E-field technology update

Electric field (E-field) sensors were introduced by Stellar Incorporated in 1975 and approved for DOE use by Sandia in 1977. Since then there have been numerous product changes and adaptations to utilize this technology to address a wide range of applications. All such products and product variants rely on the basic concept of measuring changes in capacitance between a "field" wire and a "sense" wire due to the motion of an intruder in the electric field of the wires. In all cases the biggest challenge is to differentiate between changes due to intruders and those due to environmental effects. This has lead to a number of insulator designs to try to minimize the effects of rain, snow, fog, salt spray etc. and various transformer-based balancing schemes to minimize the effects of the environment of the ground plane beneath the array of wires. Recently Senstar-Stellar engineers have embarked upon a complete rework of this technology taking advantage of field programmable gate array (FPGA) devices with high speed sigma delta analog to digital converters to extract more information from the array of wires and thereby optimize the detection process. Several features of this next generation of E-field technology described in this paper include: the ability to discern between capacitive and resistive changes, simultaneous independent measurement of the capacitance between each field and sense wire (for 4 and 8 wire arrays), notch filters to remove 50 and 60 Hz power grid noise, digital signal processing (DSP) to minimize the environmental effects on the ground plane and to differentiate between intruders approaching the sensor and those penetrating between the wires. Along with these electronically generated features the next generation product has a new insulator and support hardware design to further reduce the environmental effects and insect issues. The paper concludes with a discussion of how this technology can be utilized to address the post 9/11 threat at high security sites. While all perimeter security sensors have for years been designed to detect and delay intruders more and more emphasis is now being placed on the delay function. The stacked 8-wire barrier produced by this new design can go as high as 18 feet (5.5 meters) which certainly creates a signi- ficant intruder delay if the intruder is to penetrate the sensor undetected.