Structural Health Monitoring Using Smart Sensors - ideals ...

are saved. The 420 seconds listed at task 17 can be reduced by a factor of two.
Furthermore, only the beginning portion of the correlation function is utilized to construct
the Hankel matrix in Eq. (6.8), while the current system reports the entire correlation
function to the base station and cluster head nodes. Therefore, the report to the cluster
heads can be even shorter, though its contribution toward shortening the SHM execution is
not counted here. Subtracting the time spent for these debugging tasks from the 1,402
seconds makes the total time for the SHM strategy approximately 676 seconds; the usage
of 400 MHz mode of the CPU may further shorten the time necessary to execute the SHM
strategy.
In summary, about one minute of sensing and 9 minutes of post processing is
necessary for the current SHM system without sending debugging information to the base
station. Note that the time needed for sensing may increase if natural frequencies of a
structure appear in a lower frequency range. As for the time for post processing, the time
does not increase as the number of sensor community increases. Though tasks 12, 14 and
17 in Table 8.3 seem to be proportional to the number of sensor communities, they are
proportional only when the number of communities is small. When one cluster is out of
the communication range of another cluster, these two clusters can perform the tasks in
12, 14, and 17 simultaneously. Monitoring damage on a large structure requiring only
about nine minutes of data processing in addition to sensing time is an appealing feature of
this approach.
8.7 Battery life
The battery life of the Imote2 while running the proposed SHM strategy
implementation is estimated in this section. When Imote2s performed sensing, the change
in the supply voltage to the Imote2s is recorded. In contrast to the previous section,
sensing was repeated four times in this experiment. That is, tasks 2 to 14 in Table 8.3 were
repeated four times. The voltage values of five Imote2s are read after parameters are
initialized, after the first sensing is performed, after reporting to the base station is
completed, after the second, third, and fourth measurement finishes, and at the end of
ERA and DLV method applications.
Figure 8.12 shows the change in the supply voltage to the Imote2s. One round of
measurement consumes about 0.03 to 0.08 V. Note that sometimes sensing fails and is
repeated until successful measurement is performed. When repetition takes place many
times, the decrease in the voltage is considered significant. If sensing becomes more stable
with less sensing failures, the number of repetitions will decrease reducing the power
consumption per round of sensing.
While Imote2s repeat sensing four times for the data shown in Figure 8.12,
monitoring of full-scale structures may not need repeated measurements, thus reducing the
power consumption per monitoring event. The advantage of repeating sensing is to reduce
the effect of observation noise by increasing the number of averages in the spectral
densities in Eq (5.1). The SHM implementation explained in sections 8.2 to 8.5, as well as
that in section 8.8, measures acceleration responses of the truss only once, yielding 21
averages. From the damage detection results shown in section 8.4, 21 averages seem to be
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