04-06-2012, 03:02 PM
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).
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. It also contains one
temperature sensor (reason will be explained later). New
version of Berkeley mote, named as Mica2 [1], is used for
control and communication.
Noise Floor Test and Shaking Table Test
To see static characteristic of accelerometers, accelerome-
ter board was put to quiet place (from vibration and sound)
with constant temperature. This test shows noise °oor which
is shown in Table 1. For Silicon Designs 1221L, range was
-0.1G 0.3G. Then to see dynamic behavior of accelerome-
ters, we performed shaking table test with constant temper-
ature. Even though test site was not completely free from
vibration and sound noise, it was quiet enough for a dynamic
range of shaking table to dominate noise. Results are shown
in Figure 3. Left ¯gure is result of ADXL 202E, right one
is result of Silicon Designs 1221L, and driving frequency is
0.5Hz. Data are read from both channels at the same time.
For this test, channel for Silicon Designs 1221L had range
of -2G 2G.
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. And it also shows reference reading
from very sophisticated accelerometer in the vault, which is
used for seismic research. System with Silicon Designs 1221L
shows 20dB higher noise level.