Structural Health Monitoring Using Smart Sensors - ideals ...

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Correlation Coefficient
0
-1
-4.8 -4.7 -4.6 -4.5 -4.4
Time Shift (sec)
Figure 7.14. Correlation coefficients plotted against time shift.
keeping the stopband cutoff frequency lower than the Nyquist frequency and completely
eliminating the aliasing components, the cutoff frequency is set so that the aliasing
components are above 100 Hz. For example, when sampling frequency is converted from
560 to 280 Hz with the passband cutoff frequency of 100 Hz, the stopband cutoff
frequency is set to 180 Hz instead of 140 Hz. Note that the actual sampling frequency of
the sensor board may have 10 percent variation and is not exactly 560 Hz. The stopband
cutoff frequency normalized to the upsampled sampling rate is set to be smaller than 180/
560L, allowing the variation in the denominator. L is the upsampling factor. Likewise, the
normalized passband cutoff frequency is set to be larger than 100/560L. In this way, a
filter allowing aliased components outside of the frequency range of interest is designed
with fewer filter coefficients than a filter completely eliminating aliased components.
The length of the measured data is subject to the RAM size limitation. The sensing
function on the Imote2 first stores data in the on-board RAM; the size of RAM limits the
sensing duration. For example, a three-axis acceleration measurement using a 16-bit data
type for each axis and having 11,264 data points occupies about 67 kB of RAM. Because
sensing is first performed at a higher sampling frequency and then downsampled, the
RAM originally occupied by the measured acceleration data is about 134 kB. 256 kB of
RAM on the Imote2 does not allow continuous measurement of acceleration for an
indefinite amount of time. Once the memory space becomes full, sensing is stopped, the
stored data is copied to Flash memory, and sensing can be started again.
Acceleration signals from the Siglab spectrum analyzer and Imote2s are compared
with each other to examine the sensing capability of the Imote2. Because the sampling
frequency of the reference signals is different from that of Imote2 signals, which is set at
280 Hz, the reference signals are resampled to 280 Hz. The resampling algorithm
described in section 5.3 is utilized. Also, the Siglab system is not synchronized with
Imote2s, making direct comparison of the two time domain signals difficult. Signals from
the two data acquisition systems are synchronized using the resampling algorithm
discussed previously. To estimate the offset, the reference sensor signals are shifted on a
time axis by a specified offset and the correlation coefficient between the two signals is
calculated. Figure 7.14 provides a plot of the correlation coefficient against the time shift.
The offset between the two sets of clocks is determined as the time shift giving the
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