It is well known to provide electronic article surveillance systems to prevent or deter theft of merchandise from retail establishments. In a typical system, markers designed to interact with an electromagnetic field placed at the store exit are secured to articles of merchandise. If a marker is brought into the field or "interrogation zone," the presence of the marker is detected and an alarm is generated. On the other hand, upon proper payment for the merchandise at a checkout counter, either the marker is removed from the article of merchandise or, if the marker is to remain attached to the article, then a deactivation procedure is carried out which changes a characteristic of the marker so that the marker will no longer be detected at the interrogation zone.
In one type of widely-used EAS system, the electromagnetic field provided at the interrogation zone alternates at a selected frequency and the markers to be detected include a magnetic material that produces harmonic perturbations of the selected frequency on passing through the field. Detection equipment is provided at the interrogation zone and is tuned to recognize the characteristic harmonic frequencies produced by the marker, and if such frequencies are present, the detection system actuates an alarm. According to one conventional practice, the marker includes a first type of high permeability magnetic material which exhibits a relatively smooth hysteresis loop characteristic. One example of this first type of material is known as "Permalloy." A disadvantage of this type of material is that the harmonic signals produced by this type of material are not always readily distinguishable from harmonic disturbances caused by coins, keys, belt buckles, metallic articles of merchandise, or other non-marker items that may be brought into the interrogation zone.
U.S. Pat. No. 4,660,025 (issued to Humphrey and commonly assigned with the present application) proposes a second type of material for use in EAS markers. The second type of material has a hysteresis loop characteristic with a substantial discontinuity and represents an improvement as compared to the first type of material, because, for a given strength of interrogation signal, the second type of material generates detectable amplitudes of substantially higher harmonics than the first type of material. These higher harmonics are not likely to be produced by non-marker materials, so that the detection equipment can be tuned in such a manner that it detects the second type of material without generating false alarms in response to non-marker material. Markers incorporating the second type of material are widely used in EAS systems marketed under the trademark "AISLEKEEPER" by the assignee of the present application.
U.S. Pat. No. 4,980,670 (issued to Humphrey and Yamasaki and commonly assigned with the present application) proposes a third type of magnetic material for use in EAS markers. The third type of material is processed to fix the locations of the walls of magnetic domains in the material so that the material exhibits a hysteresis loop characteristic which, somewhat similarly to the characteristic of the second type of material, has a step change in magnetic flux. The third type of material generates a signal that is rich in high harmonics like the signal generated by the second type of material and thus shares the advantages of the second type of material, while providing certain additional advantages including additional convenience in deactivation.
The disclosures of the above-referenced '025 and '670 patents are incorporated herein by reference.
One of the difficulties encountered in electronic article surveillance is that the amplitude level of the interrogation signal varies from point to point within the interrogation zone. Also, the path along which the article of merchandise with the marker attached is transported through the interrogation zone cannot be practically controlled, so that it is far from certain that the marker will be placed at a point in the interrogation zone where the interrogation field is at a maximum amplitude. Furthermore, the variation in field strength from one point to another in the zone can be quite large, and the harmonic signal generated by a marker present at a point of maximum field strength may be much greater than the harmonic signal generated by a marker which traverses the interrogation zone along a path which avoids the point of maximum field strength. It is therefore necessary, in order to provide reliable detection of all markers of interest, to set the detection equipment to detect relatively low amplitudes of the harmonics produced by the marker. However, as indicated at FIG. 10 of '025 patent, the first type of magnetic material, if exposed to a field of sufficient amplitude, may generate high harmonics at a detectable level and therefore mimic the signature characteristic of the second and third types of material. Of course, a retail establishment using an EAS system designed to detect markers incorporating the second and third types of materials (hereinafter "second and third types of markers") would not intentionally affix a marker including the first type of material (hereinafter "first type of marker") to articles of merchandise sold at the establishment. However, there is an increasing trend in the field of electronic article surveillance for a marker to be incorporated in or packaged with an article of merchandise by the manufacturer or distributor so that the retailer is not required to apply markers to the merchandise. As a result of this practice (known as "source tagging") there may be cases in which a retailer who uses an EAS system designed to detect the second and third types of markers receives in his inventory items that already have the first type of markers incorporated therein. If the retailer is not aware of the presence of the incorporated marker, or for other reasons is not able or willing to deactivate or remove the marker, then false alarms may be occasioned when it happens that the first type of marker is placed in a position in the interrogation zone which results in mimicking of the signature of the second and third types of markers. Such a scenario may also take place, for example, when a customer brings into the store goods purchased at another location and having incorporated therein an active marker of the first type.
Thus, it would be desirable to provide an EAS system in which different types of markers can be reliably distinguished from each other, notwithstanding a tendency of one type of marker to mimic another type of marker under certain circumstances.
It would also be desirable to provide an EAS system which can be set to selectively recognize the presence of only one of two or more types of marker. A retail establishment which had such a system installed would then have flexibility in selecting the type of marker to be used with the system.
More generally, it is desirable that EAS systems be provided which can discriminate with greater precision between signals generated by markers that are of interest and other signals, including noise signals and signals generated by metallic items that are not markers.