Apparatus and method for detecting a magnetic marker

This invention relates to an apparatus and method for detecting the presence of a magnetic marker in an interrogation zone. A dual frequency magnetic field is generated causing a magnetic marker to produce amplitude modulated side band signals at harmonics of the higher frequency field. These side band signals are readily distinguishable from amplitude noise.

BACKGROUND OF THE INVENTION 
In a typical magnetic electronic article surveillance (EAS) system for 
detecting magnetic markers, a magnetized marker is placed in a 
interrogation zone in which an oscillating magnetic field is generated at 
a frequency "f" (kHz). The EAS system includes a generating coil for 
generating the magnetic field and a receiving coil for detecting signals 
generated by the markers. As the field passes a critical value of about 
0.1 oersteds (Oe), the magnetic dipole moment of the marker switches and 
emits a signal. This causes a pulse of voltage to be produced in the 
receiving coil. Half a cycle later the dipole moment switches back causing 
a second pulse of the opposite polarity to be produced in the receiving 
coil. Because the marker is designed to give sharp pulses, the generated 
signal contains high harmonics, i.e., signals at all multiples of the 
frequency of the field f. An alarm is set off using a threshold of the 
higher harmonics, e.g., 9f kHz, 10f kHz . . . 25f kHz. The shortcomings of 
prior systems are that the signal at these high hrmonics is very small and 
the amplifier also generates signals of the harmonics due to amplifier 
non-linearity. A relatively expensive and precise amplifier is needed to 
isolate the signal from coherent amplifier noise. 
SUMMARY OF THE INVENTION 
It has been found that generating dual frequency, overlapping magnetic 
fields of two substantially different values results in a greater ability 
to detect a marker in an interrogation zone. The first magnetic field is 
generated with a relatively high frequency and relatively large amplitude 
and the second overlapping magnetic field is generated with a 
substantially lower frequency and smaller amplitude. The phase of the 
higher frequency field at which the marker switches, oscillates at the 
frequency of the lower field, resulting in side bands being induced around 
the harmonics of the signal generated by the marker. These side bands are 
distinct from amplifier noise and are easy to detect. Viewed from the time 
domain rather than the frequency domain, amplitude of the even harmonics 
are modulated at the frequency of the lower field. Such amplitude 
modulated signals easily can be detected by inexpensive and accurate AM 
radio techniques.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring initially to FIGS. 1, 2, 5A and 5B, a system is shown generally 
at 10 wherein a dual frequency magnetic field may be generated to detect 
the presence of a magnetic marker. This system includes a pair of opposed 
gates 12 which create an interrogation zone therebetween as will be 
explained hereinafter. Each of the gates 12 includes a first magnetic 
field generating circuit 13 that has a coil 14. This coil 14 is connected 
to a first field generator 16 and will generate a magnetic field of 
relatively high frequency, in the range of 5-20 kHz, with a relatively 
high amplitude, 3-18 Oe. Also included in the first magnetic field 
generating circuit 13 are a plurality of capacitors 18. Located adjacent 
to and overlapping the first coil 14 are a pair of coils 22 that form part 
of a second field generating circuit 20. The coils 22 are connected to a 
second generator 24 with the current flowing in opposed directions as 
indicated by the arrows. The second field generating circuit 20 will 
generate a magnetic field of relatively low frequency, i.e., 0.05-3.0 kHz 
and an amplitude in the range of 0.1 to 0.5 Oe. The ratio of the frequency 
of the first field to that of the second field can vary from 2:1 to 100:1 
with the preferable ratio being approximately 20:1. The ratio of the 
amplitude of the first field to that of the second field can vary from 2:1 
to 30:1, the preferable ratio being about 5:1. Also associated with the 
interrogation zone is a pair of receiving coils 26 which are connected to 
a detector 28 (see FIG. 5C). As seen in FIG. 2, the detector coils 26 
overlap both the high frequency coil 14 and low frequency coils 22 with 
the current flow clockwise in one coil 26 and counter-clockwise in the 
other. The detector 28 may be any of a number of commercially available 
devices for detecting AM radio signals and may take the form of a buzzer, 
siren, light and the like. 
An article 30, which may be a package, article of clothing or the like, 
having a marker 32 connected thereto is shown within the interrogation 
zone. The interrogation zone is defined as the area between the two gates 
12. The magnetic marker 32 is made of a solid material 34 that supports a 
ferromagnetic material 36 capable of inducing a sharp electrical pulse in 
the pick-up coil 26 in response to the generated magnetic fields. An 
example of such a ferromagnetic material is permalloy. Normally the 
ferromagnetic material will be in the form of a strip approximately two to 
three inches long, having a width of approximately of one-quarter of an 
inch and a thickness of about 10 mils. This magnetic strip 36 is placed 
within or into the support material 34. This support material can be paper 
or plastic which may take the form of a label, ticket or tag. This marker 
32 can be attached to or located within any type of article 30 for which 
surveillance is required. 
As shown in FIG. 2, the coils 14, 22, and 26 overlap one another and are 
contained within a gate 12, there being two gates opposed and parallel to 
one another. An alternative arrangement is shown in FIG. 3 wherein the 
high frequency coil 14 and detector coils 26 are supported within opposed 
gates 12 and the low frequency field is generated by a coil 22 located in 
the floor at a location between the gates. The field generated by this 
coil 22 will have the same frequency and amplitude characteristics as the 
two coils shown in FIG. 2. 
Still another embodiment of the invention is shown in FIGS. 4 and 5C 
wherein the low frequency field generation and detection device are 
contained within one circuit 40. This circuit includes a low frequency 
generator 24, a choke 42, a pair of coils 44, a pair of RC filters 46 and 
a detector 28. With this circuit 40, the coils 44 will serve both as a low 
frequency field and to respond to a signal generated by a marker 32 to 
activate the detector 28 in cooperation with the RC filters 46. 
The field H(t) generated by the generating coils 14, 22 in the 
interrogation zone is defined by the equation: 
EQU H(t)=H.sub.d cos(2.pi..sub.fd t)+H.sub.m cos(2.pi.f.sub.m t) 
where 
Hd=the amplitude of the field generated by the first coil 
f.sub.d =the frequent of the field generated by the first coil 
H.sub.m =the amplitude of the field generated by the second coil 16, and 
f.sub.m =the frequency of the field generated by the second coil 16. 
Referring now to FIGS. 6A to 6D, graphs are included showing various modes 
of operation. FIG. 6A shows signals produced in an interrogation zone with 
no marker present and a frequency field generated only by the first field 
generator 13 at a frequency of 10 kHz. The signals represented in the 
graph at the various frequencies are produced by amplifier noise only. 
FIG. 6B is a graph similar to FIG. 6A except that it shows the effect of 
introducing a magnetic marker 32 into the interrogation zone. It will be 
noted that the signals in the interrogation zone are stronger but the 
signal from the marker 32 is generally indistinguishable from amplifier 
noise. 
FIG. 6C demonstrates the effect of creating a dual frequency field by 
enabling both the first field generator 13 and the second field generator 
20 and with a marker 32 in the interrogation zone. In this example, the 
second field generator 20 creates a magnetic field of one kHz and an 
amplitude of 0.2 Oe which is added to the field created by the first field 
generator 13, i.e. 10 kHz and 5 Oe. It will be noted that side bands are 
created about any harmonic of the frequency of the field generated by the 
first generator 16 that are readily detectable by an AM demodulator or 
receiver 28. These side bands are readily detectable because they are 
distinct from the field noise since modulation is not present in amplifier 
noise. As is known, harmonics are integer multiples of the frequency of 
the field. 
FIG. 6D is a graph showing an expansion of the ninth harmonic. The 90 kHz 
signal includes both the ninth harmonic and amplifier noise. The side 
bands, i.e., 87-89 kHz and 91-93 kHz contain no amplifier noise and, as a 
consequence, the presence of these side bands evidences the existence of a 
marker 32 in the interrogation zone. 
The advantages of this detection system are as follows: 
(1) the amplifier harmonics do not interfere with signal detection so that 
a low cost amplifier can be used; 
(2) the signal is an amplitude modulated sign wave at each harmonic of the 
high frequency field "f", i.e., 2f kHz, 3f kHz, 25f kHz, and 
(3) the signature of the marker signal is clearly distinguished from 
coherent noise sources so fewer false alarms will result.