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Structural Health Monitoring of the Golden Gate Bridge

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Title: Structural Health Monitoring of the Golden Gate Bridge


1
Structural Health Monitoringof the Golden Gate
Bridge
  • Sukun Kim, David Culler
  • James Demmel, Gregory Fenves, Steve Glaser
  • Thomas Oberheim, Shamim Pakzad
  • UC Berkeley

NEST Retreat Jan 15, 2004
2
Structure Monitoring
Data Acquisition
Data Collection
Processing Feedback
3
Overview
  • Low cost structure monitoring - Monitor
    structure, and analyze the health of structure
    based on sensed data at low cost
  • For Golden Gate Bridge, monitor vibration of
    bridge, and detect unusual behavior by wind,
    earthquake, or local damage
  • Extend reach of Wireless Sensor Network in a
    different direction high fidelity sampling
  • High accuracy, high frequency with low jitter,
    large amount of data

4
Table of Contents
  • Overview
  • Data Acquisition
  • Accelerometer Board
  • High Frequency Sampling Jitter
  • Data Collection
  • Large-scale Reliable Data Transfer
  • Signal processing System Identification
  • Conclusion
  • Challenges Future Work

5
Accelerometer Board
  • Crossbow donates manufacture and test
  • On board 16bit ADC
  • Low-pass filter

6
Table of Contents
  • Overview
  • Data Acquisition
  • Accelerometer Board
  • High Frequency Sampling Jitter
  • Data Collection
  • Large-scale Reliable Data Transfer
  • Signal processing System Identification
  • Conclusion
  • Challenges Future Work

7
HighFrequencySampling
  • Made by David Gay
  • Up to 6.67KHz with 4 bytes sample
  • MicroTimer Supports one timer, micro second
    level granularity
  • BufferLog Has two buffers. One is filled up by
    upper layer application while the other buffer is
    written to flash memory as a background task

8
Jitter Test (1KHz, 5KHz, 6.67KHz)
  • Peak to Peak is time to fill up buffer
  • Spiky portion is time to write buffer to flash
  • Can sample as long as the former is larger than
    the latter

9
Jitter Test Histogram(1KHz, 5KHz, 6.67KHz)
  • Jitter is within 10µs
  • Peak at 625ns Wakeup time from sleep mode

10
Jitter Analysis
11
Table of Contents
  • Overview
  • Data Acquisition
  • Accelerometer Board
  • High Frequency Sampling Jitter
  • Data Collection
  • Large-scale Reliable Data Transfer
  • Signal processing System Identification
  • Conclusion
  • Challenges Future Work

12
Large-scale Reliable Data Transfer
  • 4Byte of data and 4Byte of time stamp at 100Hz in
    100 nodes, transfer 40pkt/s Sample data for 5
    minutes, and collect data for more than 5
    hours!!!
  • Efficient and reliable data transfer is crucial
  • RAM to RAM one-hop transfer is implemented as a
    building block - LRX

13
LRX component (continued)
  • Explicit open handshake - Data description and
    size of cluster is sent as a transfer request
  • Data transfer is composed of multiple rounds. In
    each round, sender sends packets missing in the
    previous round
  • Tear-down is implicit

14
  • Throttle for data packet is fixed at 10 pkt/s
  • Optimal case window size is infinite
  • For the case with window size 16, throughput is
    88 of optimal case.
  • Considering loss rate of 3, actual relative
    throughput is 91, which is higher than 85 of
    channel utilization ratio. This is because
    control packets do not follow 10 packets/s.

15
  • As loss rate increases, retransmission increases,
    and throughput decreases

16
Channel Utilization
  • LRX (data only) is the theoretical limit of LRX
    (when window size is infinite)
  • Usage LRX lowers channel utilization by 15

17
Table of Contents
  • Overview
  • Data Acquisition
  • Accelerometer Board
  • High Frequency Sampling Jitter
  • Data Collection
  • Large-scale Reliable Data Transfer
  • Signal processing System Identification
  • Conclusion
  • Challenges Future Work

18
Signal Processing
  • As an analog signal processing low-pass filter is
    used, which filters high frequency noise
  • For accelerometer board, low-pass filter with
    threshold frequency 25Hz is used. Then ADC should
    sample at frequency much higher than 50Hz by
    Nyquist theorem, and imperfect low-pass filter
  • As a digital signal processing, averaging is
    used. If noise follows Gaussian distribution, by
    averaging N numbers, noise decreases by a factor
    of sqrt(N)

19
System Identification
  • Identifying model of target system
  • By matching input to system and output from
    system, we can construct a mathematical system
    model.
  • Usual process is (1) fitting a general
    Box-Jenkins multi-input multi-output model to
    sampled data. (2) And natural frequencies,
    damping ratios and mode shape are then estimated
    using the estimated Box-Jenkins model.
  • Most part of system identification is under
    development on civil engineering side.

20
Table of Contents
  • Overview
  • Data Acquisition
  • Accelerometer Board
  • High Frequency Sampling Jitter
  • Data Collection
  • Large-scale Reliable Data Transfer
  • Signal processing System Identification
  • Conclusion
  • Challenges Future Work

21
Conclusion
  • New challenges are analyzed which are brought by
    structure monitoring to wireless sensor network
  • High accuracy accelerometer, high frequency
    sampling with low jitter, low-pass filter,
    averaging, large-scale reliable data collection

22
Table of Contents
  • Overview
  • Data Acquisition
  • Accelerometer Board
  • High Frequency Sampling Jitter
  • Data Collection
  • Large-scale Reliable Data Transfer
  • Signal processing System Identification
  • Conclusion
  • Challenges Future Work

23
Challenges Future Work
  • Calibrating acceleration value to temperature
  • Time synchronization RBS, TPSN
  • To maximize utility of channel, we need to
    monitor channel quality (loss rate), and throttle
    packet injection rate accordingly
  • Using LRX as a building block, multi-hop data
    collection need be implemented
  • TASK

24
At the Golden Gate Bridge
25
Questions
  • http//www.cs.berkeley.edu/binetude/ggb

26
Backup Slides
27
Cost Comparison
  • Conventional piezoelectric accelerometer with PC
    system costs 40,000
  • Budget for structure monitoring budget is
    1,000,000 level
  • Wireless sensor network with MEM accelerometer
    costs 500
  • Cheaper by a factor of 100

28
Shaking Table Test
  • Silicon Design 1221L is more quite, but less
    sensitive to dynamic movement

29
Noise Floor Test
  • Blue Seismic Vault
  • Red McCone Hall

30
Jitter Analysis (continued)
  • T(i) execution time of atomic section i
  • X(i) a random variable uniformly distributed in
    0, T(i)
  • C context switch time
  • F(i) frequency of occurrence of atomic section i
  • Assume that the probability of timer event
    occurring at any point in atomic section i is
    same, then jitter will follow CX(i).
  • Since jitter distribution of every atomic section
    begins from C, the frequency is highest near C
    and decreases as moving farther. And frequency
    drop at CT(i) by F(i), since atomic section i
    will not have any distribution beyond CT(i).
  • Actually there is a peak at C, because when
    program is in preemptible section, it will
    immediately service timer event after context
    switch time C.

31
Calculation of Transfer Timer
  • Let us assume each node store 4Byte of data and
    4Byte of time stamp at 100Hz. And assume there
    are 100 nodes, radio throughput is 1.2KB/s, and
    data is collected to one base station. If
    acceleration data worthy 5 minutes is collected,
    each node will transfer 240,000Bytes. 100 nodes
    will transfer 24,000,000Bytes. Since the end link
    to base station is a bottleneck, it will take
    more than 5 hours. We can see bandwidth is narrow
    compared to aggressive data sampling. Even if we
    alleviate this problem using multi-channel or
    multi-tier network, still we will be in short of
    bandwidth.

32
LRX component
  • Transfers one data cluster, which is composed of
    several blocks.
  • One block fits into one packet, so the number of
    blocks is equal to window size.
  • Each data cluster has a data description. After
    looking at data description, receiver may deny
    data (receiver already has that data, or that
    data is not useful anymore).

33
Sender
34
Receiver
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42
Why Sender times out
  • There are two reasons why only sender times out
    and stimulate receiver for Ack. The first reason
    is shown in Figure 16. If sender doesnt time
    out, for a receiver to make sure Ack is delivered
    to sender, receiver should get acknowledgement
    from sender for Ack itself. This is not good. So
    it is clear that sender should timeout. Given
    that sender times out, timeout of receiver makes
    no difference except that channel is wasted by
    unnecessary Ack from receiver. So timeout in only
    sender side is desirable. As a second reason, if
    receiver times out, in case like Figure 18 (if
    first Data after Ack is lost), second Data always
    collide with resent Ack of receiver. This is not
    a good phenomenon. Therefore, after sending last
    packet in each round, if acknowledgement does not
    come, sender sends the last packet in that round
    again to stimulate acknowledgement. However, this
    does not mean receiver has no timeout. Receiver
    waits sufficient amount of time, and if nothing
    happens, it regards the situation as a failure.

43
Imperfect Low-pass Filter
44
Time Synchronization
  • Temporal jitter is handled by high frequency
    sampling component. Spatial jitter should be
    solved by time synchronization. ITP 8 is a time
    synchronization protocol widely used in Internet.
    In wireless sensor network, there were several
    studies. In RBS 9, synchronization is done
    among receivers, eliminating senders jitter in
    media access. TPSN 10 put time stamp after
    obtaining channel. This gives even better
    synchronization accuracy than RBS (10µs compared
    to 20µs). Still there is a source of jitter at
    receiver side. As we saw in jitter for sampling,
    handling interrupt by radio can be delayed by
    atomic section of other activity. As suggested in
    10, putting time stamp at MAC layer in receiver
    side will eliminate this jitter.

45
Table of Contents
  • Overview
  • Data Acquisition
  • Accelerometer Board
  • High Frequency Sampling Jitter
  • Data Collection
  • Large-scale Reliable Data Transfer
  • Signal processing System Identification
  • Conclusion
  • Challenges Future Work

46
Acknowledgement
  • This work is supported, in part, by the National
    Science Foundation under Grant No. EIA-0122599.
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