Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
2002-07-30
2004-03-16
Le, N. (Department: 2862)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S239000, C340S551000, C340S572600
Reexamination Certificate
active
06707295
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for distinguishing between a semi-soft magnetic materialand a soft magnetic material. The semi-soft material and/or the soft magnetic material may be used as a security feature in or on the substrate of a security article.
BACKGROUND OF THE INVENTION
Soft magnetic security features are well known in the art of electronic article surveillance systems (EAS) and are often called anti-pilferage tags. The EAS systems make use of the non-linear magnetic properties of the B-H loop of the soft magnetic material. Small activating fields typically drive the soft magnetic material into saturation. Sensitivity to small fields is required here because it is difficult to generate a large magnetic field at a distance from a source, and typical EAS systems need to interrogate as large a volume as possible, e.g. the public access routes in and out of shops. The security features used here are therefore commonly based upon very soft magnetic materials such as the amorphous Metglas® or Vitrovac® or thin films such as made of a Co
a
Fe
b
Ni
c
Mo
d
Si
e
B
f
alloy, where a to f are atomic percentages and a ranges between 35% and 70%, b between 0% and 8%, c between 0% and 40%, d between 0% and 4%, e between 0% and 30%, f between 0% and 30%, with at least one element of each of the groups (b, c, d) and (e, f) being non zero. Such a Co
a
Fe
b
Ni
c
Mo
d
Si
e
B
f
composition is hereinafter referred to as a CoFeNiMoSiB composition. CoFeNiMoSiB films are marketed under the name of Atalante®. The term “thin ” here refers to a film having a thickness, which is smaller than 10 micrometer. These materials have a very low coercivity and a high magnetic permeability.
Within the context of the present invention, the terms “soft magnetic material ” typically refer to materials having a low magnetic saturation field H
s
, i.e. those materials require a magnetic field ranging between 3 A/m and 100 A/m (measured at 1 kHz) to saturate.
Using non-linear magnetic properties for the authentication of objects could also be an attractive approach because of simplicity and sensitivity. However, the approach would be of little use if the security elements set-off the alarms of the gates commonly used for EAS. The approach would also be of little use unless the security elements were difficult to obtain or copy.
Patent applications WO-A-98/26378 and WO-A-98/26377 disclose how to solve the above problem. The security element used comprises small, elongated magnetic particles which require a magnetic field greater than 100 A/m, and preferably greater than 300 A/m, to saturate. This property is chosen to ensure that the magnetic hardness of the particles is sufficiently high that they will not be driven into saturation at the field strengths commonly used in EAS gates. The security feature used here will therefore not set-off the alarm of the EAS gates.
In addition it is desirable to keep the magnetic field required for saturation well below that at which more commonly available Ferro-magnetic materials will saturate and to keep it at a sufficiently low level that the particles can be saturated, and therefore detected, at short ranges from a compact reading apparatus. In general this implies magnetic fields of less than about 3000 A/m.
Within the context of the present invention, the terms “semi-soft magnetic material ” refer to magnetic materials typically having a magnetic saturation field H
s
ranging from 100 A/m to 3000 A/m, e.g. from 200 A/m to 3000 A/m, preferably from 300 A/m to 3000 A/m (measured at 1 kHz).
Although the generation of high harmonics at low magnetic field strengths is particular to the soft magnetic materials in the case of EAS and to the semi-soft magnetic materials in the case of authentication, the inventors have discovered, however, that there is no clear difference between the harmonics generated from these types of materials. This is particularly true if the orientation of the security element is varied relative to the magnetic field.
Another problem with soft magnetic materials and semi-soft magnetic materials is that soft magnetic materials may be looked as semi-soft magnetic materials at a great distance between the drive coil and the material.
Moreover the drive field at which the security element will saturate, will vary with the orientation of the security element in the field. These problems can be solved by making the authentication method a contact one or by ensuring that the spatial orientation of the drive coil and material are fixed. However, for hand-held applications it is most convenient to validate the security element with a non-contact reading where the spatial orientation between drive coil and material is not fixed.
Still another problem is that there may be a magnetic field, external to the field generated by the drive coil, which could bias the total field.
EP-A1-0295085, EP-A2-0366335 and U.S. Pat. No. 5,204,526 all disclose magnetic material in the form of thin films or in the form of thin strips or wires used as markers or identifiers in detection or recognition systems. All documents suggest the use of magnetic material with two or more different coercive forces. These documents, however, are silent with respect to the difference between soft magnetic and semi-soft magnetic materials.
SUMMARY OF THE INVENTION
It is an object of the present invention to avoid the problems of the prior art.
It is a further object of the present invention to provide an authentication system, which can discriminate between various types of soft magnetic and semi-soft magnetic materials.
It is a further object of the present invention to provide a non-contact and a hand-held method for authentication.
It is also an object of the present invention to provide a compact low cost reading apparatus, which can be used to detect the special markers at distances up to a few centimeters.
According to the invention there is provided a method for distinguishing between a semi-soft magnetic material and soft magnetic material. The method comprises following steps:
(a) emitting an electromagnetic drive signal of one or more particular frequencies to an article so that any present semi-soft magnetic or soft magnetic material in the article go into saturation for both positive and negative magnetic fields;
(b) detecting an electromagnetic detection signal emanating from the article;
(c) measuring time or relative phase delays between one or more reference points of the drive signal and points at which positive and negative peaks of the detection signal occur;
(d) comparing the measured time or relative phase delays with values, which are typical for semi-soft magnetic material in order to make a decision whether the material is soft magnetic or semi-soft magnetic.
The method may comprise a further step of measuring the heights of the positive and negative peaks. The height of the peaks of the detection signal gives an indication about the distance or the orientation of the article. In a preferable embodiment only measurements which fall within a predetermined range of the heights are further processed. The time or relative phase delay between a reference point of the drive signal and a point at which the peaks occur give, together with the height of the peaks, an indication of the magnetic softness of the article.
Due to the fact that an indication is given about the distance or orientation of the material, the detection method can be a non-contact method, and more particularly a hand-held method. Within the context of the present invention, the terms “hand-held method” refer to the use of a small and light weigth detection apparatus with sizes not much greater than sizes of current available palm top organizers or portable telephones. A hand-held method is a method which can be applied outside a dedicated laboratory. The hand-held method can be applied everywhere, e.g. at the point of sales or point of transaction, in order to check magnetic security features in articles.
In a preferable embodiment of the invention, following steps occur:
(a)
Fisher John
Robertson Paul
Te Lintelo Johannes
Aurora Reena
Foley & Lardner
Le N.
N. V. Bekaert S.A.
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