Structural Health Monitoring Using Smart Sensors - ideals ...

4
set value: 560 Hz
3
Count
2
1
0
520 530 540 550 560 570 580 590 600 610 620
Sampling frequency (Hz)
Figure 5.22. Sampling frequencies of 14 sensor boards.
(Frequency calibration was conducted by Jennifer A. Rice.)
were calibrated on a shake table (see Figure 5.22). Differences in the sampling frequencies
among the sensor nodes result in inaccurate estimation of structural properties unless
appropriate post processing is performed. If signals from sensors with nonuniform
sampling frequency are used for modal analysis, one physical mode may be identified as
several modes spread around the true natural frequency (Nagayama et al., 2006a).
4. Fluctuation in sampling frequency over time
A nonconstant sampling rate was observed with the Imote2 sensor boards, which if
not addressed, results in a seriously degraded acceleration measurement. The Imote2
receives the digital acceleration signal once a block of data is available from the
accelerometer. The block size is set at 110 data points. The Imote2 puts a time stamp on
the data once a block is available. By comparing differences between two consecutive
time stamps, the sampling frequency of the accelerometer is estimated. The difference
between the time stamps, shown in Figure 5.23, provides an indication of the variation in
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