US 5,436,613 AGranted

System for detecting human presence in hazardous situations

AssigneeInstitut de Recherche en Sante et en Securite du Travail

InventorsKalyan Ghosh, Tapan K. Bose, Raymond Courteau, Pierre Gibello

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Abstract

The disclosure relates to a system for the detection of human presence in hazardous situations near moving machinery; appropriately shaped electrodes are placed in an area to be safeguarded and a radio-frequency signal of appropriate frequency and voltage is applied on the electrodes creating an alternating electric field around them. A bridge circuit is used to measure the capacitive impedance created by the electrodes and a dissipation factor is calculated from the real and imaginary parts of the impedance. If a foreign object is introduced into the electric field, the characteristics of the dielectric are modified creating a change of the value of the dissipation factor. The dissipation of energy in the high frequency region is much greater if this foreign object is a biological tissue as compared to other types of materials, such as conductors or insulators. Thus, the system detects human presence by noting the increase of the dissipation factor above a threshold value, and the movement of the appropriate machine part can then be stopped, thereby eliminating the danger of injury to the person.

Claims7

Dependent claims

A system for detecting the human presence in a given area, comprising:

an arrangement of at least one electrode located in said area defining a condenser having a capacitance C.sub.x, an impedance Z.sub.x and a dissipation factor D, whose values depend on the nature of the objects located in said area;
means in electrical connection with said arrangement of at least one electrode for applying a radio frequency signal to said arrangement of at least one electrode an alternating electric field around said at least one electrode whereby human presence in said field causes a change of the characteristics of the dielectric of said condenser;
impedance bridge means, in electrical connection with said arrangement of at least one electrode, including a known reference impedance Z.sub.r, a fixed voltage source S.sub.f and a source of adjustable voltage S.sub.a ;
said sources S.sub.f and S.sub.a producing a sinusoidal voltage at an operating frequency f.sub.op ;
said known reference impedance Z.sub.r said fixed voltage source S.sub.f and said adjustable voltage source S.sub.a being in electrical connection with each other and with said arrangement of at least one electrode;
vectorial detector means for measuring real and imaginary parts of the complex amplitude of a signal V.sub.L between each arms of said bridge:
circuit balancing means or continuously adjusting the voltage produced by S.sub.a to reduce said real and imaginary parts of V.sub.L to zero;
means measuring said dissipation factor D from outputs of said vectorial detector means,
including means for determining a human presence in said area when a change of said dissipation factor exceeds a predetermined threshold value.

6 more elements

Dependencies are inferred from the claim text.