EP0846313A1

EP0846313A1 - Deactivation apparatus for an article surveillance tag

Info

Publication number: EP0846313A1
Application number: EP96927073A
Authority: EP
Inventors: Heikki SEPPÄ; Olli Jaakkola
Original assignee: TUOTESUOJA SIRPA JAERVENSIVU KY; TUOTESUOJA SIRPA JAERVENSIVU K; Tuotesuoja Sirpa Jarvensivu Ky
Current assignee: TUOTESUOJA SIRPA JAERVENSIVU Oy
Priority date: 1995-08-23
Filing date: 1996-08-22
Publication date: 1998-06-10
Also published as: WO1997008670A1; FI953959A0; AU6702696A; FI953959A; FI100491B

Abstract

The present invention relates to a deactivation apparatus of an article surveillance tag, the deactivation apparatus comprising a first (1) and a second (3) oscillator means, whereby the first oscillator means (1) is arranged to produce a signal (Sr) whose frequency sweeps in the vicinity of a resonance frequency of the article surveillance tag, and the second oscillator means (3) is arranged to produce a constant-frequency signal (Sm); mixer means (2) for producing from output signals (Sr, Sm) of the first and the second oscillator means two signals (S1, S2) having different frequencies for simultaneously producing a first and a second electromagnetic field by means of an antenna means (ANT), the frequencies (S1, S2) of the fields sweeping in the vicinity of the resonance frequency, of the article surveillance tag; detector means (10) for detecting the presence of an article surveillance tag in the first and the second field; and means (1, 11) for locking the frequency (Sr) of the first oscillator means and increasing the field strength of the electromagnetic field for deactivating the article surveillance tag in response to detecting the article surveillance tag.

Description

Deactivation apparatus for an article surveillance tag

The present invention relates to a deactivation apparatus for an article surveillance tag, comprising: means for generating a first electromagnetic field having a predetermined frequency, said frequency being arranged to sweep in the vicinity of the resonance frequency of the article surveillance tag, detector means for detecting the presence of an article surveillance tag, and deactivation means for increasing the field strength of said first electromagnetic field onto a predetermined level on which the article surveillance tag is deactivated.

This invention is particularly associated with theft alarm devices used in stores and shops and based on LC resonator circuits i.e. article surveillance tags attached to the articles. The resonance frequency of said circuits may be e.g. 8.2 MHz. When an object carrying a resonator enters the electromagnetic field of a theft alarm device, i.e. an electronic article surveillance system (EAS) , said field being produced by the transmitter of the system and an antenna attached to it, the resonator will cause a change in the field strength. On the basis of that change, the receiver of the alarm system detects the presence of the surveillance tag. The transmitter and the receiver of the system are placed in such a location or such locations in which they are expected to detect unauthorized removal of an article carrying a tag, e.g. at the exit of the store.

In order to avoid such a situation in the theft alarm system described above in which wares that the customer has already paid for at the cash desk would later cause an alarm upon the customer leaving the store, it must be possible to deactivate the article surveillance tags. Deactivating the article surveillance tags is based on the idea that the article surveillance tags are brought into an electromagnetic field whose frequency equals to the resonance frequency of the article surveillance tag and whose field strength is sufficient in order that some of the components of the article surveillance tag would be damaged. Depending on the type of the article surveillance tag, damaging of a component described above usually causes either the resonance frequency of the article surveillance tag to shift to such a frequency range which is not detected by the theft alarm system, or the article surveillance tag to be destroyed so severely that it no longer has any actual resonance frequency at all.

An apparatus for deactivating an article surveillance tag is previously known, wherein the transmitter of the deactivating apparatus is arranged to sweep constantly the frequency of a sinusoidal signal in the vicinity of the resonance frequency of the article surveillance tag. In other words, the transmitter constantly changes the frequency so that it goes through all the frequencies within the frequency range (e.g. 7.0 - 9.0 MHz) in a specific time period. In said prior art solution, the deactivation apparatus is arranged to constantly produce a sufficiently strong field in order that the article surveillance tag entering the field would certainly be destroyed. The major drawback of this prior art solution is that the field generated by the deactivation apparatus endangers the health e.g. of a cashier who constantly stays within the field owing to its constantly high field strength. Another drawback is the fact that the article surveillance tag cannot necessarily be deactivated with this apparatus every time because the frequency of the field sweeping sinusoidally is not necessarily equal to the resonance frequency of the article surveillance tag so long as to ensure the deactivation of the article surveillance tag. In other words, if the frequency of an electromagnetic field varies too rapidly during deactivation, no sufficient energy is induced in the resonator in order that it would be destroyed. Assume that the resonance frequency f of the article surveillance tag in question is e.g. 8.0 MHz and its quality factor Q is e.g. 80, the frequency of the field should be equal to the resonance frequency of the tag with the accuracy f/Q=100 kHz in order that a sufficient amount of energy would be induced in the tag for destroying it. A deactivating apparatus having detector means for detecting the presence of an article surveillance tag is also previously known. The deactivation apparatus in question produces a field that has a relatively low field strength and that sweeps sinusoidally, the frequency of said field varying in the vicinity of the resonance frequency of the tag, e.g. 7.0 - 9.0 MHz, detector means continuously measuring the field strength of said field for detecting an article surveillance tag. This prior art deactivation apparatus comprises measuring devices responsive to the detector means, arranged to measure the resonance frequency of an article surveillance tag detected by the detector means. This prior art deactivation apparatus further comprises a signal processor which is arranged to generate a strong electromagnetic field required for deactivating the article surveillance tag on the frequency that has been found by means of measuring means to be the actual resonance frequency of the article surveillance tag. The major drawback of the prior art solution mentioned above is the fact that is requires the use of a microprocessor, for which reason the apparatus in question has a relatively complicated structure, in addition to which the price of the apparatus is also high.

The object of the present invention is to solve the problems mentioned above and provide a cheaper and more reliable deactivation apparatus as compared with the prior art solutions. This is achieved with a deactivation apparatus of the invention, which is characterized by the apparatus comprising a first and a second oscillator means, whereby the first oscillator means is arranged to produce a signal that sweeps in the vicinity of the resonance frequency of the article surveillance tag, and the second oscillator means is arranged to produce a constant-frequency signal whose frequency is substantially lower than the resonance frequency of the article surveillance tag; the apparatus comprising mixer means for producing from output signals of the first and the second oscillator means two signals having different frequencies, whereby the mixer means are arranged to supply said signals to an antenna means for producing a first and a second electromagnetic field simultaneously, the frequencies of the fields sweeping in the vicinity of the resonance frequency of the article surveillance tag; the detector means being arranged to detect the presence of the article surveillance tag in the first and the second field on the basis of changes in the field strength caused in the fields by the article surveillance tag; and the deactivation apparatus comprising means responsive to detection means for locking the frequency of the first oscillator means and for producing an electromagnetic field responsive to the output signal of the first oscillator means at the field strength at which the article surveillance tag is deactivated, in response to detecting the article surveillance tag.

The invention is based on the idea that when the deactivation means is provided with two oscillators one of which sweeps through a predetermined frequency range and the other one of which generates a constant- frequency signal, and when the signals of said oscillators are modulated with each other in a suitable manner for generating two electromagnetic fields whose frequencies sweep in the vicinity of the resonance range of the article surveillance tag, the presence of the article surveillance tag may be detected easier than heretofore from the real or the imaginary part of the demodulated signal, i.e. the difference signal obtained from the antenna. When the deactivation means is then arranged to lock onto the resonance frequency of the article surveillance tag, it is ensured that the article surveillance tag will be deactivated.

The invention makes use of the fact that when two or more frequencies sweep past the resonance circuit, the presence of the resonance is indicated in that there is a difference in the transfer function of two frequencies due to the resonator. In other words, if it is assumed that the transfer function of the resonator circuit appearing in the antenna of the deactivation apparatus takes the form H(w), wherein w is the frequency. The transmission function is complex, whereby a real part and an imaginary part may be separated from it. If two frequencies are generated from one frequency wO (e.g 7 - 9 MHz) by means of modulation (e.g. a complete amplitude or phase modulation) , the frequencies wO-wm and wO+wm are obtained from the output (wm being e.g. 50 kHz). When these two frequencies are supplied through the resonator circuit, signals having the transmission functions H(wO-wm) and H(w0+wm) are obtained from the output. When these signals are re-modulated with the frequency wm, a signal having the frequency wO is obtained. The effective transfer function of the system from the input signal on frequency wO into the output signal on the frequency wO is the sum or the difference of the transfer functions H(wO-wm) and H(wo+wm) . The phases of the modulation signals determine whether the sum or the difference is obtained. One can also imagine that the derivative of the transfer function is obtained by feeding two frequencies instead of one through the circuit. If the modulation frequency is selected to be as high as the assumed band of the resonance circuit (wr/Q, wherein wr is the resonance frequency and Q is the estimated quality factor of the resonator) , a strong response is achieved only for the resonators in question. Because the real part of the transfer function is symmetric (the derivative is zero at resonance) , the difference of the real parts may be used for locking the frequency w of the deactivation apparatus onto the resonance frequency of the article surveillance tag.

The major advantages of the deactivation apparatus of the invention are thus its simple structure due to which the price of the apparatus is relatively low, the ability of the deactivation apparatus to lock the frequency of the oscillator to be used for producing the deactivation peak precisely onto the resonance frequency of the detected article surveillance tag, whereby the article surveillance tag is sure to be deactivated, and the fact that the time during which the deactivation apparatus generates a strong electromagnetic field (the peak required for deactivation) is very short, whereby the health risks caused by the apparatus are extremely small. The preferred embodiments of the deactivation apparatus of the invention appear from the attached dependent claims 2 - 5. In the following, the invention will be disclosed in closer detail by means of a few preferred embodiments of the deactivation apparatus of the invention with reference to the attached figures, in which

Figure 1 illustrates a first preferred embodiment of the deactivation apparatus of the invention,

Figure 2 illustrates a signal appearing to a comparator in Figure 1,

Figure 3 illustrates a second preferred embodiment of the deactivation apparatus of the invention,

Figure 4 illustrates a signal appearing to a comparator in Figure 3, and

Figure 5 illustrates an embodiment of the antenna of the deactivation apparatus. Figure 1 illustrates a first preferred embodiment of the deactivation apparatus of the invention. The deactivation apparatus shown in Figure 1 comprises a first oscillator means 1, which may be e.g. a voltage controlled oscillator. Said oscillator generates a signal Sr whose frequency sweeps sinusoidally within the frequency range 7 - 9 MHz. The oscillator is controlled during the detection of the article surveillance tag by a triangle wave generator 17 by means of switches 14 and 15. The deactivation apparatus of Figure 1 further comprises a second oscillator means 3, which may be e.g. a voltage controlled oscillator that generates a constant-frequency signal Sm, whose frequency may be e.g. 50 kHz. The first output of oscillator 1 is connected to a mixer 2, to which signal Sr is thus supplied. Correspondingly, signal Sm is supplied from the first output of oscillator 3 to the mixer 2. The output of the mixer 2 provides a signal composed of two signal components SI and S2, whereby signal SI is a sum signal Sr+Sm and signal S2 is a difference signal Sr-Sm.

Signals SI and S2 are supplied via a switch 11 to a voltage controlled power supply 4, to the output of which an antenna ANT is connected. Two electromagnetic fields having different frequencies are thus achieved by means of the antenna ANT, the frequencies of the fields being Sr+Sm and Sr-Sm.

The antenna ANT is shown as a loop in Figure 1, but in practice any type of prior art antenna may be employed, such as an antenna arranged in connection with a bar code reader of a teller terminal. Alternatively, the antenna shown in Figure 5 may be employed.

In the case of Figure 1, the presence of the article surveillance tag in the electromagnetic fields generated by the antenna ANT is found out by monitoring the change in the real part of the impedance of the antenna. In other words, when the frequencies of two (or more) electromagnetic fields sweep past the resonance frequency of the article surveillance tag, the presence of the resonance may be detected so that there is a difference owing to the resonator between the transfer functions of two frequencies. In order to accomplish this, an amplifier 5 is connected to the antenna interface via a switch 12, the output of said amplifier being connected to a mixer 6, whose second input is supplied with a signal S3 obtained from the oscillator 3. Signal S3 corresponds to signal Sm, but there is a phase shift of 90° between said signals, that is, if Sm=sin(wm*t) , then S3=cos(wm*t) . A second mixer 8 is connected to the output of mixer 6, signal S4 obtained from the oscillator 1 being supplied to said mixer 8. Signal S4 corresponds to signal Sr, but there is a phase shift of 90° between said signals, that is, if Sr=sin(wr*t) , then S4=cos (wr*t) . The output of the mixer 8 thus provides a difference signal S5, on the basis of whose voltage the deactivation apparatus decides when the article surveillance tag is located in the fields produced by means of the antenna. The behaviour of the difference signal is described in greater detail in Figure 2.

The difference signal S5 is filtered by means of a low-pass filter 9 and supplied to a comparator 10. The comparator 10 compares the difference signal with the threshold signal ref (or -ref) . Provided that the comparison proves that there is an article surveillance tag in the fields generated by means of the antenna, the comparator produces a control signal CNT of a specific duration (e.g. 0.2 s) and supplies it to a switch 15. Switch 15 thus shifts its position, whereby the connection between the triangle wave generator 17 and the oscillator 1 is interrupted simultaneously when the output of the filter 9 is directed to the oscillator 1. Due to this feedback, the oscillator 1 is locked onto the frequency of the article surveillance tag. Locking may be furthered e.g. by arranging an integrating adjustment device (not shown in the figure) between the filter 9 and the oscillator 1.

The comparator also supplies a control signal CNT to a delay circuit 13. By means of the delay circuit, a control signal CNT2 is supplied to switches 11, 12 and 14 e.g. for the duration of about 10 ms. The duration of the control signal is dimensioned so that the oscillator 1 is locked properly onto the resonance frequency of the article surveillance tag. The control signal 2 "freezes" the control signal of the oscillator 1, in addition to which it causes the switches 11 and 12 to change their position so that the output signal Sr of the oscillator 1 is supplied to the antenna ANT. Since the frequency of the oscillator 1 is frozen onto that frequency on which it had remained after locking to the resonator, the frequency intended for deactivation is very close to the resonance frequency of the article surveillance tag. The amplifier 5 has also been switched off from the output of the antenna ANT so that it would not be destroyed due to the strong signal.

Once the time of the delay circuit 13 has expired, switches 11, 12, 14 and 15 are returned into their initial positions and the detection of the article surveillance tag continues. It is thus advantageous that switches 14 and 15 first connect the triangle wave generator 17 to the oscillator 1, and only thereafter switches 11 and 12 connect the output of mixer 2 to the antenna ANT and the output of the antenna to the amplifier 5. Thereafter, detection of the article surveillance tag continues and deactivation is activated again if the presence of the tag is detected. The process is repeated by itself until the tag is deactivated.

In the case shown in Figure 1, the output of the filter 9 may also be directed to an indicator 16 if desired. This is illustrated with a broken line in Figure 1. By means of the indicator 16 it may be communicated to the user of the teller terminal with sound or light signals that an article surveillance tag has been detected. Electronics indicating whether the deactivation was successful or not may also be connected to the indication. No microprocessors or similar components are thus required for the deactivation apparatus shown in Figure 1, for which reason the apparatus becomes very simple and reliable, and its price remains relatively low. Figure 2 illustrates a signal S5 appearing to the comparator 10 in Figure 1 (prior to low-pass filtering) in a case where an article surveillance tag is located in the vicinity of the deactivation apparatus, the tag having the frequency of 8 MHz. The horizontal axis of the figure represents the frequency Sr of the oscillator 1 in Figure 1 and the vertical axis represents the voltage of the difference signal S5. It appears from Figure 2 that at the resonance frequency (8.0 MHz) of the article surveillance tag the voltage of the difference signal S5 is zero.

Figure 3 illustrates a second preferred embodiment of the deactivation apparatus of the invention. The embodiment of Figure 3 corresponds to the case shown in Figure 1 to a great extent, but in the case of Figure 3 the presence of the article surveillance tag in the electromagnetic fields produced by the antenna ANT is found out by monitoring the change in the imaginary part of the impedance of the antenna (not in the real part as is the case in Figure 1) . This is achieved so that the output signal Sr of the first oscillator 1 (not signal S4, as in the case of Figure 1) is supplied to the oscillator 8 of Figure 3. The presence of the article surveillance tag is thus decided on the basis of the voltage of a (filtered) difference signal S6 describing the imaginary part. The behaviour of the difference signal Sβ is described in Figure .

Figure 4 illustrates a signal S6 (prior to low-pass filtering) appearing to the comparator 10 in Figure 3 in a case where an article surveillance tag is located in the vicinity of the deactivation apparatus, the tag having the frequency of 8 MHz. The horizontal axis of the figure expresses the frequency Sr of the oscillator 1 in Figure 3 and the vertical axis expresses the voltage of the difference signal S6. It appears from Figure 4 that at the resonance frequency of the article surveillance tag the voltage of the difference signal S6 is negative. In other words, when the comparator 10 detects that said negative voltage is reached, it will generate a control signal CNT for indicating the detection of the article surveillance tag.

Figure 5 illustrates a preferred embodiment of the antenna of the deactivation apparatus. Because the signal required for deactivating the article surveillance tag is extremely strong, it is advantageous to feed it via a gradiometer. The gradiometer may e.g. consist of two circular windings inside each other whose radius may be e.g. rl=10 cm and r2=20 cm, the number of turns in the smaller winding

(Ll) being larger than that in the outer winding (L2) .

The areas Al and A2 of the windings as well as the numbers of turns nl and n2 must be chosen so as to minimize the strength of the magnetic field far from the antenna (e.g. in the distance of 1 m) . This can be achieved in accordance with the invention by dimensioning the gradiometer so that the condition nl x Al=n2 x A2 is fulfilled. The phenomenon that the field strength would be unnecessarily high in the vicinity of the antenna may thus be avoided, whereby the possible health hazards may also be minimized.

The deactivation apparatus may be implemented so that either of the windings (pins AB or AC) shown in Figure 5 are employed for detecting the article surveillance tag, but the destroying is carried out by means of the gradiometer (pins B and C) . The detection range of the antenna thus becomes long, but the field intended for destroying does not disturb the environment. It must be understood that the above explanation and the figures associated therewith are only intended to illustrate the present invention. Different variations and modifications of the invention will be obvious to persons skilled in the art without deviating from the scope and the spirit of the invention set forth in the attached claims.

Claims

Claims :

1. A deactivation apparatus for an article surveillance tag, comprising: means (1, 4, ANT) for generating a first electromagnetic field having a predetermined frequency, said frequency being arranged to sweep in the vicinity of a resonance frequency of the article surveillance tag, detector means for detecting the presence of the article surveillance tag, and deactivation means (11 - 15) for increasing the field strength of said first electromagnetic field onto a predetermined level on which the article surveillance tag is deactivated, c h a r a c t e r i z e d by the apparatus comprising a first (1) and a second (3) oscillator means, whereby the first oscillator means (1) is arranged to produce a signal

(Sr) whose frequency sweeps in the vicinity of the resonance frequency of the article surveillance tag, and the second oscillator means (3) is arranged to produce a constant-frequency signal (Sm) whose frequency is substantially lower than the resonance frequency of the article surveillance tag, the apparatus comprising mixer means (2) for producing from output signals (Sr, Sm) of the first and the second oscillator means two signals (SI, S2) having different frequencies, whereby the mixer means are arranged to supply said signals to an antenna means

(ANT) for producing a first and a second electromagnetic field simultaneously, the frequencies

(SI, S2) of the fields sweeping in the vicinity of the resonance frequency of the article surveillance tag, the detector means (5, 6, 8, 9, 10) being arranged to detect the presence of the article surveillance tag in the first and the second field on the basis of changes in the field strength caused in the fields by the article surveillance tag, and the deactivation apparatus comprising means (11, 14, 15, ANT) responsive to detection means (5, 6, 8, 9, 10) for locking the frequency of the first oscillator means (1) and for producing an electromagnetic field responsive to the output signal (Sr) of the first oscillator means (1) at the field strength at which the article surveillance tag is deactivated, in response to detecting the article surveillance tag.

2. A deactivation apparatus as claimed in claim 1, c h a r a c t e r i z e d in that the first oscillator means (1) is a voltage- controlled oscillator which is arranged to produce an output signal that sweeps sinusoidally in a frequency range 7 - 9 MHz, the second oscillator means is a voltage- controlled oscillator (3) which is arranged to produce an output signal having a frequency of about 50 kHz, and the signals (SI, S2) produced by the mixer means (2) and having different frequencies are composed of a sum signal and a difference signal (Sr+Sm, Sr-Sm) of the output signals of the first and the second oscillator means.

3. A deactivation apparatus as claimed in claim 1 or 2, c h a r a c t e r i z e d in that the detector means (5, 6, 8, 9, 10) are arranged to produce a reference signal (S5, S6) responsive to the real part or the imaginary part of a signal obtained from the antenna means (ANT) , and to detect the presence of the article surveillance tag when the voltage of said reference signal exceeds a predetermined threshold value (ref) .

4. A deactivation apparatus as claimed in any one of the preceding claims 1 - 3, c h a r a c t e r i z e d in that a gradiometer is employed as the antenna means, said gradiometer consisting of two windings (Ll, L2) inside each other, the inner winding having a smaller number of turns than the outer one.

5. A deactivation apparatus as claimed in claim 4, c h a r a c t e r i z e d in that the deactivation apparatus is arranged to use only one of the windings (Ll or L2) for detection and both of the windings (Ll and L2) for deactivation.