Structural Health Monitoring Using Wireless Sensor Networks



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INTRODUCTION

Wireless sensor network enables low-cost sensing of envi-
ronment. Many applications using wireless sensor networks
have low duty cycle and low power consumption. However
the ability of wireless sensor networks can be extended in re-
verse way. Enhanced TinyOS, and new components opened
possibility for more aggressive applications. Structure mon-
itoring is one example of such applications. To monitor a
structure (e.g. bridge, building), we measure behavior (e.g.
vibration, displacement) of structure, and analyze health
of the structure based on measured data. Figure 1 shows
overall system. Each component can have multiple subcom-
ponents. In our case, sensor is accelerometer which will be
discussed in Section 2, and analog processing has low-pass
¯lter (Section 6.) Digital processing includes averaging (Sec-
tion 6), data collection (Section 5), and system identi¯cation
(Section 6). Low-jitter control contains high-frequency sam-
pling (Section 4).


RELATED WORK

Habitat monitoring is a leading application of wireless sen-
sor network. And it is an example application with low duty
cycle. ZebraNet[6] uses PDA-level device with 802.11b wire-
less network. Great Duck Island[8] uses Berkeley mote, and
watch ducks without disturbing them at low cost. For struc-
ture monitoring, there are tremendous amount of research
using conventional wired way. GPS was used combined with
wired data collection[10, 3], however at a high cost. There
is an approach using wireless network for data collection[2],
which has great advantage over wired network. However,
it uses large hardware platform (in terms of size, power,
and cost) which diminishes bene¯t of wireless approach.


DATA ACQUISITION

Data acquisition is composed of mainly two parts: data sam-
pling, and data collection. Structure monitoring requires
high ¯delity data sampling. Accurate, high frequency sam-
pling, and low jitter are main requirement for high quality
sample. Accuracy is discussed in this section, and high fre-
quency sampling with low jitter will be covered in Section 4.
And data collection will be discussed in Section 5. In struc-
ture monitoring, acceleration signal is very week. Detecting
even moderate earthquake requires to measure 500G accel-
eration.



Accelerometers

It has two kinds of accelerometers: ADXL 202E, Silicon
Designs 1221L. Table 1 shows characteristics of each ac-
celerometer combined with entire system. Accelerometer
board contains 1 of ADXL 202E, and 2 of Silicon Designs
1221L, and 4 16bit analog to digital converter (ADC). There
are two channels for ADXL 202E, and two channels for Sil-
icon Designs 1221L with same orientation. One is parallel
to gravity, and the other is vertical to gravity. Initially both
accelerometers had range of -2G 2G, but for better sen-
sitivity, range of Silicon Designs 1221L is change to -0.1G
0.1G. Channel with axis parallel to gravity has 1G o®set
to compensate for o®set by gravity.


Tilting Test and Vault Test

To measure linearity of accelerometer value, we performed
tilting test with help of Bob Uhrhammer. By changing tilt-
ing degree of accelerometer, we can obtain line showing ac-
celeration value read versus real acceleration. Only channel
vertical to gravity is measured of Silicon Designs 1221L. For
this test, range was -0.1G 0.3G. Deviation from minimum
mean square error line is within 60G For a better noise °oor
test, we went to a vault in Lawrence Berkeley Laboratory.
Figure 5 shows how quiet inside of vault is compared to nor-
mal o±ce environment.


HighFrequencySampling component

This component is written by David Gay for sampling at
KHz level frequency. Pre-existing components can sample
only up to 200Hz. There are two major sub-components
which enable high frequency sampling. MicroTimer is a new
timer component which directly accesses hardware timer,
and does not provide multiple abstract timers. This is very
simple and quick to process timer events. Bu®erLog is a
°ash memory writer. It has two bu®ers. One is ¯lled up
by upper layer application while the other bu®er is written
to °ash memory as a background task.