WO1996010241A1

WO1996010241A1 - Identification method and identification apparatus

Info

Publication number: WO1996010241A1
Application number: PCT/FI1995/000509
Authority: WO
Inventors: Heikki SEPPÄ; Olli Jaakkola
Original assignee: Tuotesuoja Sirpa Järvensivu Ky
Priority date: 1994-09-26
Filing date: 1995-09-19
Publication date: 1996-04-04
Also published as: NO971414L; FI95514C; CA2200075A1; AU3389795A; EP0783744A1; FI95514B; NO971414D0; JPH10506210A; FI944449A0

Abstract

This invention relates to an identification method and an identification apparatus for identifying an object that enters a certain zone, comprising transmitter means (1, 7, ANT) for generating a first electromagnetic field with a predetermined frequency in the zone, and measurement means (ANT, AMP, 2, 3, 6, 7) for measuring the first field in said zone. For accomplishing an apparatus reliable and immune to disturbances, the apparatus further comprises transmitter means (1, 7, ANT) for generating a second electromagnetic field of a predetermined frequency in said zone, the frequencies of the first and the second field deviating from each other, measurement means (ANT, AMP, 2, 3, 6, 7) for measuring the second electromagnetic field in said zone, and a control circuit (3) for comparing the values measured by the measurement means (3) with predetermined reference values, and for identifying, on the basis of said comparison, an object that enters the zone.

Description

Identification method and identification apparatus

This invention relates to an identification method for identifying an object that enters a certain zone, comprising the steps of: generating a first electromagnetic field with a predetermined frequency in the zone. The invention further relates to an identification apparatus for identifying an object that enters a certain zone, comprising transmitter means for generating a first electromagnetic field with a predetermined frequency in said zone, and; measurement means for measuring the first field in said zone. Measuring a field refers herein to measuring both a real and an imaginary part of the field. This invention is generally associated with surveillance systems, and especially with theft alarm devices used in stores and shops. Prior art theft alarm devices are based either on 8.2 MHz LC resonators or magnetic strips which are read at a low frequency. This invention primarily relates to a theft alarm device that may be used for detecting an LC resonant circuit. When an object carrying a resonator enters the electromagnetic field of a theft alarm device, i.e. an electronic article surveillance system (EAS), provided by a 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 resonator. The transmitter and the receiver of the alarm system are placed in such a place or places in which they are believed to detect illegitimate removal of a product carrying a resonator, e.g. at the exit of a store.

Prior art theft alarm devices are based on scanning, i.e. they are adapted to sweep the frequency of a sinusoidal signal past the resonant frequency of the resonant circuit. In other words, the transmitter and the receiver constantly change the frequency so that in a certain interval they go through all frequencies belonging to a frequency range (e.g. 8.0 - 8.8 MHz). Outputting the signal and the electromagnetic field generated from it activate the oscillation of a resonant circuit located in the vicinity. The receiver of the theft alarm device measures the change caused in the field strength (amplitude) by the resonant circuit. If the measured change exceeds a certain threshold value, the theft alarm device signals an alarm, usually with sound and/or light signals.

The most significant weakness of the prior art theft alarm device described above is its sensitivity to metal objects. In other words, if a product to which a resonant circuit is attached is fitted into a bag with an aluminium foil lining or into a tin box, the theft alarm device is not usually able to detect it. A metal object as such, however, will cause such a change in the electromagnetic field generated by the theft alarm device that is detected by the alarm device. In most cases, however, theft alarm devices are adjusted so that changes caused in the field strength by metal objects do not activate the alarm, since in other cases, for example, passing metal shopping carts through the field of a theft alarm device would cause continual false alarms. Thus, thieves have learnt that if the objects to be stolen are hidden in metal cases, bags or similar, the resonant circuits attached to the objects will not activate the theft alarm devices located in the store.

The object of this invention is to solve the above mentioned problem and provide and identification method by means of which objects entering a pre¬ determined zone may be identified better and in a more reliable way than heretofore, whereby necessary measures PCI7F195/00509

may be taken after identification. The identification method of the invention is characterized by generating a second electromagnetic field with a predetermined frequency in the zone simultaneously with the first field, whereby the frequency of the second field is selected so that it deviates from the frequency of the first field; monitoring the first and the second field, and; identifying an object entering the zone by comparing the changes caused by it in the first and the second field with predetermined reference values.

The invention is based on the idea that when at least two electromagnetic fields with different frequencies are generated in the zone, the objects that have entered the zone may be identified on the basis of the changes caused by them to the different fields. Changes refer herein to the fact that both the real and the imaginary parts of the fields are changed. Since different objects result in different changes in the fields within the zone, each object has an individual "profile" by means of which the object may be identified. The more frequencies, i.e. the more electromagnetic fields are used for identification, the more individual the profile of each object will become. The number of the frequencies to be used is advantageously about 7 - 15. Since the objects may be identified very individually, e.g. an alarm apparatus no longer needs to be adjusted to omit giving an alarm of all metal objects, but it is possible to define very specific information in the alarm apparatus on what kind of metal objects are not to cause an alarm and, in turn, what kind of objects are to cause an alarm. The most significant advantages of the method of the invention are the reliability of identification and the immunity of the method to external electric disturbances. The identification method of the invention may further be applied to access control, by making e.g. identification cards individual by fitting resonant circuits with various resonance frequencies into them. Each identification card thus has an individual profile, i.e. it causes a different change in electromagnetic fields within the zone, enabling identification of the identification card on the basis of the change caused by it. When the method of the invention is applied e.g. in connection with a locking mechanism of a door, it is possible to provide the door with such a locking mechanism that opens automatically when a certain identification card enters the electromagnetic fields surrounding it. The invention further relates to an identification apparatus with which the method of the invention may be applied. The apparatus of the invention is characterized by further comprising transmitter means for generating a second electromagnetic field with a predetermined frequency in said zone simultaneously with the first field, the frequencies of the first and the second field deviating from each other; measurement means for measuring the second electromagnetic field in said zone, and; a control unit for comparing the values measured by the measurement means with predetermined reference values and for identifying an object entering the zone on the basis of said comparison.

The preferred embodiments of the method and the apparatus of the invention appear from the attached dependent claims 2 - 3 and 5 - 8. In the following, the invention will be described in closer detail by way of example with reference to the attached figures, in which

Figure 1 shows a block diagram of a first preferred embodiment of the identification apparatus of the invention, Figure 2 illustrates signals to be fed to the antenna of Figure 1,

Figures 3a - 3d illustrate measurement results obtained from various objects, and Figure 4 illustrates a sum signal to be fed to the antenna of Figure 1.

Figure 1 shows a block diagram of a first preferred embodiment of the identification apparatus of the invention. The identification apparatus shown in Figure 1 is suited for use e.g. as a theft alarm device in stores. The theft alarm device shown in Figure 1 comprises a combined transmission and reception antenna ANT, which is designed as a gate large enough for a person to go through. The theft alarm device may naturally also be implemented by means of two separate antennas if necessary. The gate shown in Figure 1 may be arranged e.g. at the exits of stores, whereby the customers must go through the antenna loop when leaving the store. In the case shown in Figure 1, two LC/C circuits for reducing the electric field are installed in the antenna. Depending on the location and number of the LC/C circuits, the electric field of the antenna is distributed in a different way in the middle of the antenna loop ANT. In Figure 1, a switch S and a known component

4 (coil or resistor) are further arranged in connection of the antenna ANT. By means of the known component, a measurement and control unit 3 obtains a reference both for the amplitude and the phase. This facilitates calibration of the gate at the installation site.

The transmitting part of the theft alarm device in Figure 1 comprises a signal generator 1, which comprises a digital-to-analog converter, which is adapted to generate a sum signal. The sum signal may, for instance, be composed of seven different frequency components the lowest of which fl is e.g. about 1 MHz and the other six components f2 - f7 are evenly around 8.2 MHz. The signal generator 1 feeds the sum signal generated by it via its output B and via a regulating component 7 to the antenna ANT. The regulating component 7 is advantageously composed of a coil and an adjustable capacitor connected in parallel. Said component 7 may be used for "simulating" the effect of metal on the antenna ANT. If a large metal object enters the immediate vicinity of the antenna, the measurement and control unit 3 may, by means of the regulating component 7, rapidly compensate the effect of the metal object. Thus, it is possible to increase the amplification of an amplifier AMP, and the sensitivity of the apparatus as to detecting resonant circuits is thus improved in situations in which a large metal object complicates detecting.

The antenna ANT forms of the sum signal fed to it an electromagnetic field consisting of seven signal components with different frequencies. The different frequency components of the sum signal are synchronized with each other so that the peak value of the sum signal is minimized. Minimizing the peak value enables a maximal value of single signal components, which, in turn, results in high sensitivity without the field strength of the field generated by the theft alarm device exceeding the limits set by authorities. In an optimal case, the sum signal resembles noise (compare Figure 4). The theft alarm device in Figure 1 receives signals from its antenna ANT regardless of whether there are objects within the antenna loop ANT (i.e. gate) or not. The signal received from the antenna depends on the gate itself and metal objects located in the vicinity. In the case shown in Figure 1, a compensation signal is fed to a second input of a summing means 6 from output G of the signal generator 1. The compensation signal resembles the sum signal fed to the antenna. Owing to the compensation signal, a signal for compensating the inductance and resistance of the antenna, and the fixed metal objects in the vicinity is fed from the output of the amplifier AMP to a mixer 2. The required compensation signal is achieved by installing the gate to the site where it is used, subsequent to which the measurement and control unit 3 measures the signal coming from the antenna, and commands the signal generator 1 to generate compensation signal corresponding to it. Owing to said arrangement, the amplification of the amplifier AMP (and, for instance, that of the amplifiers possibly placed after the mixer 2) may be increased by coefficient 10 - 100, which improves the sensitivity at the receiving end. This improves the dynamics of the device.

Signals obtained from the output of the amplifier AMP and signals obtained from output R of the signal generator 1 are fed to the mixer 2, the frequencies of the signals being fl+fa, ... ,f7+7fa. Signals having frequencies fa, 2fa, 3fa, ... ,7fa are thus obtained from the output of the mixer. By selecting the frequency fa to be sufficiently small, for instance, 1 KHz, the output signals of the mixer 2 may be compressed to the audio-frequency range, where the information included in them may be read by means of an audio analog-to-digital converter preceding the measurement and control unit 3. The measurement and control unit 3 measures the amplitude and phase (real and imaginary part) of each signal, e.g. by utilizing FFT (Fast Fourier Transform) technique. After this, the measurement and control unit 3 compares the reference values stored in a memory 4 with the measured values and identifies the object located in the gate on the basis of the comparison.

The measurement and control unit 3 having identified the object in the gate on the basis of the reference values stored in the memory 4, it will read procedure data on the object in question from the memory 4, and feed this procedure data to an indicator unit 5. The indicator unit 5 will act after this on the basis of the content of the procedure data fed to it. In other words, if the identified object is e.g. a resonant circuit (article surveillance circuit), the indicator unit will give an alarm e.g. by means of sound and light signals in response to the content of the procedure data. If the identified object, again, is e.g. a combination of a person and a metal object (for instance, a thief carrying a beer can in his pocket), the indicator unit 5 may give an alarm by transmitting a radio message to the radio receiver unit of a store detective in response to the content of the procedure data.

The theft alarm device of Figure 1 may be calibrated e.g. so that each object to be identified (shopping cart, person, article surveillance circuit, person + beer can etc. ) is passed by turns into the antenna loop ANT, i.e. the gate subsequent to which the values measured by the measurement and control unit (compare Figures 3a - 3d), or the values describing the profile of the object in question are stored in the memory means 4 by means of the procedure data on the object in question. The objects listed above and the procedures caused by the identification are naturally given by way of example, since there is a vast number of objects and combinations of objects to be identified.

Figure 2 illustrates signals to be fed to the antenna of Figure 1, i.e. the components of the sum signal. In Figure 2, the signals are shown in frequency space. The frequency of signal fl is about 1 MHz, and it may thus be utilized for detection of metal. The frequencies of signals f2 - f7 used for identifying resonant circuits are approximately 7.5 - 9.0 MHz. If there is a resonant circuit in the vicinity of the antenna, the real part Asin(φ) (A=the amplitude of the signal, φ * the phase of the signal) and the imaginary part Acos (φ) of the measured frequency components behave in the way shown in Figure 3a. If the measured signals form curves corresponding to Figure 3a with a sufficient accuracy and sufficiently many times successively, the measurement and control unit infers that there is a resonator near the gate, and gives an alarm. The inference algorithm of the measurement and control unit is advantageously arranged so that it accepts only the curve forms corresponding to the quality factor (Q factor) of the resonant circuit, and sharp changes or inclinations are not interpreted as being caused by a resonant circuit.

Figures 3a - 3d illustrate measurement results obtained from various objects, i.e. the profile of the objects. In Figures 3a - 3d, the upper curves show the behaviour of the real part of the measurement result, and accordingly, the lower curves show the behaviour of the imaginary part. The horizontal axes in Figures 3a - 3d represent frequency. In Figures 3a - 3d, the measured values are illustrated with points. The point located leftmost illustrates a measurement to be carried out in the frequency range 1 MHz, the other six points illustrate measurements to be carried out in the frequency range 7.5 - 9.0 MHz.

Figure 3a shows a situation in which a resonant circuit with a resonant frequency of approximately 8.2 MHz is located in the gate of Figure 1. It appears from Figure 3a that a resonator affects both the real and the imaginary part in the range of 8.2 MHz.

Figure 3b shows a situation in which a metal object is located in the gate shown in Figure 1. It appears from Figure 3b that a metal object affects the imaginary part both at high and low frequencies.

Figure 3c shows a situation in which a person is located in the gate shown in Figure 1. It appears from Figure 3c that a person has only a little effect on the imaginary part at low frequencies. At high frequencies, instead, a person affects both the real and the imaginary part.

Figure 3d shows a situation in which a resonator and a person are located in the gate shown in Figure 1.

On the basis of the curves in accordance with Figures 3a - 3d it is thus possible to identify relatively unambiguously an object located in the field, subsequent to which the necessary measures can be taken. Figure 4 illustrates the sum signal to be fed to the antenna of Figure 1. The horizontal axis of Figure 4 represents time and the vertical axis the amplitude of the signal. It appears from Figure 4 that the phases of the single frequency components of the sum signal are synchronized with each other so that the sum signal resembles noise.

It should be understood that the above description 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. An identification method for identifying an object that enters a certain zone, comprising the steps of: generating in the zone a first electromagnetic field with a predetermined frequency, c h a r a c t¬ e r i z e d by generating a second electromagnetic field of a predetermined frequency in the zone simultaneously with the first field, whereby the frequency of the second field is selected so that it deviates from the frequency of the first field; monitoring the first and the second field, and; identifying an object that enters the zone by comparing the changes caused by it in the first and the second field with predetermined reference values.

2. A method as claimed in claim 1, c h a r a c- t e r i z e d by selecting the frequency of the first field so that the presence of a predetermined resonant circuit in the field causes a measurable change in the first field, and selecting the frequency of the second field so that it substantially deviates from the resonant frequency of said resonator.

3. A method as claimed in claim 1 or 2, c h a- r a c t e r i z e d by generating, in addition to the first and the second electromagnetic field, several other electromagnetic fields in the zone, selecting their frequencies near the resonant frequency of said resonant circuit, monitoring said other fields, and identifying an object that enters the zone by comparing the changes caused by it in said electromagnetic fields with predetermined reference values.

4. An identification apparatus for identifying an object that enters a certain zone, comprising transmitter means (1, 7, ANT) for generating a first electromagnetic field with a predetermined frequency in said zone, and; measurement means (ANT, AMP, 2, 3, 6, 7) for measuring the first field in said zone, c h a r a c t- e r 1 z e d by further comprising transmitter means (1, 7, ANT) for generating a second electromagnetic field of a predetermined frequency in said zone simultaneously with the first field, whereby the frequencies of the first and the second field deviate from each other; measurement means (ANT, AMP, 2, 3, 6, 7) for measuring the second electromagnetic field in said zone, and; a control unit (3) for comparing the values measured by the measurement means with predetermined reference values and for identifying an object entering the zone on the basis of said comparison.

5. An apparatus as claimed in claim 4, c h a- r a c t e r i z e d in that the transmitter means comprises a signal generator (1) which is adapted to generate a sum signal, which is composed of at least two sinusoidal signal components with different frequencies, the signal components being synchronized with each other for minimizing the peak value of the sum signal, and antenna means (ANT) for generating the first and the second electromagnetic field in response to the sum signal.

6. An apparatus as claimed in claim 4 or 5, c- h a r a c t e r i z e d in that a memory means (4) is arranged at the control unit (3), reference values and procedure data on several predetermined objects being stored in said means, that the control unit (3) is adapted to feed the procedure data on a certain object to an indicator unit (5) when the values measured by the measurement means correspond to the reference values concerning said object, stored in the memory means (4) and that the indicator unit (5) is adapted to indicate the presence of the identified object in response to the procedure data.

7. An apparatus as claimed in any of the claims 4 - 6, c h a r a c t e r i z e d in that said identification apparatus is a theft alarm device, the frequency of the first electromagnetic field generated by the transmitter means (1, 7, ANT) being approximately 7.5 - 9.0 MHz, and the frequency of the second electromagnetic field being approximately 1 MHz, and the indicator unit (5) being adapted to detect an alarm in response to the procedure data, preferably by means of sound and/or light signals when the values measured by the measurement means (ANT, AMP, 2, 3, 6, 7) correspond to the reference values regarding a predetermined resonant circuit or a metal object of a predetermined size stored in the memory means (4).

8. An apparatus as claimed in any of the claims

4 - 7, c h a r a c t e r i z e d in that the transmitter means (1, 7, ANT) are adapted to generate, in addition to the first and the second electromagnetic field, several other electromagnetic fields of various frequencies in the zone, that the measurement means (ANT, AMP, 2, 3, 6, 7) are adapted to measure all fields generated in the zone, and that the control unit (3) is adapted to identify an object entering the zone by comparing the values measured by the measurement means with predetermined reference values.