Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1989-04-10
1990-07-10
Orsino, Joseph A.
Communications: electrical
Condition responsive indicating system
Specific condition
340572, G08B 1324
Patent
active
049409662
DESCRIPTION:
BRIEF SUMMARY
This invention relates to magnetic devices and to their use in article recognition and/or detection.
The use of magnetic devices to act as antipilferage tags is well known. Typically, the tag is a magnetic medium which is detected when it (and the article to which it is attached) passes through a detection system, typically in the form of a walk-through framework which emits an alternating magnetic interrogation field. This field is designed to interact with a tag which has not been subjected to the routine deactivation procedure and to respond by, for example, triggerring a warning signal in the event that detection of a non-deactivated tag occurs. The tag can be deactivated when a shop assistant carries out the routine procedure at the time of effecting a sale. Deactivation is usually effected by applying a magnetic field to the tag which itself includes a deactivation layer, generally in the form of a magnetically semi-hard material with a high coercive force, located close to the active element in the tag. The semi-hard deactivating layer can be magnetised by a strong magnetic field and as a result the magnetised deactivating layer prevents the magnetically soft active layer from responding when subjected to an alternating magnetic field. Such deactivation prevents detection of the magnetic tag by the detection system.
The shape of the material making up the active element of the magnetic marker strongly affects the magnetization response to an external magnetic field, because of the demagnetization factor N, which is dependent on the shape. Known markers take the form of amorphous metal ribbon ferromagnets which are formed by melt-spinning or similar techniques. These ribbons are relatively thick, generally over 10 microns and often about 25 microns in thickness.
The shape-dependent demagnetizing field .DELTA.H is equal to the product of the shape-dependent demagnetization factor N and the intensity of magnetization M.
The effective permeability (.mu..sub.e) of the tag can be derived approximately by the following formula: ##EQU1## where .mu..sub.i is the intrinsic permeabilty of the magnetic coating, and N is the demagnetisation factor; this (N) can be calculated as a function of the shape of the article. The inverse of the demagnetisation factor can be termed the shape factor (1/N).
The effective permeability of the active component of a tag thus depends not only on the intrinsic permeability of the material of which it is formed, but also on its shape. The lower the demagnetisation factor, the closer is the effective permeability to the intrinsic permeability. Low demagnetisation factors are also desirable since they permit a lower intensity interrogation field to be used.
We have discovered that the use of similar magnetic devices can be extended to detection/recognition systems, by means of which different types of magnetic device can be distinguished and hence also the article or class of article on which they are carried may be distinguished and identified, or the location of a given type of article may be found.
In particular, several elements with either different ferromagnetic material compositions and hence a variety of magnetically nonlinear characteristics, or several different magnetically soft and/or hard and/or semi-hard elements in proximity to one another can be used. The hard or semi-hard magnetic material acts (to a greater or lesser extent depending on magnetic and physical parameters such as coercivity, volume and spacing) as a clamp which holds the soft material in a fixed magnetic state in which there is a specific response to an interrogating (generally alternating) field. Various combinations of the different elements thus characterise different objects, and give rise to specific magnetic signatures.
According to one aspect of the present invention, there is provided a method of recognising and/or locating preselected categories of articles, which comprises applying to the articles a plurality of magnetic elements in predetermined associations (e.g. with predetermined numbers
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Chambers Stephen H.
Crossfield Michael D.
Davies Dafydd G.
Pettigrew Robert M.
Mullen Jr. Thomas J.
Orsino Joseph A.
Scientific Generics Limited
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