Structural Health Monitoring Using Smart Sensors - ideals ...

Figure 7.13. Amplifier (Gao, 2005).
Eight-pole elliptical AA filters are employed for both the input and output measurements
with a cutoff frequency of 100 Hz. The sampling rate is 256 Hz. Other equipment used
during the testing includes:
• Amplifier: A Sony STR-D315 amplifier (see Figure 7.13) uses the input signal from
the Siglab spectrum analyzer to drive the magnetic shaker.
• Signal conditioner: PCB four-channel signal conditioners (model 441B104) have
been used in this experiment.
Using the synchronized sensing middleware service, the Imote2 is programmed to
capture acceleration responses of the truss. The LIS3L02DQ accelerometer
(STMicroelectronics, 2007) on the Imote2 sensor board applies an AA filter and yields
digital outputs. The cutoff frequency of the AA filter is fixed with respect to the sampling
frequency (see Table 7.2). Because the structural modes of the truss whose natural
frequencies of interest are as high as 100 Hz need to be analyzed, the sampling frequency
is set as 560 Hz. After measurement, signals are resampled at 280 Hz, with the resulting
usable bandwidth of the signal being a little over 100 Hz.
Table 7.2. Cutoff and Sampling Frequencies of LIS3L02DQ Accelerometer
Decimation factor Cutoff frequency (Hz) Sampling frequency (Hz)
128 70 280
64 140 560
32 280 1120
8 1120 4480
The digital lowpass filter employed in the resampling approach is designed using
Matlab (The Mathworks, Inc., 2007). The passband cutoff frequency needs to be higher
than the frequency range of interest, i.e. 100 Hz, while the stopband cutoff frequency is
usually lower than the Nyquist frequency of the resampled signals. The filter design with
these two cutoff frequencies being much lower than the sampling frequency of the
upsampled signal and close to each other requires a large number of filter coefficients. To
alleviate this filter design difficulty, the stopband cutoff frequency is increased. Instead of
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