Structural Health Monitoring Using Smart Sensors - ideals ...

Correlation function (g 2 )
8 x10-3 Imote2
Matlab
4
0
-4
Figure 7.20. Correlation function estimates.
0 1 2
Time (sec)
about -1,500. Note that the full range of a 12-bit signed integer is from -2,048 to 2,047.
The integer data is then injected to the group of Imote2s using the reliable communication
protocol for 16-bit integer data type described in section 5.2.3.
The acceleration response data is first split into several blocks and then injected. A
block of 11,264 data points is injected to a node at a time, and this injection is repeated
three times to each node. Eventually each node receives 33,792 data points per sensing
channel. Dividing time histories into several blocks is incorporated to simulate the Imote2
sensing process, which must be performed repeatedly, as explained in section 7.2.1, to
collect long records of data.
NExT is performed on the data injected to the Imote2s corresponding to nodes 2 to 7
(see Figure 7.7). Node 4 is designated as the cluster head, organizing distributed
correlation function estimation. The number of data points used in the FFT calculations is
1,024, resulting in 21 time averages in estimation of the spectral densities for the 11,264
point data records and 63 time averages for the 33,792 point data records. Then the
spectral densities are converted to correlation functions by the inverse FFT. Figure 7.20
shows the cross-correlation function between the vertical acceleration responses at nodes
3 and 4 (see Figure 7.7) calculated on the Imote2 as well as that calculated on the PC. The
estimate from the PC is overlaid by the estimate from the Imote2. Note that only the first
256 data points, or one second, of the correlation function estimate on the Imote2 is
reported to the base station and the cluster head to reduce communication requirements.
The subsequent modal analysis using ERA usually utilizes only a portion of the
correlation function, eliminating the need to transfer the entire correlation function. Figure
7.21 shows the difference between a correlation function estimated on the Imote2 and the
corresponding correlation function estimated on the PC. From these figures, NExT on the
Imote2 is considered to be numerically equal to that on the PC with the precision of a
double data type.
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