Structural Health Monitoring Using Smart Sensors - ideals ...

While a number of sensor boards for the Berkeley Mote have been designed for
generic applications and are commercially available (see Table 2.1), available sensors are
not optimized for use in civil infrastructure applications. Acceleration and strain are
among the most important physical quantities to judge the health of a structure. While
acceleration measurements are essential to obtain global responses of a structure,
structural strains provide an important indicator of local structural behavior. Studer and
Peters (2004) demonstrated that multiscale sensing yields better results than single-scale
measurements for damage identification. Available sensor boards have accelerometers,
though their applicability to civil infrastructure is limited. Ruiz-Sandoval (2004)
examined an acceleration sensor board, the MTS310CS, from the civil engineering
perspective; he found the accelerometer's performance is deficient, particularly in the low
frequency range, and developed a new sensor board, 'Tadeo,' with high sensitivity and low
noise level. The sensor boards of the Berkeley Mote can be conveniently customized
according to the applications.
The Berkeley Mote uses a open-source operating system, TinyOS, which is available
online (http://www.tinyos.net). TinyOS is a component-based operating system designed
for sensor network applications on resource-constrained hardware platforms, such as the
Berkeley Mote. More specifically, it is designed to support concurrency intensive
operations such as are required by networked sensors with minimal hardware
requirements; TinyOS fits in about 4 kB of memory space. TinyOS is, however, not a realtime
operating system. TinyOS has only two levels of priority, each of which corresponds
to hardware interrupt and tasks. Users cannot assign priority arbitrarily to specific tasks or
assume highly precise scheduling. This limitation needs to be well considered in designing
a system using TinyOS.
A wide community uses TinyOS to develop and test various algorithms and
protocols. The operating system has been ported to over a dozen platforms employing
numerous sensor boards. The autonomous characteristics of the smart sensor can be
realized by developing programs under TinyOS and then running these programs, along
with the operating system, on the on-board microprocessor. Similar software for the
Berkeley Mote and other platforms includes MANTIS (Bhatti et al., 2005) and EmStar
(Girod et al., 2004). Because of the open nature, numerous researchers have contributed to
TinyOS enhancement; users can take advantage of the wealth of previous studies.
Open-interface smart sensor platforms developed by companies, such as Ember
(2007) and Intel (2007), have similar potential. Users have access to the programming
interface and can program the sensor nodes as needed. Ember nodes must be programmed
on their own OS, but users can implement necessary functionalities through an open
interface. Ember proposed its own algorithm to construct the mesh-network. Though the
ready-to-use mesh networking is advantageous for some applications, the patented nature
of networking may result in an inflexible network system design. Intel recently developed
the Intel Imote2, which runs TinyOS or Linux (Adler et al., 2005). The Imote2 provides
enhanced computation and communication capabilities that allow resource-demanding
sensor network applications, such as SHM of civil infrastructure, to be supported, while
low-power operation and small physical size are still among the objectives. The wealth of
studies using TinyOS can be implemented directly to the Imote2; this commercial open-
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