Resonant tag and deactivator for use in an electronic security system

(54) DESIGNATION Resonant label circuit including means for destroying its resonant properties and security system intended for such label circuit (56) QUOTE PUBLICATIONS: US A 3 624 631 (340/280), US A 3 913 219 (29/592),

US A 3,967,161,317 / 101R) (579 SUMMARY:

A resonant label circuit (L1 Cl; L2, C2) having at least one resonant frequency operating in an electronic security system in which the label circuit is sensed and electronically deactivated to destroy the resonant characteristics of the label circuit at the detection frequency. The label circuit is designed so that the capacitor plates (10. 12. 22. 24) are closer together at a selected point (20. 32). This provides a collapse mechanism within the resonant structure of the label. An deactivator provides electromagnetic energy at the resonant frequency of the label circuit and of sufficient energy to effect electrical breakthrough at the selected point (20, 30). This deactivates the label circuit by causing evaporation of a conductive region or shorting the capacitor plates to destroy the resonant properties of the circuit.

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Technical area

This invention relates to electronic security systems for detecting a resonant label circuit in a monitored area and more particularly to a label circuit and device for electronically deactivating the label circuit.

Background of the Invention

Electronic security systems are known for detecting unauthorized removal of items from a detection area. Such systems have been used especially for use in retail stores to prevent theft of articles from the store and in libraries to prevent theft of books. Such electronic security systems generally contain an electromagnetic field which is provided in a monitored area through which articles must pass through the protected premises. A resonant label circuit is attached to the articles and the presence of the label circuit in the monitored area is sensed by a receiving system to indicate the unauthorized removal of the article. The label circuit is removed by authorized personnel from an item that leaves the premises in an orderly manner to allow passage of the item through the controlled area without alarm activation.

Systems are also known for electronically deactivating a resonant circuit such that the activated circuit can remain on the article that properly leaves the premises. Such a system is disclosed in US Patent 3,624,631 in which a fusible link is in series with an inductor and is fired by a high power radio frequency transmitter. The resonant circuit is interrogated by a swept radio frequency, the presence of this circuit in the monitored area providing energy absorption at the resonant frequency, which is detected by a receiver for subsequent alarm operation. When applying a swept frequency with higher%

460 809 energy than that used for detection, the fusible link of the resonant circuit can be destroyed to deactivate the tuned circuit so that no detection is possible. The deactivation must be accomplished by a swept radio frequency transmitter operating at sufficiently low radiation levels to meet the requirements of the Federal Communications Commission, and therefore the fusible link must be extremely small and made of a material that allows low power melting. The small fusible link has a high resistance which occurs in series with the inductor of the resonant circuit. The series resistance decreases the Q value of the resonant circuit, thereby reducing the sensitivity of the circuit to be detected. The current levels at which the fusible link melts are determined by the geometry of the link as well as by the thermal conductivity of the materials surrounding the melt only link. Therefore, the melting current is to a large extent influenced by the material covering and supporting the fusible link.

Another electronic security system is disclosed in US patent

3 810 147 with the same inventor as the present invention, in which a resonant circuit is utilized having two distinct frequencies, one for detection and one for deactivation. A small fusible link is utilized in the deactivation circuit which also contains a second capacitor to provide the distinct resonance frequency for deactivation.

The resonant circuit may have a resonant frequency that will vary within an area depending on manufacturing tolerances. The deactivation frequency is at a fixed frequency and therefore the resonant circuit cannot be precisely matched to the fixed deactivation frequency. The serial impedance of the inductor and the capacitor at the intended deactivation frequency must be as small as possible to allow maximum current to flow through the fusible link to effect the firing of the link. Therefore, the capacitor would have a value as large as possible and the inductor a value as small as possible. In practical construction, the inductor is designed as a single revolution and the capacitor is formed of plates as large as possible

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460 809 which is compatible with the financial and physical constraints of the particular label circuit. The size of the capacitor increases the cost and size of the total resonant circuit.

Summary of the Invention

The present invention provides a resonant label circuit having at least one resonant frequency and operating in an electronic security system in which the label circuit is sensed and electronically deactivated to destroy or alter the resonant characteristics of the label circuit at the detection frequency. The resonant label circuit is deactivated electronically through a mechanism that breaks down and operates within the resonant structure of the label without the need for a fusible link and without affecting or decreasing the Q value of the resonant circuit. The resonant label circuit is of a flat type having a flat coil formed on a surface of a thin plastic substrate film and at least one capacitor formed by capacitor plates on respective opposite surfaces of the substrate. Energy is coupled to the label circuit at or near the resonant frequency and means in the conductive regions define a path in which an electrical breakdown is primarily apt to occur upon application of an electromagnetic field at a deactivation frequency, as defined in claim 1. The resonance structure includes means for ensuring that the breakdown almost always occurs in a predetermined region of the substrate. In response to applied energy of sufficient size, an electric arc is formed through the substrate film to effect evaporation of an ambient or adjacent conductive region, thereby destroying the resonant properties of the circuit. Alternatively, the electrical breakdown through the substrate film can cause the formation of a plasma and deposition of metal between the capacitor plates along the discharge path, thereby forming a permanent short circuit between the capacitor plates which destroys the resonant properties of the circuit.

Summary of the drawings

The invention will be more fully understood by the following detailed description taken in combination with the following

460 809 of the accompanying drawings, in which:

Fig. 1 is a schematic diagram of a resonant label circuit embodying the invention;

Figure 2 is a schematic diagram of a label circuit with two resonant frequencies utilizing the invention;

Figures 3 and 4 show views of respective sides of the resonant label circuit of Figure 1;

Figures 5 and 6 are views of respective sides of the resonant label circuit of Figure 2;

Fig. 7 is a block diagram of an electronic security system utilizing the invention;

Fig. 8 is a schematic diagram of an alternative embodiment of a single resonant frequency circuit;

Fig. 9 is a schematic diagram of an alternative embodiment of a label circuit with two resonant frequencies;

Figures 10, 11 and 12 are schematic presentations of the electrical breakdown mechanisms utilized in the invention; Fig. 13 is a diagram of electronic apparatus for determining the resonant frequency of a label circuit to be deactivated;

Figs. 14 and 15 are waveforms useful for illustrating the operation of the device of Fig. 13; and Fig. 16 is a block diagram of an electronic deactivator providing deactivation energy during a controlled interval.

Detailed description of the invention

In FIG. 1 shown in schematic form a resonant label circuit containing a capacitor C1 formed by capacitor plates 10 and 12 on respective opposite surfaces of a substrate 14 which is of a dielectric or electrically insulating material, and an inductor L1 in series with the capacitor to provide a single resonant frequency , the inductor is coupled at one end to capacitor plate 10 and has the other end connected to an electrical path 16 through the substrate 14 which is coupled to capacitor plate 12 via a conductive path 18. The inductor and capacitor plate 10 are formed as a unit on

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460 809 a side of the substrate. Typically, the inductor is shaped like a flat rectangular coil on the substrate surface. Similarly, capacitor plate 12 and associated connecting path are integrally formed on the opposite substrate surface. The flat label construction will be described in detail below. A portion 20 of the conductive path 18 facing the capacitor plate 10 is recessed or otherwise formed to be separated from the capacitor plate 10 at a distance less than the distance between the plates 10 and 12. When sufficient electrical energy is coupled to the label circuit at or near the resonant frequency of the circuit, the voltage across the capacitor plates 10 and 12 increases until electrical breakdown occurs at the burn-off point caused by the recessed portion 20 of the conductive path. If this portion provides the shortest distance between the capacitor plates, electrical breakdown always occurs at this point. The electric arc formed at the breakdown is maintained by the energy which is continuously connected to the resonant circuit through an external energy source. The electric arc evaporates metal in the vicinity of the breakdown region 20 which destroys the conductive path 18 and thereby permanently destroys the resonant properties of the label circuit.

An alternative embodiment of the resonant label circuit is shown schematically in Figure 2 in which the label circuit has two resonant frequencies. In addition to capacitor C1 formed by plates 10 and 12 and inductor L1, the circuit of Fig. 2 contains a second capacitor C2 formed by plates 22 and 24 and an inductor L2. The junction between the inductors L1 and L2 is coupled to the capacitor plate 22. The other end of the inductor L2 is coupled to a through connection 26 in the substrate and which is connected via a conductive path 28 to the capacitor plate 24. A conductive path 30 connects capacitor plates 24 and 12 and this conductive path contains a designated burnout portion 32 provided in a capacitor plate 22 reversed contact.

A resonant frequency is used to detect the label with

460 809 an associated electronic security system and the second resonant frequency are utilized to deactivate the label. Usually, the deactivation rate is chosen to be one of the frequencies assigned by Federal Communications

Commission (FCC) to be in the Industrial, Scientific and Medical Application (ISM) band so that the radiated energy for label deactivation can be at a relatively high power without a special federal license. The detection frequency is usually selected to be in one of the frequency bands 10 assigned to field interference sensors. A detection frequency of 8.2 MHz is typical.

Capacitor C2 and inductor L2 are the primary components which form a resonant tuned circuit at the deactivation frequency, while inductor L1 in combination with the capacitor C1 are the primary components which form a resonant tuned circuit at the detection frequency. Depending on the reciprocal coupling, all components interact to provide the exact frequencies for detection and deactivation. When sufficient energy is coupled to the circuit at the deactivation frequency, the voltage across the capacitor plates 22 and 24 increases until there is a breakdown in the substrate film at the burn-off point 32. Again, the breakdown always occurs at the burn-off point because this point or area 32 provides the shortest distance between the capacitor plates 22. The electric arc arising from the breakdown is maintained by the energy coupled to the resonant circuit from the external energy source and this arc causes gasification of metal in the vicinity of the breakdown region, including the adjacent portion of the conductive path 30. When the external energy is cut off, the electric arc. The resonant properties of the label at the detection frequency are permanently destroyed as there is no longer an electrical connection between the capacitor plate 24 and the capacitor plate 12.

The resonant circuits of Figures 1 and 2 do not require the use of a small narrow fuse and therefore there is no additional resistance placed in series of inductor and condensation.

460 809 tor elements in the circuit. Therefore, there is no deterioration in the Q value of the resonant circuit. Furthermore, since the electric arc occurs between the capacitor plates and does not affect the surface, the materials covering or in contact with the capacitor plate's surfaces do not significantly affect the electric arc's ability to gasify the metal in the vicinity of the arc. In order to maximize the voltage developed across capacitor plates 22 and 24, the capacitance of capacitor C2 should be as small as possible and the inductance of inductor L2 be as large as possible to resonate at the intended deactivation frequency. Capacitor C2 can be made physically very small and will not significantly increase the total size and cost of the label circuit with two frequencies according to Fig. 2.

The resonant label circuit of Figure 1 is illustrated in typical construction of Figures 3 and 4 which depict respective opposite planar surfaces of the label. According to Fig. 3, the inductor L1 is formed as a flat coil 40 on the surface of the thin plastic film substrate 42. The plastic film serves partly as a dielectric for the parallel plate capacitor and partly as the supporting substrate for the circuit. The helical web extends between an outer conductive region 44 and an inner conductive region 46. The inner conductive region 46 serves as capacitor plate 10. On the opposite side of the label shown in Fig. 4, conductive regions 48 and 50 are arranged opposite the conductive regions 44 and 50, respectively, and are connected by a conductive path 52.

The conductive region 50 serves as the capacitor plate 12 and thus the capacitor C1 is made up of the facing leads 30 of the regions 46 and 50. A conductive connection 54 interconnects the conductive regions 44 and 48 to complete the circuit.

The conductive region 50 includes recesses 51 adjacent the connecting region between the conductive region 50 and the conductive path 52. This region includes a recessed portion 56 to provide a conductive region of the path 52 which faces the conductive region 46 and which is closer this than the distance between the conductive regions 46 and 50. This recess 56 provides the firing point at which

460 809 electrical breakdown will occur in response to the supply of energy from an external source at the resonant frequency of the label circuit and with sufficient power to cause breakdown.

The two-frequency label circuit of FIG. 2 is shown in typical construction of FIGS. 5 and 6, which depict the respective opposite planar surfaces of the label. The inductor L1 is formed as a flat coil 60 on the surface of the plastic film 62, which coils extends between conductive regions 64 and 66. The inductor L2 is formed by a flat coil 68 on the film surface and which extends between conductive region 64 and conductive region 70. On the opposite side of the film substrate shown in FIG. 6, conductive regions 72, 74 and 76 are arranged opposite the conductive regions

64, 66 and 70, respectively, on the second substrate surface. The conductive regions 72 and 74 are connected by a conductive path 78, while the conductive regions 72 and 76 are connected to a conductive path 80. A burn-off point is provided in the conductive path 78 by recessing a portion 82 of the path facing the conductive path 78. the capacitor C2 of Fig. 2 is provided by the conductive regions 66 and 74, while the capacitor C2 is provided by the conductive regions 64 and 72. A conductive internal connection 84 between the conductive regions 70 and 76 is provided through the substrate film to complete the circuit.

The circuit is operative in the manner described to effect destruction of the resonant properties of the label circuit at the detection frequency by burn-out or evaporation of the conductive path near the burn-off point 82 as a result of an electric arc.

The resonant labels described herein are similar to those of Patent 3,810,147 to the present inventors. The design of the label circuits is advantageously carried out according to the fabrication process for flat circuits which is the subject of patent 3 913 219 with the present inventor.

Fig. 7 shows equipment for use in deactivating the resonance characteristics of the label circuits described above.

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This equipment includes an antenna 90 operable to sense the presence of a resonant label circuit 92 and coupled to a label sensing system 94 which provides an output to a label presence indicator 96 and a label deactivation indicator 98. The tag sensing system 94 also provides a control signal to a tag deactivation system 100 which includes an antenna 102. The tag deactivation system can also be manually activated by a manual control 104. In detecting the presence of the label circuit 92, the label sensing system 94 is operative to set the deactivation system 100 to provide radiation through the antenna 102 of radiation at the resonant frequency of the label circuit and at a sufficient energy level to provide electrical breakdown of an electric point junction at the label circuit 15. In the event that it is a label with two frequencies detected, the deactivation system provides energy at the deactivation frequency of that label. Visual or other indications may be provided by indicators 96 and 98 for the presence and deactivation of the label.

If a label circuit is a single resonant circuit shown in Fig. 1, the label sensing system 94 is operative to determine the resonant frequency of the particular label 92 which is sensed and to provide a control signal to the deactivation system 100 representative of the measured resonant label frequency. In response to the control signal, the deactivation system will provide radiation at this resonant frequency and efficient coupling to the label circuit to disrupt its resonance characteristics. The label sensing system may contain the devices shown in Fig. 13 for determining the approximate resonant frequency of the label circuit. A voltage controlled oscillator 150 drives the label sensing antenna 90, which oscillator is controlled by the magnitude of a microcomputer 152 via a digital-to-analog converter 154. The microcomputer stores digital values ​​which, after conversion to analog form of the converter 154, drive the oscillator 150 to generate a frequency sweep. The antenna signal is applied to one

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analog-digital converter 156 whose digital magnitude is supplied to the microcomputer 152 which stores such digital magnitudes.

The operation of the equipment of Fig. 13 will be explained in connection with the waveforms of Figs. 14 and 15. The magnitude of the voltage-controlled oscillator 150 is shown in Fig. 14 and includes frequency steps, with each step occurring for a corresponding time interval or step number. Fig. 15 illustrates the current through the antenna 90 as a function of time. Without any resonant circuit present, the current through the antenna decreases as the frequency of the oscillator increases as illustrated by the straight portion of the waveform of FIG. as illustrated in Fig. 15. The current through the antenna is converted to digital values ​​by the transducer 156 and these digital values ​​are stored in the memory of the microcomputer 152. The step number corresponding to the minimum value of the stored current values ​​corresponds to the approximate resonant frequency of the label20 circuit. The stored digital value representing the resonant frequency is converted into an analog signal for controlling the oscillator 150 to provide an output at the resonant frequency for actuation of the deactivation system 100 (FIG. 7) to destroy the resonant properties of the label circuit 92.

The deactivation energy is supplied for a predetermined period of time determined in accordance with the intended time for label deactivation. After deactivation, the label sensing system 94 is operative to sense label presence and if label 30 has been deactivated, indicator 98 will be turned on to indicate that deactivation has occurred. If the label 92 is still operating at its resonant frequency as sensed by the system 94, the deactivation system 100 will again be started for another deactivation cycle. The deactivation cycle will be repeated a set number of times until deactivation occurs. If deactivation has not occurred after a predetermined number of cycles, a signal may be switched on to notify the operator that the particular label has not

460 809 activated. The operator can then manually manipulate the deactivation system to deactivate the label or take other measures to deactivate or destroy the label. Alternatively, upon detecting a resonant label circuit 92, the tag sensing system 94 may cause the deactivation system 100 to operate the antenna 102 with a relatively high power signal whose frequency is slowly swept past the resonant frequency of the tag 92. The equipment may be operative to alternatively sense label presence and activate the deactivation field in a cyclic manner until the label is deactivated. Again, an operator may be alerted by appropriate notification in the event that a particular label has not been deactivated.

In case a dual frequency label circuit is utilized, the label sensing system 94 is operative to detect the resonant detection frequency of the label while the deactivation system 100 is operative to provide energy at the resonant deactivation frequency of the label. Equipment suitable for deactivating and sensing the label with two frequencies is disclosed in patent 3,938,044 with the same inventor as the present invention.

Resonance circuits of alternative construction are shown in Fig. 8 and will be recognized as similar to the circuits in Figs. 1 and 2, respectively. In the embodiments of Figs. 8 and 9, countersinks are made at any selected point or points on one or both capacitor plates to reduce the thickness of the dielectric film at this recess, thereby reducing the required voltage to provide an arc across the capacitor plates. In the embodiment of FIG. 8 the recess is shown in capacitor plate 12a. In the embodiment of Fig. 9, the recess is shown in capacitor plate 24a. When applying energy at the resonant frequency of the label of sufficient size, electrical breakdown occurs through the dielectric film at the recessed point and as energy is supplied to the label, the arc tends to be maintained and forms a plasma between the capacitor plates. Because of the Q value of

460 The 809 resonant circuit consumes very little energy in the resonant circuit itself and the energy is consumed in the arc formed between the plates. The energy in the arc rapidly heats up the plasma and causes vaporization of the metal which forms the capacitor plates. The evaporated metal causes the arc to become conductive and shortens the capacitor plates which temporarily destroys the resonant properties of the circuit and causes the current through the arc and voltage across the arc to disappear rapidly. Therefore, the arc cools, causing deposition of the previously vaporized metal between the capacitor plates.

If a short circuit is formed, the label is permanently destroyed. If a short circuit is not formed, the voltage is rebuilt over the capacitor plates in accordance with the applied energy and the process is repeated. Since the plastic film has already broken and weakened at the breaking point, the arc will normally be formed again at the same point and additional material will evaporate and deposit until a permanent short circuit occurs. The deactivation sequence is illustrated in FIG. 10-12. In Fig. 10, the initiation of a voltage break through the plastic film 110 and between the plates 112 and 114. is shown. The formation of the plasma after the arc discharge is shown in Figs.

and the final deposition of metal along the discharge path for shorting the capacitor plates is illustrated in Figure 12.

If the deactivation power is too high, it is possible to burn off a portion of the capacitor plate without shorting the plate. This will effect some change in the resonant frequency with each build and collapse of the arc until an arc can no longer be formed, although the label will still exhibit a resonant frequency. The deactivation effect should be carefully controlled or the deactivation process electronically monitored to turn off the deactivator shortly after the first arc is formed. The deactivator can be switched on again on a cyclic basis as described until a permanent short circuit has been developed over the capacitor plates. Since the deactivation 460 809 in the antenna is connected to the label circuit, the label circuit's impedance is reflected back into the deactivation antenna. In forming a discharge path, the impedance of the resonant circuit is reversed and this change is reflected directly back into the deactivation antenna and can be detected by the deactivation system and utilized for accurate control of the deactivation system. Therefore, in detecting an abrupt change of current in the deactivation antenna caused by impedance change in the resonant label circuit in response to a two-breakthrough deactivation system, it can be switched off and cyclically resized to cause arc formation and deposition of metal along the discharge path along the discharge path. controlled way to deactivate the resonant properties of the label circuit.

The deactivation system 100 can be controlled in the manner illustrated in Fig. 16. The label sensing system 94 provides a label signal in response to the swept radio frequency signal passing past the resonant frequency of the label circuit, and this label signal is applied to a high pass flutter 160 with steep cut. Filter 160 filters out the modulation components and essentially all components of the label signal spectrum. When a discharge path is formed across the capacitor plates, the result is a relatively large and abrupt change in current through the antenna 90 and this signal will pass through the high-pass filter 160 to a threshold detector 162, which initiates a timing circuit 164 which determines the time interval during which the label activation system 100 operates. The duty cycle f can be repeated if necessary to deactivate the resonant properties of the label circuit.

The invention is not limited to what has been specifically shown and described but to what is apparent from the appended claims.

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