Structural Health Monitoring Using Smart Sensors - ideals ...

The estimated correlation functions are reported back to the cluster heads and base
station. While cluster heads need correlation function estimates from their leaf nodes, the
base station does not need the estimates except for the debugging purposes. Reporting to
the base station can be omitted once the system is fully developed.
Cluster heads then apply ERA to the correlation function estimates. Natural
frequencies and mode shapes are calculated. Based on these modal parameters, mass
normalization constants are estimated, or damage localization is performed. During
monitoring in the local sensor communities, the DCS logic is applied to the outcome of the
DLV procedure. The results are then sent back to the base station. Some of these results
are injected to cluster heads at the beginning of the next set of measurement. For example,
the estimated mass normalization constants are injected to the cluster heads at the
beginning of the monitoring process by the local sensor communities. This reporting is
also not necessary, once the system is fully developed. The outcome of one set of
measurements can be kept on RAM or nonvolatile memory for the next set of
measurements, eliminating the need for the parameter injection.
In the implementation of DCS for SHM, the frequency scalable feature of the Imote2
is utilized. The microprocessor on the Imote2 can operate at multiple frequencies. Because
the driver on TinyOS supports only the 13 and 100 MHz modes at the time, the
operational frequency is switched between only these two values. Numerical calculations
in NExT, ERA, and DLV, as well as sensing, are all performed at 100 MHz. Other tasks
such as communication are performed at 13 MHz.
All of the tasks in DCS for SHM need to be performed in the proper order by relevant
nodes. A 1-Byte variable containing instruction for the next task is utilized in organizing
all of the tasks. All of the communication packets have 1-byte variable to describe the
instructions. At the end of transmission and reception, this instruction is processed in a
task named “ProcessInstruction”. “ProcessInstruction” then looks for the block of codes
following the “switch” statement corresponding to the instruction value. Operations
described in the block of codes are executed. At the end of the block, new values can be
assigned to the instruction byte to execute the next task, or a communication packet
carrying new values for the instruction byte can be sent out. Components of DCS for SHM
explained in this chapter are synthesized in this manner. Table 7.7 summarizes the
instructions, associated operations, and information accompanying the communication
packet. The implementation on the Imote2 is summarized from Figures 7.26 to 7.30.
7.3 Summary
This chapter described the implementation of DCS for SHM on a network of Imote2s.
The validity of the system is numerically examined in a component-by-component
manner. In the next chapter, the Imote2s are installed on a three-dimensional truss and the
algorithms are experimentally verified.
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