DEACTIVATABLE FERROMAGNETIC MARKER FOR DETECTION OF OBJECTS HAVING MARKER SECURED THERETO AND METHOD AND SYSTEM OF USING SAME

• US 3,747,086 A
• Filed: 11/24/1971
• Issued: 07/17/1973
• Est. Priority Date: 03/22/1968
• Status: Expired due to Term

• Current Assignee: Glen Peterson
• Original Assignee: Glen Peterson
• Inventors: Peterson, Glen
• Primary Examiner(s): Trafton, David L.
• Application Number:
  US05/201,687
• Time in Patent Office: 601 Days
• Field of Search: 340/258R,258C,280,224 325/8,105 343/6.5SS,6.8,787,788 179/82 235/61.12M
• US Class Current: 340/572.2
• 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: Y10T 29/49075 including permanent magnet or core

Abstract

A deactivatable ferromagnetic marker useful in tagging objects to allow selective detection of tagged objects depending upon the activation state of the marker. The marker comprises a plurality of ferromagnetic elements including a first element capable of generating a signal containing harmonics of an exciting oscillatory interrogating field and a second element having a coercive force greater than the first element and capable of retaining a state of magnetization when exposed to the interrogation field, such that when so magnetized a magnetic bias is imposed on the first element to prevent the generation of the harmonic signal. The system includes the marker attached to selected objects, an interrogation field, detection means, and deactivation means wherein the second element becomes magnetized.

Claims

1. A marker for being secured to an object to enable detection of the presence, identity or status of the object within an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency, said marker comprising:

a first elongated ferromagnetic element being dimensioned to be secured to said object, said first element having low coercivity capable of generating a detectable signal containing harmonics of said fundamental frequency when the object bearing said first element is placed in said zone, and a second ferromagnetic element disposed adjacent and generally aligned with said first element, said second element having a coercivity greater than said first element, wherein magnetization of said second element alters the harmonic content of the detectable signal produced by said first element when in said interrogation zone.

2. A marker according to claim 1 wherein said second element has a total magnetic moment capable of saturating said first ferromagnetic element to prevent said first element from generating a detectable signal.

3. A marker according to claim 1 wherein said marker further comprises:

a third ferromagnetic element having a coercivity greater than that of said second ferromagnetic element, said second and third elements being disposed adjacent to and generally aligned with said first ferromagnetic element, and said second and third ferromagnetic elements each being individually incapable of supplying a sufficient number of magnetic lines of flux to saturate said first ferromagnetic element, the magnetic lines of flux supplied by both said second and third elements when magnetized in the same direction being sufficient to saturate said first ferromagnetic element to thereby prevent said first element from generating said harmonics.

4. A marker according to claim 3 wherein said second ferromagnetic element has a coercivity of approximately 15 oersteds and said third ferromagnetic element has a coercivity of approximately 100 oersteds.

5. A marker according to claim 1 wherein said ferromagnetic elements are supported within thin insulating material.

6. A marker for being secured to an object to enable detection of the presence, identity or status of the object when the object is in an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency, said marker comprising:

a first elongated ferromagnetic element for being secured to said object and having low coercivity capable of responding to said magnetic field when said object is placed in said interrogation zone to generate signals containing harmonics of said fundamental frequency in excess of the 20th order, and at least a second elongated ferromagneTic element disposed close to and generally aligned with said first element and having a coercivity greater than said first element, said second element when magnetized capable of altering the harmonic content of the signals produced by said first ferromagnetic element when in the interrogation zone.

7. A method of making a marker for being secured to an object to enable detection of the presence, identity or status of the object when the object is in an interrogation zone such as a doorway, said zone having a magnetic field periodically varying at a predetermined fundamental frequency, said method comprising:

selecting a first elongated ferromagnetic element of low coercivity capable of generating a detectable signal containing harmonics of said fundamental frequency when placed in said zone, selecting a second ferromagnetic element having a coercivity greater than said first element and which when magnetized alters the harmonic content of the detectable signal produced by said first element when in said interrogation zone, and disposing said first and second element adjacent to and generally aligned with one another whereby when said first and second elements are secured to an object and said object is disposed in said interrogation zone, the external magnetic field produced by magnetization in said second element acts on said first element to alter the response of said first element to said magnetic field of said zone.

8. A method of making a marker according to claim 7 and further comprising:

disposing a third ferromagnetic element having a coercivity greater than said second ferromagnetic element adjacent said first and second elements, said second and third elements each being indivudually incapable of supplying a sufficient number of lines of magnetic flux to saturate said first ferromagnetic element, the lines of magnetic flux supplied by said second and third elements when magnetized in the same direction being sufficient to saturate said first element, whereby the external magnetic field produced by magnetization in said second and third elements acts on said first element to alter the response of said first element produced when said disposed elements are subjected to said magnetic field of said zone.

9. A system for detecting a characteristic 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 for being secured to an object comprising a first elongated ferromagnetic element of low coercivity capable of generating a detectable signal containing harmonics of said fundamental frequency when placed in said zone, and at least a second ferromagnetic element disposed close to and generally aligned with said first element and having a coercivity greater than said first element, said second element when magnetized capable of altering the harmonic content of the detectable signal produced by said first element when in said interrogation zone, radiating means for producing within said interrogation zone said magnetic field, receiving means for detecting the harmonic content of the signal produced by said first element, and security readout and communication means coupled to said receiving means responsive to said signal to indicate said characteristic of an object in accordance with said harmonic content.

10. The system of claim 9 wherein said magnetic field zone has a peak intensity greater than the coercivity of said first ferromagnetic element but less than the coercivity of said second ferromagnetic element.

11. A system for detecting the characteristic of an object according to claim 9 wherein said first ferromagnetic element is further characterized by a permeability of at least 400,000, a coercivity of in the range of 0.02 oersteds, and dimensions including a ratio of the length to square root of cross-sectional area of about 200, said second ferromagnetic element bEing further characterized by a coercivity of about 15 oersteds, and wherein said marker further comprises a third ferromagnetic element having a coercivity greater than that of said second ferromagnetic element, said second and third elements being disposed close to and generally aligned with said first ferromagnetic element, said second and third ferromagnetic elements each being incapable of supplying by itself a sufficient number of lines of magnetic flux to saturate said first ferromagnetic element, but the lines of flux supplied by both said second and third elements being sufficient to saturate said first ferromagnetic element to thereby prevent said first element from generating said harmonics.

12. A system according to claim 9 further comprising:

means for altering the magnetization state of said marker prior to said marker being placed in said interrogation zone, such that said alteration is recognizable by the harmonic content of the signal produced upon passage of said marker through said interrogation zone.

13. A system according to claim 9 wherein said receiving means further comprises an electronic device for selecting even and odd harmonics of said fundamental frequency present in said signal and for delivering to said security readout and communication means a voltage proportional to the ratio of selected even and odd harmonics of said fundamental frequency.

14. A system for detecting a characteristic of an object comprising:

means for generating a magnetic field varying at a predetermined fundamental frequency, marker means for being attached to an object and comprising first and second elongated ferromagnetic elements having different coercivities, means for altering the magnetization state of said marker means prior to being disposed in said magnetic field, and means for detecting the harmonic content of the signal produced upon passage of said marker through said magnetic field to indicate a characteristic of said object.

15. The system of claim 14 and further comprising:

a third ferromagnetic element adjacently disposed to said first and second elements and having a higher coercivity than said first and second elements.

16. A method for detecting a characteristic of an object when the object is in an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency, said method comprising:

securing to an object a marker comprising a first elongated ferromagnetic element of low coercivity capable of generating a detectable signal containing harmonics of said fundamental frequency when placed in said zone, and at least a second ferromagnetic element disposed adjacent to said first element and having a coercivity greater than said first element, said second element when magnetized altering the harmonic content of the detectable signal produced by said first element when in said interrogation zone;

radiating within said interrogation zone said magnetic field having a peak intensity greater than the coercivity of said first ferromagnetic element but less than the coercivity of said second ferromagnetic element;

receiving said detectable signal produced by said first element upon excitation by said varying magnetic field; and

responding to said signal to produce an indication of the characteristic of an object in accordance with said harmonic content.

17. A method for detecting a characteristic of an object according to claim 16 wherein said first ferromagnetic element is further characterized by a permeability of at least 400,000 and dimensions including a high ratio of the length to square root of cross-sectional area, said second ferromagnetic element being further characterized by a coercivity of about 15 oersteds.

18. A method according to claim 16 and further comprising the step of placing said marker in a magnetizing field having a peak intensity greater than the coercivity of said second ferromagnetic mAterial whereby said second ferromagnetic material becomes magnetized and thereby capable of altering said detectable signal produced by said first ferromagnetic element when said marker is in said interrogation zone.