SECURITY TAG

Abstract: A tag for use in an Electronic Article Surveillance system is disclosed. Such tags are typically provided internally with a transponder circuit comprised of first and second resonant circuits, each provided with inductive components between which there is at least some mutual magnetic coupling and/or electrical coupling. The tag according to the invention is identified by the angle subtended between the axes of the inductive components of the first and second resonant circuits, as it has been found by the applicants that this angle substantially affects the "pick rate" of a tag, i.e. the number of times a particular tag is actually detected by a system compared to the number of times the tag passes through the system. The longitudinal axes of said first and second inductance coils ideally subtend an angle of theta DEG , where 90 DEG < theta DEG < 180 DEG , theta DEG < > 90 DEG , 180 DEG .

Description: Security Tag The invention relates to a security tag for the protection of articles, and more particularly to a security tag for use in an Electronic Article Surveillance (EAS) system. Such systems are usually installed in clothing, book, pharmacy, hi-fi, white good, and some music retail outlets to prevent unauthorised removal of articles therefrom. The systems typically comprise one or more transmitters and detectors in the form of pedestals located at the customer entry/exit points of a retail outlet. These pedestals transmit a signal to a tag attached to the article which responds by re-radiating a second signal which is detected by the said pedestals thus triggering an alarm to indicate an attempt at unauthorised removal of articles. The tags may be permanently or releasably attached to the article. At the point of sale of the article, the tag is either removed from the article, or is deactivated if permanently attached thereto. EAS tags and systems are designed around different operating parameters, and the primary systems currently in use are Split Frequency (SF) , Radio Frequency (RF) , Acousto-magnetic (AM) , Microwave Frequency (MF) , Electromagnetic (EM) . This invention is primarily concerned with the SF and EM systems in which the tag is releasably attached to an article such that it can be reused after removal therefrom. In the SF and EM systems, an alternating magnetic field is transmitted between the pedestals and the tag is typically provided with a pair of LC resonant circuits which are electrically and/or magnetically coupled such that the second resonant circuit re- radiates a signal at a different frequency to that transmitted by the pedestals . This allows the pedestals to detect an active tag on an article as it passes therebetween. An important consideration in the design of any EAS system pedestals and tags is that the "pick rate" of the system, i.e. the percentage of tags moving between the pedestals which activate the system is as high as possible, as tags which pass undetected between the pedestals allow the unauthorised removal of the article to which they are attached. As tags may be attached to a large number of articles within the retail premises where the EAS system is installed, there is a requirement that the tags are inexpensive, and therefore the inclusion of complex circuitry in the tag or the provision of a battery powered circuit are precluded. The fact that many currently available EAS systems have a pick rate which is substantially below 100% (~65-70% is common) is a result of the relative weakness of the signal which is re-radiated by the tag as it passes between the transmission/ detection pedestals . A further disadvantage of the magnetically coupled resonant circuits incorporated within certain tags is that there is a often a mathematical restriction on the mutual coupling between the first and second circuits. Circuits which are too effectively mutually coupled fail to re-radiate a signal at a second and different frequency from that transmitted by the pedestals. Furthermore, it has be shown that certain resonant circuits become more sensitive and as a result more difficult to "tune" as the mutual coupling therebetween is increased. More specifically, the mutual coupling between the inductance coils of a resonant circuit affects the resonant frequency of each of said resonant circuits because the mutual inductance between the coils is reflected as an additional impedance in each of the resonant circuits . Also, the extent to which the inductance coils are disposed on a ferromagnetic core linking the two coils can affect both the mutual inductance between the coils and the self-inductance value within a resonant circuit. It will thus be appreciated by persons skilled in the art that by adjusting the position of the resonant circuits on the ferromagnetic core, the circuits can be tuned to provide a maximum response characteristic to magnetic radiation transmitted by the pedestal and received by the tag at a first frequency. A yet further disadvantage of current tags which incorporate magnetically coupled resonant circuits is their generally linear configuration which results from the linear nature of the ferromagnetic core which often couples the two circuits within the tag. When it is considered that the pedestals transmit a substantially uniaxial magnetic field, and that it is essential that this field induces a current in a inductance coil provided in at least one of the resonant circuits within the tag, the particular orientation of the tag as it passes through the field becomes important. More particularly, if a linear core and henceforth the longitudinal axis of the inductive coil thereon is perpendicularly orientated to said transmitted uniaxial field, then no current will be induced in the coil, and the tag will fail to re-radiate a second signal for detection. It will be understood by those skilled in the art that the correct functioning of the tag is therefore a question of the degree to which the longitudinal axis of a particular inductive coil incorporated in the tag is orientated with respect to the transmitted field. This factor is very influential on the correct functioning of the tag and contributes significantly to the reduced pick rate. It is the primary object of this invention to provide tag of similar electrical and magnetic configuration to those described above with an improved pick rate which nevertheless is neither more difficult nor expensive to manufacture. According to the invention there is provided a tag for use in an EAS system, said tag being provided with first and second resonant circuits between which there is at least some mutual magnetic coupling and/or electrical coupling, said first resonant circuit comprising a first inductance coil and said second resonant circuit comprising a second inductance coil, characterised in that the longitudinal axes of said first and second inductance coils subtend an angle of θ °, where 0°< θ °< 180°, θ°< >90°. It is preferable that a ferromagnetic core enhances the coupling between the first and second inductance coils . Preferably the longitudinal axes of the first and second inductance coils subtend an angle of θ °, where 90°< θ °< 180°. Further preferably the first and second inductance coils are at least partially disposed around either end of a single ferromagnetic core which couples first and second inductance coils, the ends of said ferromagnetic core being inclined towards one another by an amount θ °, where 90°< θ °< 180°. The "cranked" configuration of the tag according to the invention mitigates the problems associated with inducing a current in at least one of the resonant circuits because regardless of the orientation of the tag, at least one of the ends of the ferromagnetic core will always receive at least some magnetic flux from the transmitted uniaxial magnetic field. Furthermore, the mutual coupling between the respective first and second inductance coils is retained with this configuration. As much research has already been conducted into mutually coupled resonant circuits, the behaviour of tags incorporating circuits with such coupling is already well understood, and the efficacy of the tag as a result of the coupling and optionally the tuning of the resonant circuits can be maximised quickly. This reduces the overall cost of the tag. In addition, the cost of manufacture of tags according to the invention is only marginally increased on account of the reshaping of the moulds from which the tag bodies are produced. Furthermore, it is a simple matter to produce a "cranked" ferromagentic material core for use in the tags as opposed to a linear core, and thus it can be seen that the cost of tags according to the invention, and currently available tags will be only fractionally different. A further advantage of the invention arises from the facility for providing a simple transmitting system within the pedestals at the entry/exit points of the retail outlet in which the system is installed, as there is no need for sophisticated systems which transmit magnetic signals in a variety of differently orientated planes. A description of the invention is now provided by way of example only with reference to the accompanying drawings, wherein: Figure 1 shows a pair of resonant circuits disposed around a ferromagnetic core shaped according to the invention and for incorporation in an EAS tag; Figure 2 shows a perspective view of a tag according to the invention provided internally with the core arrangement of Figure 1 , and Figure 3 demonstrates the efficacy of the tag which positioned in a uniaxially applied magnetic field. Referring firstly to Figure 1 , a "cranked" ferromagnetic core 2 is provided at its ends 4, 6 with inductance coils LI , L2 (the numerals representing first and second resonant circuits which are formed with the said inductance coils by capacitors Cl , C2 respectively) which are wound on hollow bobbins 8, 10, the hollow of which is shaped according to the cross-sectional shape of the ferromagnetic core 2. In order to reduce the mutual coupling between the first and second inductance coils LI , L2 provided by the core 2, it is possible to enlarge the cross-sectional shape of the hollow within the bobbins 8, 10 in order that less of the magnetic flux produced by either inductance coil is concentrated within the core and thus transferred to the other coil. This may be required to ensure that the mutual coupling between the circuits does not exceed the threshold value discussed above. The longitudinal axes of the inductance coils are denoted by LI * , L2 * , and it can be seen from the Figure that these axes subtend an angle of θ °. It is the intention of the invention that this angle is not 90°, for if this were the case, the inductance coils LI and L2 would not be mutually coupled. Once the first and second resonant circuits have been tuned such that a signal of different frequency is re-radiated by second circuit on receipt by the first circuit of a signal of a particular frequency, then the core and circuits disposed thereon may be inserted into a tag of the type shown in Figure 2. From Figure 2, a tag 12 is of essentially two-piece construction having an upper half 14 and a lower half 16, each of said halves being provided with cavities (not shown) capable of receiving the upper and lower half of the core shown in Figure 1 and the inductance coil bobbins 8, 10 shown in Figure 1. The core 2 can thus be sandwiched between the said upper and lower halves l 4, 16 and encased thereby. The said upper and lower halves are ideally formed of a rigid plastics material. Additionally the tag 12 is provided with a protrusion 18 which houses a clutch arrangement (not shown) for receiving a pin shank 20 and securing same therein. Such a clutch arrangement is disclosed in United States Patent No. 4523356 to Lincoln Chariot, and comprises a spring loaded anvil which forces a set of ball bearings into a cup. The anvil may be displaced against the action of the spring under magnetic means thus releasing the pin shank 20. Additionally, the pin shank 20 has reduced diameter portions 22 on which the ball bearings within the clutch mechanism engage and prevent release of said pin shank. Referring finally to Figure 3, a pair of tags 2', 2" are shown in different orientations . Corresponding pairs of orthogonal axes x, y are shown in the Figure to indicate the primary advantage of the tag. A pair of pedestals are diagrammatically indicated at 32, 34 at comprise in their simplest form loops of wire 36, 38 through which is passed an alternating current via electrical connections 40, 42. The loops of wire 36, 38 shown are transmission loops. The loops of wire 36, 38 effectively form inductive coils through which is generated a magnetic flux B, of alternating frequency identical to that of the current passed through the loops. The tags detect this magnetic signal and re-radiate a second magnetic signal of different frequency, and it is this signal which is detected by detection loops which are provided adjacent with or internally of the transmission loops . In the interests of clarity the detection loops are not shown. It should be pointed out that Figure 3 is a schematic representation provided only to aid understanding of the invention. It will be appreciated that the particular orientation and powering of the transmission and detection loops provided within the pedestals may be complex, and as it is not the subject of the invention, specific disclosure is not provided herein. It will be seen from the two different orientations of the tags 2', 2" and in particular the orientation of the "cranked" ends of the tags that a directional component of the magnetic flux density B can always be resolved in a direction parallel to at least one such cranked end of the tag. It is therefore the case that regardless of the orientation of the tag, a current will be induced in at least one of the resonant circuits within the tag as a result of the changing directional component of magnetic flux passing therethrough. The mutual coupling between the first and second inductance coils of the circuits ensures that a signal is retransmitted by the tag. In the circumstance where a ferromagnetic core is provided within the tag, the directional component of magnetic flux is concentrated by the core, and the magnitude of the current induced in a particular resonant circuit is increased. This increases the efficacy of the tag in re-radiating the second signal at the second frequency which is detected by the pedestals. It is to be pointed out that the resonant circuits shown in Figure 1 , which are simple LC circuits, are provided only to illustrate the working of the invention. Any particular desired resonant circuit configurations may be adapted for use in this invention. For example circuits incorporating variable reactance or inductance elements, and circuits which are electrically coupled may be employed without departing from the spirit or exceeding the scope of the invention.

Claims: CLAIMS: 1. A tag for use in an EAS system, said tag being provided with first and second resonant circuits between which there is at least some mutual magnetic coupling and/or electrical coupling, said first resonant circuit comprising a first inductance coil and said second resonant circuit comprising a second inductance coil, characterised in that the longitudinal axes of said first and second inductance coils subtend an angle of θ °, where 0°< θ °< 180°, θ °< >90°. 2. A tag according to claim 1 characterised in that a ferromagnetic core enhances the coupling between the first and second inductance coils. 3. A tag according to any of the preceding claims characterised in that the longitudinal axes of the first and second inductance coils subtend an angle of θ °, where 90°< θ °< 180°. 4. A tag according to any of the preceding claims characterised in that the first and second inductance coils are at least partially disposed around either end of a single ferromagnetic core which couples first and second inductance coils, the ends of said ferromagnetic core being inclined towards one another by an amount θ °, where 90°< θ °< 180°.

Inventors: PARFITT ANTHONY, PARFITT, ANTHONY
Applicants: CLAN HOLDINGS LTD, PARFITT ANTHONY, PARFITT, ANTHONY
Application year: 1999
Main classification: G08B13
IPC codes: G08B13