AU695143B2

AU695143B2 - Electronic article surveillance system

Info

Publication number: AU695143B2
Application number: AU34213/95A
Authority: AU
Inventors: Ian Thompson Bell; Nicholas David Swales
Original assignee: Esselte Meto International GmbH
Current assignee: Meto International GmbH
Priority date: 1994-10-15
Filing date: 1995-10-12
Publication date: 1998-08-06
Anticipated expiration: 2015-10-12
Also published as: ATE177551T1; ES2127981T3; DE59505271D1; DE4436978A1; EP0707295A1; EP0707295B1; AU3421395A

Abstract

An electronic security system to prevent theft of articles uses a hanger (9), to which the article is attached, that is located within an electromagnetic field. The hanger contains an inductance and capacitance coupled as a resonator device and is located between transmitting (8) and receiving (10) aerials. A voltage controlled oscillator (2) is controlled to produce a range of frequencies that are mixed (3) with the output of a second oscillator (18) and a multiplier (19). The receiver output is mixed with a half the frequency of the VCO output and is used in the detector. The presence of the hanger triggers the alarm.

Description

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name of Applicant: Actual Jnventors: Address of Service: ESSELTE MIETO INTERNATIONAL GmbH- Nicholas David Swales and Ian Thompson Bell SHELSTON WATERS 60 MARGARET STREET SYDNEY NSW 2000 "ELECTRONIC ARTICLE SURVEILLANCE SYSTEM"

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SC'1.A Invention Title: The following statement is a fall description of this invention, including the best method of performing it known to us:-

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.4 -la- Electronic Article Surveillance System The invention relates to an electronic article surveillance system.

Electronic article security systems are known in prior art and, in particular, are employed in sales units, guarding survei:.'ance zones so as to detect the presence therein of security tags which are provided with a resonant circuit (coil and capacitor). Since the respective resonant frequencies of the individual security tags are subject to a manufacturing tolerance of typically 10%, it is necessary to vary (sweep) the transmission frequency over a defined range so as to be sure that all security tags can be detected. In order to accomplish this, a voltagecontrolled oscillator (VCO) is usually employed as a means of providing the desired range of frequencies. A sine-wave voltage, delta voltage or staircase voltage is supplied to its input, and a frequency proportional to the input voltage will be present at its output. Alternatively, digital-to-analog converters can also serve as the frequency-generating means. The oscillator is typically connected to a final output stage which, operating in a pulse mode, supplies a transmitting antenna which in its turn generates a magnetic field in the surveillance zone.

A separate receiving

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-2antenna is employed for receiving the disturbances that a security tag creates in the surveillance zone. This antenna is connected to a receiving circuit which is sensitive to the entire range of frequencies swept by the transmitter, while the output of the receiver is connected to a detector which triggers an alarm in the event of the presence of a security tag in the surveillance zone, so that the sales unit staff or comparable personnel can, if appropriate, prevent an act of theft.

Detection is usually accomplished in a manner such that an alarm is given in the event of a change in the received power due to the presence of a surveillance tag at a defined, unchanging transmission frequency, i.e. at a constant point in time during the sweep cycles.

A generic-type system was disclosed in EP 565 481 Al.

In order to be able, on the one hand, to improve the security tag detection probability by increasing the sensitivity and, on the other hand, to reduce the probability of a false alarm, the receiver is provided with a mixer, the first input of which is connected to the receiving coil. The transmission frequency is applied to the second input. As a result, I'o :a constant voltage signal will be present at the output of the mixer, and this voltage will change in the event 25 of the presence of a security tag in the surveillance zone. This voltage is supplied to a bandpass filter and an amplifier, and finally to an analog-to-digital With such a system, the fact that the output frequency of the mixer must remain relatively low is to be regarded as disadvantageous. Low frequencies can be amplified and filtered, but only with difficulty. The consequences are high manufacturing costs and high sensitivity to interfering signals.

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.4 5 -3- The problem underlying the invention is, starting from the prior art, to devise an electronic article surveillance system which features low sensitivity to interfering signals and a high detection probability.

The problem is solved, according to the invention, by what is specified in claim 1.

The essential idea is to apply the superheterodyne principle to the generation of the (intermediate frequency) signal that is to be amplified prior to being finally supplied to the detection arrangement. While the signal that is received by the antenna, corresponding to the transmission frequency, is present at the first input of the mixer, the second input of said mixer is supplied with a signal that has a frequency corresponding either to the sum of the transmission frequency and a second, fixed frequency, or to the difference between these frequencies. Since the output from the mixer carries the two input frequencies and, in addition, their sum and their difference, it is possible, in both cases, to obtain an intermediate frequency which is constant with respect to 20 time, which is indepe:ndent of the transmission frequency, and which can be as high as may be desired, with the result that amplification will be easy. If the frequency corresponding to the sum of the transmission signal frequency and the fixed frequency is present at the second input of the mixer, the difference frequency present at the output of the mixer will be constant with respect to time i.e. it will be independent of the sweep frequency. Analogously, the sum frequency present at the output will be constant if the frequency corresponding to the difference between the transmission frequency and a fixed frequency is present at the second input of the mixer. This voltage is now supplied to the intermediate frequency amplifier, which amplifies only signals in a narrow band centered on the chosen intermediate frequency, while all other frequencies are suppressed with the consequence that undesired interfering signals are suppressed as well.

The advantages of the invention reside principally in the fact that the intermediate frequency which can be as high as may be desired allows easy amplification, with the result that a high sensitivity and a high detection probability are achieved. The sensitivity with respect to interfering signals is substantially reduced on account of the fact that the amplification is selective.

In practice, it proves expedient to provide a second mixer, for the purpose of generating the transmission frequency. When this form of design is adopted, the output of the circuit for generating the variable frequency i.e. the voltage-controlled oscillator or *.iJ the digital-to-analog converter is connected to one 20 input of this second mixer, while the other input is connected to a fixed frequency generator. It is obvious that the frequencies of the generators are selected so as to ensure that the sum or difference frequency present at the output of the mixer, namely the frequency that is 25 supplied to the transmitting antenna the difference frequency being preferred corresponds to the resonant frequency of the security tag. In order to generate the intermediate frequency for the receiver, the output of the circuit for generating the variable frequency is 30 connected to the second input of the mixer associated with the receiver.

In principle, it would be possible to arrange for the fixed frequency generator to operate at the intermediate frequency. The circuit for generating the variable frequency would then oscillate at the sum or difference frequency deriving from the transmission frequency and the frequency of the fixed frequency generator. However, undesirable disturbances (possibly even hunting) would be the disadvantageous consequence of the fact that the intermediate frequency amplifier would be operating at a high amplification factor. It is accordingly proposed that two frequency dividers, with equal division ratios, be employed (or two frequency multipliers).

One is connected between the output of the second mixer and the transmitting antenna, while the other connects the output of the circuit for generating the variable frequency to the second input of the mixer associated with the receiver. As a result, the fixed frequency generator now operates at a multiple (or fraction) of the frequency of the intermediate amplifier, so that there is absolutely no risk of interference being caused by incoming external radiation.

It is obvious that an infinite number of combinations *of frequency offset, division ratio and intermediate frequency are possible. If the intermediate frequency ep pp o° is denoted by f, the division ratio of the frequency dividers by N and the sweep range by s, and if the 25 signal that is supplied to the transmitting antenna has a frequency corresponding to the difference between the frequencies that are input to the mixer associated with lt! the transmitter, the circuit for generating a variable frequency will sweep a frequency range that corresponds to the expression N (f s)

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~it i The output of the fixed frequency generator then carries an output voltage with the frequency N f 6- (j -6- Since the difference frequency is transmitted, the difference frequency produced by the mixer associated with the transmitter is N (f s) N f N s The frequency divider associated with the transmitter then produces the desired sweep range N s N s The output of the frequency divider connected between the circuit for generating a variable frequency and the mixer associated with the receiver then supplies an output frequency that corresponds to N (f s) N =f s Finally, the mixer associated with the receiver supplies the intermediate frequency f s s f i.e. the intermediate frequency that is desired.

Moreover, the fixed frequency generator can be provided with a low-frequency oscillator, generating an output signal which is subjected to frequency multiplication.

When this form of design is adopted, care must be taken to ensure that the multiplication factor which applies here is as different as possible from the division factor of the frequency dividers which are respectively connected to the mixers associated with the transmitter and 25 receiver, since if this were not the case the oscillator So frequency and the intermediate frequency would coincide, and undesired external radiation might consequently be able to enter the intermediate frequency amplifier.

9 Since not only the amplitude of the received signal changes in the event of the presence of a security tag in the surveillance zone, but the phase of this signal also

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OF o "Vr 0C -7changes, it is preferable to employ a phase-sensitive synchronous detector in the detection arrangement. The first of its two inputs is connected to the receiver, i.e. to the output of the intermediate frequency amplifier, while its second input is connected to the fixed frequency generator.

The synchronous detector is thus rendered capable of inspecting the phase difference between the radiated signal and the received signal. In this process, the fixed frequency generator serves as the source of the phase information, since this is the only available phase information that is independent of the instantaneous sweep frequency.

A so-called IQ detector, known per se, is preferably employed as the synchronous detector. It has an output at which the in-phase content of the transmitted and received signals will 15 be present. That portion of the received signal which is shifted through 90' relative to the transmitted signal will be present at a second output. The decision whether to trigger an alarm can now be made by reference to two items of information (amplitude and phase), and is subject to a lower error probability than in the case of conventional systems which evaluate only the amplitude.

In what follows, an embodiment of the invention is explained in greater detail by reference to the drawing, which is a diagrammatic representation of an electronic article surveillance system.

The system according to this embodiment of the invention comprises a digital-to-analog converter which serves as a sweep generator supplying a voltage-controlled oscillator with an output signal which may, e.g. be of saw-tooth, delta, sinusoidal or staircase form. The frequency that is output by said oscillator is 8 proportional to the voltage input to it. In the embodiment shown, the frequency of the voltage-controlled oscillator is varied over a range covering twice the width of the frequency range desired in the surveillance zone. Furthermore, the output frequency of the voltage-controlled oscillator is offset relative thereto by a fixed amount 90 MHz). In the case of a conventional electronic article surveillance system, the resonant frequency of the security tags is 8.2 MHz 10%, so that the transmission frequency has to be varied between 7.4 MHz and 9.0 MHz. The sweep generator varies the output frequency over twice this range (between 14.8 and 18 MHz) and with an offset (of 90 MHz).

The output frequency of the voltage-controlled oscillator accordingly varies between 90 14.8 MHz (104.8 MHz) and 18 MHz (108 MHz).

A separate oscillator operating at 30 MHz and preferably crystal-controlled, controls the frequency multiplier which has a multiplication factor of 20 3 and accordingly generates a frequency of 90 MHz.

The outputs of the frequency multiplier (19) and voltagecontrolled oscillator are connected to a mixer its output frequency corresponding to the input frequencies, to their sum, and to their difference. Its output accordingly carries the 90 MHz generated by the frequency tripler frequencies in the range 104.8 to 108 MHz, corresponding to the voltage-controlled oscillator frequencies in the range 194.8 to 198 MHz, resulting as sum frequencies, and lastly frequencies in the range 14.8 to 18 MHz, resulting as difference frequencies between the voltage-controlled oscillator and the frequency tripler The output of the mixer is connected to a low-pass filter which cuts off frequencies above 20 MHz. As a result, only frequencies in the range 14.8 to 18 MHz are present at the output 9 of the low-pass filter these being the difference frequencies between those of the voltage-controlled oscillator and the frequency tripler (19).

I o ki H te "'o v uloe lao r r~a, The output of the low-pass filter i. connected to the input of a frequency divider which halves the frequency. Its output voltage accordingly varies within the range 7.4 to 9.0 MHz. However, since the frequency divider also produces higher harmonics of its input frequency, its output is connected to a further low-pass filter which allows only frequencies below 9.0 MHz to pass.

The oucput from the low-pass filter is then supplied to a final outpu stage which amplifies the signal to a level suitable for driving a transmitting antenna The output of the voltage-controlled oscillator the frequency of which varies within the range 104.8 to 108 MHz, is additionally connected to a frequency divider (13) which halves the frequency, so that it varies within the range 52.4 to 54 MHz. The output of the frequency divider (13) is connected to the mixer (12) associated with the receiver, possibly via an (optional) low-pass filter (14) which eliminates the higher harmonics.

The signals received by a receiving antenna 25 possibly originating from a security tag with a resonant circuit, are also supplied to the receiver mixer via a preamplifier The output of the mixer (12) accordingly carries the input frequencies, as well as their sum and their difference. In the practical embodiment in question, these frequencies are in the range 52.4 to 54 MHz, from the low-pass filter (14), and in the range 7.4 to 9 MHz, from the preamplifier a.

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10 Since the voltage-controlled oscillator serves to generate both the frequencies input to the mixer (12), their difference is constant. If the output froquency of the voltage-controlled oscillator is, 104.8 MHz, the output frequency of the low-pass filter (14) will be 52.4 MHz and the frequenry input to the receiver will be 7.4 MHz. The sum of these frequencies is 59.8 MHz and their difference is 45 MHz. Analogously, if the output frequency of the voltage-controlled oscillator (2) is 108 MHz, the output of the low-pass filter (14) will be carrying 54 MHz and the output of the preamplifier .11) will be carrying 9.0 MHz. The sum of these frequencies is 63 MHz, while their difference is 45 MHz. It is obvious that the difference frequency is always 45 MHz, irrespective of the output frequency of the voltagecontrolled oscillator The system according to the invention is thus taking advantage of the the well known superheterodyne principle.

output of the mixer (12) associated with the receiver is connected to the intermediate frequency amplifier which amplifies only signals within a narrow band MHz 5 kHz in the case of the present embodiment).

As a result of the combination of the superheterodyne U *..principle with the narrow bandwidth characteristic of the intermediate amplifier all interfering frequencies that depart from the transmission frequency by more than 10 kHz are suppressed. The result is a substantial improvement in the suppression of interfering signals, and this allows some lowe-ing of the detection threshold, 30 with the consequence that a higher detection probability is achieved.

Due to the fact that the intermediate frequency amplifier operates at a high amplification factor, care must be taken to ensure that the 45 MHz, corresponding to its xS~ 11 amplification frequency, are not generated in any portion of the remainder of the system. This is achieved by upwardly offsetting the voltage-controlled oscillator by twice the intermediate frequency of 45 MHz, i.e. by 90 MHz, and by arranging for it to genera a bandwidth corresponding to twice the sweep bandwidth.

Furthermore, the frequency of the fixed oscillator (18) is, first of all, tripled, so that in no case will it correspond to the intermediate frequency. Moreover, in no case will the frequency that the voltage-controlled oscillator supplies to the receiver mixer (12) correspond to the intermediate frequency.

A person with appropriate technical knowledge will recognize that the process of doubling the sweep frequency, adding an offset and then halving the resulting frequency successfully prevents the generation of the intermediate frequency at locations where it would be undesirable.

It remains to be stated that the output of the frequency tripler (19) is connected to a further frequency doubler I which consequently supplies 180 MHz in the case of the illustrative embodiment presented here. This frequency is supplied to the synchronous detector as is also the output voltage from the intermediate frequency amplifier Said synchronous detector (16) .,internally divides the 180 MHz supplied by the frequency doubler (20) by 4, so as to present, at its outputs, the in-phase and quadrature information contained in the transmitted signals and the received signals.

The in-phase content of the transmitted and received 7 signals is thus presented at one output of the synchronous detector while the quadrature portion i.e. the portion shifted through 900 relative to the transmitted signal is presented at the other output.

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12 It is known that the amplitude of the received signal corresponds to the square root of the sum of the squares of the two output voltages from the synchronous detector These two voltages are supplied to a circuit (17) which triggers an alarm in the event of defined threshold values being infringed.

Since the phase of the signal which is radiated by the transmitting antenna is determined by the phase of the voltage-controlled oscillator and by that of the frequency multiplier and since the phase of the two output signals from the synchronous detector also depends on the voltage-controlled oscillator any change in the phase and/or amplitude of the received signal due to the presence of a security tag in the surveillance zone can be detec'ed by reference to the two outputs from the synchronous detector with the result that the system according to the invention enables a substantial improvement to be achieved in the ability to discriminate between security tags, other S: 20 objects and interference effects.

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Claims

1. Electronic article surveillance system, with a transmitter which radiates an electromagnetic field into a surveillance zone, and with a receiver which is assigned to the transmitter, the output signal from said receiver being supplied to a detection arrangement which activates an alarm signal in the event of the presence of a security tag in the surveillance zone, said security tag containing a resonant circuit consisting of an inductance and a capacitor, the frequency of the transmitter being varied over a range which covers the resonant frequency of the resonant circuit associated with the security tag, and the receiver being provided with a mixer, the first input of which is connected to a receiving antenna and the second input of which is connected to the transmitter, wherein the second input of the mixer carries a signal that has a frequency corresponding either to the sum of the transmission frequency and a fixed frequency, or to the difference between these frequencies, and wherein an 20 intermediate frequency amplifier is connected to the output of the mixer and its output is connected to the detection arrangement, said amplifier amplifying only signals in a narrow band centered on a frequency which corresponds to the difference formed from the two frequencies when the sum of the two frequencies is input to the mixer, or which corresponds to the sum of these two frequencies when the difference of the two frequencies is input to the mixer.

2. System according to Claim 1, wherein the output of a circuit for generating the variable frequency, in particular the output of a voltage-controlled oscillator or -14- of a digital-to-analog converter, is connected to a second mixer which drives the transmitting antenna, and the second input of which is connected to a fixed frequency generator, and wherein the output of the circuit for generating the variable frequency is connected to the second input of the mixer associated with the receiver.

3. System according to Claim 2, wherein a frequency divider is connected between the output of the second mixer and the transmitting antenna, and a further frequency divider is connected between the output of the circuit for generating the variable frequency and the second input of I the mixer associated with the receiver. 0 ie4. System according to Claim 2 or 3, wherein the fixed *frequency generator is provided with an oscillator, generating an output signal which is supplied to a frequency multiplier.

5. System according to any one of Claims 1 to 4, wherein i: the detection arrangement is provided with a phase- sensitive synchronous detector, its first input being K 20 connected to the output of the intermediate frequency i sensitive synchronous detector, its first input being Kamplifier, and its second input being connected to the 9 t fixed frequency generator.

6. System accordin~g to Claim 5, wherein the synchronous detector has an output at which the in-phase content ot the transmitted and received signals is presented, and it has an output at which that portion of the received signal which is shifted through 900 relative to the transmitted signal is presented. IIS

7. An electronic article surveillance system substantially as herein described with reference to the accompanying drawing. DATED this 22nd day of June, 1998 ESSELTE METO INTERNATIONAL GmbH Attorney: PETER R. HEATHCOTE Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS Ir ci r 1 Ir r ABSTRACT Electronic article surveillance system, with a transmitter which radiates an electromagnetic field into a surveillance zone, and with a receiver which is assigned to the transmitter. The output signal from the receiver is supplied to a detection arrangement which activates an alarm signal in the event of the presence of a security tag in the surveillance zone. The security tag contains a resonant circuit consisting of of of an inductance and a capacitor, the frequency of the transmitter br'ni ited over a range which covers the resonant fr-equency of the resonant circuit asso -i h the security I~ tag The receiver is provided with a mixer the first input of which is connected to a receiving antenna (10) and the second input of which is connected to the transmitter. No 1: The second input of the mixer (12) carries a signal that has a frequency corresponding either to the sum of the transmission frequency and a fixed frequency, or to the difference between these frequencies. An intermediate frequency amplifier (1 *connected to the output of the mixer (12) and its outputt is connected to the detection .4,,arrangement. The amplifier (15) amplifies only signals in a narrow band centered on a too. frequency which corresponds to the difference formed from the two frequencies that are input to the mixer or which corresponds to the sum of these two frequencies.