Designing of Wireless Sensor Network Nodes to Detect
Vibrational changes for Structural Health Monitoring
Application
K. S. Raju 1, Sarika Lamba 2, A.H. Ansari 2
1Digital System Group Lab
2Department of Computer Science and Engineering 1Central Electronics Engineering Research Institute, Pilani (Raj.)
2Suresh Gyan Vihar University, Jaipur (Raj.) 1Sarika.lamba23@gmail.com
The Structural Health Monitoring (SHM) for building applications using wireless sensor networks is gaining a lot of interest now a day. The low power consumption, low cost and extendable network is a great challenge for designing and monitoring the building applications. Zigbee based on IEEE 802.15.4 characteristics are best suitable for SHM applications. In this paper Zigbee node is transferring the sensor data from sensor to the processing computer. A network has been designed with the coordinator and end device. End device will collect the data from the sensor and send the sensor data to the coordinator, through the coordinator data will be displayed by the processing computer. The prototype works as milestone for achieving the goal of transferring the sensor data with low power consumption.
Keywords: IEEE 802.15.4, Structural Health Monitoring, Wireless Sensor Network, ZigBee.
I. INTRODUCTION
Structural health monitoring application plays a very important role in every person’s everyday’s life. The destruction of structure is due to the use or misuse of building/bridges as well as the catastrophic changes for example earthquake causes the deterioration of the structure. Health monitoring can be done in two ways, one is alarm warning i.e. notification of disaster for explosion, tsunami etc. and another one is continuous health monitoring i.e. from continuous vibration, wind speed measurement etc. [1]. The destruction of building results in the loss of property may become cause for the loss of precious human life as well. The financial loss will far exceed to rebuilding or to repair the structure that has been damaged due to the catastrophic events in the environment. To prevent the destruction and to save the life, the continuous health monitoring can be considered as a challenge. The reliability of a structure is an essential requirement to make a safe and healthy structure and it can be done by the prediction of the hazard our event, detection of the damage, detection of the location of the damage as well as measuring the intensity of the damage according to that we can find out the state of the structure and then apply an appropriate security [2], [3].
SHM provides the reliability to the civil structure by applying the sensors that will sense environmental changes. These environmental changes can make hazards to the building in absence of monitoring system. To monitor the system we can make a network that will cover the whole building. The sensors can be connected through wires and send the sensor data to the base station that is connected to data processing computer. There are some disadvantages in this infrastructure, firstly the sensors will cover a large area network so it becomes a very complex and costly structure due to the use of wires and the second, it makes a centralized network and the central node will be overburdened with sensor data as well as with the computational tasks, it results in a slow speed computation. To overcome such limitations of wired networks, the wireless networks come into light. WSNs (Wireless Sensor Networks) use the distributed computing and detect the damage location even if it is in critical location [4], [5]. WSNs are easy in installation and maintenance, consume low power, much lower cost, and provide flexibility in joining and leaving the network by the node.
system, high-frequency sampling with low jitter and time synchronized sampling. F Federici et al., show a design example of damage detection and show that this damage indicator can be useful in columns damage Detection [2]. The authors conclude that the analysis of proposed procedure can effectively locally diagnose the occurrence of damage. Xiang-dong Jiang et al., introduce software and hardware design of wireless sensing system for large scale buildings such as bridges [3]. They analyze the performance of the system under multi-hop network topology, and claim that the introduced system can monitor the health of large scale structure very well. Jianjun Niu et al., proposed a SHM system based on Wireless data transfer technology [4]. The authors proposed a system having low latency and high throughput by the TDMA approach. Jian Wu et al. developed two tier network systems that can monitor the position of applied load and position of loose bolt [5]. The authors give the guidance to develop a large scale SHM system based on WSN for composite engineering structures by the embedding pattern matching method. The restricted bandwidth and memory poses big challenge to transmit the synchronously sampled sensor data to the processing center by reducing the data collision. D Liaw et al. developed a network topology, source routing mechanism and flexible beacon synchronization for SHM monitoring by using AWSP platform [6].
The rest of the paper is organized as follows: section 2 describes the node topology and system design, validation and system analysis is given in section 3, and the results are discussed in section 4, finally conclusion is given in section 5.
II SYSTEM DESIGN AND NODE TOPOLOGY
The objective of the study is to realize a prototype of WSN for SHM applications and to validate the system experimentally. The prototype is composed of zigbee nodes: zigbee coordinator and zigbee end device, accelerometer sensor, processing computer.
Figure. 1 Experimental setup of Hardware components
Wireless sensor networks are scalable with respect to the number of nodes in the network, self organization, self healing, energy efficient, having a sufficient degree of connectivity among nodes, low complexity and small size of nodes [8]. The zigbee nodes for system design are programmed with these characteristics of wireless sensor network.
The zigbee coordinator is designed to coordinate the other nodes of the system and it is the only link between the processing computer and the network of zigbee nodes. It will initialize the channel and PAN ID, allow an end device to join the network and give the local address to the end device for further communication. When the end device sends the sensor data, it will retrieve the sensor data and neighbor information of end device from the end device and then send it through serial data communication to the processing computer. The processing computer will display the sensor data with the neighbor information of coordinator and end device through the hyper terminal. The hyper terminal will display the whole information send by the coordinator continuously after every 10 ms.
COORDINATOR
END DEVICE
The zigbee end device is mainly designed to relay the data from accelerometer sensor to the coordinator. End device collects the sensor data from sensor, convert the analog data to the digital format through the ADC and will send the data to the coordinator in voltage form. The sampling of the sensor data is done at every zigbee node and transferred to the processing computer synchronously. End device is linked with the coordinator wirelessly and sensor is in contact with the end device physically, coordinator is connected through the USB with the processing computer, end device is taking supply from battery power supply.
III VALIDATION OF SYSTEM AND ANALYSIS
In this system, the coordinator and end device are communicating through the established network. The end device sends the sensor information to the coordinator and end devices will relay the sensor information from sensor to the coordinator or from one end device to the coordinator. To test the connection first, connect the coordinator, end device to the processing computer in which the programming code has been written via the USB then download the binary (.bin) file in the chip flash. To download the program, we will select the communication port then reset the device and put it into programming mode. Once the program has been downloaded then disconnect the device from the PC and power cycle the device. Now connect the coordinator with the hyper terminal that will show the results of the sensor via the processing computer. To connect the coordinator with the hyper terminal we have to select the bit rate i.e. 19200 bit per second, baud rate 8, stop bit 1 and the flow control is through the hardware. We make the neighbor table to stop the man-in-middle attack i.e. by the coordinator and the end device as well as the MAC addresses and the physical addresses of the devices are also there to make the system secure from the attackers.
A. Hardware-
The Network is composed of a coordinator, end device, accelerometer and a processing computer. The coordinator and end device are the jennic 1023 JN5139 devices which work on Vcc 3.0 V, the accelerometer shown in fig 2 which we have used is ADXL322, and JN5121/JN513x wireless RF(WSN) microcontroller is used. The ADXL322 accelerometer produced by Analog Device which is a small, thin, low power, complete, dual-axis and measure acceleration with a full-scale range of ±2 g (typical). The ADXL322 is available in a 4 mm × 4 mm × 1.45 mm, 16-lead, plastic LFCSP. It is available with wide supply voltage range 2.4 V to 6.0 V and low power 340 µA at Vcc= 2.4 V. ADXL322 is available in most of the application like Cost-sensitive motion, tilt-sensing applications, Smart hand-held devices, Mobile phones, Sports and health-related devices, PC security and PC peripherals [8].
Figure.2 Accelerometer ADXL322 has 2 axes in a single chip
The JN5121/5139x microcontroller is suitable for the IEEE 802.15.4 standard and for the ZigBee applications. It is low power, low cost microcontroller which integrates a 32 bit RISC processor, 96 kb of RAM, 192 kb of ROM, 2.4 GHz radio, IEEE 802.15.4 MAC, O-QPSK modem and a mixture of analog and digital peripherals. The coordinator works as the initiator of the Network that will initialize the Network, hardware, BOS, software etc. and it is directly connected with the processing computer that is used to take action to make the safe civil structure based on the sensor’s information. The end device is used to relay the information from sensor to the coordinator and the
end device also maintains the table that will store the information of neighbors that are attached with that end device.
B. Software-
we add some function in the existing ZigBee coordinator, end device and end device node’s program, these are as following:
− vTxSerialDataFrame: to transfer the data to the processing computer i.e. connected to the coordinator via USB,
− vReadNeighbourTable: used to read the information about the neighbors of the coordinator that are connected to the coordinator,
− getNeighborData: store the neighbors information like physical address of the neighbor, the parent and the child of that node,
− vSendData: used to send the data to the parent node, − vInitSensors: initialize the sensor hardware, − vReadSensors: read the sensors reading,
To compile the programming code and developing applications in IDE (Integrated Development Environment), we use CODE::BLOCKS IDE provided by jennic. It is an open source, full featured and is available free of charge. To produce the binary (.bin) file, we have to build the application in CODE::BLOCKS IDE and then download the .bin file to the flash chip on the JN51xx module to run the application. To download the files connect the chip to the Processing unit via USB port using the JN51xx Flash Programmer. Processing computer will show the result on the hyper terminal by selecting the appropriate port setting.
Figure. 3 Characteristic measurements of ADXL322 [9]
The estimated theoretical results of ADXL322 accelerometer sensor are shown in fig. 3[9]. These are earth surface v/s orientation of accelerometer sensor results. When sensor is parallel to earth surface, it is showing acceleration of X axis Xout is 1.08V and acceleration of Y axis Yout is 1.50V. If we rotate accelerometer with 90, 180,270 degree from its mean position then accelerometer reading are given in above diagram.
IV. RESULTS
Display on Hyper Terminal-
Figure. 4 A snapshot of display on Hyper Terminal
The nodes are so programmed that every time a node sends its sensor data, it first identifies itself by mentioning its short address (which follows the start sequence). After this, the node gives information of its neighbors i.e. the device type of the neighbor coordinator or end device and the relationship of the neighbor with this node –parent, child, sibling, none neighbor or leaving. All this information is obtained by a node from its neighbor table and the function getNeighborData () obtains the information from the neighbor table.
#KC0x0 EC0x796F*KR0x796F CP0x0*Mac address = 0x158d00 0xa6862
Address = 0x796F
ACCELERATION_Y=1505 ACCELERATION_X=1503
#K is the start sequence characters which are useful in capturing all the characters sent by a node. Thus, the characters between ‘#K’ and ‘*’ (which is sent by another node) gives all the information of a particular node in the network. ’K’ is followed by ‘C0x0’ which means that a Coordinator of the short address 0x0 is sending the information of its neighbors.
The coordinator is so programmed that every time the information of a node is sent to Computer, coordinator gives the information of its own neighbors. The characters EC0x796F indicate that the node having the short address
0x796 is an End device Child of the node 0x0. The character ‘*’ is another delimiter which indicates that the characters that follow it are sent by another node. KE0x796F indicate that a node0x796F is sending its neighbors’ information now. CP0x0 indicate that the node 0x0 is a Coordinator Parent of this node 0x1.
Here 0x158d00 is the MAC address of Coordinator, 0xa6862 is the MAC address of end device and 0x796F is the End device network address that is short address given by Coordinator.
In the result, end device is continuously sending the sensors information to coordinator with certain sampling period. If more ZigBee nodes want to join in the network it will show their MAC address, Network address in the hyper terminal and it will show the sensor readings.
Figure. 5 Accelerometer reading when sensor is parallel to earth surface
The experimental results of accelerometer sensor obtained by the designed nodes are acceptable when compared with the characteristic measurement of ADXL322. The results in fig. 5 shows that Y axis of accelerometer sensor senses a reading 1547 which is equivalent to the 1.547 V and according to the characteristic measurement, it should be 1.5 V. Similarly, Experimental results of the X axis of accelerometer sensor showing 1084 reading which is equivalent to the 1.084 V. According to the characteristic measurement the value should be 1.08 V. So the designed node is capable of measuring almost accurate vibration for structural health monitoring applications.
The sensor data at 90, 180, 270 degrees are taken and are compared with the characteristic measurement of ADXL322. These results are accurate enough for our purpose. The data at different angle are also taken and then a graph has been plotted between the output gain in form of voltage and the input given by us by changing the angle in from of degrees. The graph shows the accelerometer non linear output in both X and Y axis of ADXL322. The blue and red lines show the X and Y axes data of accelerometer respectively. The graph of fig. 6 shows that the voltage changes as we change the angle of the accelerometer.
V. CONCLUSION
Structures like bridges and buildings needs continuous monitoring for safety purpose. This monitoring can be done by different sensors. These sensors should transfer data to Base station to know the current state of system health. To fulfill this agenda the wireless network has been designed that consists of spatially distributed nodes; these nodes can communicate via radio frequency. Each node can communicate up to 100 meters in open space. In closed space it can communicate up to 20 to 30 meters. The end device as transmitter and coordinator as receiver are designed to transfer the vibrational changes inside the building by using the accelerometer.
We have successfully designed zigbee nodes that can transfer ADXL 322 accelerometer sensor data using jennic JN-5139 Zigbee module for measuring vibration from sensors. The vibration sensor data is sampled and synchronized at every zigbee node. The ADXL322 generates the analogue voltage depending on orientation of sensor. This analogue voltage is given to A/D converters of zigbee node. This zigbee node transfers the data to the base station through coordinator. Nodes are designed like after some modification in zigbee nodes programming code, we can transfer the temperature, humidity and displacement etc. sensor data as well. To monitor the structural health system of a building application through wireless sensor network this can be used.
The end device is designed to save the power by adding sleep mode for some time when data has been send. After sending data from end device node to its parent node, every end device goes to sleep mode. After some time it wake up from sleep, sense the sensor data and send this data to the coordinator. so the power consumption will be reduced during the sleep mode. By reducing the power consumption the cost will also be reduced.
The result of accelerometer shows that the designed network nodes are working correctly; the correct data has been transferred from sensor to the coordinator via child nodes of the coordinator. The graph shows a non linear output of the hyper terminal result, this shows that as the angle of the accelerometer changes, the acceleration also changes. This change in voltage depends upon the X and Y axes of accelerometer as well as the orientation of accelerometer. The graph shows the non linearity in voltage, so due to this analysis the ADXL322 can be used in tilt sensing applications, cost sensitive motion applications, smart hand held devices, health related devices, mobile phones etc.
VI. REFERENCES
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