Method and system for detecting an indicating device

(74) Prosecutor Civil Service. Jens FC Langfeldt, Bryns Patentkontor A / S, Oslo.

(30) Priority requested 15.02.84, SE, No. 8400826,

19.09.84, SE, No. B404691.

(54) DESCRIPTION OF THE INVENTION PROCEDURE AND CLEANING FOR A DETECTED PRESENCE OF AN INDICATING DEVICE IN A LIMITED DISEASE ZONE.

(57) Summary

A method of detecting the presence of an indicating device in a restricted examination zone, including detecting an indicating device (31) of a high permeable material upon emitting and receiving a magnetic field, wherein two coils (1, 2) are caused to emit a high frequency magnetic alternating field, and a pre-ote of the coils (1) emits an alternating field with a frequency (f) that deviates from the frequency (f1 of the field transmitted by the other of the coils (2), and wherein at least one differential and / or sum frequency nf ^ + mf ^ occurring during intermodulation by means of the indicating device (31), where n and m are positive or negative integers, is received by one or more coils (9). According to the invention, a third low-frequency magnetic alternating field is transmitted & transmitted in said zone at a frequency (e.g.₃) which is substantially lower than the frequencies f ^ fj, where the low-frequency field is caused to have a sufficient field strength, independent of the fields with the frequencies f₁ and f₂ setting the indicating device (31) in saturated state to the frequency of the low-frequency field. Hereby, a field transmitted from the indicating device (31) is detected as a periodic intermodulation occurring between the frequencies f 1 and f 3 with the low frequency field frequency by means of a detector device.

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(56) Publications cited none.

BACKGROUND OF THE INVENTION The present invention relates to a method for detecting the presence of an indicating device in a restricted examination zone and a device for carrying out the method.

The area of ​​application is primarily intended to be the surveillance of goods within the trade, especially the grocery trade, but can also be used for indication of other kinds, for example for the indication of persons.

More particularly, the invention relates to such a method which comprises the steps of detecting an indicating device of a high permeable material upon emitting and receiving a magnetic field, two coils being caused to emit a high frequency magnetic alternating field, and a first of the coils emitting an alternating frequency with a first frequency. (fi), which differs from a second frequency (fg) in the field transmitted by the second of the coils, and wherein at least one differential and / or sum frequency n 'f ^ + m · δ β occurring during intermodulation by means of the indicating device, where n and m are positive or negative integers, is received by one or more coils.

The device for carrying out the method includes coils and associated feeding equipment for emitting and receiving magnetic alternating field, thereby detecting an indicating device of a preferably high permeable material, two coils being provided, a first of which being capable of emitting a high frequency alternating field with a frequency f and where a second of the coils is capable of emitting a high frequency alternating field with a frequency fg that deviates from said first frequency, and wherein a detector means includes a receiver coil and is capable of detecting the occurrence of at least one differential or sum frequency n · f} + m · fg, where n and m are positive or negative integers formed by intermodulation by means of said indicating device. Such a system can detect the presence of indicating devices in the form of, for example, a narrow and thin but relatively long strip of a high permeable material, such as material marketed under the name Permalloy. The presence of such strips is detected by means of coils emitting and receiving magnetic fields as well as devices for detecting received signals.

Such a strip, if exposed to a relatively weak outer magnetic field 1 in its longitudinal direction, has a magnetic flux density that can be on the order of 20,000 times greater than in the outer magnetic field. The presence of such strips is detected by means of a detector device 1 in which variations in induced voltage in a receiver coil are detected.

Said intermodulation is caused by the non-linear magnetic properties of the inlay device.

However, this is even the case with even objects of iron that are normally in the study zone, such as shopping carts, tin boxes, etc., giving rise to intermodulation. Furthermore, interference with other magnetic fields or objects of iron may cause the detection of received signals to be perceived as an detection device.

For example, a tin box located near a coil that emits a high frequency signal upon intermodulation may give rise to a received signal whose characteristic corresponds to a signal from a strip of high permeable material which is far from said coil.

For this reason, for example, different-order intermodulation products are required to distinguish whether a received signal originates either from a display device in the form of a strip or from another object in the examination zone.

That silk surveillance systems should not trigger alarm due to other objects a special locking device is an essential requirement.

BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for transmitting magnetic fields which give rise to a received signal which can be readily analyzed in order to distinguish an extension device from another object present in the examination zone.

The present invention thus provides a method and a device, in which the probability of such false alarm 1 is substantially eliminated, with respect to objects normally located in the examination zone.

The present invention thus relates to a method of the type mentioned in the introduction and characterized in that a third low frequency magnetic alternating field is emitted in said zone with a third frequency fg substantially lower than said first and second frequencies fg, fg, which low-frequency field is caused to have a field strength sufficiently high, independently of the fields of said first and second frequencies (ff and fg), to set the indulgence device 31 in saturated state 1 with the frequency of the low-frequency field, and that a field retransmitted from the indexing device is caused to be detected as a periodically occurring Intermodulation between the first frequency fj and the second frequency fg by the detector device with the frequency of the low frequency field.

Furthermore, the invention relates to a device of the type mentioned in the introduction and characterized in that the feeding equipment includes means for transmitting a third alternating field which is low frequency with a third frequency fg which is substantially lower than said first and second frequencies fj, fg, which means are capable of emitting the low-frequency field with a field strength sufficiently high for, regardless of the fields with said first and second frequencies f | and, to set the grounding device 1 saturated state to the frequency of the low-frequency field, and the detector means being able to receive by means of a receiver coil, thereby detecting a field transmitted by the grounding device, which includes a periodic low-frequency field. 3 appearing modulation between said first and second frequencies f1 and fg, and, upon the occurrence of such a periodically occurring intermodulation, to send a signal to an alarm device.

One problem that is solved by the above patent is that the survey zone can be clearly limited in terms of geometric extent, which is essential for the alarm not to be triggered by an indicator device or other object outside the relevant survey zone, which can, for example, consist of once between two adjacent boxes. . This limitation occurs by placing a coil on each side of the survey zone, and each coil emits a magnetic field with one of two different frequencies.

When the two outer fields are generated by each coil, located on both sides of the survey zone, the product of the field strengths has a large value midway between the coils, which value decreases steeply outside the coils. The strength of the lighter modulation products emitted by the inverter device is roughly a measure of the product of the field intensity of the two outer alternating fields. This means that said personal space can be a monitoring zone with a coil located on each side of the personal space. Thus, a relatively high product of the field strengths is achieved in the monitoring zone, while the product is low outside the monitoring zone.

For example, at a checkout counter 1 a self-service shop, where the customer passage may have a salvage of 70 cm, among other things, goods that do not pass on the checkout conveyor belt which is provided with a strip, must cause the ingress of such a strip to occur. By placing a coil on either side of the monitoring zone, the incorporation of such a strip on the conveyor belt is prevented.

This advantageous solution is also utilized in the method and apparatus of the present invention.

The features of the invention are set forth in the appended claims and in the following description with reference to the drawings.

The present invention is described in more detail in connection with an embodiment of the invention shown in the accompanying drawing, in which:

Follow. 1 schematically shows a device according to the invention,

Follow. 2 shows the basic appearance of a received signal after a bandpass filtering line 1 shows a voltage-time diagram,

Fig. 3 shows the basic appearance of a received signal after bandpass fluttering, amplitude detection including a low-pass flicker in a voltage-time diagram, where the received signal is derived from the influence of one inlay device and another ferromagnetic object, such as a tin box,

Follow. 4 shows the basic appearance of a received signal following the treatment described above for FIG. 3 in a voltage-time diagram, but where the signal originates only from an inverter device; and

Follow. 5 shows an alternative detector device according to the invention.

An embodiment of the invention is shown in Fig. 1. The device of Fig. 1 includes a coil 1, 2 for transmitting an electromagnetic alternating field on each side of a probe zone 3. One of said coils 1 is supplied with an alternating current of frequency f from a supply device which includes a first oscillator 4 and a summing amplifier 5. The second of the coils 2 is fed with an alternating current of frequency f₂ from a second oscillator 6 and summing amplifier 5. The other of the coils 2 is fed with an alternating current of frequency f₂ from a second oscillator 6 and buzzer 7 belonging to the feeder. In addition, the feeder includes a third oscillator 8 which is arranged to produce an alternating voltage of the frequency f₃. Said third oscillator is connected to said two buzzer amplifiers 5, 7, which are arranged to emit an alternating current consisting of the frequency of₃ in the coil 1 consisting of the frequency f₂ frequency f ^ and emitting superimposed on superimposed on an AC frequency frequency f₃ in the coil 2.

When using a strip of high permeable material, which is also long relative to its cross-sectional surface, only a relatively weak field strength is required. The diameter of the coils can be varied within relatively wide limits, but should be of the same order of magnitude as the distance between them, ie for a normal checkout counter 1 a self-service shop approx. 70 cm.

The two coils 1, 2 shall be positioned so that the alternating fields emitted by the coils are substantially parallel in each point 1 of the monitoring zone 3.

A problem encountered in monitoring the type to which the present invention relates is that the system does not scan strips oriented perpendicular to the field lines.

In order to obtain 100% detection, one may have three coils arranged in such a way that the fields generated by the respective coil pairs are substantially perpendicular to each other. This enables one coil pair to be activated at a time. When the switching between the coil pairs is sufficiently rapid, a strip passed through the examination zone will be detected regardless of its orientation.

In the following, an article 30 is provided with an indication device 31 in the form of a strip of high permeable material.

In order, on the one hand, to obtain sufficiently high energy content in a signal obtained by a receiver coil 9 emanating from a magnetic field transmitted from the strip, the frequencies f | and fg be relatively high. For, on the other hand, that the magnetic fields should not be shielded by objects in the survey zone, the frequencies f ^ and fg should be chosen low. The high frequencies f1 and fg should therefore be lower than approx. 5 kHz and higher than approx. 1 kHz.

In the present invention, the strip's woolen properties are used to create sum and difference frequencies of frequencies fj and fg.

According to an embodiment of the invention, the differential frequency and the sum frequency are analyzed by at least the first order. These are preferably of the same order of magnitude.

Therefore, the frequencies fj and fg may preferably be such that fj is 1.5 kHz and fg is 2.5 kHz, in which case the sum frequency is 4 kHz and the difference frequency is 1 kHz.

As only the frequencies f1 and fg are emitted and caused to produce intermodulation products which are detected, as mentioned above, there is no possibility to directly determine whether an intermodulation product was produced by a strip or by a ferromagnetic object, such as a tin box, with much larger mass and a slightly curved magnetization basket. It is necessary in this case to compare Intermodulation products of different order to distinguish the strip from a box. This requires a relatively complicated and relatively expensive signal processing equipment.

A suitable field strength for the goods monitoring device of the type in question is of the order of 1 Gauss. Fields of this magnitude are not sufficient to saturate a tin box or other ferromagnetic objects that normally appear in the study zone, e.g. shopping trolleys, metal frames and other personal items. However, a field of such intensity is quite sufficient to saturate a strip of said type.

According to the present invention, a strip can be separated from a tin box in a much simpler and safer way than when only two frequencies are utilized. According to the present invention, as mentioned above, a third magnetic alternating field is produced which varies with a frequency which is substantially lower than said higher frequencies fj and fg. This low frequency field frequency is lower than approx. 100 Hz, preferably about 25 Hz. A suitable frequency is, for example, 20 Hz. This field has a field strength sufficiently high, regardless of the fields of frequencies fj and f2, together with static fields, such as the earth's magnetic field, to put the strip in saturated state in both directions. The low frequency field is preferably brought to a strength which exceeds the combined strength of the high frequency fields by a factor of 2 to 6, preferably by a factor of 4.

When only the two frequencies f 1 and f 2 are used, such as in the above patent, the composite field of f 1 and f 2 swings around a working point on a strip's magnetization curve. This gives rise to continuous intermodulation products.

By moving, according to the present invention, said work point with frequency f3 by means of the low frequency field, a shed of intermodulation products is obtained for each half period of fg. Thus, intermodulation products will be formed and retransmitted by the strip during the time periods that the strip is not saturated by the low-frequency field.

A ferromagnetic object, such as a tin box, does indeed create intermodulation products between the frequencies f1 and f2, but these are formed continuously. The strength of the low-frequency field is considerably lower than the strength required to measure, for example, a can. For this reason, the strength of the received signal resulting from intermodulation in a can box is essentially independent of the low-frequency field, since this only negligibly moves the work point along the magnetization curve of a can.

According to the aforementioned patent, a suitable strength for the emitted magnetic field is, for example, a maximum of 5 Gauss, to ensure that the composite field runs through the wool-like part of the magnetic device curve of the indulgence device. By applying a field of 20 Hz according to the present invention with a lowest strength of about 1 Gauss in the survey zone, it is ensured that said working point for the two high frequency fields passes through the woolen part of the Inlection Device's magnetic reading curve 40 times per second. second. Thus, it is sufficient that the strength of the high frequency fields is approximately one quarter of the strength of the low frequency field. Pg a. transmitting the low frequency field from two sides, the coils generating the field can be performed less cheaply than in the case when the low frequency field is emitted from only one side of the survey zone.

Thus, it is only necessary, in order to distinguish a strip from a tin box, to study the periodic variation in the received signal arising from the effect of the low-frequency field on the strip, i.e., the pulses.

Thus, in a simpler embodiment of the invention, it is sufficient by means of a detector arrangement, including an amplitude detector, to amplify the received signal at a frequency, e.g., f 4 + f the double frequency f 3 or not.

Said amplitude detection is performed in an amplitude detector which may be performed in a manner known per se.

For example, in the device 1, FIG. 1 shows a detector device, including a bandpass filter 10 containing coils and capacitors, which filter at frequencies given above as example passes through 4 kHz with, for example, a bandwidth of 500 Hz. After the bandpass filter 10 there is an amplitude detector 11 which includes, for example, a diode, after the amplitude detector 11 there is a low pass filter 12, which includes, for example, an RC circuit where frequency components of 4 kHz and higher are filtered away. For example, the lanter modulation products produced by a tin box, after amplitude detection and low pass filtering, give rise to essentially an auxiliary voltage, as well as low frequencies, primarily 40 Hz of low amplitude. The intermodulation products produced by a strip act as pulses.

After the bandpass filter 10, the signal originating from a box and a strip has the 1 shown in principle 2 with pulses 25. After the lowpass filter 12, the signal originating from a box and a strip has the 1 FIG. 3 shows the fundamental appearance of a signal originating only from a strip, after the low-pass filter 12.

Thus, when both a tin box and a strip are present in the survey zone, after the low-pass filtering, a signal occurs where said pulses originating from the strip are above the level originating from the tin box, see Fig. 3.

Thus, as mentioned above, by the present invention, in its simplest embodiment, it is sufficient in a pulse detector 13 of known type to detect the possible occurrence of pulses, thereby detecting any occurrence of a strip in the examination zone. Preferably, it is detected that pulses occur 40 times per minute. second to eliminate the impact of interference on the detection result. When such pulses are detected, it is considered that an induction device exists in the examination zone. The pulse detector 13 is capable of transmitting a signal to an alarm device 24 of suitable construction.

The low frequency fg is selected sufficiently low that the pulses which can pass the bandpass filter 10 should be short compared to the period of the low frequency. Therefore, if the bandwidth of the bandpass filter is 500 Hz, a suitable frequency is f 3 20 Hz.

However, the low frequency f3 should be selected sufficiently high to be able to generate a sufficient number of pulses for detection and assessment within a reasonable time. A suitable number of pulses for detection may be 4. When three transmitter and receiver devices are available for scanning a survey zone in three directions perpendicular to each other, switching between the various transmitters and receiver systems can be carried out, for example 5 times per second. second. Thus, a frequency f3 of 20 Hz is suitable. However, other suitable frequencies than those given above can be selected for frequency f3 as well as frequencies fj and fg.

However, according to a preferred embodiment, the signal is carefully analyzed, compared to only detecting the number of pulses. According to this embodiment, the sum frequency and / or the diffrance frequency are analyzed with respect to both phase position relative to the magnetic field originally emitted and the amplitude.

However, when only the presence of pulses is detected, the frequencies f_lt fg and f3 are increased compared to the values ​​given above to obtain more pulses per second. second, thereby achieving greater security against interference.

According to this embodiment, the bandpass signal received by the receiver coil 9 is band-filtered, which is a measurement signal, about at least one of the differential or sum frequencies generated by the intermodulation. A reference signal of the same frequency is formed by mixing the high frequency (fj, fg) signals which are generated to emit the high frequency fields. The measurement signal and the reference signal thus formed are applied to a phase and amplitude detector, preferably a quadrature detector capable of emitting in a manner known per se a signal corresponding to the amplitude (A) of said pulses and the phase difference (φ) between the measurement signal and the reference signal. .

A detector device for carrying out a method according to this embodiment is described below with reference to FIG. 5.

The detector device comprises a means, e.g. a mixer 17 capable of generating a reference signal consisting of at least one differential or sum frequency for the frequencies f₃ and fg. The mixer is fed via respective conductors 18 and 19 from the first oscillator 4 and second oscillator 6, respectively.

A bandpass filter 20 is provided to select only the selected difference or sum frequency, which e.g. can be 4 kHz. This reference signal is applied to one input detector 26 of a square detector 15.

A signal received from the receiver coil 14, i.e. the measurement signal, is passed through a bandpass field 16, which filters out said differential or sum frequency, which is equal to the frequency of the reference signal. The bandpass filter 4 is thus tuned, for example, for 4 kHz with a bandwidth of 500 Hz. After the bandpass filtering, the measurement signal is applied to the second input 27 of the quadrature detector.

The square detector 15 is designed in a known manner and includes two mixers, which mix the measurement signal with the reference signal.

In a mixer, the reference signal, which has the phase angle φ, is mixed_κ, with the measurement signal, which has the phase angle φ ^. Before the second mixer, the phase vortex measurement and / or reference signal is used so that the difference between their phases is &lt;Pr - Φμ + 90 ° or φ_κ ΦΜ - 90 °. The square detector 15 also includes a low pass filter for filtering out frequencies 4 Hz and higher. The low-frequency pulses obtained from the quadrature detector 15 Contain information on the phase and amplitude of the pulses of intermodulation products.

The quadrature detector 15 is capable of emitting a signal in two channels 21, 22, in which one channel signal corresponds to A · snow where A is the amplitude of the measurement signal and φ is φρ - and where the signal in the other channel corresponds to A · οοβφ. For example, in a microprocessor 23 of suitable type, two or more consecutive pulses in the channels 21 and 22 are compared with respect to A and φ. In the case where there is a predetermined degree of similarity with respect to A and φ, which exists between two or more consecutive pulses, it is considered that a strip is present in the examination zone. The microprocessor 23 is capable of transmitting a signal to an appropriate alarm device.

To further enhance the security of the detection system, the time course of the 20 Hz sleep signal can be scanned by known means and fed to the microprocessor 23. Said microprocessor 23 is capable of assessing whether pulses occur in accordance with the 20 Hz signal. This suppresses the impact of any other externally varying magnetic field or other interference.

Said microprocessor 23 and to this associated circuit for analyzing amplitude A and phase angle φ are of suitable known type, and are not in themselves embraced by the present invention. They are therefore not described in detail here.

Above is described.

quadrature detection of sum frequency

Instead, of course, the differential frequency kHz can be processed, as well as both the differential and sum frequencies. In the latter case, of course, a further increased security is obtained in the analysis of a received signal.

In addition to the previously mentioned sum and difference frequencies, a large number of frequencies n * fj + m 'f₂ where n and m are positive or negative integers. Of course, analyzing more than one or two of these frequencies further increases the sensitivity of the system.

All frequencies fj, f₂ and f3 can advantageously be obtained by frequency sharing from a common oscillator (not shown). It is thus easy to generate four-phase signals for use in the aforementioned quadrature detection.

It is clear that the present invention eliminates the disadvantages mentioned above in the introductory part. Thus, the present invention primarily provides the advantage that from a received signal, the indicating device can be easily and securely separated from, for example, a tin box or other object of magnetic material in the examination zone. As mentioned above, the high-frequency fields can be made weaker than is the case with the prior art where a further low-frequency field is not utilized. Because of the possibility of reducing the strength of the high-frequency fields increases the specificity of the received signal. This, in combination with the increased ability to separate a strip from a tin box by applying a third field, can be utilized in such a way that the indicator device is reduced. Making the indicating device, i.e. the strip, smaller than usual is particularly advantageous in cases when the strip is to be integrated with a price tag.

Thus, the present invention is particularly advantageous for such use.

Various embodiments have been described above. However, the invention may be varied without renouncing the idea of ​​the invention. For example, the number of coils for broadcasting the high-frequency and low-frequency fields can be varied, and further the method and means for generating desired fields can be varied. It is also possible to provide multiple receiver coils.

Thus, the invention should not be considered to be limited to the above embodiments, but may be varied within the scope of the appended claims.

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