Structural Health Monitoring Using Smart Sensors - ideals ...

The lowest frequencies of interest for the truss structure used in the experiments were
around 20 Hz, and the Imote2 sensing capability was shown to be sufficient for the DCS
around these natural frequencies. However, natural frequencies of full-scale buildings and
bridges are usually below 10 Hz. Three-dimensional sensing characteristics need to be
examined in the low frequency range.
Future smart sensors used for ambient vibration measurement need to have better
resolution. The least significant bit of the digital output of the Imote2 accelerometer is
about 1 mg. Therefore, the resolution is at best 1 mg. While this resolution is sufficient for
this research, ambient vibration measurement of civil infrastructure needs better
resolution. An accelerometer chip with a low noise level and higher resolution is needed
for ambient vibration measurement; if the accelerometer does not have digital outputs, the
ADC also needs to have high resolution.
While smart sensors are often equipped with accelerometers, other types of sensors
often utilized in civil engineering applications need to be developed for the Imote2.
Though a strain sensor board for the Berkeley Mote has been developed, the Imote2 does
not have a strain sensor board. A strain sensor board for the Imote2 will be beneficial to
civil engineers.
Another problem is failure in sensing. Under the current design of the system, all of
the sensors repeat sensing if one or more sensors fail to start sensing. The system can be
modified so that sensing failure of a node makes only the sensors in the same community
retake data. Even though the expected number of repetitions can be reduced in this way,
this repetition should preferably be avoided by decreasing the chances of sensing failure.
9.2.2 Damage detection capability
The damage detection algorithms sometimes fail to detect damage. Repeated false
damage detection was observed, though previous work on the DLV method did not report
such false detection. Because the DCS for SHM on simulated numerical truss responses
does not show false-negatives or repeated false-positive damage detection, such false
damage detection in the Imote2 experiments is considered to be due primarily to causes
specific to smart sensors. One possible cause is observation noise from imperfect
measurements. As stated before, sensing hardware for the Imote2 still has room for
improvement. Inaccurate sensing results in inaccurate damage detection. Another possible
cause is a discrepancy between the physical truss and the model assumed in DCS for
SHM. NExT, ERA, and the DLV methods all assume a linear model. If linearity is not
satisfied, damage detection may become faulty. Errors in the numerical truss model used
in the stress analysis possibly introduced inaccuracy in the damage detection, too. Before
the application of the SHM system to the full-scale structures, the conditions under which
damage detection becomes faulty need to be clarified, and the reliability needs to be
improved.
The influence of other factors, such as temperature and humidity, should be
accommodated in the damage detection strategy. For example, natural frequencies of a
structure change along with temperature. The effects of temperature and humidity changes
on the DLV methods have not yet been studied.
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