Demagnetization apparatus for magnetic markers used with electromagnetic article surveillance systems

• US 4,689,590 A
• Filed: 07/21/1986
• Issued: 08/25/1987
• Est. Priority Date: 07/21/1986
• Status: Expired due to Term

• Current Assignee: 3M Company
• Original Assignee: 3M Company
• Inventors: Heltemes, Eugene C.
• Primary Examiner(s): Harris, George
• Application Number:
  US06/887,721
• Time in Patent Office: 400 Days
• Field of Search: 335/284, 361/149, 361/151, 361/267, 340/572, 324/259, 324/260, 324/261
• US Class Current: 335/284
• CPC Class Codes: H01F 13/006 Methods and devices for demagnetising of magnetic bodies, e.g. workpieces, sheet material for erasing of information on magnetic record carriers G11B5/00

Abstract

A demagnetization apparatus for use with magnetically based electronic article surveillance systems having a dual status anti-theft marker containing at least one demagnetizable control element which when demagnetized allows the marker to be detected by the system when the marker is present in an interrogation zone. The apparatus includes an elongated magnetic section contained within a housing which exhibits a succession of fields of alternate polarity and a portion of which exhibits generally decreasing intensities at the working surface of the housing along that portion of the section. The section and a cover plate are orientated such that the external fields near the working surface are sufficient in intensities to demagnetize the demagnetizable element of the marker positioned proximate thereto while being rapidly attenuated a short distance from the section. Accordingly, magnetically sensitive articles, such as for example, prerecorded magnetic cassettes, to which the markers are affixed, are not adversely affected.

Claims

1. An apparatus which in movement relative to an article, having affixed thereto a dual status anti-theft marker including at least one remanently magnetizable control element having a coercive force of a predetermined value, demagnetizes said control element to change the status of the marker, said apparatus comprising:

a housing having a working surface relative to which an article may be moved, and an elongated section of a permanent magnetic material associated with said housing;

said elongated section having a plurality of closely spaced poles, said poles exhibiting at the working surface a succession of fields of alternating polarity and a first portion of which exhibits at the working surface fields of generally decreasing intensities along at least that portion of said elongated section;

each pole extending across the width of said elongated section and the succession of poles extending along the length of said elongated section; and

wherein the field intensity at said working surface associated with the most intense pole in said succession is approximately one and one-half times said predetermined value of the coercive force of the control element, whereby movement of said article relative to the working surface of said housing from a position adjacent the field associated with the most intense field past each generally successively weaker field of opposite polarity, will expose the marker affixed to the article to fields of alternate polarities and generally decreasing intensities, thereby substantially demagnetizing the control element of said marker, and the close spacing of the alternate poles results in a rapid decrease in the intensities of the fields above the working surface so as not to adversely affect a magnetically sensitive object contained within the article.

2. The apparatus defined in claim 1, wherein the pole to pole spacing along the length of the elongated section is no more than 0.25 inch.

3. The apparatus defined in claim 1, wherein said first portion is inclined at a predetermined angle relative to said working surface of said housing to exhibit said succession of fields of alternate polarity and of decreasing intensities at said working surface.

4. The apparatus defined in claim 3, wherein said elongated section further comprises a second portion associated with that end of said first portion which exhibits the most intense field at the working surface of said housing, said second portion includes more than one pole, said poles exhibit at the surface of said housing a succession of closely spaced poles of alternating polarity and of approximately uniform intensities, and the maximum intensity of the fields at said working surface associated with said second portion is approximately one and one-half times the predetermined value of coercive force of said control element.

5. The apparatus defined in claim 3, wherein said elongated section further comprises a third portion associated with that end of said first portion which exhibits the least intense field at the working surface of said housing, and said third portion includes a plurality of closely spaced poles, said poles exhibit at said working surface a succession of fields of alternating polarity and of generally decreasing intensities at a greater rate along said third portion than along said first portion.

6. The apparatus defined in claim 1, further comprising a thin non-magnetic plate covering the working surface of said housing to protect said elongated section while providing a durable wear surface allowing the magnetic lines of flux to extend therethrough substantially unattenuated.

7. The apparatus defined in claim 6, wherein said housing further comprises a recess opening onto the working surface of said housing within which said elongated section is positioned.

8. The apparatus defined in claim 1, wherein said elongated section comprises a plurality of permanent magnets.

9. The apparatus defined in claim 8 wherein said permanent magnets are injection molded.

10. The apparatus defined in claim 1, wherein said elongated section includes a permanent magnet material which has been magnetized with approximately uniform alternating poles.

11. The apparatus defined in claim 1, wherein said poles of alternate polarity are adjacent one another.

12. The apparatus defined in claim 1, wherein said poles are within the range of 8 poles per inch to 4 poles per inch.

13. The apparatus defined in claim 1, wherein the average field intensity at the working surface of said housing associated with each pole of said first portion intermediate said most intense pole and said least intense pole decreases in the range of 5 to 20 percent between any two adjacent fields.

14. The apparatus defined in claim 1, wherein the field above said most intense pole is within the range of 350 to 420 oersteds.

15. An apparatus capable of substantially demagnetizing at least one control element of a dual status marker affixable to an article containing a magnetically sensitive tape, said apparatus comprising:

a housing having a working surface relative to which the article may be moved;

an elongated section of a permanent magnetic material associated with the working surface of said housing having a plurality of closely spaced poles from one end of said elongated section to the other;

said poles exhibiting at the working surface a succession of magnetic fields of alternating polarity and at least a first portion exhibiting a plurality of magnetic fields of generally decreasing intensities at said working surface along that portion of said elongated section; and

the most intense field at approximately 0.030 inch above said working surface being no more than 450 oersteds such that when said article is moved relative to the working surface of said housing from a position adjacent the most intense field at the working surface past each generally successively weaker field of opposite polarity, exposing the marker affixed to the article to said fields of alternate polarities and generally decreasing intensities, thereby substantially demagnetizing the control element associated with said marker, and whereby the close spacing of the alternate poles results in a rapid decrease of the intensity of the fields away from the working surface so as not to affect the magnetically sensitive tape contained within the article.

16. The apparatus defined in claim 15, wherein said first portion is inclined at a predetermined angle relative to said working surface to exhibit said succession of fields of generally decreasing intensities at said working surface.

17. The apparatus defined in claim 15, wherein a second portion of said elongated section associated with one end of said elongated section approximate to said most intense field of said first portion exhibits at the working surface more than one field of substantially uniform intensity and of approximately equal intensity to said most intense field.

18. The apparatus defined in claim 17, wherein a third portion of said elongated section associated with the other end of said elongated section approximate to said least intense field of said first portion exhibits at the working surface a plurality of magnetic fields of generally decreasing intensity at a greater rate than said first portion of said elongated section.

19. The apparatus defined in claim 15, wherein each pole extends across the width of said elongated section and the succession of poles extends along the length of said elongated section.

20. The apparatus defined in claim 15, further comprising a thin non-magnetic plate covering the working surface of said housing to protect said elongated section and to provide a durable wear surface allowing the magnetic lines of flux to extend therethrough substantially unattenuated.