Demagnetization apparatus for magnetic markers used with article surveilliance systems

• US 4,752,758 A
• Filed: 06/24/1987
• Issued: 06/21/1988
• 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:
  US07/061,545
• Time in Patent Office: 363 Days
• Field of Search: 335/284, 335/306, 340/572, 343/787, 361/149, 361/151, 361/267, 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 peak intensity of the outermost field is controlled to have a peak intensity less than that of adjacent regions. The section and a cover plate are oriented 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 permanent magnetic material associated with said housing,

said elongated section having a succession of alternately polarized permanently magnetized regions extending along the length of the section, each of which regions extends across the width of said section, said regions exhibiting at the working surface a succession of fields of alternating polarity, wherein said section has a first portion which exhibits at the working surface fields of generally decreasing intensities to thereby expose a said marker affixed to a said article moved along said length to fields of alternate polarities and generally decreasing intensities and thereby substantially demagnetize the control element of said marker, and wherein said 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 including a succession of alternately polarized permanently magnetized regions of approximately equal peak intensities and an outermost region having a peak intensity less than that of the other regions, thereby ensuring that the peak intensity at the working surface of the outermost field is not greater than that associated with the other regions.

2. An apparatus according to claim 1, wherein said outermost region extends approximately parallel to the working surface and at about the same height relative thereto as the remainder of the second section, and has an intrinsic field intensity approximately one-half that provided by the remainder of the regions thereof.

3. An apparatus according to claim 1, wherein said elongated section of permanent magnet material has substantially the same composition throughout and the outermost region of said second portion thereof comprises a piece of said material of smaller dimensions than that associated with each pole in the remainder of the section.

4. An apparatus according to claim 1, wherein said elongated section comprises an assembly of discrete pieces of permanent magnetic material, each piece other than an outermost piece being magnetized to provide substantially the same intrinsic field intensity, and the outermost piece being magnetized to provide a peak intensity less than that provided by the other pieces.