This paper discusses the development of an Unattended Ground Sensor based on an array of pressure sensors designed
to be buried in the ground. This sensor array, along with the required software (still under development), will have the
ability to distinguish between humans and animals based on the size and shape of the foot print. The technology may
also be applied to determine the weight and type of vehicle traveling on a road. The sensor array consists of pressure
sensitive resistors (piezoresistors) on 0.8 inch centers printed on a sheet of polyimide film. Although very large arrays
might one day be screen printed, the arrays for this study have been printed using a syringe dispenser and a precision x-y
computer controlled table. For the preliminary development, the array has been sized to 8X10 inches. The piezoresistive
properties of the sensors are discussed and preliminary test data is presented. It is shown that the piezoresistive gauge
factor (ΔR/R/ΔL/L) is roughly 10 times that of conventional metal strain gauges. Because the change in resistance is
large compared to metal strain gauges, lower cost electronics can be used. The small net size and low mass enables
sensing elements with fast response time. The fact that these piezoresistive elements are directly printed, as opposed to
being adhesively attached to a surface, eliminates many of the issues associated with bonded discrete sensors. It is
anticipated that the piezoresistive sensor approach presented in this paper will be well suited to extremely rugged
environmental conditions compared to the commercially available sensor arrays which rely on surface contact resistance
or capacitive sensors which can be easily destroyed by moisture. Environmental testing will be done in a future phase of
the project. The final system, which is still under development, will consist of a sensor array, information processing,
and RF signal transmission. The system is anticipated to be low cost and environmentally rugged.
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