Magnetic detection for an anti-shoplifting system utilizing combined magnetometer and gradiometer signals

Claim 2

In an anti-shoplifting apparatus employing magnetic detection of a magnetic marker affixed to an object for detection of unauthorized passage of the magnetically marked object through a surveillance station: sensor means to be disposed proximate the region to be monitored for sensing the magnetic field of the magnetic marker to which said sensor is exposed to derive first and second outputs, said first output being a function of a first component of the magnetic field of the marker oF a certain magnetic order, said second output being a function of a second component of the field of the magnetic marker of a magnetic order higher than that of said first magnetic field component; means responsive to said first and second outputs to derive third and fourth outputs, said third output being a function of the magnitude of the magnetic moment of the magnetic field component produced by the marker, if any, to which said sensor means is exposed, and said fourth output being a function of the range from said sensor means to said magnetic marker.

Claim 3

The apparatus of claim 1 wherein said sensor means includes a pair of magnetic sensors spaced apart to derive said first and second outputs, and wherein said first output is proportional to the magnitude of a magnetic field intensity component of the magnetic marker to which said sensor means is exposed, and wherein said second output is proportional to the magnitude of a gradient component of the magnetic field intensity component of the magnetic marker to which said sensor means is exposed.

Claim 4

The apparatus of claim 2 wherein said pair of magnetic sensors are each fluxgate type magnetic sensors, and means for taking the difference of the outputs of said pair of spaced fluxgate sensors to derive said second output.

Claim 5

The apparatus of claim 1 wherein said sensor means includes, means for sensing said first component of the magnetic field of the marker in a plurality of different directions, and means for selecting the largest one of said plurality of sensed components as said first output.

Claim 6

The apparatus of claim 1 wherein said sensor means includes, means for sensing said second component of the magnetic field of the magnetic marker in a plurality of different directions, and means for selecting the largest one of said plurality of sensed components as said second output.

Claim 7

The apparatus of claim 2 wherein said means responsive to said first magnetic field intensity output and said second magnetic field gradient output to derive said third and fourth outputs includes, means responsive to said first and second outputs to derive respective logarithmic outputs proportional to the respective logarithms of the amplitudes of said first and second outputs, and means for taking the difference of said pair of logarithmic outputs to derive said fourth output which is a function of range.

Claim 8

The apparatus of claim 2 wherein said means responsive to said first magnetic field intensity output and said second magnetic field gradient output to derive said third and fourth outputs includes, means responsive to said first and second outputs to derive respective logarithmic outputs proportional to the respective logarithms of the amplitudes of said first and second outputs, and means for differently weighting said respective first and second logarithmic outputs according to the ratio of more parts of said first logarithmic output than the parts of said second logarithmic output, and means for taking the difference of said weighted first and second logarithmic outputs to derive said third output which is a function of the magnitude of the magnetic moment of the field to which said sensor is exposed.

Claim 9

The apparatus of claim 1 including, means for comparing said third output, which is a function of the sensed magnetic moment, with a reference determinative of a minimum magnetic moment to be detected to derive an output determinative of whether the sensed magnetic moment is greater than or less than the reference minimum magnetic moment.

Claim 10

The apparatus of claim 1 including, means for comparing said third output, which is a function of the magnitude of the sensed magnetic moment, with a reference determinative of a maximum magnetic moment to be detected to derive an output determinative of whether the sensed magnetic moment is greater than or less than the reference maximum magnetic moment to be detected.

Claim 11

The apparatus of claim 1 including, means for comparing said fourth output, which is a functiOn of range to the sensed magnetic moment, with a reference determinative of maximum range to be detected to derive an output determinative of whether the range of the sensed magnetic moment is greater than or less than the reference maximum range to be detected.

Claim 12

The apparatus of claim 2 including, means for comparing said gradient second output with a reference determinative of the minimum gradient to which the system is to be responsive to derive an output determinative of whether the gradient of the sensed magnetic field component is greater than or less than the reference minimum gradient to be detected.

Claim 13

The apparatus of claim 1 including, means for comparing said third and fourth outputs with respective reference quantities determinative of minimum and maximum magnetic moment and maximum range to derive three respective fifth outputs determinative of whether the sensed magnetic moment is within a prescribed range of values and whether the range to the sensed magnetic moment is less than a prescribed value, and means responsive to said three respective fifth outputs for deriving a sixth output indicative of whether the sensed magnetic moment is within a predetermined prescribed window of magnetic moment value and the range to the sensed magnetic moment is less than a predetermined value.

Claim 14

The apparatus of claim 12 including, means for sequentially sampling and storing said sixth output, and means for analyzing the sequence of said stored sixth outputs to derive a seventh output when a certain predetermined number of said stored sixth outputs are true within a predetermined sequence of a given number.

Claim 15

The apparatus of claim 1 wherein said sensor means includes a pair of sensors disposed proximate the region of space to be monitored and each producing a respective pair of said first and second outputs derived from said common region of space under surveillance, means for multiplexing said respective pairs of said first and second outputs to said means responsive to said first and second outputs to derive multiplexed respective pairs of said third and fourth outputs, and means for comparing said multiplexed respective pairs of third and fourth outputs with reference quantities determinative of minimum and maximum magnetic moment and maximum range to derive three pairs of multiplexed respective fifth outputs determinative of whether the respectively sensed magnetic moment is within a prescribed range of values and whether the range to the sensed magnetic moment is less than a prescribed value, and means responsive to said pair of three respective fifth outputs for deriving a respective pair of sixth outputs indicative of whether the sensed magnetic moment as sensed by the respective sensor is within a predetermined prescribed window of values and the range to the sensed magnetic moment is less than a prescribed value.

Claim 16

The apparatus of claim 14 including, means for sequentially sampling and storing said pair of sixth outputs, and means for analyzing a sequence of said stored sixth outputs to derive a seventh output when a certain predetermined number of said stored sixth outputs fall within a predetermined sequence of a given number.

Claim 17

The apparatus of claim 15 including, means for comparing said respective pairs of said first outputs with a reference quantity corresponding to a sensed first output produced by a referenced magnetic moment disposed generally centrally of the common region of space under surveillance and generally midway between said pair of sensors to derive an additional fifth output determinative of whether at least one of said pair of sensors has a first output of a magnitude greater or less than said compared reference quantity, and wherein said means for producing said sixth output is also responsive to said additional fifth output.

Claim 18

The apparatus of claim 13 including, means responsive to a ''''false'''' output of said third output comparator means for inhibiting said seventh output for several seconds.

Claim 19

The apparatus of claim 17 wherein said second output is proportional to a gradient component of a magnetic field vector component to which said sensor is exposed and including, means for comparing said gradient second output with a reference determinative of the minimum gradient to which the output of the system is to be responsive to derive an output determinative of whether the sensed gradient component is greater than or less than the gradient reference, and wherein said inhibiting means is additionally responsive to a true output of said second gradient output comparator means for inhibiting said seventh output for several seconds.

Claim 20

The apparatus of claim 12 including, means responsive to said first and second outputs to derive a polarity signal output determinative of the direction to the sensed magnetic moment from said sensor means, and wherein said means responsive to said fifth output for producing said sixth output is additionally responsive to said polarity signal output for causing said sixth output to be false when said sensed magnetic moment is not in a predetermined selected direction relative to said sensor means.

Claim 21

The apparatus of claim 3 wherein each of said fluxgate magnetic sensors includes, a pair of magnetic field sensing windings having their respective magnetic axes substantially angularly displaced relative to each other.

Claim 22

The apparatus of claim 20 wherein said magnetic axes of said field sensing windings are orthogonally displaced.

Claim 23

The apparatus of claim 3 wherein each of said fluxgate magnetic sensors includes, three magnetic field sensing windings having their respective magnetic axes orthogonally angularly displaced relative to each other.

Claim 24

In a magnetic surveillance system for selective detection of magnetic moments of a predetermined magnitude within a surveillance region: sensor means to be disposed proximate the region to be monitored for sensing the magnetic field to which said sensor is exposed to derive first and second outputs, said first output being a function of a first component of the magnetic field of a certain order, said second output being a function of a second component of the field of an order higher than that of said first component; means responsive to said first and second outputs to derive third and fourth outputs, said third output being a function of the magnitude of a magnetic moment of a magnetic field, if any, to which said sensor means is exposed, and said fourth output being a function of the range from said sensor means to said magnetic moment.

Claim 25

In a magnetic surveillance system for selective detection of magnetic moments of a predetermined magnitude within a surveillance region: sensor means to be disposed proximate the region to be monitored for sensing the magnetic field to which said sensor is exposed to derive first and second outputs, said first output being a function of a first component of the magnetic field of a certain order, said second output being a function of a second component of the field of an order higher than that of said first component; means responsive to said first and second outputs to derive third and fourth outputs, said third output being a function of the magnitude of a magnetic moment of a magnetic field, if any, to which said sensor means is exposed, and said fourth output being a function of the range from said sensor means to said magnetic moment.