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Publication numberUS3820104 A
Publication typeGrant
Publication dateJun 25, 1974
Filing dateDec 15, 1972
Priority dateDec 15, 1972
Publication numberUS 3820104 A, US 3820104A, US-A-3820104, US3820104 A, US3820104A
InventorsE Fearon
Original AssigneeStop Loss Inc
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method and system for detecting an object within a magnetic field interrogation zone
US 3820104 A
Abstract  available in
Images(7)
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Claims  available in
Description  (OCR text may contain errors)

United States Patent [191 Fearon June 25, 11974 METHOD AND SYSTEM FOR DETECTING AN OBJECT WITHIN A MAGNETIC FIELD INTERROGATION ZONE Edward R. Fearon, Richardson, Tex.

[73] Assignee: Stop-Loss Incorporated, Atlanta,

22 Filed: Dec. 15, 1972 21 Appl. No.: 315,320

[75] Inventor:

Primary ExaminerDavid L. Trafton Attorney, Agent, or Firm-Richards, Harris & Medlock [57] ABSTRACT This specification discloses a marker for being secured to an object to enable the detection of the object within an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency. The marker includes a first elongated ferromagnetic element for being secured to the object. The first ferromagnetic element has a relatively low coercivity and is operable to generate a detectable signal containing harmonics of the fundamental frequency when placed in the interrogation zone. The marker further includes a second ferromagnetic element disposed adjacent to the first element and having a coercivity greater than the first element. Deactivation structure is provided to selectively magnetize the second ferromagnetic element to impose a plurality of pairs of alternate magnetic poles on the first element in order to deactivate the marker. The deactivated marker does not generate a detectable signal containing the desired harmonics when the object passes through the interrogation zone. In the preferred embodiment of the'invention, the second ferromagnetic element is constructed so as to be magnetized by a predetermined magnetic field oriented in any direction to deactivate the marker. Also in the preferred embodiment, deactivation structure is provided which checks to insure that the marker has been deactivated prior to allowing the energization of an auxiliary device such as a cash register or the like.

27 Claims, 22 Drawing Figures PATENTEDJIIII25 IIIII SHEET 5 BF 7 390 VOLTAGE PHASE I INPUT COMPARATOR I"I I-| r-L l VCO OUTPUT 224 30 58 310 IL 308 312 UNIT INPUT FROM 0 J/ #0 AMPLIFIER ll TO GNDO---- 314 W POWER OUTPUT b 316 CIRCUITS OU8UT A 342 COIL UNIT 330 336 FROM DRIVER 33a cIRcuIT voi 7h w PATENTEHJUH 25 1974 SHEET 8 [1F 7 G of COMMON Cr FIG. IS

TO RECEIVERCF FIG. i9

T m T U O ml 0 4 O 2 4 1w G T L RB Ma F. Mfg A 2 O f 4 4 4 .LHAV W w T A t; 3 14 4 7 A" w m 4 my m of Ar a 1 D V A R N MM E 6 DG L H 2 R E n H METHOD AND SYSTEM FOR DETECTING AN OBJECT WITHIN A MAGNETIC FIELD INTERROGATION ZONE FIELD OF THE INVENTION This invention relates to methods and systems for detecting objects within an interrogation zone, and more particularly relates to methods and systems wherein markers are applied to objects for generating detectable harmonic signals when placed in an alternating magnetic field interrogation zone.

THE PRIOR ART Systems have been heretofore developed wherein ferromagnetic strips or markers are applied to objects and a magnetic field is imposed at an interrogation zone. Passage of the ferromagnetic strips through the zone generate detectable signals which may be utilized to indicate the presence of the object within the interrogation zone. These previously developed systems have been utilized to prevent shoplifting in retail stores and to prevent unauthorized removal of such objects as books from libraries and the like. Further, techniques have been previously developed for deactivating such ferromagnetic strip markers such that an alarm will not be triggered when an object which has been properly paid for or checked out is passed through the interrogation zone. Such prior deactivation techniques have included physically breaking or degrading a portion of the ferromagnetic strip and have further included the use of a second ferromagnetic strip which may be magnetized in order to alter the harmonic signal generated by the first ferromagnetic strip. Examples of such prior systems are disclosed in U.S. Pat. No. 3,63 l ,442, issued Dec. 28, 1971 and U.S. Pat. No. 3,665,449, issued May 23, 1972. A deactivatable marker is also described and claimed in copending U.S. Pat. application Ser. No. 201,687, filed Nov. 24, 1971, now U.S. Pat. No. 3,747,086, a divisional of U.S. Pat. No. 3,631,442.

While such previously developed detection systems utilizing ferromagnetic strips have generally worked well in practice, it has been found important to provide an easily deactivatable marker which includes safeguards to prevent an improperly deactivated marker from operating an alarm. It will be apparent that the operation of an alarm by a person who has properly purchased or checked out an object can result in considerable embarrassment and difficulty. Such previously developed strips and markers have not been completely satisfactory in providing complete deactivation of a marker, and previously developed methods and systems have not provided adequate safeguard features to insure that a marker has been completely deactivated.

SUMMARY OF THE INVENTION In accordance with the present invention, a marker is provided for being secured to an object to enable detection of the presence, identify or status of the object within an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency. The marker includes a first elongated ferromagnetic element for being secured to the object and having low coercivity capable of generating a detectable signal containing harmonics of the fundamental frequency when placed in the interrogation zone. The marker further includes a second ferromagnetic structure disposed adjacent to the first element and having a coercivity greater than the first element. The second ferromagnetic structure is operable to impose a plurality of pairs of alternate magnetic poles on the first element when placed in a predetermined magnetic field to thereby prevent the generation of the detectable signal.

In accordance with yet another aspect of the invention, a deactivatable marker is provided for being secured to an object to enable detection of the object when the object is placed in an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency. The'marker includes a first elongated ferromagnetic element of low coercivity capable of responding to the magnetic field to generate signals containing harmonics of the fundamental frequency. The marker further includes a second ferromagnetic structure disposed adjacent to the first element and having a coercivity greater than the first element. The second ferromagnetic structure is operable to be magnetized by a predetermined magnetic field oriented at any direction relative to the first element for substantially altering the harmonic content of the signals produced by the first ferromagnetic element when in the interrogation zone.

In accordance with another aspect of the invention, a system is provided for detecting a characteristic of an object when the object is in an interrogation Zone having a magnetic field periodically varying at a predetermined fundamental frequency. The system includes a marker for being secured to the object and includes a first elongated ferromagnetic element of low coercivity capable of generating a detectable signal containing harmonics of the fundamental frequency when placed in the interrogation zone. The marker further includes a second ferromagnetic structure disposed adjacent to the first element and having a coercivity greater than the first element. The second ferromagnetic structure when magnetized is capable of imposing a plurality of pairs of alternate magnetic poles on the first element to alter the harmonic content of the detectable signal produced by the first element when the object is in the interrogation zone. Deactivation circuitry is provided to selectively magnetize the second ferromagnetic structure. Radiating coils produce the magnetic field within the interrogation zone. Receiving circuitry is provided to detect the harmonic content of the signals produced by the first element. A security readout circuit is coupled to the receiving coil and is responsive to the harmonic signal generated by the first ferromagnetic element when the second ferromagnetic structure is not magnetized. The security readout circuit does not provide an indication of the characteristic of the object when the second ferromagnetic structure is magnetized.

In accordance with yet another aspect of the invention, a system is provided for detecting an object when the object is in an interrogation zone. A coil generates in the interrogation zone a magnetic field periodically varying at a predetermined fundamental frequency. A marker is provided for being secured to an object and includes a first elongated ferromagnetic element of low coercivity capable of generating a detectable signal containing harmonics of the fundamental frequency when placed in the interrogation zone. Circuitry detects the detectable signals to indicate the presence of the object. The marker further includes a second ferromagnetic structure adjacent to the first ferromagnetic element and operable when magnetized to alter the harmonic content of the detectable signal such that the detecting circuitry will not indicate the presence of the object. Deactivation circuitry is provided to sense the presence of the first ferromagnetic element and in response thereto for magnetizing the second ferromagnetic structure. Utilization means such as a cash register is operable to be energized only when the second ferromagnetic structure has been successfully magnetized to alter the harmonic content of the detectable signal.

DESCRIPTION OF THE DRAWINGS For more complete understanding of the present invention and for further objects and advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a somewhat diagramatic illustration of a typical store installation of the present detection system;

FIG. 2 is a block diagram of the marker detection portion of the system shown in FIG. 1;

FIG. 3 is a block diagram of the deactivation circuitry of the system shown in FIG. 1;

FIG. 4 is a perspective view, partially broken away, of one of the transmitting and receiving coil units placed adjacent to the interrogation zone according to the invention;

FIG. 5 is a schematic illustration of the interconnection of the transmitting coil units of the present invention;

FIG. 6 is a top view of one embodiment of a deactivatable marker according to the invention;

FIG. 7 is a perspective view illustrating the assembly of the marker shown in FIG. 6;

FIG. 8 is a top view illustrating the deactivation of the marker shown in FIG. 6 by a magnetic field directed parallel to the axis of the marker;

FIG. 9 illustrates the deactivation of the marker shown in FIG. 6 when the magnetic field is imposed at an angle to the axis of the marker;

FIG. 10 is a top view of a deactivatable marker according to a second preferred embodiment of the invention which may be deactivated by the application of a magnetic field oriented at any direction relative to the marker;

FIG. 11 is an exploded side view of the marker shown in FIG. 10;

FIG. 12 is an illustration of the deactivation provided by the marker shown in FIG. 10 in response to a magnetic field directed parallel to the length of the marker;

FIG. 13 is an illustration of the deactivation of the marker shown in FIG. 10 in response to a magnetic field directed perpendicular to the length of the marker;

FIG. 14 is a schematic of the phase lock oscillator circuitry of the system shown in FIG. 2;

FIG. 15 is a block diagram of a portion of the phase lock oscillator shown in FIG. 14;

FIG. 16 is a schematic diagram of one of the driver circuits of the system shown in FIG. 2;

FIG. 17 is a schematic diagram of one of the power amplifiers of the system shown in FIG. 2;

FIG. 18 is a schematic diagram of the dropout circuit shown in FIG. 2;

FIG. 19 is a schematic diagram of the receiver filter of the system shown in FIG. 2;

FIG. 20 is a schematic diagram of the receiver detector for the system shown in FIG. 2;

FIG. 21 is a schematic diagram of the oscillator for the deactivation system shown in FIG. 3; and

FIG. 22 is a schematic diagram of the deactivation gaussing circuitry of the system shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a somewhat diagramatic view of a typical installation of the present system is illustrated. The system includes a pair of coil units 10 and 12 disposed on opposite sides of a path leading to the exit 14 of a store. Detection circuitry and an alarm system is mounted within a cabinet 16. A plurality of objects 18, which may comprise items for retail sale or books to be checked out, are displayed within the store. Each of the objects 18 has attached thereto a marker 20 constructed in accordance with the present invention. As will be subsequently described, the marker 20 includes elongated ferromagnetic strips which normally are in the activated mode. When in the activated mode, the marker 20 will cause an alarm to be emitted from the cabinet 16 when placed between the coil units 10 and 12. The markers 20 thus prevent unauthorized removal of the objects 18 from the store.

A deactivator system 22 is disposed on the checkout table adjacent to a cash register or other auxiliary device 24. The deactivator device is interconnected to cash register 24 via wire 26. After an object 18 has been properly paid for or otherwise checked out, the object 18 is disposed within an aperture 28 withinthe deactivation system 22, and a similar field as is generated by coil units 10 and 12 is applied to the marker 20. Detection circuitry within the deactivation system 22 then detects harmonic signals generated by the marker 20 and activates a gaussing circuit. The gaussing circuit applies a high magnetic field to the marker 20 to place the marker in a deactivated mode. The deactivation device 22 then senses to insure that the marker has been deactivated, and then generates an electrical signal via the wire 26 to allow the cash register 24 to be opened. The object 18 and the deactivated marker 20 may then be carried between the coil units 10 and 12 without the generation of an alarm signal from the cabinet 16. As will be subsequently described in greater detail, the safeguard features provided by the deactivation unit 22 prevent an improperly deactivated marker from being carried through the exit 14, thus eliminating the embarrassment and attendant problems created by a false alarm.

FIG. 2 illustrates in block diagram form the marker detection system of the invention. A phase lock oscillator 30 generates a prescribed fundamental frequency suitable to resonate the coil units 10 and 12, which in the preferred embodiment comprises 2,500 KI-Iz. The frequency is passed through a frequency amplifier 32 and a power amplifier 34 and is applied to the transmitting coils 36 which are disposed within the coil units 10 and 12. The transmitting coils 36 operate in resonance to generate an alternating magnetic field having a fundamental frequency of 2,500 Hz. This field is applied between the coil units 10 and 12 to form an interrogation zone through which all persons exiting the secure area must pass.

A dropout circuit 38 is connected between the transmitting coils 36 and the phase lock oscillator 30 to disable the oscillator 30 in case a heavy metal object is placed in the interrogation zone in order to prevent damage to the system. A power supply 40 provides power to the transmitting system.

A marker 20 is illustrated as being disposed between the transmitting coils 36 and the receiving antenna 42.

The receiving antenna 42 is also located within the coil units and 12 in a manner to be subsequently described. The receiving antenna 42 detects signals generated in response to the marker having high order harmonics of the fundamental frequency transmitted by the transmitting coils 36. In the preferred embodiment, the harmonic signals detected by the receiving antenna 42 are in the range of about 80 250 KHz. The transponding operation of the marker 20 is disclosed in greater detail in U.S. Pat. No. 3,631,442 and No. 3,665,449, previously identified.

The harmonic signals received by the receiving antenna 42 are applied through a filter 44 to eliminate noise and the filtered signals are applied through an amplifier 46. The amplifier signals are applied to an integrator 48 and subsequently to a detector 50. If a signal having a prescribed frequency content persists a determined length of time, the detector 50 activates an alarm 52 which generates either an audible or visual alarm to indicate the presence of an unauthorized object within the interrogation zone. Although not illustrated, a logic computer may be provided which requires a valid signal to be present for 10 consecutive cycles to prevent a false alarm. A power supply 56 provides power to the detection circuit.

FIG. 3 illustrates a block diagram of the circuitry of the deactivation system. Basically, the deactivation system comprises a small replica of the detection circuitry shown in FIG. 2, in combination with a gaussing circuit for deactivating the label. The system includes a phase lock oscillator 60 which generates a 2,500 Hz signal which is amplified by an amplifier 62. The amplified signal is applied to a field coil 64 located in the aperture 28 of the deactivator system 22. In the preferred embodiment, the amplifier 62 may comprise anyone of a number of well known amplifier circuits including the Crown D-lSO High Fidelity Amplifier. The field coil 64 in the preferred embodiment comprises 35 turns of No. 6 wire. The field coil 64 thus generates in the aperture 28 of the deactivator system 22 a magnetic field oscillating about a fundamental frequency of 2,500 l-lz. When a marker 20 is placed in the field, the marker generates harmonics of the fundamental frequency in the manner previously described. In the preferred embodiment, the marker 20 generates harmonic signals within the range of 80 to 250 KHZ.

The harmonic signals are received by an antenna and filter 66 which are also mounted within the aperture 28 of the deactivation system 22. The harmonic signals are applied through a tuned eirnplifier 68 and are applied to a detector 70. The output of the detector 70 is applied to a gausser circuit 74 which generates an electrical signal for application to a gaussing coil 76. The gaussing coil 76 applies a high magnetic field to the marker 20 within the aperture 28 of the deactivation system 22 to deactivate the label 20. In the preferred embodiment, the gaussing coil 76 generates a magnetic field in the range of 100 1,000 oersteds. A manual switch 78 may be utilized to manually operate the gausser 74 if required.

The output of the detector is also applied to a cash register solenoid 80 which prevents operation of a key or the drawer of the cash register when locked. The unlock signal applied from the detector 70 is not generated until the gausser 74 has been energized and until a signal is no longer received and detected by the detector 70. When no signal is received from the detector 70, thus indicating that the marker 20 is deactivated, the cash register solenoid is unlocked to allow operation of the cash register 24. A key or drawer operated microswitch 82 is provided on the cash register to rearrn the solenoid 80 when the key or drawer of the cash register is closed. Once the drawer of the cash register is closed, the cash register may not again be utilized until the solenoid 80 is again unlocked.

FIG. 4 illustrates a prospective view, partially-broken away, of the coil unit 12. The coil unit includes an upright housing including a rectangular open-ended frame with decorative screen portions 92 and 94 covering the ends. The housing extends vertically for a distance approximating the height of an individual.

A plurality of wooden brackets 96, 98 and 100 extend across the width of the housing 90 for support of the transmitting and receiving coils of the invention. The transmitting coil comprises a plurality of loops of rigid metal pipe 102 having air filled interiors. In the preferred embodiment, the pipe comprises three loops of copper tubing or pipe having a 1 inch inner diameter and 1% inch outer diameter. The loops of pipe comprise a plurality of linear portions joined together by 90 elbows. In the preferred embodiment, the transmit ting coil extends vertically for approximately 60 inches and has a width of approximately 35 inches in order to encompass an area in which one of the tagged objects would usually be carried to the interrogation zone. The loops of rigid pipe 102 extend through apertures within the wooden shelves 96, 98 and 100 and are thus rigidly supported within the housing 12. A plurality of capacitors 104 are tied across the ends of the loops of pipe to provide an L-C' circuit. In the preferred'embodiment, the capacitors comprise ten 20-mf capacitors.

The receiving antenna or coil 106 in the preferred embodiment comprises 15 loops of No. 22 wire also rigidly supported within the housing 12 by the brackets 98 and 100. As previously described, the source of alternating electrical energy is connected to the rigid pipes 102 and a magnetic field is thus established in the interrogation zone between the coil units 10 and 12. The harmonic signals generated by the presence of a marker are detected by the receiving coil 106.

FIG. 5 illustrates how the transmitting coils within coil units 10 and 12 are interconnected. The first set of loops of rigid metal pipe is termed 102a and is disposed in the coil unit 12, with the second set of loops of rigid metal pipe being termed 102b and is disposed within the coil unit 10. A capacitance 104a of 200 mf is connected across the loops 102a, while a similar capacitance 104b is connected across the loops 102b. Like terminals of the loops 102a and 102b are interconnected to terminals 110 and 112 such that the loops are connected in an aiding configuration.

FIGS. 6 and 7 illustrate the construction of one embodiment of a marker according to the invention. The marker comprises an elongated strip of ferromag- Supermalloy. In the preferred embodiment, the strip 120 has a length of 3 inches, a width of 43 inch and a thickness of approximately 0.7 mil. As previously noted, when strip 120 is excited by an alternating magnetic field having a fundamental frequency of 2,500 Hz, the strip generates a signal having harmonics in the range of 80 to 250 KHZ.

A second ferromagnetic structure 122 is disposed over the strip 120 to enable deactivation of the strip 120. Structure 122 is comprised of ferromagnetic material having a higher coercivity than the strip 120 and comprises in the preferred embodiment the low permeability ferromagnetic material manufactured and sold under the tradename Vicalloy. In the preferred embodiment, the strip 122 is provided with a length of 3 inches, a width of inch and a thickness of l to 2 mils. Circular apertures 124 are formed through the ferromagnetic l22 for reasons to be subsequently described. In the preferred embodiment, the apertures 124 comprise circules having diameters of inch.

FIG. 7 illustrates the construction of the marker in its final form. The strips 120 and 122 are aligned in the manner illustrated and are sandwiched between two paper strips 128 and 130. The edges of the paper strips 128 and 130 may be glued together in order to hold the ferromagnetic strips in place. Adhesive may be applied to one of the paper strips in order to allow the marker to be secured to an object.

FIGS. 8 and 9 illustrate the deactivation mode of the present system. When the strip 122 is not magnetized, the strip 120 operates as a transponder in the manner previously described. However, when the strip 122 is magnetized, as by a magnetic field directed generally parallel to the length of the marker as indicated by the arrow 134, the apertures I24 cause a plurality of pairs of alternate poles to be formed on the strip 122 in the manner illustrated in FIG. 8. Due to the relative magnetic properties of the strips 120 and 122, the strip 122 imposes upon the strip 120 the plurality of pairs of alternate poles, thereby substantially altering the transponder or transformer core-like. characteristics of the strip 120. Hereinafter, when the deactivated markeris placed in the interrogation zone of the system, the strip 120 will thus no longer generate signals having the required harmonics for operation of the detection system. The present deactivation system may then be utilized to deactivate the markers after sale or valid checkout of an object.

FIG. 9 illustrates the formation of a plurality of pairs of alternate magnetic poles on the strip 122 due to the apertures 124 upon the application of a magnetic field disposed along a line indicated by the arrow 136. In this mode of magnetization of the marker, the magnetic poles formed on the strip 122 are skewed, but the strip 120 is still deactivated due to the provision of a plurality of poles along the length thereof.

However, it may be seen that care must be taken in the use of the marker shown in FIGS. 6 9 not to place the marker completely perpendicular with respect to the deactivating magnetic field. In such case, only two poles would be formed on opposite sides of strip 120. The strip 120 actually would lie between the two poles of a magnet. The poles are not alternate since they are not on the strip, thereby allowing the strip to operate in some instances so as to generate harmonics which generate an alarm when passed through the present interrogation zone. The marker shown in FIGS. 10 13 is thus provided to eliminate the requirement of carefully positioning the marker within the deactivation unit 22, as this marker maybe deactivated when placed at any angle relative to the magnetizing field.

This preferred embodiment of the marker comprises an elongated ferromagnetic strip constructed from Supermalloy in an identical manner as the previously described strip 120. A plurality of rectangular discrete elements 142 146 formed from a low permeability and high coercivity ferromagnetic material 'such as Vicalloy is positioned at spaced-apart locations along the length of the strip 140. Elements 142 and 146 are placed on the top side of the strip 140, while element 144 is placed on the bottom side of strip 140. A lower strip of paper 148 is illustrated in FIG. 10, with the upper strip of paper being removed in this view. Referring to FIG. 11, which is a slightly exploded view of the marker, the relationship of the strip 140 and elements 142 146 is illustrated relative to the lower paper strip 148 and an upper paper strip 150.

Referring again to FIG. 10, each of the rectangular elements 142 146 has a width equal to twice the width of strip 140. The rectangular elements 142 and 146 are disposed with their edges 152 and 154 coincident with the edge of the strip 140 and extend laterally from the strip 140 in the manner illustrated. The discrete element 144 is positioned with its edge 156 coincident with the other edge of the strip 140 and extends laterally from the strip 140 in the opposite direction from elements 142 and 146. The elements 142 146 enable the marker to be deactivated by a magnetic field disposed in any direction relative to the marker.

For example, FIG. 12 illustrates the magnetization of the elements 142 146 by a magnetic field directed along in the direction of the arrow 160. In this mode of operation, north and south poles are formed on each of the elements 142 146 in the manner illustrated. The magnetizedelements 142 146 thus impose upon the strip 140 three spaced-apart pairs of alternate northsouth poles to alter the transpondent characteristics of the strip 140. Further, FIG. 13 illustrates magnetization of the elements 142 --146 by a magnetic field disposed in the direction of an arrow 162. In this embodiment, each of the elements 142 146 is provided with north and south poles in the manner illustrated, and the strip 140 has imposed thereon two north poles and a'south pole. The transpondent characteristics of the strip 140 are thus altered in a manner such that the detection circuitry of the invention will not detect the presence of the marker. It will also be understood that a magnetic field having a vector perpendicular to both of the above vector arrows 160 and 162 shown in FIGS. 12 and 13 will also deactivate marker strip 140.

It will be understood that the present invention also encompasses the positioning of all of the elements 142, 144 and 146 on either the top or the bottom of strip 140 (not shown) rather than staggered configuration shown in FIG. 11. This alternate construction provides simplicity of manufacture, but enables the resulting marker to be deactivated only by magnetic fields oriented within the plane defined by the marker. Magnetic fields oriented perpendicular to the plane defined by the marker would not deactivate this particular marker.

Referring to FIG. 14, the phase lock oscillator and the preamplifier 32 previously noted in FIG. 2 are illustrated in schematic detail. One of the transmitting coils 190 is illustrated in schematic form, with a capacitance 192 connected thereacross to form an L-C circuit. One terminal of the L-C is connected through a voltage divider comprising resistors 194 and 196 and is applied to an input of a phase-lock loop circuit 200. in the preferred embodiment, the circuit 200 comprises the NE565A Phase-Lock Loop manufactured and sold by Signetics. As will be later described in great detail, the phase-lock loop 200 generates a square wave signal which is applied through a decoupling capacitor 202 and through resistors 204 and 206 to an input of an operational amplifier 108 connected as an active Butterworth filter.

The voltage input applied through resistors 194 and 196 is applied to the external input to the voltage comparator section of the phase-lock loop 200. Resistor 210 provides a balanced dc bias to the comparator section. Resistors 212 and 214 are dropping resistors to reduce the supply voltage to the required bias voltage for the phase-lock loop 200. Capacitors 216 and 218 provide a power supply decoupling. Capacitor 220 is a timing capacitor which determines, along with resistor 222, the free running frequency of the voltage control oscillator of the phase-lock loop 200. Capacitor 224 is the external element of a low pass filter on the error voltage output of the voltage comparator section. The

operation of capacitor 224 and an internal resistor in the phase-lock loop 200 is to smooth out the error signal from the voltage comparator section and apply it to the control voltage point of the voltage control oscillator in such a manner as to force the frequency of the voltage control oscillator to track'the frequency of the locking voltage signal. The variable resistance 222 provides a variable frequency control for adjusting the free-running frequency of the voltage control oscillator.

A resistor 230 and resistor 204, along with capacitor 232, form a voltage dividing network and phase correction network for the input signal of the active filter 208. Resistors 236 and 238, along with capacitors 240 and 242, provide power supply decoupling for the active filter.

A resistor 244 and a capacitor 246 determine the attenuation characteristics of the active filter by providing an ac feedback path for the cancellation of the higher frequency. Resistor 248 provides negative feedback for stability of the operational amplifier 208. Output of the active filter, which operates to chop off the higher harmonics to provide an output sine wave, is ap-. plied to a relay switch element 250 which is movable into and out of contact with a relay contact 252. Operation of the relay member 250 is controlled by a relay coil 254 which is operated by the dropout circuit to be subsequently described. When the relay switch member 250 is closed against the contact 252, the output from the active filter is applied through a resistance 256 and a variable resistance 258 to an input of a voltage amplifier 260, which may comprise, for example, an N553 1T amplifier manufactured and sold by Signetics. Resistor 258 may be adjusted to provide a variable gain control.

The output of the phase-lock loop 200 is also applied via lead 262 to an operational amplifier 264 connected to provide an amplified square wave for subsequent gating functions in the detecting circuitry. The output of the voltage amplifier 260 is applied to the input of a current amplifier 266, which in the preferred embodiment may comprise an MC1438R amplifier manufactured and sold by Motorola Corporation. The network comprised of resistance 268 and capacitor 270 provides power supply decoupling for the voltage amplifier 260. A capacitor 272 provides high frequency compensation for the voltage amplifier 260. The network comprised of resistor 274 and capacitor 276 provides high frequency compensation for the amplifier 266. The voltage amplifier and current amplifier are tied together in a feedback network including resistors 280 and 282. This feedback network provides negative voltage feedback for both gain control and stability of the amplifier.

The network formedby resistor 284 and capacitor 286 at the output of amplifier 266 provides a high frequency roll-off network to eliminate higher order harmonics or noise which may be present on the output of the amplifier 266. The amplified output of amplifier 266 is applied to the driver and power output circuits 290 which will be subsequently described in FIGS. 16 and 17. The power amplified signals are then applied directly to the terminal of the coil and capacitor 192 in order to drive the coil 190 to provide the desired alternating magnetic field within the interrogation zone of the invention. The opposite terminals of the coil 190 and capacitor 192 are applied to the input of the amplifier 260 and are also applied to the circuit ground through resistor network 294. The voltage developed by load current flowing through the resistor network 294 appears at resistor 296 and provides a current feedback signal which is developed across resistors 296 and 284. Thus, the present amplifier system includes both voltage and current feedback in order to stabilize the circuit.

As will be subsequently described in greater detail, the output applied from the driver and power output circuit 290 is less than full wave drive, thus providing an alternating magnetic field generated by coil 190 which has improved distortion characteristics which enable the coil to be driven by more economical and efficient driving circuitry.

FIG. 15 illustrates in greater detail the interconnection of the phase-lock loop 200 previously described with respect to FIG. 14. As is known, the phase-lock loop circuit includes a phase comparator 300 and a voltage controlled oscillator (VCO) 302. The voltage received from the coil 190 and capacitor 192 are applied to the phase comparator 300 and the phase of the input voltage is compared with the output current from the VCO 302. The resulting error signal from the phase comparator 300 is applied at the output 304 of the comparator and is applied to the low pass filter capacitor 224 and is applied to the input of the VCO 302. The output of the phase comparator 300 thus controls the frequency generated by the VCO 302. The circuitry shown in FIG. 15 thus continually maintains the output square wave signal at a frequency which provides resonance of the L-C circuit formed by the coil 190 and the capacitor 192. In this manner, the L-C circuit is operated at the desired resonance and provides the most efficient operation of the system. As previously noted, in

the preferred embodiment, the circuit operates at a nominal center frequency of 2,500 Hz, but the operation of the circuitry shown in FIG. 15 may slightly vary this frequency to continually operate the LC circuit of the transmitting coils at resonance at all times under various operating and loading conditions.

FIG. 16 illustrates the driver circuit of the invention. The signal generated by the amplifier 266 shown in FIG. 14 is applied through a decoupling capacitor 308 and through a transformer 310 to the base of a driver transistor 312. A terminal of the primary winding of the transformer 310 is connected to ground, with the terminal of the secondary of the transformer being connected to a source of negative bias voltage. The transformer 310 is a one-to-one transformer and shifts the level of the signal downward in order that the transmitting coil of the invention may be tied to circuit ground. A transformer load resistor 314 is connected across the secondary of the transformer 310. The transistor 312 is connected in an emitter follower configuration and generates an output signal through resistor 316 for application to the power output circuitry shown in FIG. 17. The collector of the transistor 312 is connected through a fuse 318 which operates to protect the transistor and transformer of the system. The collector of the transistor 312 is connected to a terminal 320 which is connected to the transmitting coil 190 shown in FIG. 14.

FIG. 17 illustrates one stage of the plural stage power output circuit of the system shown in FIG. 14. In the preferred embodiment, the power output of the circuit includes 10 stages identical to the one shown in FIG. 17 connected in parallel to provide the desired power output; The output from the driver circuit shown in FIG. 16 is applied to terminal 330 and to the base of a transistor 332. A balancing resistor 334 is connected between the emitter of the transistor 332 and a source of negative bias voltage. The collector of transistor 332 is connected to a fuse 336 which is connected across an indicator lamp 338 and a resistance 340. If the fuse if blown, the indicator lamp 338 becomes energized and indicates the blowing of the fuse. The output of the collector of the transistor 332 is applied to terminal 342 for output directly to the transmitting coil unit comprising coil 190 and capacitor 192. v

The outputs of each of the parallel connected power output circuits are tied to the common summing node and are applied to the transmitter coil.

An important aspect of the invention is that the current applied at terminal 342 may be less than full wave. In the embodiment shown, the current applied at terminal 342 is a half wave signal, with the positive current peak eliminated. Thus, the transmitting coil 190 in FIG. 14 is driven only by the negative going portions of the current. Due to the fact that the LC circuit comprising the coil 190 and the capacitor l92has a relatively high Q, the L-C circuit tends to generate a continuous mag netic field by a fly-wheeling" operation. This driving of the transmitting coil 190 by spaced-apart current pulses results in a magnetic field generated by the coil 190 which has better distortion characteristics than a system wherein the coil is continuously driven. Driving systems for the coil 190 inherently produce distortion of the generated magnetic field, and by reducing the amount of time that the coil is directly driven, the distortion in the magnetic field is reduced. Moreover, the use of less than full wave current drive of the transmit- While the circuitry shown in FIG. 17 is directed to I half wave current drive of the transmitting coil, it will be understood that improved distortion characteristics of the magnetic field may also result in less than half wave current drive. For example, it has been found that driving the transmitting coil with a 60 current signal for alternate cycles results in a magnetic field having improved distortion characteristics.

FIG. 18 illustrates in schematic detail the dropout circuitry of the invention. The 2,500 Hz signal generated by the oscillator of the invention is applied at ter minal 350 and is aplied through an ac coupler capacitor 352 and through a voltage divider 354 to the inverting input of an operational amplifier 356. The operational amplifier is interconnected with diodes 358 and 360 and associated circuitry to provide a precision rectifier circuit. The output of the rectifier circuit is applied as a dc signal to a filter comprising a resistor 362 and a capacitor 364. The filter signal is applied to the input of a comparator 366, the output of which is applied to a resistor 368 to the base of a transistor 370. A reference voltage is defined by resistors 372 and 374 and is applied to the second input of the comparator 366. The output of the comparator 366 is thus an error signal when the dc voltage applied via resistor 362 differs from the voltage level applied as a reference level. Thus, when the variable dc voltage drops below the reference dc voltage, a negative pulse is generated from the comparator 366 which turns transistor 370 off.

The collector of transistor 370 is connected through a resistance 378 to the base of a transistor 380. When transistor 370 is turned off, transistor 380 is turned off. I

The collector of transistor 380 is connected to the coil 254, previously noted as the dropout relay coil in FIG. 14. When the transistor 380 is turned off, the normally closed relay switch 250 is opened due to energization of the coil 254 and the transmitting system of the invention is disconnected from the transmitting coil. The operation of the dropout circuit thus operates to protect the present system is case of the passage of an extremely heavy metal object through the interrogation zone of the invention which would tend to load the transmitting coil down and change the transmitter frequency. Passage of such a very heavy metallic load through the interrogation zone, without the use of the dropout circuit, would in some instances provide a false alarm or cause damage to the power amplifier of the circuit.

The emitter of a unijunction transistor 390 is connected through a resistance 392 to the collector of the transistor 370. A capacitor 394 is connected across circuit ground and the main terminal of the resistor 392 to form an R-C timing network. The emitter of transistor 380 is tied to a base of the unijunction transistor 390. The other base of the transistor .390 is connected through a resistor 396 to the base of a transistor 398. The collector of a transistor 370 is connected directly to the collector of the transistor 398.

The unijunction transistor 390 and transistor 398 operate to form a timing and reset circuit. When the transistor 370 is turned off, the charging voltage is applied through resistor 378 and 392 and capacitor 394 is charged. When the voltage across capacitor 394 reaches a predetermined level, the transistor 390 fires and turns on transistor 398. Transistor 398 operates to turn transistor 380 back on for a time interval determined by the time constant of the circuit. The coil 254 is then energized to close relay switch member 250 against the contact 254 such that the circuit tends to turn on again. If the conditions which cause the circuit to originally drop out still exist, the transistor 380 will again be turned off and the relay will be dropped out to again disconnect the circuit.

FIG. 19 illustrates the receiver filter 44 shown in FIG. 2 in schematic detail. The receiving antenna 42 comprises a plurality of loops of wire and detects the harmonic signal of the fundamental frequency generated by the transmitting coil. The harmonic signals are applied to a twin T notched filter comprising resistors 400, 402, 404, 406 and capacitors 408, 410 and 412. The notched filter operates to eliminate the 2,500 l-Iz fundamental frequency generated by the transmitting coil. Resistor 404 is also connected to a tap of a transformer 414, a terminal of which is applied via lead 416 to the receiver circuitry. An important aspect of the invention is that a capacitor 418 operates in conjunction with the notched filter and the transformer 414 to provide a high pass filter for eliminating the fundamental frequency and lower order harmonics which are not required for the invention.

FIG. 20 illustrates in schematic detail the amplifier integrator and detector of the system shown previously in FIG. 2. Filters accordingto the circuitry shown in FIG. 19 are connected to each one of the receiving coils located in the coil units 10 and 12. The outputs of the filters are added at a summing node 420 and are applied to the inverting input of an amplifier 422. The output of amplifier 422 is applied to a low pass filter comprising a resistor 424 and capacitor 426 in order to eliminate noise such as radio stations and the like. The signals are then applied through a high pass filter comprising capacitor 428 and a shielded inductance 430 in order to eliminate unwanted frequencies. The signal is then applied to an input of an amplifier 432 whichgenerates an amplified signal which is applied through a resistor 434 to the collector of the transistor 436. An important aspect of the invention is that the transistor 436 is gated by the signal data gate applied at terminal 438. The data gate signal is derived from the amplifier 264 previously shown in FIG. 14.

The gated signal is then applied through a diode 440 and a resistor 442 to an integrator comprising capacitor 444 and resistor 446. The integrator operates to average out the random noise and to prevent false triggering of the device. The integrated signal is then applied to an input of a comparator 448. The reference voltage is applied to the comparator 448 through a variable resistor 450. An alarm output is generated by the comparator 448 when the integrated value rises above the preselected reference voltage. The alarm signal may be utilized to operate an audible noise such as a horn or.

the like or alternatively may operate a visual alarm, such as a lamp or the like.

FIG. 21 is a schematic diagram of the oscillator 60 of the deactivation system shown in FIG. 3. The oscillator is operated in a similar manner as the oscillator previously described in FIG. 14, in that the frequency gener- 14 ated by the oscillator is maintained by a phase-lock loop in order to maintain the transmitting coil of the deactivater system in resonance, regardless of the load on the field imposed'in the aperture 28 of the deactivation circuit. The signal applied from the transmitting coil of the deactivation circuit is applied to terminal 452 and through voltage dividers 454 and 456 to the input of a phase-lock loop 458. The phase-lock loop 458 operates in the same manner as the circuit shown in FIG. 16 to generate an output through a capacitor 460 having a frequency to cause resonance of the transmitting coil.

The signal is applied through a filter network includ ing an operational amplifier 462 and is then applied through resistor 464 and capacitor 466 as a sine wave oscillator output for amplification and for driving of the field coil. The field coil thus generates a magnetic field within the deactivation zone in order to sense the presence of a marker in order to enable the marker to be deactivated. The output from the phase-lock loop 458 is also applied to an imput of an amplifier 468 which generates a buffered sync signal which is utilized to gate the receiver of the deactivator circuitry in a similar manner as that previously described in FIG. 20.

FIG. 22 illustrates in schematic detail the deactivator gaussing circuitry of the invention. A relay switch member 500 is selectably movable against a relay contact 502 by a relay coil 504. The relay coil 504 is attached to the output of the detector of the deactivator circuitry shown in FIG. 3. As previously noted, the amplifier 68 and detector 70 as shown in FIG. 3 are identical to the detecting circuitry of the system shown in FIG. 2. When a marker is detected with the deactivation unit by the detector, a signal is applied to the relay coil 504 and the relay member 500 is pulled out of contact with the contact 502. Voltage is then applied to the relay coil 506 in order to operate the relay switch member 508. Normally the relay switch member 508 is closed against the relay contact member 510. Upon operation of the energization of coil 506, the relay switch member 508 is closed against contact 512.

A line plug 514 is operable to be plugged into a source of V ac which applies alternating current through a transformer 516 to relay coil 506 when desired; Alternating current is applied to a capacitor 520 through diodes 522 and 524 to charging capacitors 526, 528 and 530. Capacitors 520, 526, 528 and 530 and diodes 522 and 524 comprise a conventional voltage doubler circuit. Relay contact 510 is connected to a voltage divider and safety bleeder resistor circuit 532. Contact 510 is also connected through a resistor 534 to the source of alternating current. The relay switch member 508 is connected through a capacitor 538 to the source of alternating current. The contact 512 is connected through a resistor 540 to the gate of an SCR 542.

The anode of the SCR 542 is connected to a gaussing coil 546, with the other terminal of the gaussing coil 546 being connectedv to the terminals of the capacitors 526 530. In the preferred embodiment, the degaussing coil 546 comprises 35 turns of No. 6 wire.

In operation of the deactivation system shown in FIG. 22, when the detector in the deactivation system detects the presence of an active marker, voltage appears at the coil 504 and the relay member 500 is actuated in order to apply voltage to the relay coil 506. The relay switch member 508 is moved against the contact 512 in order to connect the charged capacitor 538 to the gate control of the SCR 542. The SCR 542 is then turned on and the energy stored in capacitors 526, 528 and 530 is connected across the gaussing coil 546 to provide an extremely large energy pulse to the deactivator chamber. The deactivating pulse magnetizes the high coercivity ferromagnetic member in the manner previously described to deactivate the marker.

When the relay member 508 again closes against contact 510, the charging cycle is again initiated and capacitor 538 charges to a voltage across resistor 534. The capacitors 526, 528 and 530 are charged approximately twice the voltage of the ac line. If the marker within the deactivation chamber is not deactivated, the sensor again senses the presence of marker and again initiates the generation of a magnetic field from the gaussing coil 546 to insure that the marker is deactivated. The peak intensity of the magnetic field generated by the coil 546 is at least 100 oesteds, and in the preferred embodiment in the range of 1,000 oesteds. If it is desired to manually operate the system, a foot switch 552 is provided which may be manually closed by the foot of the operator against a contact in line 550 to energize coil 506. The SCR 542 is thus triggered to cause the discharge of the capacitors 526 530 across the coil 546.

Whereas the present invention has been described with respect to specific embodiments thereof, it will be understood that various changes and modifications will be suggested to one skilled in the art, and it is intended to encompass such changes and modifications as fall within the scope of the appended claims.

What is claimed is:

l. A marker for being secured to an object to enable detection of the presence, identity of status of the object within an interrogation zone having a magnetic field periodically varying at a predetermined'fundamental frequency, said marker comprising:

a first elongated ferromagnetic element for being secured to said object and having low coercivity capable of generating a detectable signal containing harmonics of said fundamental frequency when placed in said zone,

a secons ferromagnetic means disposed adjacent said first element and having a coercivity greater than said first element, said second ferromagnetic means including structure for imposing a plurality of pairs of alternate magnetic poles on said first element when placed in a predetermined magnetic field to thereby prevent the generation of said detectable signal.

2. The marker of claim 1 wherein said second ferromagnetic means comprises an elongated strip of ferromagnetic material having a plurality of apertures formed therethrough.

3. The marker of claim 2 wherein said apertures comprise circular cutout portions spaced along the length of said strip.

4. The marker of claim 1 and further comprising means for enabling said second ferromagnetic means to be oriented at any angle to said predetermined magnetic field to impose said plurality of alternate magnetic poles on said first element.

5. The marker of claim 4 wherein said second ferromagnetic means comprises discrete ferromagnetic elements laterally offset from one another and spaced apart along said elongated first element.

6. The marker of claim 5 wherein said discrete ele ments comprise rectangles having widths twice as wide as the width of said first element, said discrete elements positioned to extend on alternate opposite sides of said first element. A

7. A deactivatable marker for being secured to an object to enable detection of the object when the object is placed in an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency, said marker comprising:

a first elongated ferromagnetic element of low coercivity capable of responding to said magnetic field to generate signals containing harmonics of said fundamental frequency, and

second ferromagnetic means disposed adjacent said first element and having a coercivity greater than said first element, said second ferromagnetic means operable to be magnetized by a predetermined magnetic field oriented at any direction relative to said first element for imposing a plurality of pairs of alternatemagnetic poles on said first element in order to substantially alter the harmonic content of the signal produced by said first ferromagnetic element when in the interrogation zone.

8. The deactivatable marker of claim 7 wherein said second ferromagnetic means comprises a plurality of discrete ferromagnetic elements spaced along the length of said first elongated element.

9. The deactivatable marker of claim 8 wherein said discrete elements are laterally offset from said first elements such that a plurality of alternate magnetic poles are imposed upon said first element regardless of the orientation of said predetermined magnetic field to said discrete elements.

10. The deactivatable marker of claim 9 wherein said discrete elements are rectangular with widths equal to twice the width of said first element, alternate ones of said discrete elements extending from opposite sides of said first element.

11. A deactivatable marker for being secured to an object to enable detection of the object when the object is placed in an interrogation zone having a magnetic field therein, said marker comprising:

first and second elongated paper strips,

a first elongated ferromagnetic strip having a reduced .width but the same length as said paper strips and mounted between said paper strips, said first strip.

operable to respond to said magnetic field to generate signals containing harmonics of the frequency of said magnetic field,

a second elongated ferromagnetic strip having substantially the same width and length as said paper strips and mounted between one of said paper strips and said first ferromagnetic strip, and

said second ferromagnetic strip having a greater coercivity than said first ferromagnetic strip and including discrete apertures formed therethrough and spaced along the length of said second ferromagnetic strip.

12. A deactivatable marker for being secured to an object to enable detection of the object when the object is placed in an interrogation zone having a magnetic field therein, said marker comprising:

first and second elongated paper strips,

a first elongated ferromagnetic strip having a reduced width but the same length as said paper strips and mounted between said paperstrips, said first strip operable to respond to said magnetic field to generate signals containing harmonics of the frequency of said magnetic field,

a plurality of rectangular discrete ferromagnetic elements having greater coercivity than said first ferromagnetic strip and mounted between one of said paper strips and said first ferromagnetic strip, said discrete elements having widths equal to twice the width of said first ferromagnetic strip and said discrete elements mounted along said first ferromagnetic strip such that approximately half of such discrete element extends laterally out from said first ferromagnetic element.

13. A system for detecting a characteristic of an object when said object is in an interrogation zone having a magnetic field periodically varying at a predetermined fundamental frequency, said system comprising:

marker means for being secured to the object comprising a first elongated ferromagnetic element of low coercivity capable of generating a detectable signal containing harmonics of said fundamental frequency when placed in said zone, and second ferromagnetic means disposed adjacent said first element and having a coercivity greater than said first element, said second means when magnetized capable of imposing a plurality of pairs of alternate magnetic poles on said first element to alter the harmonic content of the detectable signal produced by said first element when in said interrogation zone,

deactivation means for selectively magnetizing said second ferromagnetic means,

radiating means for producing within said interrogation zone said magnetic field, receiving means for detecting the harmonic content of the signal produced by said first element, and

security readout means coupled to said receiving means responsive to said signal to indicate said characteristic of said object when said second ferromagnetic means is not magnetized, said security readout means providing no indication of said characteristic when said second ferromagnetic means is magnetized. I v

14. The system of claim 13 wherein said second ferromagnetic means comprises an elongated strip of ferromagnetic material having a plurality of apertures formed therethrough.

15. The system of claim 13 wherein said second ferromagnetic means may be oriented at any angle to said deactivation means for being magnetized to impose a plurality of pairs of alternate magnetic poles of said first element.

16. The system of claim 13 wherein said second ferromagnetic means comprises discrete ferromagnetic elements laterally offset from one another and spaced apart along said elongated first element.

17. A system for detecting and deactivating a marker secured to an object comprising:

marker means comprising first and second ferromagnetic elements having different coercivities, said first element operable to generate harmonic signals 6 when placed 1n an interrogation zone and said second element operable when magnetized to alter the generation of said harmonic signals,

means for detecting the presence of said first element,

means operable in response to said detecting means for magnetizing said second element, and

means for generating a signal when said second element has been magnetized.

18. The system of claim 17 wherein said means comprises a coil for generating a magnetic field alternating about a fundamental frequency and a pickup coil for receiving said harmonic signals.

19. The system of claim 18 wherein said magnetizing means comprises a gaussing coil across which a charged capacitor may be discharged.

20. The system of claim 19 wherein said generating means comprises circuitry for generating an electrical energizing signal after said charged capacitor has been discharged across said gaussing coil and after said pickup coil no longer detects said harmonic signals.

21. The system of claim 20 and further comprising:

auxiliary equipment energized by said electrical energizing signal.

22. A system for detecting an object when said object is in an interrogation zone comprising:

means for generating in said interrogation zone a magnetic field periodically varying at a predetermined fundamental frequency,

marker means for being secured to said object and including a first elongated ferromagnetic element of low coercivity capable of generating a detectable signal-containing harmonics of said fundamental frequency when placed in said interrogation zone,

means for detecting said detectable signal to indicate the presence of said object,

said marker means further including second ferromagnetic means adjacent said first ferromagnetic element and operable when magnetized to alter the harmonic content of said detectable signal such that said detecting means will not indicate the presence of said object,

deactivation meansfor sensing the presence of said "first ferromagnetic element'and in response thereto for magnetizing said second ferromagnetic means,

and utilization means operable for being energized only when said second ferromagnetic means has been successfully magnetized to alter the harmonic content of said detectable signal.

23. The system of claim 22 wherein said deactivation means comprises:

means for generating a second magnetic field periodically varying at a predetermined fundamental frequency,

second means for detecting said detectable signals,

and

gaussing means responsive to said second means for generating a magnetic field to magnetize said second ferromagnetic means.

24. The system of claim 23 wherein said utilization means is energized only after the operation of said gaussing means and when said second means no longer detects said detectable signal.

25. The system of claim 24 wherein said utilization means comprises a cash register.

26. The method of deactivating a marker having a first ferromagnetic strip operable to generate detectable harmonic signals when placed in an alternating magnetic field and a second ferromagnetic element operable when magnetized to alter the detectable harmonic signal, the method comprising:

applying an alternating magnetic field to said marker,

sensing said detectable harmonic signals and in response thereto applying a magnetizing field to magnetize said second ferromagnetic element,

trical signal unlocks a cash register.

7 3 v UNITED STATES PATENT OFFICE CERTIFICATE OF QORRECTION Patent No. 332 Dated June 5, 97 4 Inventor( Edward R. Fcaron It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1, line 59, "identify" should be -identity-.

Col. 7, line 2 4, "oircules" should be -cir'cles.

Col. 9, line 17, "amplifier 108" should be --amplifier 208--.

Col. 11, line 40, u use if" should be -fuse is-.

Col. 12, line 19, aplied" should be -applied--.

C01. 1 4, line 21, ."imput" should be input-. 7

Col. 15, line 35, "of status" should be o1- status;

line M, "a secons" should be --a second.

Col. 17, line 12, "half of such" should be -half of each-;

line 5M, "poles of said" should be -poles on said-.

Signed arid sealed this 8th day of October 1974.

(SEAL) Attest:

c. MARSHALL DANN Commissioner of Patents MCCOY I. GIBSON JR. Attesting Officer

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Classifications
U.S. Classification340/572.2
International ClassificationG08B13/24
Cooperative ClassificationG08B13/2437, G08B13/2408, G08B13/2474, G08B13/2477, G08B13/2431
European ClassificationG08B13/24B1F, G08B13/24B7A2, G08B13/24B3C, G08B13/24B7A3, G08B13/24B3M