Structural Health Monitoring Using Smart Sensors - ideals ...

commands; execution timing cannot be arbitrary controlled. This feature of TinyOS needs
to be well-considered when designing a system
3.2.3 Middleware services
Middleware services are developed to provide functionalities that TinyOS does not
offer but that are needed by SHM applications. The middleware services considered
herein are model-based data aggregation, reliable communication, and synchronized
sensing. Model-based data aggregation utilizes application-specific knowledge to
efficiently collect information from measured data in a network. Model-based data
aggregation can save communication resources and contributes to scalability of a smart
sensor network. The reliable communication service enables communication without loss
of information by sending packets repeatedly. This service addresses the problem of lossy
communication. Synchronized sensing utilizes a time synchronization service and obtains
synchronized signals from a network of smart sensors. Synchronized clocks on smart
sensors in a network does not mean measured signals are synchronized, because smart
sensors cannot necessarily control sensing task timing precisely based on their clocks.
This service contributes to accurate measurement in terms of sensing timing. These
middleware services are realized on the smart sensor platform.
3.2.4 Damage detection algorithm
The damage detection algorithm employed in this work is an extension of the
Distributed Computing Strategy (DCS) for SHM proposed by Gao (2005). Smart sensors
form local sensor communities. Local sensor communities measure acceleration responses
of a structure and perform modal analysis. Locally determined modal parameters are then
utilized to construct a portion of the flexibility matrix of the structure. Changes in the
flexibility matrix before and after damage are then analyzed with Singular Value
Decomposition (SVD) to estimate the Damage Locating Vectors (DLVs). DLVs are
applied to the numerical model of the structure as input static force, and elements with
small stress are identified as potentially damaged elements. This damage detection is
performed in each local sensor community. Cluster heads communicate with each other in
order to exchange information about damaged elements. The details of this approach are
explained later in Chapter 6. By distributing and coordinating data processing, the DCS
for SHM and the proposed extension offers a solution for SHM system employing a dense
array of smart sensors.
3.3 Summary
This chapter describes desirable characteristics for smart sensor SHM systems and the
SHM architecture employed in this report. The difficulties, desirable characteristics, and
approaches in this report are summarized in Tables 3.1 and 3.2. The next chapter
demonstrates customizability of sensor boards, which leads to availability of appropriate
sensors.
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