485 interface; Crystal frequency stabilization, built-in digital phase-locked loop, frequency point can be flexibly set in the range of 300-1000mhz according to user needs; Automatically filter the noise, simplify the programming of user interface, and make it as convenient as wired; "Receive" and "send" are automatically switched, and there is no need for a dedicated transceiver control line. When data is not sent, it is in the normal "receive" state; When sending data, it will automatically switch to the "send" status, and automatically return to the "receive" after the "send"; Micro transmission power: the maximum transmission power is 10MW. The selectivity and sensitivity index of srwf-501 exceed the requirements of IEEE802.15.4 standard, which can ensure the effectiveness and reliability of short-range communication

3.2 system software platform

choose the TinyOS system development environment developed by the University of California Berkeley. TinyDB is the query processing system of TinyOS, which can extract data and information from sensor nodes in wireless networks. TinyOS provides TinyDB with a visual Java API window for real-time queries

3.3 group types

in this paper, the wireless sensor network adopts a star topology (as shown in Figure 2), with a network coordinator as the central node, which can communicate with any ordinary node. Ordinary nodes contain ultrasonic sensors to measure and sample the ultrasonic signal intensity parameters in the surrounding environment, send the collected data to the central node, and analyze and process the data and commands sent by the central node to complete the corresponding operations. If two ordinary nodes want to transmit data, they must pass through the central node, and the central node transmits the data to the corresponding node

3.4 group process

wireless sensor network is a self-organizing network. If a fully functional node is activated, it may establish a network and set itself as the network coordinator, and other ordinary nodes can apply to join the network [6]. In this way, a wireless sensor network with star topology can be built. The wireless sensor network in this paper supports superframe structure. After energy scanning and active channel scanning, the network coordinator sends beacon frames periodically according to the set parameters. After energy scanning and passive channel scanning, the ordinary node obtains the parameters containing network characteristics in the beacon frame, such as beacon sequence number, superframe sequence number and network label number. Synchronize with the network coordinator through the synchronization request, and then associate with the network coordinator through the matching request. In the process of associating with the network coordinator, the network coordinator associates each request. 7. Force measurement method: load sensor force measurement; Ordinary nodes of assign 16 bit short addresses [7]. In this way, the short address can be used for communication in the future data transmission, so as to improve the communication efficiency and reduce the energy consumption in the transmission, so as to prolong the service life of the network

3.5 data transmission mechanism

3.5.1 data format

four frames are defined in IEEE 802.15.4 standard, namely beacon frame, data frame, command frame and confirmation frame [2]

(1) beacon frame: it is used for the network coordinator to broadcast beacons to its adjacent nodes in the first time slot supporting the superframe structure. When the nearby nodes receive the beacon frame, they can apply to join the network

because the wireless sensor network system in this paper adopts a relatively simple star topology, the structure of the beacon frame is different from the IEEE802.15.4 standard: the address domain of the beacon frame only contains the network label and short address of the source node, and does not contain the information of the destination node (because it is sent by broadcast)

(2) data frame: used to transmit data containing ultrasonic information

load the sample locally. The address field includes the network label and short address of the source node and the destination node. There are two transmission directions of data frames: from the ordinary node to the central node and from the central node to the ordinary node

(3) command frame: used to establish wireless sensor network, transmit synchronous data, etc. Command frames are not much different from other types of frames in format

(4) confirmation frame: used to confirm that the target node has successfully received the data frame or command frame. When the target node successfully receives the data frame or command frame, it sends an acknowledgement frame to the sender. The sender receiving this confirmation frame indicates that the transmission is successful. If the confirmation frame is not received within the specified time, resend the data frame or command frame to introduce the wheel radial impact testing machine in detail

the type of frame is defined as confirmation frame in the frame control domain. The sequence number of the confirmation frame should be the same as that of the confirmed frame, and the load length should be zero. The confirmation frame is sent immediately after the confirmed frame, and it is not necessary to use CSMA-CA mechanism to compete for the channel [8]

3.5.2 transmission process

in the whole wireless sensor network, ordinary nodes regularly read the ultrasonic data on their sensors and send the ultrasonic data to the central node. The central node processes the received data and transmits it to the corresponding node to control the parking space setting mark on it. First, the network coordinator checks the received data frame. The "central node determination" in Figure 2 is to determine whether it is the sensor data of the specified node. If the received data is the data on the specified node, the data is compared with an ultrasonic threshold to set the control variable (used to control the switching state of the parking space) [9]. Otherwise, the sending operation will not be carried out. Then, judge whether the node with idle join the network. If a node with idle is found in the network, the central node sends the control variable to it as a data frame load. On the contrary, data frames with control variables are not sent

4 Conclusion

in the wireless sensor parking space control system we designed, the ordinary node sends the ultrasonic data it collects to the network coordinator, and the network coordinator sends the data frame containing the control variables to the contact with the parking space duty mark. At the same time, the ultrasonic data can also be transmitted to the computer through the serial port with good sealing performance. Through the background software on the computer, the change of ultrasonic intensity signal can be monitored. The occupation of parking space can be judged from the ultrasonic sensor

this paper discusses the design and implementation of wireless sensor network from the aspects of the formulation of wireless transmission protocol and the control of transmission process. In practice, as long as the specific sensor is replaced, it can be applied to a variety of sensor networks. Because the wireless sensor system group is flexible and adopts modular design, it has good portability and scalability. With the improvement of people's living standards, this system has broad application prospects in the field of traffic monitoring in the future. It has broad prospects in the field of traffic monitoring [10], intelligent household appliances and intelligent regulation of home environment in the future. (end)

Copyright © 2011 JIN SHI