HK1176730B - Method and system for reducing effect of interference in integrated metal detection/electronic article surveillance systems - Google Patents

Description

Method and system for reducing interference effects in an integrated metal detection/electronic article monitoring system

Technical Field

The present invention relates generally to electronic article surveillance ("EAS") systems, and more particularly to methods and systems for reducing the effects of interference in integrated EAS/metal detection systems.

Background

Electronic article surveillance ("EAS") systems are commonly used in retail stores and other environments to prevent unauthorized removal of articles from a protected area. Typically, a detection system is disposed at the exit of the protected area, the detection system including one or more transmitters and an antenna ("pedestal") capable of generating an electromagnetic field (referred to as an "interrogation zone") between the two sides of the exit. The protected article is attached to an EAS marker that generates a response signal when the EAS marker passes through the interrogation zone while active. An antenna and receiver in the same or another "base" detects the response signal and generates an alarm.

Metal detection systems are also useful in detecting unauthorized removal of metal items from a protected area. Although there are many metal detection systems, there have been no successful attempts to combine EAS detection systems with metal detection systems. While others have located metal detection systems in proximity to EAS systems, none have provided any mechanism for effectively and inexpensively combining the two systems into one system.

Part of the difficulty in combining two systems into one system is the problem caused by interference of the transmit signals of other adjacent EAS transmitters. If a metal detector is combined with an acousto-magnetic ("AM") EAS system and both use the same transmit electronics and tune the antenna coil to transmit EAS and metal detection signals, adjacent EAS transmitters will interfere with the metal detection receiver. Even if the metal detection frequency is different from the EAS frequency, the sidebands of adjacent EAS transmitters will have a significant impact on the metal detection receiver. The metal detection frequency cannot be too different from the EAS frequency because the metal detection transmission amplitude will be greatly reduced. This is because the antennas of the system are tuned to the EAS transmission frequency.

What is needed, therefore, is an integrated EAS/metal detection system for reducing the effects of interference that may be caused by adjacent EAS transmitters.

Disclosure of Invention

The present invention advantageously provides a method and system for reducing the effects of interference in an integrated electronic article surveillance ("EAS")/metal detection system. The system includes a transmitter operable to transmit an EAS interrogation signal, wherein the EAS interrogation signal establishes an interrogation zone and is used to detect EAS markers and metallic objects within the interrogation zone. The EAS interrogation signal is transmitted at a first frequency during an EAS detection period and at a second frequency during a metal detection period. The system includes a receiver operable to detect a signal received from an EAS marker, and a metal detector module operable to detect a metal object in proximity to an integrated Electronic Article Surveillance (EAS)/metal detection system, wherein the metal detector module includes a filter tuned to a first transmission frequency.

In one aspect of the invention, the invention provides an integrated Electronic Article Surveillance (EAS)/metal detection system. The system includes a transmitter operable to transmit an interrogation signal, wherein the interrogation signal establishes an interrogation zone and is used to detect EAS markers and metallic objects within the interrogation zone. The interrogation signal is transmitted at a first frequency during the EAS detection period and at a second frequency different from the first frequency during the metal detection period. The system also includes a receiver operable to detect signals received from the EAS marker and the metal detector module. The metal detector module includes a filter substantially centered about the first transmission frequency for filtering out received signals substantially at the first transmission frequency during metal detection.

In another aspect, the present invention provides a method for reducing interference in an integrated Electronic Article Surveillance (EAS)/metal detection system. An EAS marker interrogation signal is transmitted at a first frequency to establish an interrogation zone. The metal detection signal is transmitted at a second frequency different from the first frequency. The presence of the metal within the interrogation zone is detected, at least in part, by substantially filtering out interfering signals received from adjacent EAS transmitters transmitting at the first frequency.

In accordance with yet another aspect, the present invention provides a metal detection module in an integrated electronic article surveillance ("EAS")/metal detection system. The integrated EAS/metal detection system transmits an EAS interrogation signal at a first frequency and a metal detection signal at a second frequency different from the first frequency. The metal detection module has a filter arranged to substantially filter out received signals of the first transmission frequency and a controller operable to detect the presence or absence of metal using the second transmission frequency.

Drawings

A more complete understanding of the present invention and the attendant advantages and features thereof will be more readily obtained by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a block diagram of an exemplary acoustic electronic article surveillance ("EAS") detection system with integrated metal detection capabilities constructed in accordance with the principles of the present invention;

FIG. 2 is a block diagram of an exemplary integrated EAS/metal detection system controller constructed in accordance with the principles of the present invention;

FIG. 3 is a timing diagram illustrating a timing scheme of an acoustic EAS detection system;

FIG. 4 is a timing diagram illustrating a timing scheme for an acoustic EAS detection system including a metal detection window (window) in accordance with the principles of the present invention;

FIG. 5 is a timing diagram illustrating a hybrid cycle including interleaved metal detection windows and EAS special detection windows in accordance with the principles of the present invention; and

FIG. 6 is a flow chart of an exemplary metal detection process according to the principles of the present invention.

Detailed Description

Before describing in detail exemplary embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of apparatus components and processing steps related to implementing a system and method for reducing interference in an integrated EAS/metal detection system. Accordingly, the components of the systems and methods are represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

As used herein, relational terms, such as "first" and "second," "top" and "bottom," and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements.

One embodiment of the present invention relates to a combined EAS/metal detection system and advantageously provides a method and system for reducing the effects of interference that may be caused by adjacent EAS transmitters.

Referring now to the drawings in which like reference designators refer to like elements, there is shown in fig. 1 one configuration of an exemplary integrated EAS/metal detection system 10 constructed in accordance with the principles of the present invention and located, for example, at an entrance to a facility. The EAS detection system 10 includes a pair of pedestals 12a, 12b (collectively pedestals 12) on opposite sides of an entrance 14. One or more antennas of the EAS detection system 10 may be contained within pedestals 12a and 12b, with the pedestals 12a and 12b being located a known distance apart. The antenna located within the pedestal 12 is electrically coupled to a control system 16 for controlling the operation of the EAS detection system 10.

Referring now to fig. 2, an exemplary EAS control system may include a controller 18 (e.g., a processor or microprocessor), a power supply 20, a transceiver 22, a memory 24 (the memory 24 may include non-volatile memory, or a combination thereof), a communication interface 26, and an alarm 28. The controller 18 controls radio communication, storage of data to the memory 24, communication of stored data to other devices, and activation of the alarm 28. A power supply 20 (e.g., a battery or AC power supply) supplies power to the EAS control system 16. The alarm 28 may include software and hardware for providing a visual and/or audible alarm in response to the detection of an EAS marker and/or metal within the interrogation zone of the EAS system 10.

The transceiver 22 may include a transmitter 30 electrically coupled to one or more transmit antennas 32 and a receiver 34 electrically coupled to one or more receive antennas 36. Alternatively, a single antenna or a pair of antennas may be used for both the transmit antenna 32 and the receive antenna 36. Transmitter 30 transmits a radio frequency signal using a transmit antenna 32 to "energize" EAS markers within the interrogation zone of EAS system 10. Receiver 34 uses receive antenna 36 to detect the response signal of the EAS marker.

In one embodiment, memory 24 may include a metal detection module 38 for detecting whether metal is present within the interrogation zone. The metal detection module 38 may be implemented in hardware and/or software and may be located within the memory 24 or external to the memory 24. The operation of the metal detection module 38 will be described in more detail below. The metal detection module 38 includes a filter 39 for filtering out EAS interrogation signals from nearby EAS systems. Filter 39 can be a notch filter or any other equivalent device, or combination of devices operable to filter out a particular frequency band from a given signal. Although metal detection module 38 is shown as a software module stored within memory 24, metal detection module 38 may likewise be implemented using discrete components or may be a combination of hardware and software elements. For example, the metal detection module 38 can itself have a controller or other processing unit for performing the filtering and metal detection functions described herein in addition to or in lieu of the controller 18. Furthermore, although filter 39 is described herein as a "notch" filter, the invention is not so limited. It is contemplated that any filter having a slope sufficient to isolate the EAS signal frequency can be used. For example, a filter having a slope sufficient to filter out the EAS interrogation signal at 58kHz when the metal detection signal is 56kHz can be used in the present invention.

Referring now to FIG. 3, a timing diagram of an exemplary detection cycle 40 of an EAS system is shown. Using the characteristics of an EAS marker, the EAS detection system may transmit pulses for a period of time and then "listen" for a response signal at a subsequent time frame within one detection period 40. In one embodiment, the EAS detection cycle 40 includes four separate time periods: an emission window 42, a tag detection window 44, a synchronization window 46, and a noise window 48. An exemplary detection period 40 has a duration of 11.1 milliseconds at a frequency of 90 Hz. At the beginning of the detection period 40, a 1.6 millisecond pulse of an electromagnetic ("EM") field at 58kHz (i.e., a radio frequency signal) is emitted within the transmit window 42 to "energize" an EAS marker having the same natural resonant frequency as the frequency of 58 kHz. At the end of the transmit window 42, the EAS marker has received and stored a considerable amount of energy; thus, an actual EAS marker becomes an energy/signal source that resonates at a frequency of 58kHz as its stored energy gradually dissipates (commonly referred to as "ringing"). The transmitted EM field may be several orders of magnitude greater than the signal of the EAS marker. As a result, the receiver 34 is not operational during transmission. Receiver 34 begins to "listen" for the presence of an EAS marker signal after transmitter 30 stops transmitting EM energy. Within the tag detection window 44, the EAS marker signal can be easily detected when the background is quiet, i.e., the transmitter 30 is off. For verification, after the transmission of the EM energy pulses is complete, the receiver 34 also listens again within the synchronization window 46 and the noise window 48 (i.e., 3.9 milliseconds and 5.5 milliseconds, respectively). By this time, the energy within the EAS marker should be almost completely dissipated and not detectable. However, if the signal is still present, it may indicate whether some unknown source(s) of interference is/are present, and the alarm 28 will be disabled.

Referring now to fig. 4, in one embodiment of the present invention, a metal detection period 50 includes a metal detection window 52 in place of the emission window 42. The remainder of the metal detection cycle 50 is the same as the original detection cycle 40, i.e., the tag detection window 44, the synchronization window 46, and the noise window 48. One method for detecting metals is based on eddy currents induced during EM excitation. The induced eddy currents dissipate quickly, on the order of tens of microseconds in the case of good conductors. For poor conductors, the dissipation is poor. Even with good conductors, eddy current dissipation is about two orders of magnitude shorter than that of acoustic markers.

The EAS detection system 10 resumes marker detection after the metal detection transmission period 50 ends. In this case, the same transmitted EM excitation may be used to detect the presence of metal and an acoustic EAS marker, as shown in fig. 4. While the EAS detection signal is typically transmitted at a frequency of 58kHz, the metal detection interrogation signal is typically transmitted at a frequency of 56 kHz.

Fig. 5 shows a hybrid cycle 54, where in this example there is one metal detection cycle 50 every three cycles of the EAS special detection cycle 40. It should be noted that the order and number of interspersed metal detection cycles 50 in each EAS special detection window 40 shown in fig. 5 is illustrative only. Any combination and/or order of cycles is within the scope of the invention. Because both EAS and metal detection systems use the same transmit electronics, the frequency at which the metal detection transmit signal propagates cannot be varied by a large amount because the metal detection system is tuned to the EAS transmit frequency. In the embodiment of the invention shown in fig. 4 and 5, the metal detection transmit frequency is set to about 56kHz, which is different from the EAS transmit frequency of 58 kHz. However, because the EAS transmit and receive portions of cycle 40 are at a frequency of 58kHz, while the metal detection transmit is at a frequency of 56kHz, there is the potential for: the adjacent EAS detection system will interfere with the metal detection emissions of the metal detection portion of the hybrid EAS/metal detection system.

To counteract the interference effects that may be caused by neighboring EAS systems having EAS transmitters that transmit EAS interrogation signals at the same frequency (i.e., 58 kHz) as the signal transmitted by the integrated EAS/metal detection system 10, the metal detection module includes a notch filter 39 that is substantially centered about the EAS transmit frequency (58 kHz in this example). This will "notch" or filter interference that may occur due to adjacent EAS transmitters transmitting at a frequency of 58 kHz. In one embodiment, notch filter 39 has a bandwidth of approximately 1kHz-1.5kHz, corresponding to a 3dB attenuation.

FIG. 6 is a flow chart illustrating an exemplary process performed by metal detection module 38 incorporating the principles of the present invention. The method is based on detecting eddy currents induced during EM excitation. An embodiment of the invention will only be describedOne base is used to transmit EM energy and the other base is used as a receiving antenna 36 for metal detection. The metal detection module 38 determines a reference voltage (V) developed across the receive antenna 36 by transmitting an EM energy pulse through the transmit antenna 32 and measuring the voltage induced at the receive antenna 36 in the absence of metal within the interrogation zone_r). Induced voltage (V) generated in the absence of metal on the sensing coil of the receiving antenna 36 within the emission window 52 of the metal detection cycle 50_r) Is rather large due to the emitted EM field.

Once metal detection module 38 determines background voltage V_rThe system 10 proceeds to the metal detection cycle 50 (step S102). During the metal detection period 50, pulses of EM energy are transmitted by the transmit antenna 32 (step S104) and received at the receive antenna (step S106). The received EM energy may contain interference from surrounding EAS transmitters. Thus, metal detection module 38 filters out this interference by using notch filter 39 that is substantially centered around the frequency of the surrounding EAS transmitters (e.g., typically 58 kHz) (step S108).

Generally, if metal is present within the interrogation zone, the received signal strength is significantly less than the directly induced voltage induced during the transmission of the EM field due to eddy current effects. It is then determined whether metal is present within the interrogation zone (step S110). In one embodiment, if metal is present, the induced voltage is reduced to a value V_m. Net effective received voltage (V) due to the presence of metal_s) Is calculated as V_r-V_mThe voltage V is_sIs V_rA fraction (a few percent). If metal is detected, the metal detection module triggers an alarm (step S112). The alarm may be an audible, visual, or tactile alarm, or may notify security personnel or other authorized personnel to: the metal is detected to be carried by the EAS detection system 10. The system 10 then enters the EAS acoustic detection cycle 40 for a predetermined number of repetitions (step S114) before repeating the metal detection cycle 50 (step S102).

As described above, the induced eddy current dissipates very quickly, for example, on the order of tens of microseconds in the case of a good conductor. As a result, detection during the metal detection period 50 may be reduced as compared to detection during the detection period 40 for only EAS markers, where both pedestals may transmit simultaneously. In this case, the same transmitted EM excitation may be used to detect the presence of metal and acoustic EAS markers. Once the metal detection cycle 50 is complete, both pedestals may be used to detect the acoustic EAS marker.

Thus, the present invention provides an integrated EAS/metal detection system 10 that is capable of filtering out potential interfering signals from adjacent EAS transmitters that are transmitting their interrogation signals at substantially the same frequency as the EAS interrogation signals transmitted by the integrated EAS/metal detection system 10. By including notch filter 39 in metal detection module 38 that is tuned to the same frequency as the adjacent EAS transmitter, extraneous signals can be removed, allowing metal detection module 38 to more accurately detect the presence of metal objects within the interrogation zone without falsely triggering a metal detection alarm when no metal objects are within the interrogation zone.

The present invention can be realized in hardware, software, or a combination of hardware and software. Any kind of computing system, or other apparatus adapted for carrying out the methods described herein, is suited to perform the functions described herein.

A typical combination of hardware and software could be a specialized or general purpose computer system having one or more processing elements and a computer program stored on a storage medium that, when loaded and executed, controls the computer system such that it carries out the methods described herein. The present invention can also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which when loaded in a computing system is able to carry out these methods. Storage medium refers to any volatile or non-volatile memory device.

Computer program or application in the context of the present invention is intended to be any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) reproduced in different material forms.

Moreover, unless stated otherwise above, it should be noted that all of the accompanying drawings are not to scale. Significantly, this invention can be embodied in other specific forms without departing from the spirit or essential attributes thereof, and accordingly, reference should be had to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.

Claims (13)

1. An integrated Electronic Article Surveillance (EAS)/metal detection system comprising:

a transmitter configured to periodically transmit an interrogation signal at a first frequency and at a second frequency different from the first frequency;

a receiver configured to receive a first signal at the first frequency and a second signal at the second frequency;

a controller configured to detect an EAS marker based on the first signal received at the first frequency when the transmitter is not transmitting;

a metal detector module configured to detect metal by:

receiving an output of the receiver when the transmitter transmits to determine a reference voltage in the absence of a metal object; entering a metal detection period, receiving an output of the receiver when the transmitter is transmitting to determine a first voltage of the second signal;

filtering out signals received substantially at the first frequency;

comparing the first voltage to a reference voltage; and

determining that a metal object is present in response to determining that the first voltage differs from the reference voltage by more than a specified amount

To detect metal objects in proximity to the integrated electronic article surveillance/metal detection system.

2. The system of claim 1, wherein the metal detector module is configured to filter out signals received substantially at the first frequency using a notch filter.

3. The system of claim 1, wherein the metal detector module is configured to filter out signals received substantially at the first frequency with a filter configured to have a bandwidth between 1KHz and 1.5 KHz.

4. The system of claim 1, wherein the first frequency is substantially 58KHz and the second frequency is substantially 56 KHz.

5. The system of claim 1, wherein the controller is electrically connected to the transmitter, the receiver, and the metal detector module.

6. The system of claim 1, wherein the metal detector module is configured to determine that the metal object is present only in response to determining that the first voltage is less than the reference voltage by at least a predetermined threshold.

7. The system of claim 1, further comprising an alarm; and the metal detector module is further configured to trigger an alarm in response to detecting the presence of the metal object in the interrogation zone.

8. The system of claim 1, further comprising:

at least one transmitting antenna in communication with the transmitter, the at least one transmitting antenna being arranged to transmit the interrogation signal at the first frequency and to transmit an interrogation signal at the second frequency; and

at least one receive antenna in communication with the receiver, the at least one receive antenna arranged to receive the first signal at the first frequency and the second signal at the second frequency.

9. The system of claim 1, wherein the transmitter is housed in a first base and the receiver is housed in a second base.

10. A method for reducing the effects of interference in an integrated Electronic Article Surveillance (EAS)/metal detection system, comprising:

periodically transmitting an interrogation signal at a first frequency and at a second frequency different from the first frequency;

receiving a first signal at the first frequency and a second signal at the second frequency;

detecting an EAS marker based on the first signal received at the first frequency when a transmitter is not transmitting;

by:

receiving an output of a receiver when the transmitter transmits to determine a reference voltage in the absence of a metal object;

entering a metal detection period, receiving an output of the receiver when the transmitter is transmitting to determine a first voltage of the second signal;

filtering out signals received substantially at the first frequency;

comparing the first voltage to a reference voltage; and

determining that a metal object is present in response to determining that the first voltage differs from the reference voltage by more than a specified amount

To detect a metal object in proximity to the integrated electronic article surveillance/metal detection system.

11. The method of claim 10, wherein filtering out signals received substantially at the first frequency is performed by a filter having a bandwidth between 1kHz and 1.5 kHz.

12. The method of claim 10, wherein the first frequency is substantially 58kHz and the second frequency is substantially 56 kHz.

13. A method for a metal detection module in an integrated Electronic Article Surveillance (EAS)/metal detection system, the integrated EAS/metal detection system including a transmitter configured to periodically transmit an interrogation signal at a first frequency and a second frequency different from the first frequency, the integrated EAS/metal detection system including a receiver configured to receive a first signal at the first frequency and a second signal at the second frequency, the integrated EAS/metal detection system including a controller configured to detect an EAS marker based on the first signal received at the first frequency when the transmitter is not transmitting, the metal detection module configured to:

by:

receiving an output of the receiver when the transmitter transmits to determine a reference voltage in the absence of a metal object;

entering a metal detection period, receiving an output of the receiver while the transmitter is transmitting the interrogation signal at the second frequency to determine a first voltage of the second signal;

filtering out signals received substantially at the first frequency with a filter;

comparing the first voltage to a reference voltage; and

determining that a metal object is present in response to determining that the first voltage differs from the reference voltage by more than a specified amount

To detect a metal object in proximity to the integrated electronic article surveillance/metal detection system.