Structural Health Monitoring Using Smart Sensors - ideals ...

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Coherence function
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Imote2- Imote2
Imote2- Siglab
Siglab - Siglab
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Frequency (Hz)
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(a) longitudinal
Coherence function
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Imote2- Imote2
Imote2- Siglab
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Frequency (Hz)
(b) transverse
Coherence function
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Imote2- Imote2
Imote2- Siglab
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Frequency (Hz)
(c) vertical
Figure 7.18. Coherence functions among Imote2s and reference sensors.
coherence function between the Imote2s is smaller than that between PCB accelerometers
around 50 Hz and above 80 Hz.
To investigate the sensitivity of the other axes of the Imote2 accelerometer, the
orientation of the Imote2 is changed. The vertical acceleration again seems to be accurate,
while the longitudinal acceleration shows noticeable differences among the signals. The
transverse acceleration measurements are not as accurate as the vertical acceleration
measurements. Similar to the previous experiment, the vertical acceleration is accurate
and the longitudinal measurement seems noisy. Therefore, the accuracy of the measured
accelerations does not appear to depend on the specific channel used on the Imote2.
There are several possible explanations for why the vertical acceleration is accurate,
while longitudinal acceleration appears noisy. First, consider the magnitude of the
accelerations. Because the truss’s motion is relatively small in the longitudinal direction
(see Figures 7.15 and 7.16), the signal-to-noise ratio (SNR) is small for longitudinal
measurements. However, similar experiments with smaller excitation also show that the
vertical acceleration is accurate even when signal’s strength is small; small SNR alone
cannot explain why the longitudinal measurements are noisy. Another possible
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