ARTICLE DETECTION IN A LIMITED INTERROGATION ZONE

Abstract: A detection apparatus includes a transmitter (16) and a receiver (18) antenna positioned on each side of an interrogation zone (10) through which protected articles (42) pass. Electrically conductive shields (20) are positioned on the sides of the antennas (16, 18) facing away from the interrogation zone (10). The transmitter antennas (16) are energized at different frequencies. A summing circuit (36) is connected to receive signals from the two receiver antennas (18) and a signal processor (38) is connected to select intermodulation components in the summer output to activate an alarm (40).

Description: TITLE ARTICLE DETECTION IN A LIMITED INTERROGATION ZONE BACKGROUND OF THE INVENTION Field of the Invention This invention relates to the field of electronic article surveillance and more particularly it concerns a novel method and apparatus for detecting articles being carried through a restricted interrogation zone without detecting articles that may be near, but not within, the interrogation zone. Descrintion of the Related Art United States Patent No. 4,623,877 describes an electronic article surveillance system which is used to protect against shoplifting and other unauthorized movement of articles. Such system may also be used to protect against the unauthorized taking of books from a library. This system includes a pair of interrogation antennas which are energized to generate a continuous alternating magnetic field in the region of a doorway or exit from a store, library or other protected area. This field constitutes an interrogation zone; and when protected articles, such as books, are brought through the zone, special sensor elements on the protected articles disturb the field in a characteristic manner. Receiver antennas are also positioned at the interrogation zone. These receiver antennas are connected to receivers which produce an alarm in response to the characteristic disturbances. In order to provide satisfactory detection capability throughout the interrogation zone, a transmitter antenna and a receiver antenna are placed on each side of the zone. However, these antennas are omnidirectional; and they may cause sensor elements on nearby articles to produce an alarm even when such articles are not actually within the zone. United States Patent No. 4,704,602 discloses the idea of applying different frequencies to transmitter antennas on opposite sides of the interrogation zone so that the sensor elements on the articles to be protected will generate intermodulation components. This patent also shows the application of a third frequency to both transmitter antennas. However, this arrangement does not provide any protection against the detection of articles which are close to but outside the interrogation zone. United States Patent No. 4,623,877 discloses the use of electrically conductive shields in the form of sheets of aluminum positioned close to the antennas on opposite sides thereof from the interrogation zone. This has been found to reduce the magnetic interrogation field energy immediately outside the zone and to reduce the incidence of a false alarm from protected articles which are near but not actually within the interrogation zone. However, the high magnetic fields which are necessary to energize the entire interrogation zone nevertheless result in a certain amount of magnetic energy being present in the region outside the shields; and there is some danger that a protected article which is close to, but not actually within, the interrogation zone will be energized sufficiently to cause an alarm signal to be generated. This is especially likely when the detection system is used in close quarters where protected merchandise, books, etc., must be kept close to the exit or doorway. SUMMARY OF THE INVENTION The present invention overcomes problems caused by high magnetic interrogation fields which extend beyond the interrogation zone where detection of articles is to take place. According to one aspect of the present invention, a novel apparatus is provided for generating detection signals in response to the presence of magnetically saturable sensor elements within an interrogation zone and for avoiding the generation of detection signals in response to the presence of such elements in regions adjacent the zone. This novel apparatus comprises a pair of transmitting antennas which are positioned, respectively, on opposite sides of an interrogation zone through which protected articles bearing sensor elements are required to pass upon being taken from a protected region. A signal generator is arranged to supply alternating electrical current continuously to each antenna so that each antenna produces a continuous alternating magnetic field at a different frequency in the interrogation zone. A pair of receiver antennas are positioned adjacent the respective transmitting antennas on opposite sides of the interrogation zone to receive alternating magnetic fields which are present in the zone, including components which are produced by sensor elements present in the zone. A pair of electrically conductive shields are positioned, respectively, adjacent each transmitting and associated receiver antenna on the side thereof away from the interrogation zone. The invention is characterized in that the signal generator is arranged to supply a different frequency to each transmitting antenna and in that a signal summing circuit is connected to receive signals from both the receiver antennas, and to produce detection signals corresponding to the combined amplitudes of intermodulation components which are produced by sensor elements. These detection signals may be used to activate an alarm. Since the intermodulation components are produced by energy from both transmitter antennas, and since the energy from one of the transmitter antennas must cross the zone and then pass through the shield before energizing a sensor element outside the zone to produce an intermodulation component, the component which is produced is of such minimal amplitude that it is not detected. On the other hand, the energy of the intermodulation components within the interrogation zone is quite high throughout the zone so that sensor elements within the zone can be reliably detected. According to another aspect of the present invention there is provided a novel method for generating detection signals in response to the presence of magnetically saturable sensor elements within an interrogation zone while avoiding the generation of detection signals in response to the presence of such elements in regions adjacent the zone. This novel method comprises steps which include transmitting continuous alternating magnetic fields from antennas which are located on opposite sides of an interrogation zone through which protected articles bearing sensor elements are required to pass upon being taken from a protected region, These steps further include receiving, at receiver antennas positioned adjacent each transmitting antenna, respectively, on opposite sides of the interrogation zone, alternating magnetic fields which are present in the zone, including components produced by sensor elements present in the zone. The steps also include attenuating, by means of a pair of electrically conductive shields positioned, respectively, adjacent each transmitting and associated receiver antenna on the side thereof away from the interrogation zone, electromagnetic energy from the transmitting antennas. The invention in this aspect is characterized in that the continuous alternating magnetic fields from each antenna have different frequencies and in that signals from both the receiver antennas are received and summed to produce detection signals corresponding to the combined weighted sums of the intermodulation components. These detection signals may be used to activate an alarm. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an article surveillance system including an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1 an interrogation zone 10 is defined between a pair of antenna assemblies 12 and 14 which may be set up on opposite sides of a doorway or other exit from protected area, such as a store or library. Each antenna assembly includes a transmitter antenna 16 and an associated receiver antenna 18. Also, alongside each antenna assembly, on the side thereof away from the interrogation zone 10, there is provided an electrically conductive shield 20 in the form of a sheet of aluminum having a surface which extends slightly beyond the extent of the transmitter and receiver antennas 16 and 18. The transmitter and receiver antennas 16 and 18 are well known and may for example be as described in U.S. Patent No. 4,623,877. Also, the shields 20 may simply be a sheet of electrically conductive material, such as sheet aluminum. The shields 20 should be spaced slightly apart from their respective antenna assemblies so that loading of the antennas by the shields is minimized. A crystal controlled oscillator 22 is connected to supply timing signals to sine wave generators 24 and 26. The sine wave generators 24 and 26 produce sine waves at respectively different subharmonic frequencies of the crystal oscillator's output. In the preferred embodiment these frequencies are chosen to be approximately 7KHZ and 9KHZ respectively. Sine wave generators are well known to those who are familiar with the field of electronic surveillance systems and no particular construction or characteristic thereof is deemed best suited for this invention other than that it is preferred to have as pure and as stable a sine wave output as possible. These frequencies should be such that their sum is not equal to any harmonic of either of them. This provides an additional advantage because it avoids confusion of intermodulation components and harmonics when the signals from the two antennas are mixed as will be described herein. The outputs of the sine wave generators 24 and 26 are amplified in associated transmitter amplifiers 28 and 30; and the amplified outputs are applied to the respective transmitter antennas 16. Each receiver antenna 18 is connected to an associated receiver amplifier 32 and 34. These amplifiers also are of well known construction and no particular operating characteristic is considered suitable for the best mode of carrying out the invention. The outputs of the amplifiers 32 and 34 are applied to input terminals 36a and 36b, respectively of a conventional signal summing circuit 36. The summing circuit produces a weighted sum of each frequency component of the signals applied to its respective terminals 36a and 36b. These frequency components include the fundamental transmitted frequencies, i.e. 7KHz and 9KHz, as well as harmonics and intermodulation components of each of these fundamental frequencies. The output of the signal summing circuit 36 is a spectrum of frequencies, with each frequency having an amplitude which is the sum of the amplitudes of that frequency as applied to the input terminals 36a and 36b. This output is applied to signal processing circuits 38 which respond to the intermodulation components to produce an alarm signal. Signal processing circuits which respond to predetermined or selected frequency components are well known and need not be described herein to enable one skilled in the art to practice the invention. The alarm signal is then applied to an alarm 40. A protected article 42, has a sensor element 44 mounted thereon or embedded therein. These elements are well known in the field of article surveillance and are described for example in U.S. Patents 4,568,921 and No. 5,029,291. In general, these elements comprise a thin elongated strip of material, such as Permalloy or an amorphous alloy, having special magnetic characteristics which enable it to disturb incident cyclical magnetic fields in such a way that unique electromagnetic signals are produced whenever the sensor element passes through the magnetic fields produced in the interrogation zone 10 by the transmitter antennas 16. These unique magnetic fields include harmonics of the interrogation frequencies supplied by each of the transmitter antennas 16. Thus, in the present example, where the transmitted frequencies are 7KHZ and 9KHZ, harmonic frequencies are produced at 14, 21, 28, 35, etc. KHZ and at 18, 27, 36, 45, etc. KHZ. In addition, intermodulation components are produced. These intermodulation components comprise the frequencies Mf1 + Nf2 where M and N are positive and negative integers; and f1 and f2 are the respective transmitted frequencies, for example 7KHZ and 9KHZ. The transmitter antennas 16 are omnidirectional; and they generate alternating magnetic fields in directions away from the interrogation zone 10 as well as into it. The shields 20 attenuate, to a certain extent, but not completely, the amplitude of the interrogation fields outside the interrogation zone. Therefore, a sensor element 44 located outside the interrogation zone 10, but close to one of the transmitter antennas 16, may be energized by that antenna. However, that sensor element will not be energized to any appreciable extent by the signal from the transmitter antenna on the far side of the interrogation zone. Accordingly, the sensor element 44 will produce electromagnetic disturbances at harmonics of the nearby transmitter antenna but it will not produce appreciable disturbances at harmonics of the more distant transmitter antenna; nor will it produce appreciable intermodulation components. However, since the signal processing circuits 38 respond to the intermodulation components and not to the harmonics of the transmitted signals, the disturbances produced by the sensor elements 44 which are outside the interrogation zone will not be detected. Thus, it is possible with the present invention, to place protected articles close to an interrogation zone, such as close to the exit doorway from a store or library, without causing false alarms to be produced. This permits the detection threshold of the system to be set quite low to ensure detection of all sensor elements which pass through the interrogation zone.

Claims: CLAIMS 1. Apparatus for generating detection signals in response to the presence of magnetically saturable sensor elements within an interrogation zone while avoiding the generation of detection signals in response to the presence of such elements in regions adjacent said zone, said apparatus comprising a pair of transmitting antennas positioned, respectively, on opposite sides of an interrogation zone through which protected articles bearing sensor elements are required to pass upon being taken from a protected region, a pair of receiver antennas, each positioned adjacent one of said transmitting antennas, on opposite sides of said interrogation zone to receive alternating magnetic fields present in said zone, including components produced by sensor elements present in said zone, a pair of electrically conductive shields, each positioned adjacent one of said transmitting and associated receiver antennas on the side thereof away from said interrogation zone, characterized in that a signal generator is arranged to supply alternating electrical current continuously at a first frequency to one of said transmitting antennas and at a second frequency to the other transmitting antenna to cause the antennas to produce continuous alternating magnetic fields at said frequencies in said interrogation zone, and in that a signal summing circuit is connected to receive signals from both said receiver antennas, and to produce detection signals corresponding to the weighted sums of said intermodulation components. 2. Apparatus according to claim 1, further characterized in that signal processing circuits are connected to receive signals from said signal summing circuit and produce alarm signals in response to intermodulation components received from said signal summing circuit. 3. Apparatus according to claim 1, characterized in the sum of said first and second frequencies is not equal to any harmonic of either of them. 4. Apparatus according to claim 1, characterized in that said first frequency is about 7KHz and said second frequency is about 9 KHz. 5. A method for generating detection signals in response to the presence of magnetically saturable sensor elements within an interrogation zone while avoiding the generation of detection signals in response to the presence of such elements in regions adjacent said zone, said method comprising the steps of transmitting continuous alternating magnetic fields from antennas on opposite sides of an interrogation zone through which protected articles bearing sensor elements are required to pass upon being taken from a protected region, receiving, by means of receiver antennas positioned adjacent each transmitting antenna, respectively, on opposite sides of said interrogation zone, alternating magnetic fields present in said zone, including components produced by sensor elements present in said zone, attenuating, by means of a pair of electrically conductive shields positioned, respectively, adjacent each transmitting and associated receiver antenna on the side thereof away from said interrogation zone, electromagnetic energy from said transmitting antennas and receiving and summing signals from both said receiver antennas, characterized in that the alternating magnetic fields transmitted from each antenna have different frequencies and in that detection signals corresponding to the weighted sums of said intermodulation components are produced. 6. A method according to claim 5, further characterized in that it includes including the step of producing alarm signals in response to said detection signals. 7. A method according to claim 5, characterized in that the sum of said first and second frequencies is not equal to any harmonic of either of them. 8. A method according to claim 5, characterized in that said first frequency is about 7KHz and said second frequency is about 9KHz.

Inventors: LUNDQUIST DAVID TIETJEN, PAUL CHRISTOPHER REINARD, LUNDQUIST, DAVID, TIETJEN, PAUL, CHRISTOPHER, REINARD
Applicants: KNOGO CORP, LUNDQUIST DAVID TIETJEN, PAUL CHRISTOPHER REINARD, KNOGO CORPORATION, LUNDQUIST, DAVID, TIETJEN, PAUL, CHRISTOPHER, REINARD
Application year: 1993
Main classification: G08B13
IPC codes: G08B13