Method, system and apparatus for magnetic surveillance of articles

Claim 2

In a method for effecting article surveillance by the use of a time-varying signal establishing a magnetic field, the steps of providing first and second diverse magnetic materials, the first for application to articles under surveillance and the second for shielding said field, and preselecting said first and second magnetic materials to have respective diverse timewise responsivity to said magnetic field.

Claim 3

The method claimed in claim 1 wherein said time-varying signal has successive positive and negative excursions with a zero-crossover therebetween, said first magnetic material having principal responsivity to said magnetic field near said zero-crossover of said time-varying signal.

Claim 4

The method claimed in claim 2 wherein said second magnetic material has principal responsivity to said magnetic field near the peaks of said positive and negative excursions of said time-varying signal.

Claim 5

The method claimed in claim 3 including the further step of receiving signals indicative of such responsivities of said first and second magnetic materials to said magnetic field, and of processing such received signals in manner suppressing surveillance output alarm indication for received signal content attributable to such responsivity of said second magnetic material and generating surveillance output alarm indication for received signal content attributable to said responsivity of said first magnetic material.

Claim 6

The method claimed in claim 1 wherein said second magnetic material is configured as a shield, said second magnetic material being selected to minimize eddy current generation in said shield responsively to said magnetic field.

Claim 7

The method claimed in claim 5 wherein said second magnetic material is selected to have a saturation flux density greater than the largest flux density produced in such shield responsively to said magnetic field.

Claim 8

The method claimed in claim 1 wherein said second magnetic material is configured as a shield, the resistivity of said second magnetic material being selected to be greater than the product πd 2 uf, where d is shield thickness, u is the permeability of said second magnetic material and f is the frequency of said time-varying magnetic signal.

Claim 9

The method claimed in claim 1 wherein said second magnetic material is configured as a shield, the skin depth of said shield being a multiple of said shield thickness.

Claim 10

The method claimed in claim 1 wherein said second magnetic material is selected to have a saturation flux density greater than the largest flux density produced in such shield responsively to said magnetic field.

Claim 11

The method claimed in claim 1 wherein said second magnetic material bounds a control zone for such surveillance, said method including the further steps of providing an auxiliary shield of electrically conductive character and juxtaposing said auxiliary shield with said second magnetic material outwardly of said control zone, the material of said auxiliary shield being selected to effect eddy current generation in said auxiliary shield responsively to signals generated by said first magnetic material located outwardly of said control zone.

Claim 12

The method claimed in claim 10 wherein the resistivity of said auxiliary shield material is selected to be equal to or fractionally larger or fractionally less than the product πd 2 uf, where d is auxiliary shield thickness, u is the permeability of said auxiliary shield material and f is a frequency in said signals generated by said first magnetic material.

Claim 13

The method claimed in claim 10 wherein the skin depth of said auxiliary shield is in the range of being equal to or fractionally larger or less than the thickness of said auxiliary shield.

Claim 16

The invention claimed in claim 13 wherein said step (b) is practiced by selecting said magnetic shielding material of character responsive to said magnetic field in maximum amplitude near such positive and negative peaks.

Claim 17

The invention claimed in claim 14 wherein said step (b) is further practiced to provide a ratio of peak magnetic front field to back field of at least ten to one.

Claim 18

The method claimed in claim 13 wherein said magnetic shielding material is selected to minimize eddy current generation in said magnetic shield members responsively to said magnetic field.

Claim 19

The method claimed in claim 13 wherein said magnetic shielding material is selected to have a saturation flux density greater than the largest flux density produced in said magnetic shield members responsively to said magnetic field.

Claim 20

The method claimed in claim 13 including the further steps of providing an auxiliary shield of electrically conductive character and juxtaposing said auxiliary shield with at least one of said magnetic shield members outwardly of said control zone, the material of said auxiliary shield being selected to effect eddy current generation in said auxiliary shield responsively to signals generated by said tag magnetic material located outwardly of said control zone.

Claim 23

The system claimed in claim 19 wherein said magnetic shielding material is of character responsive to said magnetic field signal in maximum amplitude generally near such positive and negative peaks.

Claim 24

The system claimed in claim 19 wherein said magnetic shielding material provides a ratio of peak magnetic front field to back field of at least ten to one.

Claim 25

The system claimed in claim 19 wherein said magnetic shield members are operative to minimize eddy current generation in said shield responsively to said magnetic field.

Claim 26

The system claimed in claim 19 wherein the resistivity of said magnetic shielding material is greater than the product πd 2 uf, where d is shield member thickness, u is the permeability of said magnetic shielding material and f is the frequency of said time-varying magnetic signal.

Claim 27

The system claimed in claim 19 wherein the skin depth of said shield members is a multiple of the shield member thickness.

Claim 28

The system claimed in claim 19 wherein said magnetic shielding material has a saturation flux density greater than the largest flux density produced in said shield members responsively to said magnetic field.

Claim 29

The system claimed in claim 19 including further an auxiliary shield of electrically conductive material juxtaposed with at least one of said magnetic shield members outwardly of said control zone, the material of said auxiliary shield effecting eddy current generation in said auxiliary shield responsively to signals generated by article surveillance tags located outwardly of said control zone.

Claim 30

The system claimed in claim 26 wherein the resistivity of said auxiliary shield material is selected to be equal to or fractionally larger or fractionally less that the product πd 2 uf, where d is auxiliary shield thickness, u is the permeability of said auxiliary shield material and f is a frequency in said signals generated by said tags responsively to said magnetic field.

Claim 31

The system claimed in claim 26 wherein the skin depth of said auxiliary shield is in the range of being equal to or fractionally larger or fractionally less than the thickness of said auxiliary shield.

Claim 32

Shielding apparatus for use in a system for determining the presence of magnetic article surveillance tags selectively in a control zone, said system employing a time-varying signal of preselected frequency to establish a magnetic field in said control zone, said shielding apparatus comprising: (a) a first shield including magnetic material having a skin depth minimizing eddy current generation in said first shield responsively to said magnetic field; and (b) a second shield including electrically conductive material having a skin depth effecting eddy current generation in said second shield responsively to signals generated by said tags responsively to said magnetic field.

Claim 33

Shielding apparatus for use in a system for determining the presence of magnetic article surveillance tags selectively in a control zone, said system employing a time-varying signal of preselected frequency to establish a magnetic field in said control zone, said shielding apparatus comprising: (a) a first shield including magnetic material having a skin depth minimizing eddy current generation in said first shield responsively to said magnetic field; and (b) a second shield including electrically conductive material having a skin depth effecting eddy current generation in said second shield responsively to signals generated by said tags responsively to said magnetic field.

Claim 34

The apparatus claimed in claim 29 wherein the resistivity of said first shield material is greater than the product πd 2 uf, where d is first shield thickness, u is the permeability of said first shield material and f is the frequency of said time-varying signal.

Claim 35

The apparatus claimed in claim 29 wherein the skin depth of said first shield is a multiple of the first shield thickness.

Claim 36

The apparatus claimed in claim 29 wherein the resistivity of said second shield material is equal to or fractionally larger or fractionally less than the product πd 2 uf, where d is second shield thickness, u is the permeability of said second shield material and f is a frequency of signals generated by said tags.

Claim 37

The apparatus claimed in claim 29 wherein the skin depth of said second shield is in the range of being equal to or fractionally larger or fractionally less than the thickness of said second shield.

Claim 38

Apparatus for use in a system for determining the presence of a magnetic article surveillance tag selectively in a control zone, said system being of type including means for generating a magnetic field by use of alternating excitation signal having current peaks at positive and negative levels and a zero-crossover therebetween, said apparatus comprising: (a) a shielding member including magnetic shielding material of character responsive to said excitation signal in maximum amplitude other than near such zero-crossovers; (b) a transmitting antenna juxtaposed with said shielding member and adapted for establishing a magnetic field responsively to said excitation signal; and (c) a receiving antenna juxtaposed with said shielding member and adapted to be responsive to perturbations induced in said magnetic field by said shielding member and said tag.

Claim 39

Apparatus for use in a system for determining the presence of a magnetic article surveillance tag selectively in a control zone, said system being of type including means for generating a magnetic field by use of alternating excitation signal having current peaks at positive and negative levels and a zero-crossover therebetween, said apparatus comprising: (a) a shielding member including magnetic shielding material of character responsive to said excitation signal in maximum amplitude other than near such zero-crossovers; (b) a transmitting antenna juxtaposed with said shielding member and adapted for establishing a magnetic field responsively to said excitation signal; and (c) a receiving antenna juxtaposed with said shielding member and adapted to be responsive to perturbations induced in said magnetic field by said shielding member and said tag.

Claim 40

The apparatus claimed in claim 34 further including an auxiliary shielding member of electrically conductive material, said magnetic shielding material being situated between said auxiliary shielding member and said transmitting and receiving antennas.

Claim 41

The apparatus claimed in claim 34 wherein the resistivity of said magnetic shielding material is greater than the product πd 2 uf, where d is shielding member thickness, u is the permeability of said magnetic shielding material and f is the frequency of said excitation signal.

Claim 42

The apparatus claimed in claim 34 wherein the skin depth of said shielding members is a multiple of the shielding member thickness.

Claim 43

The apparatus claimed in claim 34 wherein said magnetic shielding material has a saturation flux density greater than the largest flux density produced in said shielding member responsively to said magnetic field.

Claim 44

The apparatus claimed in claim 34 including further an auxiliary shield of electrically conductive material juxtaposed with said shielding member outwardly of said control zone, the material of said auxiliary shield effecting eddy current generation in said auxiliary shield responsively to signals generated by article surveillance tags located outwardly of said control zone.

Claim 45

The apparatus claimed in claim 39 wherein the resistivity of said auxiliary shield material is selected to be equal to or fractionally larger or fractionally less than the product πd 2 uf, where d is auxiliary shield thickness, u is the permeability of said auxiliary shield material and f is a frequency in said signals generated by said tags responsively to said magnetic field.

Claim 46

The apparatus claimed in claim 39 wherein the skin depth of said auxiliary shield is in the range of being equal to or fractionally larger or fractionally less than the thickness of said auxiliary shield.

Claim 47

An electronic article surveillance system to detect the presence of a magnetic article surveillance tag in a control zone, comprising: (a) transmitter means for transmitting alternating magnetic energy into said control zone; (b) receiving means for detecting perturbations of said magnetic energy caused by the presence of magnetic tags within said control zone; (c) signal processing means connected to said receiving means to cause an alarm condition when said active tag is in said control zone; (d) a shield member located at a side margin of said control zone; (e) said shield member comprising magnetic shield material being of composition to reduce the intensity of magnetic energy in the back field located in a zone behind said shield member to lessen the probability of objects in said back field causing an alarm condition; (f) said magnetic shield material being of composition to produce principal perturbations of said magnetic energy which are detectably distinguishable from the perturbations caused by said magnetic tag; (g) said signal processing means being adapted to suppress said principal perturbations caused by said magnetic shield material to preclude an alarm condition from said magnetic shield perturbations.

Claim 48

An electronic article surveillance system to detect the presence of a magnetic article surveillance tag in a control zone, comprising: (a) transmitter means for transmitting alternating magnetic energy into said control zone; (b) receiving means for detecting perturbations of said magnetic energy caused by the presence of magnetic tags within said control zone; (c) signal processing means connected to said receiving means to cause an alarm condition when said active tag is in said control zone; (d) a shield member located at a side margin of said control zone; (e) said shield member comprising magnetic shield material being of composition to reduce the intensity of magnetic energy in the back field located in a zone behind said shield member to lessen the probability of objects in said back field causing an alarm condition; (f) said magnetic shield material being of composition to produce principal perturbations of said magnetic energy which are detectably distinguishable from the perturbations caused by said magnetic tag; (g) said signal processing means being adapted to suppress said principal perturbations caused by said magnetic shield material to preclude an alarm condition from said magnetic shield perturbations.

Claim 49

The invention claimed in claim 42 where said shield member also includes an auxiliary shield located behind said magnetic shield material to attenuate perturbations resulting from the presence of a magnetic tag in said back field thereby to lessen the likelihood of said perturbations from said backfield from generating an alarm condition.

Claim 50

The invention claimed in claim 43 wherein said auxiliary shield is comprised of aluminum.

Claim 51

The invention claimed in claim 42 wherein said magnetic shield material has a relative permeability of greater than or equal to about 60.

Claim 52

The invention claimed in claim 42 wherein said magnetic shield member has a saturation flux density greater than the largest flux density produced in the shield member due to the positive or negative peak excursions of said transmitted magnetic energy.

Claim 53

The invention claimed in claim 42 wherein said magnetic shield member has a coercivity of equal to or greater than about one oersted.

Claim 54

The invention claimed in claim 42 wherein said magnetic shield member is comprised of pressed powdered iron.

Claim 55

The invention claimed in claim 42 wherein said magnetic shield material produces principal perturbations of said transmitted magnetic energy at a time other than near the zero cross-over point of a current cycle causing said transmitted magnetic energy.

Claim 56

The invention claimed in claim 42 wherein said magnetic shield member is comprised of a ferrite.