With the aging of society, it has become an important social problem to solve the long-term chronic disease monitoring. Some sudden diseases and family health care, such as cardiovascular diseases, are the daily care of the elderly. The health care of pregnant women, fetuses and babies also needs long-term home monitoring. Due to the shortage of medical resources in China, the establishment of community medical monitoring based on public network can improve the level of medical services and reduce the burden on patients. The previous solution was to use wired. Mode or simple wireless data transmission and reception mode, it is difficult for the sensor equipment installed on the monitored person to move and access the system freely and flexibly. The emergence of Zigbee technology provides a new solution for wireless transmission of sensor signals. The coverage of Zigbee nodes is tens of meters, which can increase the coverage of routing nodes and is suitable for home and residence. At the same time, because the data transmission flow of physiological monitoring signals is not high, Zigbee with a large transmission rate of kbps can meet the requirements of physiological data transmission. Zigbee sensor nodes can join and leave the network freely and flexibly, which has the characteristics of low power consumption and low cost.
The above characteristics of Zigbee wireless sensor network make it have a good application prospect in personal physiological signal monitoring and remote home monitoring. Based on the analysis of Zigbee wireless sensor network technology, this paper studies its application in mobile monitoring.
Architecture of wireless network home monitoring system based on Zigbee
Network requirements of remote home monitoring system
Home monitoring network needs to consider energy consumption, coverage, transmission rate, Internet communication and other factors. In this study, the wireless network based on Zigbee technology is used to collect indoor physiological signals, and the physiological data are transmitted to the remote monitoring server through the Internet. The human body carries the mobile physiological signal sensor terminal to move within the network coverage, and the Zigbee network is connected to the Internet through the routing nodes in the network, which is self-organizing and dynamic. At the same time, Zigbee network considers the energy saving of nodes and reduces the computational burden of node processors. Doctors in hospitals or communities can check patients' physiological information at any time through the Internet, control the collection mode of physiological sensors, and also obtain information of other monitoring devices in wireless networks.
. network topology
There are three types of IEEE network topology: star structure, grid structure and clan structure. As shown in the figure, the grid structure and clan structure belong to the point-to-point structure ... In the network, devices can be divided into full-function devices (FFD) and reduced-function devices (fIFD) according to their communication capabilities. FFD devices and FFB devices and RFD devices can communicate directly, but RFDs can't. In IEEE. In the+network, there is an FFD device called PAN network coordinator, which is the main controller in the sensor network. There is only one master controller in each network. The network coordinator not only directly participates in the application, but also completes the functions of membership management, link state information management, packet forwarding and so on.
All nodes in the star network cannot communicate directly with the communication nodes of the central coordinator. The central node consumes a lot of energy and is suitable for network applications with few network nodes and simple network structure. In a peer-to-peer network, any two devices can communicate as long as both parties are within their radiation range. The coordinator in peer-to-peer network is mainly responsible for managing link state information, authenticating device identity and other functions. Peer-to-peer networks support Ad Hoc networks and can build more complex network structures.
In the home monitoring system, the monitored object may move in multiple rooms. In order to expand the coverage at any time and facilitate the future function expansion, the home network topology is selected, and the bridge between the purple halogen ee wireless network and Ethernet is established, and the monitoring information is transmitted to the monitoring server to realize the * * * sharing of the monitoring information.
Architecture of home monitoring network
Based on the above analysis, the architecture of the remote home monitoring network designed in this paper is as shown in the figure. Zigbee wireless system is mainly composed of Zigbee wireless sensor nodes (pulse sensor nodes), several wireless nodes with routing function and Zigbee central network coordinator (connecting home wireless bridge). Forming a wireless bridge connecting zigbee wireless network and Ethernet is the core part of home wireless network, which is responsible for the management of wireless sensor network nodes and equipment nodes. In the figure, A B C D is an FFD node with routing function, and sensor nodes and routing nodes independently form a multi-hop network.
The pulse sensor node can send data to the gateway through the ABC node. Because the monitored person can move freely at home, the routing of the sensor nodes he carries may change dynamically. The indoor communication distance of Zigbee wireless nodes is ~ m, and ABC nodes can be arranged according to the distribution of rooms to cover the active area to the greatest extent. Pulse physiological data can be transmitted to remote monitoring through home gateway. The remote monitoring server of the server is responsible for the real-time collection, display and storage of pulse physiological data. Other monitoring information, such as the status of monitoring image security equipment, can also be transmitted to the server. Hospital monitoring centers and doctors can log in to the monitoring server to check the physiological information of the monitored person, and can also remotely control the sensors and equipment in the home Zigbee wireless network, so as to find out and take rescue measures in time when the monitored patient is abnormal. Relatives of the monitored person can also log in to the monitoring server at any time to know the health status of the monitored person.
Hardware structure of Zigbee home wireless network monitoring system
For sensor nodes, it needs to have the characteristics of small size, low power consumption and strong adaptability. According to the Zigbee protocol standard, the transmission output of Zigbee equipment is ~. DBM, communication distance ~ m, according to these detection results, the transmission power of the equipment can be automatically adjusted, and the energy consumption of the equipment can be minimized under the condition of ensuring the quality of the communication link. At present, the wireless transmitting and receiving chips on the market are typical. There are products from Chipcon and Freeseale. In this paper, Freescale's RF chip is selected as the chip of the system. This chip can be combined with Freescale controller GT to form a low-power wireless module wireless sensor node. The structural block diagram is shown in the figure.
? Because the wireless sensor is portable, button cell pulse sensor and PVDF piezoelectric film are used, and its output impedance is very large. Signal amplification and filtering are realized by conditioning circuit. The interference of high frequency circuit to sensor signal is considered in the design. The separation design of antenna from high-frequency transmitting and receiving part is the key to the design of wireless module, which directly affects the communication quality and communication distance of sensor nodes. The design method of. Ghz antenna This design adopts the principle that all copper foil traces of dipole microstrip PCB antenna adopt microstrip transmission line, which reduces the transmission loss caused by reflection and obtains higher output power and higher receiving sensitivity.
The home gateway is responsible for controlling and managing the home wireless sensor network, realizing information fusion processing and transmitting information to the Internet. Home gateway has a large amount of data transmission and calculation, so it can use external power supply as power supply. Therefore, the controller adopts arm series with strong information processing ability and network function. In this paper, Samsung's S C is used as controller, Freeseale's MCl wireless controller chip is used, and GT is used to communicate with each other through SPI port. The hardware structure of the wireless gateway is shown in the figure.
Zigbee wireless network software system
Zigbee protocol stack consists of a series of hierarchical structures, each layer provides data transmission services for the upper data entities, and the management entities provide other functional services. Each service entity provides an interface for the upper layer through the service access point CsAP. The software hierarchy based on Zigtme network is shown in the figure.
IEEE defines the PHY layer and MAC layer ... standard groups. The physical layer defines the interface between the physical wireless channel and the MAC sublayer, and provides physical layer data services and physical layer management services. Physical layer data services send and receive data from wireless channels. The physical management layer maintains a database composed of data related to the physical layer.
Zigbee Alliance provides a framework of network layer, application support layer and application layer, based on ... The Zigbee network layer provides functions such as joining and leaving the network mechanism, encrypting data and frame routing. Routing protocol is responsible for forwarding data packets from the source node to the destination node through the network, and mainly completes two functions () to find the optimal path between the source node and the destination node (). Forward packets along the optimized path. In order to use energy efficiently and reduce traffic, Zigbee network allows tree routing, that is, tree structure positioning. Using the tree routing device, there is no need to save the routing table which occupies huge memory or perform additional over-the-air downloading operation to find the path, thus reducing the network traffic. Zigbee routing allows routers to find shortcuts.
The routing algorithm adopts AODV (Ad Hoc on Demand Distance Vetor) algorithm. Each router maintains a routing table and regularly exchanges routing information with its neighbors. According to the minimum routing vector, the routing table is updated, and the application layer framework defines the monitoring network node protocol.
The design model of wireless gateway connecting internal wireless network and external wired Ethernet gateway is shown in the figure. The gateway adopts arm series to run Linux operating system. The communication protocol of wireless gateway is based on Zigbee protocol framework, including device number, data flow direction data information and so on. After the system is turned on and powered on, the system self-check hardware is initialized and connected to the remote monitoring server, and then it enters the data stream relay service to realize data protocol conversion and other functions. After the remote server accepts the connection, it can receive the transmitted data at any time and save it to the database server according to the needs.
Analysis of experimental results
According to the designed zigbee wireless monitoring network platform, the pulse pressure signals carried by the human body are continuously collected and displayed on the monitoring server in real time. With the data sampling frequency Hz of bit A/D converter as the data waveform collected by campus LAN in wired network environment, as shown in the figure, it is the waveform curve of pulse signal directly collected by computer through network transmission. The sampling frequency is Hz.
By comparing the graph and the graph, we can see that the waveform of the pulse data signal collected by the home monitoring network has basically not changed, but the delay of the network makes the signal slightly jitter. When the system goes online, the delay will increase and the jitter will be more obvious. By increasing the buffer, the influence of network delay on real-time signal acquisition can be reduced. On the other hand, human activities will also bring great interference to the signal. Further measures such as filtering can be taken to reduce interference.
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