Field of the invention.
-
The present invention relates to a method for detecting the presence of a
semi-soft magnetic security feature in a 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
CoaFebNicModSieBfalloy, 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 CoaFebNicModSieBf
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.
Generally, the term "soft magnetic" typically refers to materials having a
low coercive force, e.g. a coercive force ranging between 3 A/m and 100
A/m (measured at 1 kHz).
-
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.
-
The term "semi-soft magnetic material" refers to magnetic materials
typically having a magnetic saturation field 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 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.
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 detecting the
presence of a semi-soft magnetic or soft magnetic security feature in a
substrate of a security article. The magnetic security feature is
preferably a semi-soft magnetic security feature with a magnetic
saturation field ranging from 100 A/m to 1000 A/m.
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
security features 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 the height of the peaks of the detection signal;
- (d) measuring the time or relative phase delays between a reference
point of the drive signal and a point at which the peaks occur;
- (e) Comparing the measured heights and measured time or relative
phase delays with values, which are typical for semi-soft magnetic
features.
The height of the peaks of the detection signal gives an indication about
the distance or the orientation of the article.-
-
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.
-
In a preferable embodiment the electromagnetic detection signal is
proportional to the rate of change of magnetic flux in the article (dB(t)/dt).
-
In another example the electromagnetic detection signal is proportional
to an integral of the rate of change of magnetic flux in the article (B(t)).
-
In a more elaborated example, the detection method further comprises a
step of measuring the width of the peaks of the detection signal at one or
more levels in order to discriminate semi-soft magnetic security features
from Ferro-magnetic materials such as iron.
-
The semi-soft magnetic security feature can take many forms.
-
In a typical example the semi-soft magnetic security feature comprises a
number of fibres such as disclosed in the above-mentioned patent
applications WO-A-98/26378 and WO-A-98/26377.
-
In another example the semi-soft magnetic security feature comprises a
thin semi-soft magnetic film.
-
In both examples the demagnetisation factor N of the fibres or the thin
films is very low. Preferably, the demagnetisation factor N ranges from
10-5 to 10-4. Such a low demagnetisation factor N means that the
dynamic magnetic permeability µr' is not reduced very much in
comparison with the bulk permeability µr and remains very high.
-
In a preferable embodiment of the invention, the semi-soft magnetic
security feature comprises two or more types of magnetic material with
different magnetic coercivity values, e.g. two or more different thin semi-soft
magnetic films.
In comparison with security features which only comprise a single semi-soft
magnetic material with only one coercivity value, such a security
feature with two or more different values of coercivity has the following
advantages:
- (a) it is easier to detect and to distinguish from other soft magnetic and
semi-soft magnetic materials;
- (b) it is more difficult to copy as security feature ;
- (c) The detection algorithm is more difficult to copy.
-
Brief description of the drawings.
-
The invention will now be described into more detail with reference to the
accompanying drawings wherein
- FIGURE 1 compares a dB/dt signal coming from a soft magnetic
material with a dB/dt signal coming from a semi-soft magnetic
material.
- FIGURE 2 illustrates what values can be measured on a dB/dt
curve in a detection method according to the invention.
Description of the preferred embodiments of the invention.
-
In a detector apparatus one or more coils, the drive coils, are driven with
an alternating current to drive the security elements into saturation for
both positive and negative magnetic fields. One or more coils, the
detection coils, are used to detect the returned signal which is
proportional to the rate of change of flux with time (dB(t)/dt). Signal
processing electronics is then used to process and analyse the signals
and to provide an indicator signal which may be visual or auditive, when
materials having the correct magnetic properties are situated in the drive
field.
-
FIGURE 1 shows time plots 10 and 20 of typical dB(t)/dt signals received
from two magnetic materials with different magnetic properties. It can be
seen that the two materials have both different shapes and that they
occur at different distances along the time axis, which is referenced to
the drive current in the drive coils. In this plot the peaks of the signal
correspond with the maximum slope of the B-H loop. The material
corresponding to plot 10 is a soft magnetic material; the material
corresponding to plot 20 is a semi-soft magnetic material.
-
FIGURE 2 shows a time plot 20 of a typical dB(t)/dt signal received from
a semi-soft magnetic material and of a square wave 30. Square wave
30 is derived from the sinusoidal drive current to the drive coils. This
square wave 30 is used to provide a reference to start measuring the A-value
and the B-value.
Both the A-value and the B-value are time or relative phase delays
between reference points of the drive signal and a point at which the
peaks in the dB(t)/dt occur. In terms of signal processing it has been
found practical to sum the A-value and the B-value to obtain an
indication on the magnetic hardness of the material under detection.
The height C of the peaks of the dB(t)/dt signal is also measured. Only
measurements of C within a certain range of amplitude are further
processed since measurements of C lying outside that range indicate
that the article under detection is too remote or is too close. The height
C provides information the distance or the orientation of the magnetic
material. Due to the fact that an indication is given about the distance or
orientation of the material, the detection method can be a hand-held
method. Alternatively, if increased precision of measurement is needed,
it may be used in a configuration in which the distance and orientation of
the material from the signal drive and detection means is known. The
measurement of the magnetic hardness can then reliably based on the
sum of the above-mentioned A- and B-values. Using this approach it
has been found to be possible to minimise the effect of external fields
and to give reliable discrimination between materials of different
hardness. This reliability can be explained as follows. The A-value is
the time interval between a reference and a positive peak and the B-value
is the time interval between a reference and a negative peak. Any
extraneous magnetic fields are compensated in this way.
-
It has also been found that adding into the recognition algorithm a
parameter based on the shape of the positive and / or negative pulses of
the dB(t)/dt signal can give a further improvement in the ability to
discriminate materials. One example of this parameter is the width D of
the peak at one or more levels of the dB(t)/dt signal. Measurement of D
is a good way to determine if the returned signal is from a large object of
common Ferro-magnetic materials such as iron. Magnetically hard
materials such as iron will not be saturated by the detector field but they
will return a large sinusoidal signal. The shape of dB(t)/dt signals
returned from iron is much more rounded than the signals from soft
magnetic and semi-soft magnetic materials as shown in Figures 1 and 2.
To improve the consistency of the width measurement for different
magnitudes of the returned signal it is beneficial to use a circuit which
tracks the peak value and then measures the width at one or more fixed
fractions of the peak value.
-
If the security feature is constructed from several materials with different
magnetic properties, and particularly if this property is the magnetic field
required to drive them into saturation, then the returned signal will show
changes in shape as each material goes into saturation. In fact, a
double or a triple superimposed B-H hysteresis curve is obtained,
because of the different magnetic properties. Ferromagnetic coupling
between the various magnetic materials also effects this curve, which
means that the coercivity values of the various materials taken in
isolation, will be changed due to the combination of the materials. In the
case of thin films, this ferromagnetic coupling is largely dependent upon
the thickness of the layers so that a wide variety of security features can
be obtained.
The relative positions of the shape changes of the B-H curve can be
used in the same way as for the single materials to determine
parameters proportional to the hardness of the materials.
An example of such a security element is a combination of a thin
CoFeNiMoSiB film with a thin film of an amorphous CoxZrYNbz alloy.
-
As is well known, the magnetic properties of the materials can be
strongly affected by the shape factor (the ratio of length to cross-section
area). For example, if the security feature is in the form of magnetic
fibres of high permeability material, then the field at which they will
saturate can be controlled by altering the length to diameter ratio.
However, altering the orientation of the fibres relative to the magnetic
field will also change the field at which they will saturate and so this
needs to be taken into account in interpreting the signals from the
reading apparatus. One example, if the fibres are randomly distributed
in the substrate, is to extract the minimum saturation field to determine
the type of material present. An alternative would be to orient the fibres
so that they can be aligned with the interrogating magnetic field.
-
If the security feature is made up from a combination of materials
showing a significant anisotropy between the saturation field in the hard
and soft directions, and these materials are arranged with their soft axes
at a range of discrete angles, then the signal from a relative rotation
between the detector and material will show peaks as the drive field
aligns with each soft axis direction. This approach can be used to
provide a coded signal. The reference point for the coded signal could
be one layer with a greater thickness or permeability, which would
always give a greater signal than the other layers, or it could be via an
optical security feature and associated sensor system. An alternative
would be to use the shape anisotropy of magnetic fibres, which could
then be aligned at a series of discrete angles in the substrate to give the
same effect as, described above.
