Method and apparatus for detection of targets in an interrogation zone

Claim 3

A method according to claim 1 wherein said comparison is made by algebraically combining the amplitudes of the electrical signal which occur during said time increments.

Claim 4

A method according to claim 2 wherein several of said time increments occur during each cycle of said interrogation frequency.

Claim 5

A method according to claim 1 wherein the corresponding ones of said second group of time increments are separated in time by one half cycle of said interrogation frequency from their respective time increments in said first group.

Claim 6

A method according to claim 4 wherein said comparison is made by algebraically combining the amplitudes of the signals which occur during said time increments.

Claim 7

A method according to claim 5 wherein the amplitudes of the signal which occur during each of said first group of successive time increments are stored for a duration of one half period of said interrogation frequency to be compared with the amplitudes which occur during each of said second group of successive time increments.

Claim 8

A method according to claim 1 wherein said first electrical signal is divided according to a series of successive time increments by switching said signal successively for individually storing the amplitudes of the signal which occur during the different time increments.

Claim 9

A method according to claim 1 wherein said switching is carried out in synchronism with said interrogation frequency.

Claim 10

A method according to claim 8 wherein said groups of time increments occur in successive half cycles of said interrogation frequency.

Claim 11

A method according to claim 7 wherein the amplitudes of the signal which occur during each of said first group of successive time increments are stored as voltages in associated capacitors and wherein the amplitudes of the signal which occur during the corresponding ones of each of said second group of successive time increments are also applied as voltages to said capacitors.

Claim 12

ll. A method according to claim 2 wherein, prior to dividing said first electrical signal, its amplitude variations are changed by an amount inversely proportional to the magnitude of preceeding increases in amplitude of. the signal.

Claim 13

A method according to claim 11 wherein said amplitude variations are changed only in response to the magnitude of said preceeding increases exceeding a predetermined threshold.

Claim 14

A method according to claim 1 wherein said alarm is produced in response to said alarm signal exceeding a predetermined value relative to the amplitude of said first electrical signal.

Claim 15

A method according to claim 2 wherein the amplitudes of the first electrical signals are compared for corresponding time increments in several successive half cycles of said interrogation frequency.

Claim 16

A method according to claim 13 wherein said alarm is produced in response to said alarm signal exceeding, in said several successive half cycles of said interrogation frequency, a predetermined value relative to the amplitude of said first electrical signal.

Claim 17

A method according to claim 13 wherein, prior to dividing said first electrical signal, its amplitude variations are changed by an amount inversely proportional to the magnitude of preceeding increases in amplitude of the signal which occurred within the preceeding several half cycles of said interrogation frequency.

Claim 18

A method according to claim 13 wherein said alarm signal and said first electrical signal are each integrated over several successive half cycles of said interrogation frequency to produce integrated alarm signals and integrated first electrical signals and wherein said alarm is produced in response to said integrated alarm signal attaining a predetermined value relative to said integrated first electrical signal.

Claim 19

A method according to claim 17 wherein only those portions of said alarm signal and said first electrical signal which occur when said magnetic interrogation field is at less than maximum intensity are integrated to produce said said integrated alarm signals and integrated first electrical signals.

Claim 22

A method according to claim 19 wherein said first electrical signals are produced in response to electromagnetic waves in said interrogation zone which are greater in frequency than said interrogation frequency.

Claim 23

A method according to claim 20 wherein said first electrical signals are produced in response to electromagnetic waves in said interrogation zone which are synchronous with said interrogation frequency.

Claim 24

A method according to claim 21 wherein said step of processing said first electrical signals comprises extracting from said signals the component thereof which corresponds to their asymmetry.

Claim 25

A method according to claim 19 wherein said step of processing said first electrical signals comprises dividing said signals into several successive time segments synchronized to said interrogation frequency and comparing the portions of said electrical signal which occur in corresponding time segments in successive half cycles of said interrogation frequency.

Claim 26

A method according to claim 23 wherein said step of processing said first electrical signals further comprises switching said signals into separate signal storage means during each of said successive time segments which occur in one half cycle of said interrogation frequency and thereafter, during the next half cycle of said interrogation frequency comparing the signals which occur during each time increment with the signal stored in the corresponding storage means.

Claim 27

A method according to claim 19 wherein said signals are compared by combining said signals algebraically.

Claim 28

A method according to claim 25 wherein the step of comparing said first electrical signals and said further signals is carried out by comparing the amplitudes of said signals.

Claim 29

A method according to claim 26 wherein the step of comparing said first electrical signals and said further signals is carried out by comparing the values of said first electrical signals which occur in several successive half cycles of said interrogation frequency with the values of said further electrical signals which occur in several successive half cycles of said interrogation frequency.

Claim 30

A method according to claim 26 wherein only the values of the first electrical signals which occur when the alternating magnetic interrogation field is less than a first predetermined intensity and only the values of said further signals which occur when the alternating magnetic interrogation field is less than a second predetermined intensity are compared in said step of comparing.

Claim 31

A method according to claim 28 wherein said first and second predetermined intensities are less than the maximum intensity of said alternating magnetic field.

Claim 32

A method according to claim 28 wherein the step of comparing said first electrical signals and said further signals is carried out by comparing the values of said first electrical signals which occur in several successive half cycles of said interrogation frequency with the values of said further electrical signals which occur in several successive half cycles of said interrogation frequency.

Claim 33

A method according to claim 26 wherein the step of producing an alarm signal is carried out in response to the ratio of the amplitude of said further signals to the amplitude of said first electrical signals exceeding a predetermined value.

Claim 34

A method according to claim 31 wherein said further signals are amplified in a signal amplification device whose gain is said predetermined value and wherein said alarm signal is produced when the amplitude of the thus amplified further signals exceeds the amplitude of the first electrical signals by a predetermined amount.

Claim 35

A method according to claim 20 wherein said step of processing said electrical detection signals is carried out by sampling the amplitudes of the signal in several successive time increments during each period and in synchronism with said base frequency and algebraically combining each sampled amplitude with amplitudes sampled at times displaced therefrom by one half periods of said interrogation frequency.

Claim 36

Electromagnetic type theft detection apparatus for detecting the presence of targets (30) in an interrogation zone (24), said targets comprising elements capable, when in said interrogation zone of being driven alternately into and out of magnetic saturation by an alternating magnetic field in said zone, and apparatus comprising means (62, 64, 66, 68, 71, 76, 88, 92, 160, 42, 44) for generating an alternating magnetic interrogation field in an interrogation zone (24) at an interrogation frequency and at an amplitude sufficient to drive targets (30) in said zone alternately into and out of magnetic saturation, magnetic field detection means (50, 52, 100, 102, 104, 106, 108, 110, 112) arranged to detect the magnetic fields in said interrogation zone and to produce a corresponding first electrical signal whose amplitude varies according to the intensity of the magnetic fields in said interrogation zone, averager means (124) including switch means (Sa ... Sp) arranged to be operated in synchronism with said generating means (68, 70) and connected to said detection means to divide said first electrical signal according to a series of successive time increments, comparison means (Ca ... Cp) arranged in conjunction with said switch means (Sa ... Sp) for comparing the amplitudes of the first electrical signal which occur during each of a first group of said time increments with the amplitudes of the first electrical signal which occur during corresponding ones of a second group of time increments, each of said time increments being in synchronism with said interrogation frequency and means (130, 132, 134, 136, 138, 141, 142, 146, 156) for activating an alarm (28) in response to a predetermined output from said comparison means.

Claim 37

Electromagnetic type theft detection apparatus for detecting the presence of targets (30) in an interrogation zone (24), said targets comprising elements capable, when in said interrogation zone of being driven alternately into and out of magnetic saturation by an alternating magnetic field in said zone, and apparatus comprising means (62, 64, 66, 68, 71, 76, 88, 92, 160, 42, 44) for generating an alternating magnetic interrogation field in an interrogation zone (24) at an interrogation frequency and at an amplitude sufficient to drive targets (30) in said zone alternately into and out of magnetic saturation, magnetic field detection means (50, 52, 100, 102, 104, 106, 108, 110, 112) arranged to detect the magnetic fields in said interrogation zone and to produce a corresponding first electrical signal whose amplitude varies according to the intensity of the magnetic fields in said interrogation zone, averager means (124) including switch means (Sa ... Sp) arranged to be operated in synchronism with said generating means (68, 70) and connected to said detection means to divide said first electrical signal according to a series of successive time increments, comparison means (Ca ... Cp) arranged in conjunction with said switch means (Sa ... Sp) for comparing the amplitudes of the first electrical signal which occur during each of a first group of said time increments with the amplitudes of the first electrical signal which occur during corresponding ones of a second group of time increments, each of said time increments being in synchronism with said interrogation frequency and means (130, 132, 134, 136, 138, 141, 142, 146, 156) for activating an alarm (28) in response to a predetermined output from said comparison means.

Claim 38

Theft detection apparatus according to claim 34 wherein said comparison means (Ca ... Cp) is constructed to algebraically combine the amplitudes of the electrical signal which occur during said time increments.

Claim 39

Theft detection apparatus according to claim 34 wherein said comparision (Ca ... Cp) means comprises a plurality of storage elements each associated with a different time increment.

Claim 40

Theft detection apparatus according to claim 36 wherein said storage elements are capacitors (Ca ... Cp).

Claim 41

Theft detection apparatus according to claim 37 wherein said switch means (Sa ... Sp) comprises a plurality of switches (Sa ... Sp) each arranged to connect a different capacitor to said magnetic field detection means (50, 52, 100, 102, 106, 108, 110, 112).

Claim 42

Theft detection apparatus according to claim 38 wherein said means for generating an alternating magnetic field (62, 64, 66, 68, 71, 76, 88, 92, 160, 42, 44) includes an oscillator (62) which operates at a frequency several times higher than said interrogation frequency and frequency divider means (66, 68) connected to said oscillator (62) to produce said interrogation frequency and wherein said frequency divider means (66, 68) is also connected to said switch means (Sa ... Sp) to operate each switch to connect a different capacitor (Ca ... Cp) in succession to said magnetic field detection means (50, 52, 100, 102, 104, 106, 108, 110, 112), whereby different capacitors receive said first electrical signal during different successive time intervals in each cycle of said alternating magnetic interrogation field and in sychronism therewith.

Claim 43

Theft detection apparatus according to claim 39 wherein said frequency divider means (66, 68) and said switch means (Sa ... Sp) are arranged such that said plurality of storage elements (Ca ... Cp) are connected to receive said electrical signal during successive time increments in one half cycle of said alternating magnetic interrogation field.

Claim 44

Theft detection apparatus according to claim 40 wherein said frequency divider means (66, 68) and said switch means (Sa ... Sp) are further arranged such that said plurality of storage elements (Ca ... Cp) are connected also to receive said electrical signal during corresponding successive time increments in successive half cycles of said alternating magnetic interrogation field.

Claim 45

Theft detection apparatus according to claim 34 wherein said means (130, 132, 134, 136, 138, 141, 142, 146, 156) for actuating an alarm in response to a predetermined output from said comparison means comprises a further comparison means (152, 154, 141, 142, 146) connected to receive outputs from the first comparison means (124) and from said magnetic field detection means (50, 52, 100, 102, 104, 106, 108, 110, 112, 114, 148, 150, 152, 154).

Claim 46

Theft detection apparatus according to claim 42 wherein said further comparison means (152, 154, 141, 142, 146) includes an amplifier (142) connected to amplify inputs thereto from said first comparison means (124).

Claim 47

Theft detection apparatus according to claim 43 wherein a-signal compressor (118) is connected between said magnetic field detection means (50, 52, 100, 102, 104, 106, 108, 110, 112) and said averager means (124) for changing the amplitude variations of said first electrical signal by an amount inversely proportional to the magnitude of preceeding amplitudes of the signal.

Claim 48

Theft detection apparatus according to claim 44 wherein said signal compressor means (118) comprises a variable gain amplifier (120) whose gain is inversely proportional to the amplitude of said first electrical signal.

Claim 49

Theft detection apparatus according to claim 45 wherein said further comparison means (152, 154, 141, 142, 146) comprises means (141, 152) to integrate the signals from said first comparison means (124) and the signals from said magnetic detection means (50, 52, 100, 102, 104, 106, 108, 110, 112, 114, 148, 150, 152, 154) over several half cycles of said magnetic interrogation field and to compare the integrated signals.

Claim 50

Theft detection apparatus according to claim 46 whrein said further comparision means (152, 154, 141, 142, 146) comprises signal gates (138, 150) connected to be operated in synchronism with said means (62, 64, 66, 68, 71, 76, 88, 92 160, 42, 44) for generating an alternating magnetic field and arranged to exclude from comparison those signals from said first comparision means (124) and from said magnetic field detection means (50, 52, 100, 102, 104, 106, 108, 110, 112) which occur during the intervals when the interrogation magnetic field is at maximum intensity.

Claim 51

Electromagnetic type theft detection apparatus for detecting the presence of targets (30) in an interrogation zone (24), said targets comprising elements capable, when in said interrogation zone, of being driven alternately into and out of magnetic saturation by an alternating magnetic field in said zone, and apparatus comprising alternating magnetic interrogation field generating means (62, 64, 66, 68, 71, 76, 88, 92, 160, 42, 44) arranged to generate in an interrogation zone (24) an alternating magnetic interrogation field at an interrogation frequency and at an intensity sufficient to drive targets (30) in said zone alternately into and out of magnetic saturation, magnetic field detection means (50, 52, 100, 102, 104, 106, 108, 110, 112) arranged to detect the presence of magnetic fields in said interrogation zone and to produce corresponding first electrical detection signals, signal processing means (124) connected to said magnetic field detection means to process said first electrical detection signals to produce further signals corresponding to the effects produced on said targets by a uniform continuous magnetic bias, comparison means (146) connected to said magnetic field detection means (50, 52, 100, 102, 104, 106, 108, 110, 112) and to said signal processing means (118, 124) to compare said first electrical detection signals and said further signals and an alarm actuation means (156, 28) connected to said comparison means and operative to produce an alarm upon a predetermined relationship between said first and further electrical signals.

Claim 52

Electromagnetic type theft detection apparatus for detecting the presence of targets (30) in an interrogation zone (24), said targets comprising elements capable, when in said interrogation zone, of being driven alternately into and out of magnetic saturation by an alternating magnetic field in said zone, and apparatus comprising alternating magnetic interrogation field generating means (62, 64, 66, 68, 71, 76, 88, 92, 160, 42, 44) arranged to generate in an interrogation zone (24) an alternating magnetic interrogation field at an interrogation frequency and at an intensity sufficient to drive targets (30) in said zone alternately into and out of magnetic saturation, magnetic field detection means (50, 52, 100, 102, 104, 106, 108, 110, 112) arranged to detect the presence of magnetic fields in said interrogation zone and to produce corresponding first electrical detection signals, signal processing means (124) connected to said magnetic field detection means to process said first electrical detection signals to produce further signals corresponding to the effects produced on said targets by a uniform continuous magnetic bias, comparison means (146) connected to said magnetic field detection means (50, 52, 100, 102, 104, 106, 108, 110, 112) and to said signal processing means (118, 124) to compare said first electrical detection signals and said further signals and an alarm actuation means (156, 28) connected to said comparison means and operative to produce an alarm upon a predetermined relationship between said first and further electrical signals.

Claim 53

Theft detection apparatus according to claim 48 wherein said magnetic field detection means (50, 52, 100, 102, 104, 106, 108, 110, 112) is arranged to detect magnetic fields which vary in a predetermined frequency.

Claim 54

Theft detection apparatus according to claim'49 wherein said signal processing means (118, 124) is constructed to produce further signals which are synchronous with said interrogation frequency.

Claim 55

Theft detection apparatus according to claim 48 wherein said signal processing means (118, 124) is constructed to produce further signals corresponding to the effects of the earth's magnetic field on said targets.

Claim 56

Theft detection apparatus according to claim 51 wherein said signal processing means (118, 124) is constructed to produce further signals corresponding to the asymmetry of said first electrical detection signals.

Claim 57

Theft detection apparatus according to claim 48 wherein said signal processing means (118, 124) includes a signal averager (124) which is constructed to divide said first electrical signals into several successive time segments within each cycle of said interrogation frequency and synchronized therewith and to compare the portions of said electrical signal which occur in corresponding time segments in successive half cycles of said interrogation frequency.

Claim 58

Theft detection apparatus according to claim 53 wherein said signal processing means (118, 124) further includes a compressor (118) which is constructed and connected to subject said first electrical signals to a gain which is inversely proportional to their amplitudes and to supply the thus subjected signals to said averager (124).

Claim 59

Theft detection apparatus according to claim 54 wherein said compressor (118) comprises a variable gain amplifier (120) connected to receive said first electrical signals and a rectifier and integrator (122) connected to receive the output of said variable gain amplifier (120), the output of said rectifier and integrator (122) being connected to adjust the gain of said variable gain amplifier (120) and the output of said variable gain amplifier (120) being connected to said averager (124).

Claim 60

Theft detection apparatus according to claim 55 wherein said integrator (122) has a rapid rise time constant and a slower fall time constant.

Claim 61

Theft detection apparatus according to claim 56 wherein the fall time constant of said integrator (122) extends over several cycles of the interrogation frequency.

Claim 62

Theft detection apparatus according to claim 53 wherein said signal processing means (118, 124) includes switches (Sa ... Sp) and storage elements (Ca ... Cp), said switches being constructed and arranged to be closed sequentially and alternately in synchronism with said interrogation frequency and connected so that, when closed, each switch (S) applys said first electrical signal to its respective storage element (C).

Claim 63

Theft detection apparatus according to claim 58 wherein said switches (Sa ... Sp) are each arranged to be closed once in each half cycle of said interrogation frequency in a predetermined sequence.

Claim 64

Theft detection apparatus according to claim 59 wherein said storage means (Ca ... Cp) are capacitors to which corresponding portions of said first electrical signal are applied once in each half cycle of said interrogation frequency so that said portions are combined algebraically.

Claim 65

Theft detection apparatus according to claim 48 wherein said comparison means (146) includes gating means (138, 150) synchronized with said alternating magnetic interrogation field generating means to exclude from comparison those signals generated when the magnetic interrogation field is at its maximum intensity.

Claim 66

Theft detection apparatus according to claim 61 wherein said gating means (138, 150) includes separate gates (138, 150) connected to gates said first electrical signal and said further signals.

Claim 67

Theft detection apparatus according to claim 48 wherein said comparision means (146) includes integrators (141, 152) constructed and connected to integrate the values of said first and further signals over several half cycles of said interrogation frequency.