Structural Health Monitoring Using Smart Sensors - ideals ...

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ecords is not necessarily efficient for single packet transfer. Communication protocol for long data records and single packets are, therefore, designed separately. In the SHM architecture employed in this research, most communication takes place inside a smart sensor cluster as unicast or multicast. Unicast is communication between one sender and one receiver, while multicast is communication from one sender to multiple receivers. Both unicast and multicast protocols are, therefore, developed for long data records and for short message communication. Communication protocols developed in this chapter suffice for most communication requirements in DCS for SHM with the exception being communication involved in time synchronization. 1. Communication protocol for long data records Communication protocols suitable to transfer long data records reliably are developed. The length of data assumed in this section is the acceleration record length used in DCS. The first step of DCS estimates the CSD using FFT. The estimation is frequently based on 10 to 20 times the number of data points used in the FFT. Assuming the number of FFT points is 1,024, the target length of data to be transmitted is set as 11,264. Though repeated data transmission without acknowledgment can statistically improve the reliability of communication, such protocols cannot guarantee communication success rate deterministically. If the packet loss rate is expected to be approximately constant over long time, repeated sending practically results in no data loss; the number of repetitions can be dynamically adjusted based on the packet loss rate of the last communication. When many packets are lost statistically, the number of repetitions can be increased. However, in smart sensor networks, burst packet loss may take place. RF transmission devices nearby, for example, may cause the large number of communication packets to be dropped. The fluctuation in packet loss rate is considered large, as shown in Figure 5.11. Repeated transmission is not a sufficient solution to the packet loss problem. Acknowledgment packets potentially guarantee reliable communication between nodes. However, a poorly-designed communication protocol involving acknowledgment messages can be notably inefficient. Many acknowledgment messages may be required, waiting times may be long, and the same packets may need to be sent many times. With careful consideration of efficiency, reliable communication protocols are realized in this section using acknowledgment messages. These protocols are similar to error control methods for data transmission, which are briefly reviewed and then modified for use in the proposed reliable communication protocol for SHM applications. Automatic Repeat reQuest (ARQ) is an error control method which repeats sending packets based on the request from the receiver. On reception of packets without error, the receiver replies with a positive acknowledgment (ACK). If an error is detected, the receiver sends a negative acknowledgment (NACK) and requests retransmission. There are several ARQ protocols, some of which are described in the following paragraphs. In the stop-and-wait protocol, the sender waits for an acknowledgment after the transmission of each packet. If an ACK is received, the sender transmits the next packet. 63
On reception of a NACK, the sender resends the last packet. Simplicity is a main advantage of this method. The IEEE 802.11 wireless LAN adapts the stop-and-wait protocol. This protocol is, however, not efficient in the sense that the sender needs to wait for acknowledgment of each packet. In the go-back-N protocol, the sender transmits more than one packet without waiting for acknowledgment. The receiver keeps receiving packets replying ACK until an error is detected. If an error is found in the N-th packet, the N-th packet and all subsequent packets are discarded. The receiver sends back a NACK with the number corresponding to the lost packet. The sender resends packets from the N-th packet. Discarding the received packets after the N-th packets lowers the efficiency of this protocol. Selective-repeat protocol addresses the inefficiency in the go-back-N protocol. The sender continues sending unsent packets. Once the receiver detects an error in a packet, a NACK is sent back to the sender. Packets received after detection of an error are stored in a reception buffer. Only the packet with an error is disregarded. On reception of a NACK, the sender temporary stops sending unsent packets and resends the lost packet. Then the sender continues to send the remaining packets. These concepts of ARQ can be utilized to guarantee reliable communication among smart sensors. The reliable communication protocol developed by Mechitov et al. (2004) sends a set of packets and then waits for acknowledgment. If the receiver does not receive ACK, the same set of packets is sent again. Upon reception of ACK, the sender moves on to the next set of packets. Even when one of packets sent in a group is lost, all of the packets are sent again, reducing the efficiency of the protocol. In this section, concepts from the ARQ protocol are modified for reliable wireless communication. The RF component on the Imote2 is in either a listening mode or a transmission mode. During transmission, the Imote2 cannot receive packets. However, in selective-repeat protocol, transmission and reception are deeply interwoven. The receiver may send an acknowledgment while the sender is in transmission mode. Packet loss and retransmission are expected to be more frequent if the ARQ protocol is implemented directly. Scheduling interwoven transmission and reception may result in long waiting times. In the proposed protocols, the sender transmits all of the packets before this node expects an acknowledgment packet. The receiver stores all of the received data in a buffer. Once the receiver gets a message indicating the end of data transmission, the receiver replies to the sender, indicating which packets are missing. Only missing packets are sent again. In this way, the number of acknowledgments and retransmissions can be greatly reduced. This protocol is designed to send either 64-bit double precision data, 32-bit integers, or 16-bit integers. Many ADCs on traditional data acquisition systems have a resolution less than 16 bits, supporting the need for transfer of 16-bit integer format data. Some ADCs have a resolution better than 16 bits, necessitating data transfer in 32-bit integer format. Once an acceleration record is processed, the outcome may need more bits. Onboard data processing such as FFTs and SVDs is usually performed using double precision calculations. Even when the effective number of bits is smaller than 32, debugging of onboard data processing greatly benefits from transfer of double precision 64
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NSEL Report Series Report No. NSEL-
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The Newmark Structural Engineering
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Contents Page CHAPTER 1 INTRODUCTIO
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9.2.3 Power harvesting . . . . . .
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damage/deterioration is intrinsical
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scalability to a large number of sm
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logic circuit. However, FPGAs are m
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easons, smart sensors spatially dis
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Figure 2.1. EYES project sensor pro
Page 19 and 20: Table 2.1. Smart Sensor Specificati
Page 21 and 22: While a number of sensor boards for
Page 23 and 24: need to be within the coordinator's
Page 25 and 26: eing examined (Moore et al., 2001).
Page 27 and 28: structural damage, the analysis to
Page 29 and 30: that sensor installation cost inclu
Page 31 and 32: (2004) indicated that this accelero
Page 33 and 34: different type of sensors connected
Page 35 and 36: Chapter 3 SHM ARCHITECTURE This cha
Page 37 and 38: Modal operation Several modal opera
Page 39 and 40: its spatial gradient, which is clai
Page 41 and 42: Manager node Cluster head node Leaf
Page 43 and 44: commands; execution timing cannot b
Page 45 and 46: Table 3.2. Desirable Characteristic
Page 47 and 48: Table 3.2. Desirable Characteristic
Page 49 and 50: Figure 4.1. Foil strain gage. strai
Page 51 and 52: dB A s Figure 4.4. AA filter design
Page 53 and 54: 8.32F 3V 3.45F 3V Input 1K 1K 1K 1K
Page 55 and 56: Strain ( ) 80 w/o Digital Filter 60
Page 57 and 58: performance was experimentally vali
Page 59 and 60: anges from 10 to 20. If 50 percent
Page 61 and 62: Node 1 x1 i =1,2,…,ns E x 1
Page 63 and 64: Correlation function estimate (m 2
Page 65 and 66: Figure 5.5. Effect of data loss and
Page 67 and 68: Frobenius norm. Because only a limi
Page 69: 90 80 70 60 50 Node1 Node2 Node3 No
Page 73 and 74: Sender SendLData.send() Pack parame
Page 75 and 76: If there are missing packets, the r
Page 77 and 78: Sender SendLData.bcast() Pack param
Page 79 and 80: Sender Send a packet • Sender sen
Page 81 and 82: utilized in SHM applications follow
Page 83 and 84: where R xxref is a vector consistin
Page 85 and 86: Time (sec) 80 60 40 20 0 -20 -40 -6
Page 87 and 88: Difficulties encountered in realizi
Page 89 and 90: 1.8715 x10 5 #ofdatablocks 1.871 me
Page 91 and 92: Before this decimation is applied,
Page 93 and 94: where xn is the original signal,
Page 95 and 96: Timer firing every T3 110 data poin
Page 97 and 98: locks before or after the current b
Page 99 and 100: Chapter 6 ALGORITHMS In this chapte
Page 101 and 102: The eigenproblem of the system matr
Page 103 and 104: where M 0 is the mass matrix of the
Page 105 and 106: same community, eliminating the nee
Page 107 and 108: measurement or mass perturbation ca
Page 109 and 110: these algorithms are implemented on
Page 111 and 112: 10 0 10 -2 10 -4 0 500 1000 1500 20
Page 113 and 114: Table 7.1. SVD Accuracy Check on Ma
Page 115 and 116: 7.1.4 Complex matrix inverse A comp
Page 117 and 118: Figure 7.9. Details of the joint (G
Page 119 and 120: Figure 7.13. Amplifier (Gao, 2005).
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0.08 0.2 Acceleration (g) 0.04 0 -0
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3 Imote2_2/Imote2_1 Imote2_1/Siglab
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200 3 Phase of spectral densities (
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1.5 x10-17 Time (sec) Difference in
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input forces applied at nodes in th
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CH1 CH2 CH3 …. CHn-1 CHn Initiali
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1 2 Node ID 102 RETAKE 1 3 4 INCONS
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Initialization: within local sensor
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PC Base station Manager sensor Clus
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Table 7.7. Instructions in DCS Code
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EXPERIMENTAL VERIFICATION Chapter 8
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8 x10-3 Time (sec) Correlation func
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Table 8.2. Mass Normalization Const
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1 2 Node ID 67 RETAKE 0 3 4 ID 67 #
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for a SHM strategy does not necessa
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are saved. The 420 seconds listed a
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1 1 Normalized accumulated stress m
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Table 8.5. Summary of False Damage
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Table 8.7. Summary of False Damage
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e scalability, autonomous distribut
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The lowest frequencies of interest
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structural vibration is considered
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REFERENCES Actis, R. L. & Dimarogon
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Casciati, F., Faravelli, L, Borghet
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Feldman, M. & Braun, S. (1995). “
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Kling, R. (2003). “Intel Mote: an
Page 181 and 182:
Mufti, A. A. (2003). “Integration
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Sazonov, E., Janoyan, K., & Jha, R.
Page 185 and 186:
Yi, J.-H. & Yun, C.-B. (2004). “C
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