Design of community monitoring system based on ZigBee and PLC technology

With the increasing aging of society, the concern for the health of the elderly has become an important social issue. In addition, some sudden diseases and family health care, such as cardiovascular disease, health care for pregnant women, fetuses, infants, and young children, also require long-term family monitoring. Therefore, studying family medical monitoring based on public networks, establishing community medical networks, and thus forming a hospital nursing system network can not only increase the safety of patients and improve people's quality of life, but also enable hospitals to more effectively improve the work efficiency of managers, doctors, and nurses, and coordinate the orderly work of relevant departments.

In some cases, the implementation cost of a wired monitoring system will far exceed the cost of purchasing monitoring equipment. For example, in a multi-community medical monitoring network, the transmission of real-time monitoring of patients' physiological parameters to a remote monitoring center and the remote backup of all data are huge expenses, and the maintenance cost is even more staggering. Although the introduction of a wireless monitoring network can solve some problems, the community medical monitoring system based on technologies such as Bluetooth and GPRS has corresponding shortcomings, such as the safety of wireless radiation, the rapid movement of wireless terminals, seamless roaming, and the cost of wireless networks.

The emergence of ZigBee technology provides a new solution for the wireless transmission of sensor signals. ZigBee nodes have a coverage range of tens of meters, and routing nodes can be added to expand the coverage range. In addition, it has the advantages of low wireless radiation, secure data transmission and low cost, so it is suitable for family homes. At the same time, since the data transmission flow of physiological monitoring signals is not large, ZigBee with a transmission rate of 250kbit/s can meet the requirements of physiological data transmission. ZigBee sensor nodes can freely and flexibly join and leave the network, and have the characteristics of low power consumption and low cost. In addition, power lines have the advantages of high speed (the backbone network speed has reached hundreds of Gbit/s), using power transmission lines as transmission carriers (currently my country has the second longest power transmission lines in the world), and power line carrier communication is described by foreign media as "unmined gold mine". We can see that if ZigBee and PLC technologies are organically combined, it will not only make up for some of the shortcomings of the existing community medical monitoring system and alleviate the residents' demand for medical monitoring, but also meet the requirements of China, which is not rich.

The overall framework of the community guardianship system

The community monitoring system is mainly divided into three parts, as shown in Figure 1. The first part consists of several home monitoring units based on the ZigBee wireless network, in which the wireless communication nodes are divided into wireless sensor nodes and PAN coordination nodes according to their functions. The wireless sensor nodes have the characteristics of being portable and mobile, and are mainly responsible for collecting important physiological parameters of the human body; while the PAN coordination nodes are mainly responsible for collecting important physiological parameters and sending these parameters to the monitoring center (i.e., the third part) through the second part, that is, the power line network part. In this way, professional medical staff can realize the statistics and observation of the data, provide relevant medical consultation and services, and achieve the purpose of telemedicine.

The hardware structure of the community monitoring system is divided into: wireless sensor nodes, PAN coordination nodes (including radio frequency communication modules and power line transmission modules) and monitoring center. The structural diagram of the system is shown in Figure 2.

Hardware Design of Community Monitoring System

Hardware Design of ZigBee Sensor Node

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Since the application environment of the home monitoring unit based on the ZigBee wireless network is mainly the home and the network coverage is small, a star topology can be directly adopted, that is, the network is mainly composed of 3~5 sensor nodes, namely RFD nodes (Reduce Function Device, referred to as RFD) and a master node, namely FFD node (Full Function Device). Although the specific functions of the RFD node and the FFD node are different, in order to increase versatility and facilitate maintenance, the basic circuits of the sensor nodes are the same. This article uses the MC13192 RF chip launched by Freescale that complies with the ZigBee standard. Since the RF signal of the MC13192 adopts a differential mode and the inverted F-type antenna is a single-ended antenna, a balun circuit is used between the chip and the antenna to achieve the best transceiver effect. This solution uses the LDB212G4020C chip dedicated to the balun circuit. UPG2012TK is a RF switch with an operating frequency of 0.5~2.5GHz, very low insertion loss and high isolation performance; the package form is a 6-pin short-pin small chip package, which can reduce the area occupied by the circuit board.

In addition, since the TI MSP430 series microcontroller is an ultra-low power mixed signal controller, it includes a series of devices, which are composed of different modules for different applications. These microcontrollers can be powered by batteries and have a long service life. With a 16-bit RISC structure, the 16 registers and constant generators in the CPU enable the MSP430 microcontroller to achieve the highest code efficiency; the flexible clock source can enable the device to achieve the lowest power consumption; the digitally controlled oscillator (DCO) can enable the device to quickly wake up from the low power mode and activate to the active working mode in less than 6s. It is a low-power type of microcontroller, especially suitable for battery applications or handheld devices. Among them, the MSP430F449 microcontroller integrates a 60kB FLASH storage module, 2kB RAM, 6 I/O ports, and a 12-bit high-precision analog-to-digital conversion module ADC12. The smaller package and extremely low power consumption make it ideal for combination with MC13192 as a wireless sensor network node based on ZigBee technology.

MSP430F449 and MC13192 are connected through SPI bus. The SPI interface of MSP430F449 works in master mode and is the controller of data transmission; MSP430F449 is set to slave mode. MSP430F449 reads and writes the internal registers of MC13192 through the 4-wire SPI interface, thereby completing the control of MC13192 and data communication.

Power Line Modems

Lihe Microelectronics has a patented technology for power line communication and control based on multi-carrier fast frequency hopping, which is specially designed for the domestic power grid environment, has good anti-interference performance, and realizes reliable data transmission and control of power lines. Its dedicated chip LME2200 is flexible and convenient to use, and provides an optimized chip solution for various power line communication and control applications. Therefore, this article uses the LME2200 chip, and the hardware block diagram of the modem is shown in Figure 3.

The circuit consists of a signal sending circuit, a signal receiving circuit and a zero-crossing detection circuit, and the basic circuit is shown in FIG4 .

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As can be seen from Figure 4, the transmission circuit consists of a low-pass filter and a power amplifier. The function of the low-pass filter is to filter out high-frequency signal components and smooth the waveform of the DAC output signal. The output of the power amplifier (PA) is connected to the power line through a coupling transformer. The bandwidth of the filter is determined by the carrier frequency used. The requirement for the power amplifier is to obtain a signal level of 1-2Vrms under an impedance of 2 to 100 ohms after passing through the transformer. In addition, LME2200C supports half-duplex operation mode. When sending (TX_BUSY low level), it is used to turn on the power amplifier, and when receiving (TX_BUSY is high level), the power amplifier is turned off to increase the receiving impedance. In the receiving path, the signal obtained by the coupling transformer is sent to the on-chip amplifier for amplification, and then sent to the chip RXIN input after passing through the off-chip bandpass filter, and the on-chip receiver completes the reception of the data packet. The zero-crossing detection circuit outputs a square wave signal, and its rising edge is at the zero-crossing of the power frequency signal. This signal is used as the SYNC synchronization input of LME2200C and as a reference for transmission and reception synchronization.

Software Design

① Software design of ZigBee sensor node

The ZigBee sensor node is mainly responsible for collecting the physiological data of the guardian and transmitting this data to the coordinator. At the same time, it receives relevant instructions from the coordinator and performs relevant operations according to these instructions (including automatically sending physiological parameter commands and prompting to take medicine on time). When no relevant instructions are executed, it enters the sleep mode and the node power consumption is reduced to the minimum. The software flow of the ZigBee sensor node is shown in Figure 5.

② Software design of PAN coordination node

As the only coordinator in the entire network, the PAN coordinator node can be divided into two parts according to its functions: network maintenance function and data transmission function. The network maintenance part is mainly responsible for establishing the ZigBee network, allocating network addresses and maintaining the binding table.

In terms of data transmission, it mainly forwards the commands from the monitoring center to the ZigBee sensor nodes, and the ZigBee sensor nodes send physiological data to it according to the timing requirements, and then forward it to the monitoring center. The basic process of the PAN coordination node is shown in Figure 6.

③Monitoring center software design

Since this system is still in the experimental stage, the monitoring center software is only used as a demonstration system for performance testing. Only VC++6.0 is used to program the user interface and database part of the system. It uses a circular polling method to display the physiological parameters sent by each monitoring terminal device. The main interface is shown in Figure 7.

Conclusion

The debugging results of the community monitoring system in a small-scale experiment show that the system can accurately realize the home medical monitoring function, with reliable and stable communication quality, strong anti-interference ability, low power consumption, low cost, and compact size of personal monitoring terminal equipment. In the laboratory, the distance between the network coordination equipment and the personal monitoring terminal equipment can reach 70m, and it meets the requirements of real-time and efficient data transmission. The realization of the community monitoring system in this paper provides an experimental platform for the research and development of the community monitoring system, and also lays a certain foundation for its development.

References:

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