Multifunctional Materials - Biomimetics: Biologically Inspired Technologies - page 335

braids may be woven with further reinforcing fibers to form composite fabric, or they may be placed
between the layers of a laminate composite. Conventional resin transfer molding techniques such as
VARTM may then be used to wet the composite fibers with thermoset polymer. Care must be taken
in this step to avoid damage to the sensors due to excess pressure and in some cases temperature, if
curing and/or postcuring of the polymer matrix at elevated temperature were necessary.

12.2.4.4 Data Management

As the number of sensor nodes increases, and as the data load at each sensor increases, data
management becomes a key issue. If data were processed and analyzed external to the embedded
network, the bandwidth needed to transport the data would grow to unrealizable levels for any
reasonably sized structure. The computational requirements whether in- or ex-network become
intractable as well. The size of the envisaged network of embedded sensors, even for a moderately,
sized structure, will likely generate immense quantities of data that must be coordinated, inter-
preted, and acted upon. While optimal data fusion algorithms have been developed for small,
typically unconstrained, networks of sensors (Waltz and Llinas, 1990; Goodman et al., 1997;
Varshney, 1997), less is known about data fusion in networks of large numbers of sensors.
Localized processing algorithms provide a solution for the management of large data sets, and
are appropriate for sensorized composites. In local processing algorithms, some primitive process-
ing capability must be introduced to each sensor node, such that decision making in part becomes
controlled by the local elements. The local processing scheme is entirely compatible with the low
power, low bandwidth networks that can be scalably embedded in materials. Processing schemes
from communication theory (Middleton, 1960; Gabrielle, 1966), specifically the FFT and decoding
large block length codes, suggest that efficient and near optimal solutions can be achieved with
local, hierarchical algorithms. The alternative to local processing, for example, extracting all data
and performing external processing, is a computationally infeasible procedure (NP-complete);
large-sized data sets cannot be used optimally (Tsitsiklis and Athans, 1985).
Any proposed method of data handling or processing will not be specifically applicable to all
applications. The nature of the data or problem will generally dictate the type of solution needed.

12.2.4.5 Preliminary Results

We have undertaken some preliminary work to verify the feasibility of embedding sensors in
composites. An early issue is whether the proposed sensors, as well as interconnects, can survive
composite processing. As an initial test case, we embed sensors into a layup of thermally cured
prepreg composite.
The DS18B20X 1-Wire digital thermometer was chosen as a demonstration sensor. 4 The
DS18B20X has a 1.32 by 1.93 mm footprint, and is 0.6 mm high. A microprocessor was programmed
to communicate over the micro-network. A planar substrate was chosen for the initial demonstration.
The substrate was a conventional fiberglass-based printed circuit board material (FR4), 0.010 in.
thick, 0.080 in. wide and 6 in. long with a 2 oz. Cu foil overlayer. Strips with the device land pattern
shown in Figure 12.30 were fabricated using a rapid PCB prototyping numerical controlled milling
machine. 5 While 1-Wire devices can operate in a two conductor ''parasitic'' mode, a more robust
three-wire configuration was selected to allow the power and communications to have separate lines,
sharing a common ground. Convection hot gas solder reflow was used to connect to the substrate.
Following preliminary electronic testing of the device, it was embedded in a glass fiber-
reinforced epoxy composite. Style 7781 E-glass fabric was preimpregnated with BT-250 epoxy
resin supplied by Bryte Technologies, Inc. The substrate with attached sensor was embedded in the
middle of eight layers of prepregged material. The layup was consolidated at 250 8 F for 1 h under
50 psi pressure according to the material processing specifications. The connecting wires were
passed through a silicone sealing tape that protected them from excess resin during the curing