Structural Health Monitoring Using Smart Sensors - ideals ...

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Phase of spectral densities (degree)
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Figure 7.19. Phase of cross spectral densities.
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explanation concerns crosstalk. The crosstalk specification for the Imote2 accelerometer is
a maximum of 2 percent, while that of the PCB accelerometer is 5 percent. Because
vertical acceleration is much larger than longitudinal acceleration, measured acceleration
signals for the longitudinal direction may contain substantial crosstalk. If the actual value
of the crosstalk varies from sensor to sensor within the maximum bounds, the measured
longitudinal acceleration may differ from sensor to sensor; thus, each sensor may have a
different contribution from the acceleration in the perpendicular axes. Further
investigation may confirm this conjecture.
Time synchronization of measured signals is now examined. Six Imote2s are placed
on structural nodes to measure acceleration response. Phase of cross-spectral densities
among these signals is considered to indicate the accuracy of time synchronization.
Synchronized signals are expected to have a phase of zero, while synchronization error
results in linear phase. The larger the error is, the steeper the slope of phase is. Figure 7.19
shows the phase of the cross-spectral densities between the reference node signal and
signals from the other five nodes. The phase is almost flat over the frequency range below
100 Hz. Theoretically, the phase is one degree at 100 Hz if the signals have a
synchronization error of 28 s. The phase indicates that the synchronization of the
measured signals is approximately 30 s.
7.2.2 NExT
Natural Excitation Technique (NExT) is implemented on the Imote2 using the modelbased
data aggregation strategy described in section 5.1.2. The validity of this
implementation is examined by analyzing the same set of data both on a group of Imote2s
and on the PC.
The data to be analyzed is the acceleration response of the three-dimensional truss
structure. The data analyzed by Gao (2005) is converted to integers by shifting, scaling,
and rounding. Assuming the usage of a 16-bit ADC with approximately 12 effective bits,
the conversion maps the maximum data value to about 1,500 and the minimum value to
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