Magnetic reading devices

Details Magnetic reading devices Magnetic reading devices

1999-05-24

2001-05-15

Frech, Karl (Department: 2876)

Registers

Coded record sensors

Particular sensor structure

C235S493000

Reexamination Certificate

active

06230972

ABSTRACT:

This invention relates to magnetic sensing and reading devices and, more particularly, is concerned with detectors (or readers) for reading information stored in magnetic tags or elements.
In previous patent applications (GB 9506909.2 and PCT/GB96/00823—published as WO96/31790) we have described novel techniques for spatial magnetic interrogation based on exploiting the behaviour of magnetic materials as they pass through a region of space containing a magnetic null. In particular, our earlier applications describe how passive tags containing one or more magnetic elements can perform as remotely-readable data carriers, the number and spatial arrangement of the elements representing information.
WO96/31790 (published Oct. 10, 1996) defines a magnetic null as: “a point, line, plane or volume in space at or within which the component of the magnetic field in a given linear direction is zero”—see page 3 line 34 to page 4 line 2 of that document.
In our previous applications we described a number of possible system embodiments employing either permanent magnets or electromagnets to create the magnetic null. We also described some possible system implementations which were particularly appropriate for tags employing very low coercivity, high permeability magnetic elements. These implementations detected harmonics of a superimposed low amplitude alternating interrogation field. However all the configurations described in the previous applications may also be operated with the baseband signals generated by the passage of the tag through the magnetic null, without the need for any superimposed alternating interrogation field. If required a null-scanning field may be added to permit stationary tags to be read.
Moreover, the basic technique described is not restricted to low coercivity material, and the present application is concerned with a detector which is capable of interrogating tags containing elements made from magnetic material with coercivity ranging from very low (1 A/m or less) up to high (30,000 A/m or more). This type of embodiment is particularly efficient for high coercivity material because it avoids the need for the high amplitude a.c. interrogation field which would otherwise be required for such material. Other advantages include a high data rate, and simplicity. The invention is particularly suitable for extracting information from items such as security documents and banknotes which already contain magnetic elements made from material having a wide range of coercivities. It will thus be appreciated that the term “tag” as used herein encompasses within its scope items such as security documents and banknotes.
U.S. Pat. No. 3,964,042 (D1—Garrott) describes a metal detector for detecting ferrous objects such as nuts and bolts, particularly in the context of forage harvesting machines, where it is apparently common for machine parts made of iron or other magnetic materials to break away and cause damage to the forage processing parts of the machine. The metal detector utilises permanent magnets to generate a static magnetic field, the magnets being arranged so that the lines of magnetic force extend between poles of opposite polarity in a first plane which will be intersected by the passage of extraneous metal objects into the machine. A detecting coil is arranged around the permanent magnets in a second plane which is perpendicular to said first plane. As illustrated, the coil is positioned in a horizontal plane. The coil is therefore sensitive to flux changes occurring within a vertical plane (i.e. perpendicular to that of the coil). In the illustrated embodiment, four magnetic poles are arranged in the configuration:
This is achieved by using two magnets of generally horseshoe cross section placed in contact side by side. There are thus two magnetic discontinuities, both occurring in the vertical plane which passes through the contact plane between the two magnets. Such an arrangement cannot detect the passage of a magnetic element having an axis of easy magnetisation through the magnetic null (which lies in a vertical plane with the geometry of
FIGS. 2 and 3
of D1). This is because the coil arrangement employed in D1 is sensitive to flux changes in the vertical direction, but insensitive to changes in the horizontal direction. In contrast, if a magnetic tag were to move through the magnetic arrangement in the direction of the arrow shown in
FIG. 2
of D1, there would be no change of magnetic flux in the vertical direction, whereas there would be a change of flux in the direction of motion (i.e. in a horizontal plane). Further, column 6, lines 1 to 29 of D1 make it clear that if a small symmetrical metal object were to pass through the magnetic null, it is unlikely to be detected.
EP-A-0 295 085 (D2—Scientific Generics Ltd) describes a method of detecting the presence of articles by applying to them preselected magnetic tags, these being arranged in a coded formation so that magnetic interrogation can be used to determine the presence of the article. There is no disclosure of the use of an interrogating field such as is used in the present application.
U.S. Pat. No. 5,397,985 (D3—W. David Kennedy) discloses a method of electromagnetic imaging of a conductive casing in which a transducer is introduced inside the casing and is moved along the length of the casing while being rotated. Measurements of flux density variation are made.
GB-A-2 071 336 (Doduco) discloses a position encoder which employs a bistable magnetic element positioned between two permanent magnets. The bistable magnetic element has wound around it an exciter coil which transmits an AC field and a detector coil.
According to the present invention, there is provided a detector for sensing the presence of a magnetic tag having an axis of easy magnetisation, which comprises (1) either (i) a magnet or (ii) a pair of magnets arranged in magnetic opposition, the magnet or magnets being disposed so as to define a spatial region through or across which the magnetic tag is, in use, passed, the disposition of said magnet(s) and the resultant magnetic field pattern being such as to cause a change in polarity of the magnetisation of said magnetic tag in the course of its passage through a magnetic null within said spatial region; and (2) a receiver coil or coils positioned above and in proximity to one pole of said magnet (where a single magnet is employed), or positioned between said pair of magnets (where two magnets are employed), and arranged to detect magnetic dipole radiation emitted by a magnetic tag as it passes through said magnetic null with the easy axis of magnetisation of the tag oriented in the direction of travel.
In one embodiment, the detector comprises a pair of permanent magnets arranged in magnetic opposition, i.e. with like poles directed towards one another, the space between the magnets defining said spatial region and being in the form of a slot through which the magnetic tag, in use, is passed. In another embodiment, the detector comprises a single permanent magnet, said spatial region being constituted by a region of space above one surface of the magnet, and the magnetic tag, in use, being passed over said surface of the magnet in proximity thereto.
As will be described below, it is particularly advantageous for the receiver coils to be fabricated as an assembly of printed circuit boards, the coil windings being constituted by conductive paths on the boards, and the boards being interconnected to generate a three dimensional coil.
We will now describe an arrangement which is particularly applicable to the non-contact reading of information from moving tags or elements containing medium or high coercivity material. This arrangement is also appropriate for situations where the field required to saturate the tag material is high, independent of coercivity (e.g. where the tag elements have poor shape factor, and thus, because of demagnetisation effects, low permeability). The arrangement employs a magnetic null created by one or more strong permanent magnets. The strength of the magnet(

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