Structural Health Monitoring Using Smart Sensors - ideals ...

for a SHM strategy does not necessarily imply larger power consumption, the time can be
a rough indicator of power consumption. If nodes do not enter a sleep mode or turn off
radio components during monitoring, their power consumption correlates well with the
time of operation. With respect to monotoring interval, its frequency is limited by the
length of execution time; an SHM strategy that takes four hours to complete, for example,
can be performed at most only once in four hours. Considering monitoring of structures on
a weekly or monthly basis, Imote2s do not need to complete the SHM strategy in a short
time, e.g. one or two hours. However, monitoring in a short time is preferable to allow
multiple measurements before environmental conditions such as wind velocity and
temperature change. When damage detection results from neighboring sensor
communities are inconsistent with each other, sensing and damage detection need to be
repeated. If the SHM strategy takes a long time, environmental conditions may change
between monitoring intervals, making comparison among measurements difficult.
Therefore, SHM strategies requiring a short execution time are preferable.
In this section, the time required for execution of the DCS for SHM on Imote2s is
determined. The implementation includes numerous reports sent to the base station for
debugging purposes. Considering that these tasks can be omitted in the future, the time
necessary for the SHM strategy is estimated.
The breakdown of the time spent during DCS for SHM is listed in Table 8.3. At the
beginning of monitoring, parameter initialization requires about 70 seconds. Parameters to
be initialized include: the conversion matrix from nodal force to stress in elements, modal
parameters identified before damage, sampling rate, and duration of sensing. A sixsecond-long
time synchronization activity follows the parameter initialization. Sensing
continues for approximately 50 seconds. Note that the duration of sensing can be
increased if sufficient RAM is available for temporary storage of the measured data. At
the end of sensing, the manager sensor queries each node to see if sensing was successful.
Then the resampling process is applied to the measured data; resampling takes ten
seconds. The manager node again queries each node to ensure successful resampling. The
measured data is then reported to be the base station. The communication between a leaf
node and the base station is about five seconds, while communication between the base
station and a PC takes 12 seconds. Each Imote2 node reports three axes of measurement
data. This reporting is one of the tasks requiring significant time. Then the cluster heads
multicast their own data to the leaf nodes as reference signals. NExT parameters such as
the number of averages and the number of FFT points are also multicast. This multicast of
parameters and data takes only 33 seconds for three sensor communities. Then NExT is
applied on each node. Nodes which belong to multiple sensor communities estimate
multiple sets of correlation functions. The estimated correlation functions are sent to the
cluster heads and the base station. The base station then forwards the correlation functions
to the PC. This correlation function report takes more than 400 seconds. Finally, ERA is
applied to the correlation functions, and the DLV method determines the potential
locations of damaged elements. This data processing on the cluster heads takes 150
seconds. In total, DCS for SHM takes more than 23 minutes.
Among the tasks listed in Table 8.3, some of them can be omitted when debugging is
not necessary. The raw data does not need to be sent to the base station. About 500
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