System for detecting human presence in hazardous situations

• US 5,436,613 A
• Filed: 05/13/1993
• Issued: 07/25/1995
• Est. Priority Date: 05/03/1993
• Status: Expired due to Fees

• Current Assignee: Institut de Recherche EN Sante et An Securite Du Travail Du Quebec
• Original Assignee: Institut de Recherche EN Sante et An Securite Du Travail Du Quebec
• Inventors: Ghosh, Kalyan, Bose, Tapan K., Courteau, Raymond, Gibello, Pierre
• Primary Examiner(s): Peng, John K.
• Assistant Examiner(s): WONG, ALBERT KANG
• Application Number:
  US08/061,403
• Time in Patent Office: 803 Days
• Field of Search: 340/573, 340/124, 340/904, 323/219, 324/663, 324/666, 364/461, 395/90
• US Class Current: 340/573.1
• CPC Class Codes: F16P 3/147 using electro-magnetic technology, e.g. tags or radar
• CPC Class Codes: F16P 3/148 using capacitive technology
• CPC Class Codes: G08B 13/26 by proximity of an intruder causing variation in capacitance or inductance of a circuit

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.

Claims

1. 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_x, an impedance Z_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_r, a fixed voltage source S_f and a source of adjustable voltage S_a ;

said sources S_f and S_a producing a sinusoidal voltage at an operating frequency f_op ;

said known reference impedance Z_r said fixed voltage source S_f and said adjustable voltage source S_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_L between each arms of said bridge;

circuit balancing means or continuously adjusting the voltage produced by S_a to reduce said real and imaginary parts of V_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.

2. A system as defined in claim 1, wherein the adjustment of the voltage produced by S_a consists of a feedback loop of said balancing circuit means to define the following relationship:

##EQU5##

3. A system as defined in claim 2, wherein said vectorial detector means comprise vectorial detectors VD₂ and VD₃ to measure V_H and V_r and wherein said dissipation factor D is calculated using the equation:

##EQU6## where Z'"'"'_x and Z"x are respectively said real and said imaginary parts of Z_x.

4. A system as defined in claim 3, wherein said bridge is controlled by a digital controller.

5. A system as defined in claim 1, wherein said at least one electrode is placed near a moving machine component to be guarded and are supplied with radio frequency signals by said impedance bridge through a module of reference impedance containing a reference impedance, said module being in a fixed position relative to said at least one electrode and being connected to said bridge by way of flexible coaxial cables with grounded shields.

6. A system as defined in claim 1, including a module of reference impedance mounted on a body of a machine, said body being grounded and wherein an electrode on said machine is supplied with a radio frequency signal by said impedance bridge through said module of reference impedance containing a reference impedance and two transformers, said module being in a fixed position relative to said electrode and being connected to said bridge by way of five flexible coaxial cables with grounded shields.

7. A system as defined in claim 1, wherein said known reference impedance is a capacitor whose value is equal to the capacitance C_x of said at least one electrode.