‘Magneto-acoustic marker for electronic article...

Communications: electrical – Condition responsive indicating system – Specific condition

Reexamination Certificate

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C340S572100, C148S108000

Reexamination Certificate

active

06359563

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a magneto-acoustic marker for use in an electronic article surveillance system, as well as to an electronic article surveillance system employing such a magneto-acoustic marker, and to a method for making such a magneto-acoustic marker.
2. Description of the Prior Art and Related Applications
Magneto-acoustic markers for electronic article surveillance (EAS) typically include an elongated trip of a magnetostrictive amorphous alloy which is magnetically biased by an adjacent strip of a magnetically semi-hard metal strip.
The typical requirements for such EAS markers are: a consistent resonant frequency at a given bias field which is primarily determined by appropriate choice of the length of the resonator, a linear hysteresis loop in order to avoid interference with harmonic systems, which is achieved by annealing the amorphous ribbon in a magnetic field perpendicular to the long axis of the resonator, a low sensitivity of the resonant frequency to the bias field, a reliable deactivability of the marker when the bias field is removed, and a (preferably) high resonant amplitude which persists for a sufficient time when the exciting drive field is removed.
Such resonators can be realized by choosing an amorphous Fe-Co-Ni-Si-B alloy which has been annealed in the presence of a magnetic field applied perpendicularly to the ribbon axis and/or a tensile stress applied along the ribbon axis. The annealing is preferably done reel to reel with typical annealing times of a few seconds at temperatures between about 300° C. and 420° C. Thereafter the ribbon is cut to oblong pieces which form the resonators. Such resonators, and a general background description of the physics and prior art relating to magneto-acoustic markers, are described in co-pending U.S. application Ser. No. 08/890,612 (“Amorphous Magnetostrictive Alloy with Low Cobalt Content and Method for Annealing Same,” G. Herzer), filed Jul. 9, 1997 and co-pending U.S. application Ser. No. 08/968,653 (“Method of Annealing Amorphous Ribbons and Marker for Electronic Article Surveillance,” G. Herzer) filed Nov. 2, 1997. Both of these co-pending applications as assigned to the same assignee (Vacuumschmelze GmbH) as the present application, and the teachings of both of these co-pending applications are incorporated herein by reference.
Typical markers for EAS use a single resonator which is about 38 mm long, about 25 &mgr;m and about 12.7 mm or 6 mm wide. The wider marker generally produces about twice the signal amplitude of the narrower marker, however, the narrower marker is more desirable because of its smaller size. A magnetostrictive marker employing two or more elongated strips of magnetostrictive ferromagnetic material, however, is described in U.S. Pat. No. 4,510,490. In the marker described therein, the strips are disposed side-by-side in a housing. The reason for using multiple resonator strips in this known marker is stated in the reference to be for the purpose of allowing the marker (i.e., the respective multiple strips thereof) to resonate at different frequencies, thereby providing the marker with a particular signal identity.
SUMMARY OF THE INVENTION
It is an object of the present invention is to provide a magneto-acoustic marker having reduced dimensions without degradation in performance.
More specifically it is an object of the present invention to provide a magnetostrictive amorphous metal alloy for incorporation in such a marker in a magnetomechanical surveillance system which can be cut into oblong, ductile, magnetostrictive strips which can be activated and deactivated by applying or removing a pre-magnetization field H and which in the activated condition can be excited by an alternating magnetic field so as to exhibit longitudinal, mechanical resonance oscillations at a resonance frequency F
r
which, after excitation, are of high signal amplitude.
It is a further object of the present invention to provide such an alloy wherein only a slight change in the resonant frequency occurs given a change in the bias field, but wherein the resonant frequency changes significantly when the marker resonator is switched from an activated condition to a deactivated condition.
Another object of the present invention is to provide such an alloy which, when incorporated in a marker for magnetomechanical surveillance system, does not trigger an alarm in a harmonic surveillance system.
It is also an object of the present invention to provide a marker embodying such a resonator, and a method for making a marker suitable for use in a magnetomechanical surveillance system.
It is finally an object of the present invention to provide a magnetomechanical electronic article surveillance system which is operable with a marker having a resonator composed of such an amorphous magnetostrictive alloy.
The above objects are achieved in a method for making a magneto-acoustic EAS marker wherein two (or more) short oblong pieces of a narrow amorphous ribbon are disposed in registration in a housing to form a dual (multiple) resonator, with the respective resonant frequencies of the individual resonator pieces coinciding to within about +/−500 Hz and preferably within +/−300 Hz. This can be achieved by giving these pieces the same length and width, the same composition and the same annealing treatment. As a consequence it is advantageous to put two (or more) consecutively cut pieces (cut to the same length) together. Such an inventive magnetoelastic marker is capable of producing a resonant signal amplitude comparable to a conventional magnetoelastic marker of the prior art of about twice the width.
As used herein, placing the pieces “in registration” means that the pieces are disposed one over the other with a substantial overlap, if not exact congruency. In any event, the term is intended to preclude a side-by-side arrangement as in the prior art.
For a dual resonator it is advantageous to choose an Fe-Ni-Co-base alloy with an iron content of more than about 15 at % and less than about 30 at % which is annealed in the presence of a magnetic field perpendicular to the ribbon axis and/or with a tensile stress applied along the ribbon axis. A generalized formula for the alloy compositions which, when annealed as described above, produces a dual resonator having suitable properties for use in a marker in a electronic article surveillance or identification system, is as follows:
 Fe
a
Co
b
Ni
c
Si
x
B
y
M
z
wherein a, b, c, x, y and z are in at %, wherein M is one or more glass formation promoting elements such as C, P, Ge, Nb, Ta and/or Mo and/or one or more transition metals such as Cr and/or Mn and wherein
15≦a≦30
6≦b≦18
27≦c≦55
0≦x≦10
10≦y≦25
0≦z≦5
14≦x+y+z≦25
such that a+b+c+x+y+z=100.
In a preferred embodiment the resonator assembly consists of two ribbon pieces in registration, each ribbon piece having a thickness between about 20 &mgr;m and 30 &mgr;m, a width of about 4 to 8 mm and a length between about 35 mm to 40 mm.
The objects of the invention can then be realized in a particularly advantageous way by using the following refined ranges in the above formula
20≦a≦28
6≦b≦14
40≦c≦55
0.5≦x≦5
12≦y≦18
0≦z≦2
15<x+y+z<20
such that a+b+c+x+y+z=100.
Examples for such alloys which are particularly suitable for a dual resonator which is about 6 mm wide and in a range between 35 mm to 40 mm in length are as follows. Suitable alloys which have been tested are represented by alloys Nos. 3 through 9 in Table I, namely Fe
24
Co
12.5
Ni
45.5
Si
2
B
16
, Fe
24
Co
12.5
Ni
44.5
Si
2
B
17
, Fe
24
Co
13
Ni
45.5
Si
1.5
B
16
, Fe
24
Co
12
Ni
46.5
Si
1.5
B
16
, Fe
24
Co
11.5
B
16
, Fe
24
Co
11
Ni
48
Si
1
B
16
and Fe
27
Co
10
Ni
45
Si
2
B
16
. Various further compositions were tested in order to optimize the silicon and boron content in compositions having an iron content of 24

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