CA1234892A

CA1234892A - Security tag detection system

Info

Publication number: CA1234892A
Application number: CA000447593A
Authority: CA
Inventors: Pierre Taillefer; Ezequiel Mejia; Phuc Nguyen
Original assignee: 746278 Ontario Ltd D/b/a Id Systems Canada
Current assignee: 746278 Ontario Ltd D/b/a Id Systems Canada
Priority date: 1984-02-16
Filing date: 1984-02-16
Publication date: 1988-04-05
Also published as: IT1214485B; IT8519534A0; DK66885D0; ES8603100A1; SE8500652D0; DE3505052C2; US4663612A; NL8500440A; DK66885A; NO850571L; GB2154396A; ES540470A0; GB8503768D0; FR2559914B1; NO165167B; CH668845A5; GB2154396B; JPS60193098A; SE8500652L; DE3505052A1

Abstract

ABSTRACT

Security apparatus for detection of a security tag to be hidden in an article to be detected. The digitized waveshape of a tag signature signal preferably obtained by distortion of an oscillating magnetic field, (whereby the distortion signal being detected constitutes a signature signal) is compared with a stored digital representative of a signature signal, and an alarm enable signal is generating in the event of correlation a predetermined number of times in a predetermined time period. The stored signal is modified by the specific chacteristics of the apparatus and of environmental factors. The result is a reliable detection apparatus with a very low false alarm rate.

Description

~Z3~9~

01 This invention relates to security 02 apparatus, and in particular to theft detection 03 apparatus for detecting the presence of a saturable 04 magnetic tag which is usually hidden in an article 05 which is lntended to be protected.
06 In recent years a class of theft detection 07 apparatus has become popular in which a ta~ or strip 08 hidden in an article to be protected is detected.
09 While there are variations of the kind o~ tag to be detected (some being comprised of resonant circuits 11 etc.), there have been a number of designs and 12 improvements on the designs based on the detection of 13 specific harmonics or groups of harmonics of signals 14 which are stated as being generated by the tag. ThiS
class of detector is derived from an invention by 16 Pierre Arthur Picard which is described in French 17 Patent 763,6~1, issued ~ovember 10, 1933.
18 In the invention described in that patent, 19 thin strips of material having a high magnetic permeability are repeatedly driven in and out o 21 saturation in the presence of an alternating magnetic 22 field. A receiving antenna receives magnetic fields 23 which are said to be generated by the strip of 24 material, which alternate at frequencies which are harmonics of the original transmission frequency. A
26 receiver connected to the receiving antenna is tuned 27 to detect signals at one or more of the harmonic 28 frequencies produced by the target strip, and an alarm 29 is activated when such detection takes place. The specific frequencies of the harmonics designate the 31 presence of the specific tag material.
32 Improvements to that system are described 33 in U.S. Patents 4,123,749 relating to the continuous 34 rotation of the transmitted magnetic field, 3,983,552 which relates to demagnetization of a control element 36 associated with the strip, and which detects 37 predetermined harmonics, 3,665,449 which detects .~

~L234~39~

01 signals of predetermined -frequencies and having an 02 applied field of predetermined magnitude, 3,631,442 03 which relates to generating at least two oscillating 04 electromagnetic fields at different frequencies, 05 3,790,745 which performs analysis of the harmonic 06 components of the received frequencies, 3,754,226 07 which relates to a form of transmitting antenna, 08 3,820,103 which detects a predetermined harmonic 09 signal generated by the tag or marker, 4,215,342 which utilizes a sub-resonant tag, 4,158,434 which relates 11 to the direction of the lines of magnetic saturation 12 of the tags or labels, and 4,298,862 which relates to 13 the use of amorphous ferromagnetic materials which 14 generate magnetic fields having "tones" which are retained after the marker or tag is flexed or bent.
16 In the aforenoted patented inventions, the 17 markers or tags to be affixed to the goods are 18 specifically utilized to generate signals which are 19 harmonics of the frequency of the magnetic field which is applied to a detection region or zone. Many of the 21 inventions are directed to refinements for maximizing 22 the possibility of detecting certain ones o the 23 harmonics which are said to specifically identify the 24 unique presence of the strip or tag, or to thereby reject erroneous identification o~ other metallic 26 objects such as belt buckles, jewelry, other metallic 27 articles of various constitution carried by a person, 28 etc.
29 Nevertheless it has been found that such apparatus still can generate frequent false alarms.
31 often caused by materials of similar magnetic 32 constitution as the real tag to be detected. As was 33 indicated by Picard, the size or shape of the tag 34 merely varies its amplitude; the constitution of the materials identifies the harmonic content of the 36 signal which is generated. Thus the detection of 37 similar material objects as the strip, marker or ~234~39~

01 security tag has been found to be virtually 02 unavoidable.
03 The present invention utilizes an entirely 04 new approach to detection of such tags. Rather than 05 detection of harmonics of a signal generated in or by 06 the tag as in the prior art, the tag of the present 07 invention is utilized to distort the magnetic field 08 within the detection area. A receiving antenna 09 detects -the distorted field, eliminates the carrier frequency, and presents the distortion signal as a 11 signal to be detected to apparatus which compares the 12 wave shape of the distortion signal to a stored wave 13 shape. Indeed, in the preferred embodiment the 14 received signal is correlated with the stored signal and upon finding a predetermined degree of 16 correlation, an alarm enable signal is generated.
17 Thus harmonic isolation and detection is not needed 18 and is not used.
19 In addition, it was found that in the prior art inventions, in order to isolate the 21 harmonics of the original magnetic signal, a high 22 degree of filtering was required, to identify only one 23 or a selected few harmonics. During manufacture it is 24 difficult to provide filters which are precisely identical, resulting in different units having 26 different sensitivities, due to variation in component 27 values resulting from tolerance variations. Further, 28 as the units age, and with environmental changes in 29 temperat~lre, the detector characteristics would change over time, requiring expensive alignment procedures to 31 be undertaken toward an increase in false alarm rate 32 or a decrease in sensitivity, and thus of re~liability-33 In the present invention each unit 34 "learns" the waveshape characteristics which are to be identified, the signals which are learned being 36 different for each specific unit because of the 37 variation in the characteristics of each specific :~23~
01 unit. Each unit is set up in its operating location, 02 and there it is put through a simple procedure by 03 which it "learns" the waveshape characteristics of the 04 distortion signal caused by the presence of a tag.
05 Thus not only are the characteristics of each specific 06 detection system compensated for, but variations in 07 the environment of the detection system are at the 08 same time taken into account. In addition, should for 09 some reason the detection system characteristics change (e.g. with aging), it can again be put through 11 the same initial "learning" process to again store the 12 waveshape characteristics of the tags to be detected, 13 this time the stored characteristics being modiEied by 14 the changed or aged characteristics of the detection system. Thus complex and precision filters need not 16 be precisely aligned to obtain units as identical as 17 possible, and enviromnental effects are automatically 18 compensated for.
19 Indeed, it is contemplated that with the concepts and structure described herein, that 21 different tags having characteristics different Erom 22 each other can be used and identified by the "learned"
23 stored initial signal representations, thus 24 facilitating identification of one or difference classes of tags.
26 Further, it is contemplated that the form 27 of the signal which is stored need not necessarily be 28 the waveshape of a distortion signal; it can be any 29 form of signature signal from the tag to be detected, e.g. signals from kinds of tags other than the field 31 distortion tag described herein. Thus the signal to 32 be detected will be referred to herein as a tag 33 signature signal, which is not restricted specifically 34 to a waveshape, although that is the preferred form of signal to be identified. An important aspect of the 36 invention, however, is that the signature signal 37 should be compared with a stored representation ~34t3~
01 thereof which is to be detected.
02 In general, one embodiment of the 03 invention is an apparatus for detecting a security tag 04 in a security tag detection system comprising 05 apparatus for storing a first security tag signature 06 signal, apparatus for receiving a signature signal 07 from a security tag, apparatus for comparing the 08 received signature signal with the stored signature 09 signal, and apparatus for providing an output alarm enabling signal in the event the stored signature 11 signal is similar to the compared received signature 12 signal to a predetermined degree.
13 Preferably successively received signature 14 signals from the same tag are repetitively comparea to the stored security tag signature signal, and the 16 alarm signal is enabled in the event the stored 17 signature signal is similar to the received signature 18 signal to a predetermined degree a predetermined 19 minimum successive number of times, or a predetermined minimum number o times over a predetermined period of 21 time.
22 In the "learning" phase of the apparatus, 23 an oscillating magnetic field is applied to a 24 detection region. A security tag signature signal is detected from the received magnetic field from the 26 detection region caused by distortion of the field by 27 the tag, and a memory stores a signature signal 28 derived from the received signal. It is preferred 29 that the signature signal should be comprised of at least two peaks and a trough between the peaks. It is 31 further preferred that this initial signature signal 32 qhould be stored if its peak to trough amplitude ratio 33 is at least a predetermined value and also that the 34 peaks are no greater in amplitude than the first threshold and that the trough is no smaller t~an a 36 second threshold.
37 According to a further embodiment, the 12348~

01 peaks should be classified in, for example, -three 02 amplitude level ranges. Amplitude classifications are 03 assigned to each of the thresholds. The amplitude of 04 the peaks are determined upon the peaks having 05 amplitude below predetermined thresholds, and above 06 predetermined thresholds, i.e. between three amplitude 07 ranges. The peak to trough ratio or ratios are 08 determined in the event -the amplitude of the peaks are 09 not in excess of or lower than the predetermined thresholds, and the digital representation of the 11 signature signal is stored in memory locations 12 according to the classifications.
13 As a result, when the apparatus is used to 14 detect the tag, the peak amplitude of the signature signal is established and the signature signal 16 waveform is compared with the corresponding stored 17 representation which corresponds to the maximum 18 threshold range which the received signature did not 19 exceed. As noted above, it has been ound desirable to provide three thresholds by which a low, medium and 21 high amplitude received signature signals are compared 22 with corresponding low, medium and high amplitude 23 stored representative signature signals.
24 The result of the above invention is the provision of a tag detection system which has been 26 found to have an extremely low false alarm rate, which 27 rate is believed to be significantly lower than the 28 false alarm rate of the systems which detect harmonics 29 and enable an alarm upon detection of those harmonics. It is thus believed that the present 31 invention will ind wide commerical acceptance.
32 It should be noted that the term "alarm 33 enabling signal" is intended to mean a signal which 34 indicates the presence of the tag, and is not intended to be construed only to mean a signal for causing an 36 actual alarm display or sound to turn on.
37 A detailed description of the preferred 123~39~
01 embodiment of the invention is given below, with 02 reference to the accompanying drawings, in which:
03 Fiyure 1 is a general block diagram used 04 to illustrate the concepts o~ the invention, 05 Figure 2A and 2B are representative 06 waveforms used to illustrate the operation of the 07 inven-tion, 08 Figure 3 is an exaggerated waveform of a 09 received signal, Figure 4 illustrates an idealized and 11 representative tag signature signal, 12 Figure 5 is a block diagram of the 13 preferred form of the invention, 14 Figure 6 is a detailed block diagram of the invention, and 16 Figures 7A, 7B and 7C illustrate waveforms 17 found at various points in the invention.
18 Figure 1 illustrates in block form the 19 basic conceptual structure of the preferred embodiment of the invention. A transmitting antenna 1, typically 21 in the form of a large coil, carries an alternating 22 current, usually in a resonant mode and generates an 23 alternating magnetic field in a region through which a

2~ tag 2 which is to be detected passes. The apparatus for setting up the magnetic field need not be 26 described here to understand the concepts of the 27 invention and can be as descri~ed in the aforenoted 28 French Picard patent. A receiving antenna 3, which 29 can be a large coil in figure 8 form, for example, also as described in the Picard patent (or the 31 antennae reciprocally interchanged) detects the 32 magnetic field through which tag 2 passes.
33 Preferably tag 2 is comprised of a 34 lamination of a pair of thin strips of magnetically soft materials having different coercivities but 36 similar thresholds of magnetic saturation. Short 37 strips of a third magnetizable material having high ~ j~3~8~

01 coercivity relative to the coercivities of -the 02 magnetically soft materials are disposed in fixed 03 spaced posi-tions adjacent to and along at least one 04 face of the magnetically soft materials so as to 05 magnetically bias the magnetically soft materials into 06 saturation when the third magnetizably soft material 07 has been remanantly magnetized to avoid detection of 08 the tag. Successful materials that have been used as 09 the soft magnetic materials are amorphous metallic alloys, such as Co66Fe4(Mo,Si,B)30, each strip of the 11 pair having been differently heat treated to obtain 12 different coercivities but similar magnetic saturation 13 thresholds. However different materals could be used, 14 which have the above characteristics and different materials which have different coercivities and 16 different thresholds of magnetic saturation. For the 17 purpose of this description to allow detection, the 18 strips are assumed not to be biased into magnetic 19 saturation by the third magnetizable material. The important characteristic of the unbiased material, 21 however, is that when the tag 2 is placed in the 22 magnetic field caused by transmitting antenna 1, that 23 field is distorted. This occurs when the strength of 24 the field is sufficient to repeatedly bias the magnetically soft materials into, and then out of 26 saturation, in opposite polarity directions, as the 27 alternating magnetic field increases, then reverses.
28 The distorted magnetic Eield passing 29 through receiving antenna 3 generates a signal voltage which is applied to receiver 4.
31 It is contemplated that should a signature 32 signal be generated by the security tag 2 by means 33 other than the application of the magnetic field by 34 coil 1, this would satisfy the concepts of the invention.
36 According to the preferred embodiment, 37 receiver 4 distinguishes the distortion portion of the ~23~39~

01 signal, processing it as a tag signature signal. This 02 signal is applied to a comparator 5.
03 A memory (store) 6 contains a stored 04 signal corresponding to the tag signature signal which 05 it is desired to detect. This signal is also applied 06 to comparator 5.
07 When the comparator 5 distinguishes a 08 correspondence between the form of the received 09 signature signal and the stored signal, it outputs an alarm enabling signal on the output line 7.
11 In order to store the signal to be 12 recognized initially, the apparatus is put into its 13 initialization mode, which activates an accept 14 function circuit 8. A field is established in a detection zone as before and a tag is introduced 16 therein. The signal is received in receiver 4 and the 17 resulting distortion signal, i.e. the tag signature 18 signal is applied to the accept circuit 8. Here the 19 amplitude of the tag signature signal is determined as to whether it falls between an upper and lower 21 threshold. If it does not, the acceptance operation 22 does not proceed. If it does fall between the 23 thresholds, for the two-material laminated tag, at the 24 time that the strips saturate it has been ound that there would be two analog peaks separated by a dip or 26 valley therebetween. The ratio or ratios of the peaks 27 to valley amplitude is determined, and if it is within 28 a predetermined range, the accept circuit applies the 29 tag signature waveform to the store 6 for storage.
It is preferred that the tag signature 31 signal should be digitized before application to the 32 comparison circuit 5 or the accept circuit 8. Storage 33 of the signal should preferably be in digital form in 34 a random access memory, preferably of non-volatile type. It will also become clear that the comparator 36 circuit, the accept circuit and a portion o~ the 37 receiver could be effected in a central processing 38 _ 9 _ ~34l3~

01 unit, and that the store 6 would be an adjunct memory 02 operating therewith.
03 Certain of the signals observed are shown 04 in exaggerated form in Figures 2, 3 and 4. In Figure 05 2B, short duration square waves 9 are shown which are 06 applied to transmitting antenna 1. The pulses 9, 07 which will be referred to herein as pumping pulses, 08 would typically be approximately 12 microseconds 09 long. Approximately 15 pumping pulses are applied to transmi-tting antenna 1, resulting in an increasing 11 voltage waveform 10 thereacross. With each pumping 12 pulse, the amplitude of waveform 10 increases.
13 Preferably a capacitor is connected in parallel with 14 transmitting antenna 1, to form a resonant circuit at the frequency of the pumping pulses. Typically the 16 peak amplitude obtained would be about 200 volts.
17 Th~ pumping pulses are then stopped, and 18 the current in transmitting antenna 1 continues to 19 oscillate, but with decreasing voltage amplitude as shown, eventually reaching 0. This cycle is repeated 21 continuously, the oscillating or carrier signal chosen 22 being at a frequency of 6.1 kilohertz, with peaks 23 generated about every 64 milliseconds. If two 24 transmitting coils are used with one receiving coil, the receiving coil will receive a burst of 6.1 26 kilohertz signal every 32 milliseconds.
27 It was mentioned earlier that the received 28 signal in receiving antenna 3 will be a distorted 29 representation of the transmitted signal. The distortion is caused by the presence of the tag 2, 31 causing purturbations in the lines of flux of the 32 field in the detection area. When the, or each o~ the 33 soft magnetic materials saturates, a peak in the 34 received signal results. This is shown by the representative received signal 11 in Figure 3. ~t 36 will be seen that at both the positive or negative 37 excursions of the received signal, double peak ~234~

01 distortions 12 in the received waveform are found. It 02 may be noted that when the transmitted waveform 03 reduces to such a low amplitude that the tags cannot 04 be saturated, no distortions occur.
05 Receiver 4 then detects and isolates the 06 distortion portion of the signal, which is the 07 received tag signature signal shown in idealized orm 08 as weveform 13 in Figure 4.
09 Preferably the signal is applied to a four bit analog-to-digital (A/B) converter, which has 11 sixteen levels shown in Figure 4 as levels 0-15. The 12 converter samples waveform 13, resulting in digital 13 signals which correspond to the amplitude of the 14 vertical lines over the X axis ~time) range Sl. The digitized signal is either applied to comparator S as 16 described earlier or to the accept circuitry 8, also 17 as described earlier.
18 However it has been found that the 19 idealized tag signature signal is usually not realized, and is itself dis~orted with ambient noise, 21 60 cycle or harmonics thereof, power line signals, 22 transients, etc. This non-idealized, and more normal 23 received tag signature signal is representatively 24 shown as waveform 14.
One of the reasons prior art systems often 26 are plagued with false alarms now will be evident. If 27 the system is desired to react with an idealized 28 received signal of specific harmonic frequencies, 29 assuming that the received circuitry does not age or change component values, and that all component values 31 are precisely as were designed, such prior art systems 32 might operate satisfactorily. However it has been 33 found that the received tag signal is affected by 34 environmental and other factors to a very significant effect. As an example the signature waveform 14 looks 36 vastly different from idealized tag signal 13-37 Furthermore, different installations of the same r .

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01 detection apparatus at different locations have been 02 found to result in mutually different tag signature 03 signals, and different from the theoretical ideal form 04 to such a degree that it can be understood that false 05 alarms would far from uncommon.
06 However in the present invention, the form 07 of the actual tag signature signal produced by the 08 combination of the detection system, the tag and the 09 environmental factors is determined on site and is stored in the store 6 of the apparatus. As described 11 earlier, subsequent tag signature signals which match 12 the non-idealized but real tag signature signal will 13 cause an alarm enabling signal to be generated.
14 Figure 5 illustrates a basic block diagram of the preferred form of the invention. A receiving 16 antenna 3 feeds a receiver comprised of a preamplifier 17 15, which has its output connected to the input of 18 amplifier 16. The output of amplifier 16 is connected 19 to the input of analog-to-digital tA/D) converter 17.
The data output of A/D converter 17 is connected to 21 the data bus of a central processing unit circuit 22 (CPU) 18. Control lines of CPU 18 are connected to 23 A/D converter 17 for applying timing signals for 24 controlling its operation.
CPU 18 is also connected to a buffer 26 interface 19, which has outputs connected to one or 27 more transmit antenna drivers 20. The output of each 28 dri~er 20 is connected to a parallel reson~nt circuit 29 having a transmit antenna as its inductive element.
A display 21 and a keyboard 22 are 31 connected to the CPU in a well-known manner. Other 32 input/output lines I/O are also connected to CPU 18 in 33 a well known manner.
34 In operation, the CPU applies pulse signals, as described with reference to Figure 2, 36 waveform B, to drive buffer 19. The driver 20 applies 37 amplified representations thereof to the corresponding ~Z34~

01 transmit antenna coil, which builds up its current and 02 vol-tage amplitude as described earlier with reference 03 to Figure 2, waveform A.
04 For example, ~our transmit antennae could 05 be used for a double exit to a retail establishment, 06 two transmit antennae being located on opposite sides 07 of the double sxit pathway for a customer, and two 08 other -transmit antennae being loca-ted between the exit 09 pathways and being driven by pulse groups over alternate time periods. Similarly the first two 11 transmit antannae are driven by pulse groups having 12 alternate time periods. This results in magnetic 13 field directions which sequentially change within the 14 two pathway detection zones, while providing minimum interference between the two adjacent exit pathways.
16 The representative receive antenna 3 17 receives the magnetic field, which is distorted by the 18 presence of a tag which is not previously biased into 19 saturation by a magnetized hard magnetic material in adjacency thereto. For the four transmitting antennae 21 case there would be two receiving antennae, one for 22 each pathway detection region, but for the sake of 23 simplicity of description to facilitate understanding 24 we will conform the receive apparatus description to a single channel.
26 The received signal is amplified in 27 amplifier 15, and the carrier signal is removed in 28 amplifier 16. The resulting distortion signal, i.e.
29 the received tag signature signal such as that shown in waveform 13 (idealized) or 14 (more representative 31 of the real) is digitized in A/D converter 17 and is 32 applied to CPU 18. CPU 18 at the same time receives a 33 digital stored tag signature signal from random access 34 memory RAM 23, compares it with the received signal, and if they match to a predetermined degree, generates 36 an alarm enable signal and applies it to the I/0 lire.
37 At the time of the initial installation, 1234l39~

01 the keyboard 22 is used to place the CPU into its 02 initialization mode, with the appropriate inaication 03 shown in display 21. Of course other actors such as 04 time of day, etc. can also be displayed.
05 When the CPU is in its initialization 06 mode, it causes the generation of the magnetic field 07 in the detect area as before, and when a tag is placed 08 in the detection field the resul-ting tag signature 09 signal appears on the data lines at ~PU 18 as described earlier. However in this case the CPU
11 checks the amplitude of the signal against a 12 predetermined upper and lower threshold, which is 13 stored in its original operation program signals, 14 i.e. in firmware, shown as ROM 24. If the amplitude is between the upper and lower thresholds, the CPU is 16 caused by a program signal stored in the ROM 24 to 17 check the maximum amplitude to adjacent trough ratio.
18 If desired, the next adjacent peak to preceding trough 14 ratio can also be determined as a further insurance factor. If the ratio is within a predetermined range, 21 (the range signal also being stored in ROM 24), the 22 CPU is caused to store the digitized signal in RAM 23.
23 It is preferred that the sequence that the 24 CPU should follow is to display on display 21 the commands for introduction of the tag toward receiving 26 antenna 3 in three degrees of intensity or adjacency 27 or threshold ranges. The first, or minimum degree 28 would be introduced first, i.e. having low initial 29 maximum and minimum thresholds between which the signature signal should be detected, checked for 31 ratio range and stored. The threshold range should 32 then be increased in successive trials, and resul-ting 33 digital signature signals stored in RAM 23 in a 34 similar manner as before.
Each time a successful tag signature 36 signal has been stored in RAM 23, the display 21 37 should provide an instruction to the operator to ~234~

01 proceed and introduce a tag ~urther into the magnetic 02 -field, the display 21 providing an indication as to 03 when a successful signa-ture signal storage has 04 occurred.
05 Signals from the keyboard 22 can then 06 place the CPU into its operational mode. During 07 operation when a tag is introduced adjacent -to receive 08 antenna 3, the detected amplitude of the tag signature 09 signal should be classified by degree, within threshold ranges, and the appropriate corresponding 11 tag signature signal having the next highest threshold 12 should be used to compare against it. Indeed, it will 13 be noted that since the repetition rate of t~le 14 transmit signal antenna is relatively high, ~here will be a repetitive reception of the tag signature signal 16 for comparison with the stored signature signal by CPU
17 18 as a person carrying the tag passes through the 18 detection zone. However since the person is 19 continually moving and likely turning the tag as it move past the antenna, the tag may be continuously 21 moving relative to the receive antenna, and the form 22 of the signature signal will usually vary in amplitude 23 with time.
24 Consequently it is preferred that at least three correlations with the signal stored in RAM 23 26 should be made within a period of e.g. 1/4 second in 27 order that the CPU should generate an alarm enabling 28 signal. However a predetermined number o~ successive 29 positive cor~elations could be utilized instead.
Indeed it is preferred that a correlation 31 technique should be used. For example the CPU
32 preferably operates the transfer functions 34 RK = ti Sitk (1 ) ~ o 37 RT = ~ t2i (2) 38 ~o - 15 -, ~Z34l3~

01 where ti is the received tag signature signal, 02 S is the stored tag signature signal, 03 i is the summation index, 04 K is a correlation parameter relating to a 05 signal sample number (e.g. 0.... 10) 06 and in which an output alarm enabling signal is 07 generatea in the even-t the correlation ra~io RK is 09 equal to or greater than a predetermined value.
In one successful prototype, the ratio 11 RK/RT was .75, which is expected to be a typical 12 approximate correlation value to indicate a successful 13 correlation.
14 Indeed, it is contemplated that in addition to display 21, a speech synthes.izer could be 16 used, operated by CPU 18 which voices a warning to a 17 person passing through the detection zone, who has 18 generated an alarm enabling signal, e.g. "Please 19 return to the check-out counter since the goods you are carrying have not been adequately checked out".
21 Turning now to Figure 6, a detailed block 22 schematic of the preferred embodiment of the invention 23 is shown. As described with reference to Figure 5, it 24 is preferred that there should be a plurality of transmit antennae and a plurality of receive antennae 26 which can be selected in transmit-receive antennae 27 pairs. However other configurations can be utilized.
28 In Figure 6 a transmit-receive antennae pair 30 is 29 shown which, or example, can be similar to the antennae described in the aforenoted French Picard 31 patent 763,681.

32 The receiving antenna of the pair is 33 connected to a circuit which removes the low 6.1 KHz 34 frequency carrier signal. The signal is coupled directly to one input of an adder 31, and to another 36 input of the adder through a low pass, low or no phase 37 shift, filter 32. Preferably the filter has a cut-off - 1~3413~

01 frequency of about 12 KHz and has no or very low phase 02 shift at about 6.1 KHz. Preferably ~he adder has 03 gain, and thus the circuit can be considered as an 04 amplifier.
05 The xesulting output signal from -the adder 06 is the tag signal, as shown in Figures 7A and 7B. In 07 Figure 7A, a representa-tive carrier signal at about 08 6.1 KHz is transmitted in the form of waveform 33 by 09 the transmit antenna, creating an oscillating inductive field as described earlier. A received 11 waveform 34 as shown in Figure 7B will be found to 12 have distortions 35 at the timing positions at which 13 the tag in the field saturates. These were described 14 earlier with reference to figure 3, which shows distortions 12 in waveform 11. These distortions form 16 signature signals 36, shown in Figure 7C, which will 17 be found at the output of adder 31 (along with any 18 very low amplitude residual carrier).
19 With the received waveform 34 being applied to adder 31 and to low pass ilter 32, the 21 carrier signal and accompanying noise within the 22 passband of the filter are cancelled, and the 23 resulting output signal from the adder is of the form 24 of signature signal 36 shown in Figure 7C.
It is also preferred that the carrier 26 should be eliminated in this manner rather than by 27 common high pass filtering, since the presently 28 descr.ibed carrier elimination technique has been found 29 to cause minimum changes to the signature signals. It has been found that an active high pass filter in many 31 cases is non-linear, and it thus changes the 32 characteristics of the signature signal.
33 Although not shown in this block 34 schematic, it is preferred that the output signal of adder 31 should be buffered.
36 The output tag signature signal from adder 37 31 is applied to another adder 37 through switch 38 , 1;2 3~

01 and input resistor 39. Signature signals of other 02 receivlng antennae pass via transmission paths 40A and 03 40B through switches 38A and 38B, and input resistors 04 39A and 39s to the input of adder 37. The closure of 05 any of the switches 38, 38A or 38s thus provides an 06 input signal to adder 37. If desired, some antenna 07 configurations make the combinations of the received 08 signals desirable, which can be added together at the 09 input of adder 37.
The selected signature signal of the form 11 of signal 36 (Figure 7C) or 14 (Figure 4) is then 12 passed through a high pass filter 41, in order to 13 further reduce any carrier signal, and is then applied 14 to the input of an automatic gain control circuit 42.
The resulting output signal is passed through high 16 pass filter 43, is limited in limiter 44, and is 17 applied to the input of analog-to-digital converter 18 45. The output of limiter 44 is also applied to the 19 input of a bandpass filter 46, which in a successful prototype had a center frequency of 100 K~z, and the 21 resulting output signal is passed through a high speed 22 rectifier 47, the output signal of which is in the 23 form of the envelope of the signature signal. This 24 envelope signal is applied back to the control input C
of the automatic gain control circuit 42, preferably 26 through a buffer (not shown). As a result, the 27 automatic gain control responds to the tag signature 28 signals, and not to the amplitude of the carrier 29 signal.
Switches 48 are connected in series 31 between the output of adder 37 and the input of filter 32 41, the output of filter 41 and the input of automatic 33 gain control 42, and the output of rectifier 47 and 34 the control input o:E automatic gain control 42.
Switches 49 are respectively connected in shunt 36 between the inputs of filter 41, automatic gain 37 control 4~ and the control input of automatic gain - 1234~39~

01 control 42, and ground. When switches 48 are open, 02 switches 49 are closed, and vice versa. This unction 03 both breaks the series circuit so that received 04 signals do not pass through filter 41 and AGC 42, and 05 at the same time connects the signal and control 06 inputs of filter 41 and AGC 42 to ground. This 07 operation is utilized during the time that the 08 transmitting antenna is pumped with the pulse signal 9 09 which estahlishes an increasing magnetic field. When the "pumping" is stopped, and the transmitted signal 11 is allowed to decay as described with reference to 12 Figure 2, switches 48 are closed and switches 49 are 13 opened in order to allow the circuit to operate as 14 described above to pass the signature signals.
lS The tag signature signals are converted 16 from analog to digital form in analog to digital 17 converter 45. The resulting digital parallel words 18 are passed into FIFO register 50. The output signal 19 of FIFO 50 is applied to a CPU bus Sl, which is a conventional bus communicating with central processing 21 unit 52. A data store enable control line 53 and a 22 reset signal control line 54 from the bus, operated by 23 the CPU enables the FIFO register to read data from 24 the analog to digital converter 45 and to reset the FIFO register when its stored data is applied to the 26 bus and is read by the CPU.
27 A ROM memory 55 which preferably stores 28 the operation program for the CPU for carrying out the 29 steps described herein in firmware, is connected to bus 51, as is a random access memory RAM 56 which is 31 used in a conventional way as a scratch pad memory.
32 Another random access memory RAM 56A is also connected 33 to bus 51, which stores the learned signature signals 34 against which the signature signals received during normal operation are compared.
36 The output ports P are connected to 37 individual driver circuits 57 (only one being shown 1~34~39~

01 for the sake of clarity). The output o~ each of the 02 drivers 57 is connected to a transmitting antenna o 03 receive-transmit antennae 30. The output of a driver 04 could be connected to the center tap of a transmit 05 antenna coil, the other terminals of w~ich are 06 selected at appropriate times by the CPU enabling 07 other drivers or switches connec~ed thereto. A
08 transmitting antenna can thereby be coupled to a 09 predetermined receiving antenna selected by the closure of one of the switches 38, 38A, 38B, etc-11 In this manner a transmitting antenna 12 carries current from the driver 57, and sets up an 13 oscillating magnetic field in a region through which 14 patrons of a store or a person or article carrying a tag to be sensed pass, which magnetic field is 16 received in a r~ceiving antenna which field has been 17 distorted by a tag which may be in the field, 18 resulting in the signature signal as described 19 earlier. Pairs or groups of transmitting antennae can be energized at the same time to establish the field 21 direction within the magnetic field detection region.
22 A limiter 58 is connected between the 23 output of each driver and ground, in order to reduce 24 or eliminate transients.
The output signal from each of the drivers 26 57 is applied to individual inputs of a multiplexer 27 59, the input lines of which are shown as 60, 60A, 60B
28 and 60C. The individual input lines to multiplexer 59 29 are selected via address lines 61 connected to central processing unit 52, the signal from the input lines 31 being passed via data line 62 to the CPU. Upon 32 addressing any of the input lines 60-60C, the CPU
33 detects for zero crossover of the signal across the 34 transmit antennae, and thereby obtains an indication of the exact timing of the signal passing through the 36 transmitting antenna.
37 Using the crossover signals the CPU can 1234~

01 control -the timing of switches ~8 and 49, and switches 02 38, 38A and 38s, the timing of the pump signal enable 03 signal applied to drivers 57 in order to generate the 04 pumping signals (pulses 9 of Figure 2), to ensure -that 05 no signals are processed through filter 41, AGC ~2, an 06 analog to digital converter 45 and FIFO 50 during -the 07 magnetic field increasing interval corresponding to 08 the pumping interval, and to detect the tag signature 09 signals described above which are applied to bus 51 through FIFO 50.
11 A clock 63 is connected to CPU 52 in a 12 well ~nown manner. A switch array 64 is also 13 connected to CPU 52 in a well known manner; the 14 selection of any of the switches of the array 64 selects the particular program stored in ROM 55 to 16 operate CPU 52, thus establishing the mode o~
17 operation of the entire circuit.
18 A display 65 and keyboard 66 are also 19 connected to CPU 52 in a normal manner. Keyboard 66 is used to input various parameters such as time o~
21 day, operator numbers, etc. which are stored in RAM
22 66. Display 65 is used to display the information 23 input on keyboard 66, as well as to provide a day and 24 date to the operator and to indicate instructions stored in ROM 55 to the operator, for example steps 26 the operator should take to proceed with the sequence 27 by which the circuit learns and stores the tag 28 signature signal upon installation~
29 At power up, during installation initialization, it is preferred that the firmware 31 stored in ROM 55 should cause the CPU to output a 32 signal to display 65 which states "Small Signal". At 33 the same time the CPU establishes a magnetic field as 34 described earlier. The operator puts an uncancelled tag in the field, the signature signal then being read 36 by the apparatus. The value of the amplitude should 37 be shown on display 65, and the operator inserts the -.

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Ol tag deeper into the magnetic field. The value on the 02 display 65 increases. When the value is between first 03 low level minimum and maximum thresholds, the CPU
04 causes storage of the characteristic shape of the 05 signature signal received from FIFO 50 in digital form 06 in RAM 57.
07 The CPU then causes the words "Medium 08 Signal" to show on the display 65. The operator puts 09 the tag still deeper into the magnetic field, and the same sequence proceeds as before, bu-t the minimum and 11 maximum thresholds are greater. When the amplitude of 12 the signature signal is between the second thresholds, 13 the characteristic shape of the signature signal is 14 stored in RAM 57.
The CPU causes the display 65 then to 16 display "Large Signal" on display 65. The minimum and 17 maximum thresholds are increased again, and the same 18 procedure is repeated.
19 ~nce the large signal has been stored in RAM 57, the CPU places an instruction on display 65 21 requesting that the operator should enter the date, 22 time and access code number. Preferably there should 23 be more than one access code to ensure security.
24 These access codes are stored in RAM 57, which will be the only access codes which allow operator access to 26 the system.
27 The operator then presses a "enter" key on 28 keyboard 66, the time and date are displayed on 29 display 65, and the system is operational.
A read only memory 67 is connected to bus 31 51, and stores data to operate a speech synthesizer 32 circuit 68, which operates a loudspeaker 69. Upon 33 detection of a tag passing through the magnetic field 34 as described earlier, the central processing unit 52 causes an alarm indication signal to be applied to bus 36 51, enabling ROM 67 to output signals to speech 37 synthesizer 68, resulting in a ~oiced warning to be -1234l~

01 synthesized and broadcast via speaker 69 to the person 02 carrying -the uncancelled tag. At the same time an 03 alarm signal can be applied via bus 51 and buffer 70 04 to an alarm line 71 leading to the manager of the 05 store or another person in authority.
06 It should be noted that more than one 07 memory 67 (or different plug in memories) can be used 08 if it is desired to facilitate the outputting of 09 speech in various languages. One memory 67 could thus be utilized for English, another for French, a -third 11 for German, another for Spanish, another for Italian, 12 etc.
13 It should also be noted that bus 51 can be 14 extended to an expansion module (not shown) which can be connected to a central controller which retains 16 displays, keyboards and control lines for a number of 17 circuits of the kind just described.
18 It has been found that the invention 19 described above is highly immune from false alarms, and is believed to be a significant advance in the 21 art. Since the system itself learns the signal 22 characteristics of the tag signature signal to be 23 detected, it is adaptable to a great variety of 24 environments, and can be easily reinitialized or recalibrated by unskilled personnel. Since a number 26 of different signature signals can be learned and true 27 comparison indications given to the operator, the 28 invention is not limited to the detection of thet of 29 merchandise, but can be used for other purposes such as detection of different classes of merchandise 31 carrying tags defined by different saturation 32 characteristics (signatures) of different tags.
33 Personnel carrying tags having different signatures, 34 such as doctors or nurses, can be tracked passing through various detection gates located at strategic 36 locations, such as in hospitals. Since the invention 37 incorporates a central processing unit, manipulations , .. .

~L23~

01 can be made on the numerical value of the number of 02 detections, such as summing the number of goods within 03 various classifications detected passing along a 04 conveyor belt, etc.
05 A person understanding this invention may 06 now conceive of various embodiments or alternative 07 designs using the principles described herein. A11 08 are considered to be within the scope of t~e invention 09 as defined in the claims appended hereto.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a security tag detection system, means for detecting a security tag comprising:
(a) means for storing a first security tag signature signal in a memory, (b) means for receiving a signature signal from a security tag which is distinctive of the security tag, (c) means for comparing the received signature signal with the stored signature signal, and (d) means for providing an output alarm signal indicative of the presence of the tag in the event the stored signature signal is similar to the compared received signature signal to a predetermined degree.

2. Means for detecting a security tag as defined in claim 1 including means for repetitively comparing successively received signature signals from said tag and for providing said alarm enabling signal in the event the stored signature signal is similar to the received signature signal to a predetermined degree a predetermined minimum successive number of times.

3. Means for detecting a security tag as defined in claim 1 or 2 including means for storing a plurality of different amplitudes of received signature signals from a security tag, and in which the means for comparing is comprised of means for determining an amplitude range of the received signature signal for selecting the closest amplitude range signature signal thereto from the plurality of stored signature signals, and for comparing the received signature signal therewith to determine similarity therewith.

4. Means for detecting a security tag as defined in claim 1 or 2 in which the comparing means is comprised of a correlator, and means for generating an output alarm enabling signal in accordance with the transfer functions RK = ti Sitk (1) RT = t2i (2) where ti is the received tag signature signal, S i5 the stored tag signature signal, i is the summation index, K is a correlation parameter relating to a signal sample number (e.g. 0....10) and in which an output alarm enabling signal is generated in the event the correlation ratio is equal to or greater than a predetermined value.

5. Means for detecting a security tag as defined in claim 1 or 2 further including means for storing the first signature signal in digital form, means for converting the received signature signal to digital form prior to comparison with the stored signature signal.

6. Means for detecting a security tag as defined in claim 1 or 2, further including means for storing the first signature signal in digital form, means for converting the received signature signal to digital form prior to comparison with the stored signature signal, in which the comparing means is comprised of a correlator, and means for generating an output alarm enabling signal in accordance with the transfer functions RK = ti Sitk (1) RT = t2i (2) where ti is the received tag signature signal, S is the stored tag signature signal, i is the summation index, K is a correlation parameter relating to a signal sample number (e.g. 0....10) and in which an output alarm enabling signal is generated in the event the correlation ratio is is equal to or greater than a predetermined value.

7. Means for detecting a security tag as defined in claim 1 including means for repetitively comparing successive received signature signals from said tag and for providing said alarm enabling signal in the event the stored signature signal is similar to the received signature to a predetermined degree a predetermined minimum number of times within a predetermined interval of time.

8. Means for detecting a security tag as defined in claim 7, further including means for storing the first signature signal in digital form, means for converting the received signature signal to digital form prior to comparison with the stored signature signal, in which the comparing means is comprised of a correlator, and means for generating an output alarm enabling signal in accordance with the transfer functions RK = ti Sitk (1) RT = t2i (2) where ti is the received tag signature signal, S is the stored tag signature signal, i is the summation index, K is a correlation parameter relating to a signal sample number (e.g. 0....10) and in which an output alarm enabling signal is generated in the event the correlation ratio is is equal to or greater than a predetermined value.

9. Means for detecting a security tag as defined in caim 1, in which the means for storing is comprised of (i) means for generating an oscillating magnetic field in a detection region, (ii) means for detecting a security tag signature signal from a signal received from the detection region, (iii) memory means and means for storing the signature signal in the memory means.

10. Means for detecting a security tag as defined in claim 9, in which the signature signal is comprised of at least two peaks and a trough therebetween, and in which the means for storing includes means for storing the signature signal in the event a peak to trough amplitude ratio is at least a predetermined value and also that the peaks are no greater in amplitude than a first threshold and the trough is no smaller than a second threshold.

11. Means or detecting a security tag as defined in claim 9 in which the signature signal is comprised at least two peaks and a trough therebetween, and in which the means for storing includes means for determining the amplitude of said peaks and for classifying the degree of said peaks, for reading the ratio or ratios of said peaks to said trough in the event the amplitude of said peaks is not in excess of or lower than predetermined thresholds, and for storing a digital representation of said signature in memory locations according to said classification.

12. Means for detecting a security tag as defined in claim 9 in which the signature signal is comprised of at least two peaks and a trough therebetween, and in which the means for storing includes means establishing a maximum threshold and assigning an amplitude classification to said threshold, for determining the amplitude of said peaks upon the peaks having amplitudes below said threshold, for reading the ratio or ratios of said peaks to said trough in the event the amplitude of said peaks is not less than a predetermined minimum threshold, and for storing a digital representation of said signature signal in memory locations according to said classification.

13. Means for detecting a security tag as defined in claim 9 in which the detecting means is comprised of an adder, means for applying the received signal to one input of an adder, and to the other input of the adder through a filter having a passband sufficient to pass a signal having a frequency of the oscillating magnetic field, whereby signals of said latter frequency are cancelled in the adder, resulting in an enhanced tag signature signal output therefrom.

14. Means for detecting a security tag as defined in claim 13, in which the memory means is a digital memory, further including an analog to digital converter for transforming the enhanced tag signal into digital form, and register means for temporarily storing the transformed tag signal prior to storage in the memory means.

15. Means for detecting a security tag as defined in claim 14, in which the means for storing is comprised of a digital processor, and further including a voice synthesizer means operated under control of the processor for generating a synthesized voiced warning and applying it to a speaker upon provision of said output alarm signal, for acoustical broadcasting to a user carrying said security tag.

16. Means for detecting a security tag as defined in claim 15, in which the voice synthesizer is comprised of means for generating a voiced warning in more than one language.

17. Means for detecting a security tag as defined in claim 1, 9 or 14, including means for storing a plurality of different security tag signature signals, means for comparing the received signature signals with the stored signature signals, and means for providing different output signals indicative of the detection of different ones of the received signature signals.

18. Means for detecting a security tag as defined in claim 1, 9 or 14, in which the alarm signal is an enabling signal for a detection indicating device.

19. Means for detecting a security tag as defined in claim 1, 9 or 14, in which the means for providing the alarm signal is comprised of a digital processor, the processor being adapted to store a count of the numbers of detected tags causing generation of said signal.

20. Means for detecting a security tag as defined in claim 1, 9 or 14, including means for storing a plurality of different security tag signature signals, a digital processor means for comparing the received signature signals with the stored signature signals, the processor being adapted to store a count of the numbers of detected tags of each one of the plurality of different security tag signature signals.

21. Means for detecting a security tag as defined in claim 9, in which the means for providing the alarm signal is comprised of a processor, the processor being adapted to enable cyclic and sequential operation of pairs of said generated means and means for receiving a signal from the detection region.

22. Means for detecting a security tag as defined in claim 21, including means for repeatedly causing progressive increase of the oscillating magnetic field over first time periods, and for receiving the signal from the detection region over second time periods immediately following the first time periods.

23. Means for detecting a security tag as defined in claim 22 in which the detecting means is comprised of an adder, means for applying the received signal to one input of an adder, and to the other input of the adder through a filter having a passband sufficient to pass a signal having a frequency of the oscillating magnetic field, whereby signals of said latter frequency are cancelled in the adder, resulting in an enhanced tag signature signal output therefrom.

24. Means for detecting a security tag as defined in claim 22, including means for storing a plurality of different security tag signature signals, means for comparing the received signature signals with the stored signature signals, and means for providing different output signals indicative of the detection of different ones of the received signature signals.

25. Means for detecting a security tag as defined in claim 22, 23 or 24, in which the means for providing the alarm signal is comprised of a digital processor, the processor being adapted to store a count of the numbers of detected tags causing generation of said signal.