OPEN-STRIP FERROMAGNETIC MARKER AND METHOD AND SYSTEM FOR USING SAME

• US 3,790,945 A
• Filed: 11/24/1971
• Issued: 02/05/1974
• Est. Priority Date: 03/22/1968
• Status: Expired due to Term

• Current Assignee: Edward R. Fearon
• Original Assignee: Edward R. Fearon
• Inventors: Fearon, Edward R.
• Primary Examiner(s): Trafton, David L.
• Application Number:
  US05/201,686
• Time in Patent Office: 804 Days
• Field of Search: 340/258R,258C,280,224 325/8,105 343/6.5SS,6.8,787,788 179/82
• US Class Current: 340/572.2
• CPC Class Codes: G01S 13/753 using frequency selective elements, e.g. resonator
• CPC Class Codes: G06K 7/10009 sensing by radiation using wavelengths larger than 0.1 mm, e.g. radio-waves or microwaves
• CPC Class Codes: G08B 13/2408 using ferromagnetic tags
• CPC Class Codes: G08B 13/2431 Tag circuit details
• CPC Class Codes: G08B 13/2437 Tag layered structure, processes for making layered tags
• CPC Class Codes: G08B 13/2442 Tag materials and material properties thereof, e.g. magnetic material details
• CPC Class Codes: G08B 13/2474 Antenna or antenna activator geometry, arrangement or layout loop antennae H01Q1/22

Abstract

The specification describes an open-strip marker for use in detecting the presence of an object in an interrogation zone. Each object to be detected is provided with a marker comprising an elongated ferromagnetic element having a high permeability and a high ratio of length to cross-sectional area. A periodically varying magnetic field is provided in an area through which the object is to pass. Upon passage of the object through the area, reversal of the magnetization of the marker results in a uniquely characteristic signal for each alternation of the electromagnetic field. This characteristic signal is sensed to distinguish the presence of the object.

Claims

1. A system for sensing the passage of an object through a surveillance area comprising:

means for generating an oscillating magnetic field in said surveillance area, an elongated thin open-stripe of ferromagnetic material for being secured to an object and having an aggregate saturation magnetic moment of about one pole-centimeter and producing at least one pulse of magnetic field having frequencies of very high order, extending up to and including the thousandth harmonic of the fundamental frequency of the oscillating magnetic field in response to a reversal of said field.

2. The marker of claim 1 wherein said generating means generates an oscillating magnetic field having a peak intensity of at least 3 oersteds.

3. A marker for use in a system for detecting the presence of an object within an interrogation zone wherein magnetic coils generate a magnetic field periodically varying at a predetermined fundamental frequency in the interrogation zone and a sensor operates a security readout upon detection of high harmonic frequencies of the fundamental frequency comprising:

a marker for being secured to an object to enable detection of the presence of the object within the interrogation zone, said marker comprising an elongated thin ferromagnetic strip having a maximum permeability of in the range of 400,000, said strip being about 4 centimeters in length and having a cross-sectional area of approximately 0.0004 square centimeters and a thickness not greater than about 0.00125 centimeters, said strip when disposed with its longitudinal axis generally parallel to the varying magnetic field generating very high harmonic frequencies of the fundamental frequency for detection by the sensor.

4. The marker of claim 3 wherein said ferromagnetic strip is comprised of superpermalloy.

5. The marker of claim 3 wherein said fundamental frequency equals 60 cycles per second and wherein said strip returns harmonic frequencies extending to 1.6 megacycles.

6. A system for sensing passage of objects through a surveillance area comprising:

means proximate the area for producing oscillating electromagnetic energy components in the area, a thin electromagnetically nonlinear marker having a magnetic permeability of at least 400,000 associated with each object and dimensioned to have a high ratio of length to cross-sectional area for re-radiating electromagnetic energy at very high harmonic frequencies when said components are coupled to the marker, and means for sensing electromagnetic energy in said area and for producing a signal in response to sensing energy re-radiated by a marker.

7. A system for detecting the presence of an object when said object is in an interrogation zone having a magnetic fieLd periodically varying at a predetermined fundamental frequency, said system comprising:

marker means to be secured to an object, said marker means comprising an elongated ferromagnetic element having a high permeability capable of generating signals containing harmonics of said fundamental frequency in excess of the 20th order when placed in said zone, radiating means for producing within said interrogation zone said magnetic field having an intensity sufficient to reverse the magnetization of said ferromagnetic element while in said zone, receiving means for detecting said signals containing said harmonics of said fundamental frequency produced by said ferromagnetic element, and security readout and communications means coupled to said receiving means responsive to said signal to indicate the presence of an object in accordance with the harmonic content.

8. A system according to claim 7 wherein said marker element comprises a ribbon having a maximum permeability of at least 400,000 and a ratio of length to square root of cross-sectional area in excess of 200, wherein said marker element is positioned in the zone with its axis approximately parallel with the magnetic field, and wherein said radiating means for generating said varying magnetic field radiates into said interrogation zone a 60 Hz oscillating field having a peak intensity of at least 3 oersteds.

9. A system according to claim 7 wherein said radiating means for generating said oscillating magnetic field includes:

an oscillatory power source, and a doorway coil comprising a flat wound coil to said power source to radiate an oscillating magnetic field into said interrogation zone, and wherein said receiving means comprises a loop antenna coupled to an amplifier, the output of which is coupled to said security readout and communication means.

10. A method for detecting the presence of an object when said object is in an interrogation zone having a magnetc field periodically varying at a predetermined frquency, comprising the steps of:

securing to an object a marker comprising an elongated ferromagnetic element having a high permeability capable of generating signals containing harmonics of said fundamental frequency in excess of the 20th order when placed in said zone;

radiating within said interrogation zone said magnetic field having an intensity sufficient to reverse the magnetization of said ferromagnetic element while in said zone;

receiving said signals containing said harmonics of said fundamental frequency produced upon reversal of the magnetization of said marker element when in said zone; and

energizing a security readout and communications system in response to said signals.

11. A method according to claim 10 wherein said ferromagnetic element is characterized by having a permeability in excess of 50,000, a coercivity of less than 0.1 oerstead, and dimensions such that the ratio of length to square root of cross-section area of said element is in excess of 150, and wherein said marker element is positioned in the zone with its axis approximately parallel with the magnetic field.

12. A method according to claim 10 further characterized by radiating within said interrogation zone said magnetic field having an oscillation frequency of 60 Hz, and a minimum peak intensity of at least 3 oersteds and further comprising receiving from said element said signals containing harmonics of said oscillating frequency extending up to and including those having a frequency higher than 106 Hz.

13. A system for detecting the presence of an object within an interrogation zone comprising:

coil means extending around said interrogation zone, an oscillator having a fundamental frequency connected to said coil means, marker means for being attached to an object to reflect harmonics of said fundamental frequency when said object is placed in said interroGation zone, circuit means connected to said coil means for detecting even and odd harmonics present on said coil means and for generating a voltage representative of the ratio of selected even and odd harmonics, and means responsive to said voltage for energizing object indication means.

14. A marker for being used in a system for sensing passage of objects through an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency comprising:

a marker for being secured to an object to be detected by said system, said marker comprising an elongated thin ferromagnetic element of high permeability and characterized by dimensions including a ratio of length to square root of cross-sectional area of at least 200 such that said marker generates detectable signals containing harmonics of said fundamental frequency in excess of the 20th order when said object bearing said marker is placed in said zone with the longitudinal axis of said ferromagnetic element positioned generally parallel to the varying magnetic field.

15. A marker for being used in a system for sensing passage of objects through an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency comprising:

a marker for being secured to an object to be detected by said system, said marker comprising an elongated thin ferromagnetic element of high permeability and characterized by a saturation magnetization in excess of 0.1 pole-centimeter such that said marker generates detectable signals containing harmonics of said fundamental frequency in excess of the 20th order when said object bearing said marker is placed in said zone with the longitudinal axis of said ferromagnetic element positioned generally parallel to the varying magnetic field.

16. A marker for being used in a system for sensing passage of objects through an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency comprising:

a marker for being secured to an object to be detected by said system, said marker comprising an elongated thin ferromagnetic element of high permeability and having a cross-sectional area of less than 10 3 CM2 and a length of at least 4 centimeters such that said marker generates detectable signals containing harmonics of said fundamental frequency in excess of the 20th order when said object bearing said marker is placed in said zone with the longitudinal axis of said ferromagnetic element positioned generally parallel to the varying magnetic field.

17. A marker for being used in a system for sensing passage of objects through an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency comprising:

a marker for being secured to an object to be detected by said system, said marker comprising an elongated thin ferromagnetic element of high permeability and having a cross-sectional area of in the range of 0.0004 CM2 such that said marker generates detectable signals containing harmonics of said fundamental frequency in excess of the 20th order when said object bearing said marker is placed in said zone with the longitudinal axis of said ferromagnetic element positioned generally parallel to the varying magnetic field.

18. A marker for being used in a system for sensing passage of objects through an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency comprising:

a marker for being secured to an object to be detected by said system, said marker comprising an elongated thin ferromagnetic element of high permeability and low coercive force and having a slender cross section compared to length, said marker further having a magnetic moment of a magnitude such that said marker generates detectable signals containing harmonics of said fundamental freQuency in excess of the 20th order when said object bearing said marker is placed in said zone with the longitudinal axis of said ferromagnetic element positioned generally parallel to the varying magnetic field.