Multiple frequency magnetic field technique for differentiating between classes of metal objects

Claim 2

Apparatus for detecting preselected classes of metal objects, comprising, an A.C. electromagnetic means including magnetizing means, a multi-frequency excitation circuit means operatively connected to said magnetizing means, said magnetizing means responding to excitation current signals from said multi-frequency excitation circuit means by developing at least first and second oscillating magnetic fields, said first oscillating magnetic field being a low frequency oscillating magnetic field and said second oscillating magnetic field being a high frequency oscillating magnetic field, said metal objects being subjected to said oscillating magnetic fields, detecting means for monitoring the induced magnetic fields developed by said metal objects being subjected to said low and high frequency oscillating magnetic fields and developing output signals representative thereof, said signals including components at each frequency which are in-phase and out-ofphase with said excitation current signals, and signal measuring and processing circuit means operatively connected to said detecting means, said signal measuring and processing means including first circuit means responding to said in-phase components at each frequency by generating first output signals indicative of energy loss attributable to said metal objects at each frequency, second circuit means responding to said out-of-phase components by generating second output signals indicative of whether said metal objects are ferromagnetic or nonferromagnetic, and third circuit means responsive to said first and second output signals for evaluating said signals and generating a third output signal indicative of the class of said metal objects.

Claim 3

Apparatus as claimed in claim 1 wherein said low frequency oscillating magnetic field being selected from a range of frequencies for which the skin depth of said induced magnetic field developed by said preselected class of metal objects is relatively large compared with the thickness, said high frequency oscillating magnetic field being selected from a range of frequencies for which the skin depth of said induced magnetic field developed by said preselected class of metal objects is relatively small compared to the thickness.

Claim 4

Apparatus as claimed in claim 2 wherein said magnetizing means includes a coil configuration having at least one magnetizing coil, and said detector means includes at least one detector coil for developing said output signals of said detector means as a function of the magnetic flux linking said metal objects and said detector coil at each of the frequencies of said low frequency magnetic field and said high frequency magnetic field.

Claim 5

Apparatus as claimed in claim 1 further including circuit means for substantially eliminating from the signals developed by said detector means the effect of the excitation signals of said multi-frequency excitation circuit means.

Claim 6

Apparatus as claimed in claim 2 wherein said magnetizing means includes three coil configurations operatively connected to said multi-frequency excitation circuit means to generate three substantially mutually orthogonal magnetic fields each comprised of a low and a high frequency oscillating magnetic field, said signal measuring and processing circuit means being operatively connected to said three coil configurations to provide detection and discrimination of metal objects passing through said magnetic fields regardless of orientation of said metal objects.

Claim 7

Apparatus as claimed in claim 5 wherein said three coil configurations are positioned about the path of travel of said metal objects and are disposed in a successive manner such that said metal objects are sequentially subjected to the magnetic fields of the respective coil configurations.

Claim 8

Apparatus as claimed in claim 5 wherein said three coil configurations are positioned about the path of travel of said metal objects such that the metaL objects are simultaneously subjected to the magnetic fields of the respective coil configurations.

Claim 9

Apparatus as claimed in claim 6 wherein the low and high frequency magnetic fields of each coil configuration are the same.

Claim 10

Apparatus as claimed in claim 7 wherein the low and high frequency magnetic fields of each coil configuration are different.

Claim 11

Apparatus as claimed in claim 1 wherein said out-of-phase components are approximately 90* out-of-phase with said excitation signals.

Claim 12

Apparatus as claimed in claim 1 wherein said oscillating magnetic fields are developed simultaneously.

Claim 13

Apparatus as claimed in claim 1 wherein said first circuit means determines the relative magnitudes of the signal components corresponding to energy loss at said low and high frequency, said first output signals representing the relationship of these magnitudes, said third circuit means responding to said first and second output signals for comparing said signals to predetermined values, said third output signal being generated if said first and second output signals establish a predetermined relationship with said predetermined values.

Claim 14

Apparatus for detecting preselected classes of metal objects, comprising, an A.C. electromagnetic means including magnetizing means, a multi-frequency excitation circuit means operatively connected to said magnetizing means, said magnetizing means responding to excitation current signals from said multi-frequency excitation circuit means by developing oscillating magnetic fields, said metal objects being subjected to said oscillating magnetic fields, said oscillating magnetic fields including at least one low frequency magnetic field and at least one high frequency magnetic field, detecting means for monitoring the induced magnetic field developed by said metal objects being subjected to said oscillating magnetic fields and developing output signals corresponding to said low and high frequency magnetic fields, and signal measuring and processing circuit means including first circuit means responding to said output signals of said detecting means produced by said low frequency oscillating magnetic field by developing a first output signal which is out-of-phase with said excitation current signals, second and third circuit means for developing second and third output signals indicative of components of said output signals of said detecting means which are produced by said low frequency and said high frequency oscillating magnetic fields respectively, said second and third output signals being in phase with said excitation current signals, said first output signal being an indication of whether said metal objects are ferromagnetic or nonferromagnetic, said second and third output signals being a measurement of the energy loss attributable to said metal objects at said low frequency and high frequency oscillating magnetic fields respectively, fourth circuit means responsive to said second and third output signals for determining the relative magnitudes of said second and third output signals and developing a fourth output signal indicative of this relationship, and object classification circuit means responsive to said first output signal and said fourth output signal for generating an object classification output signal indicative of a specific class of metal objects.

Claim 15

Apparatus for detecting preselected classes of metal objects, comprising, an A.C. electromagnetic means including magnetizing means, a multi-frequency excitation circuit means operatively connected to said magnetizing means, said magnetizing means responding to excitation current signals from said multi-frequency excitation circuit means by developing oscillating magnetic fields, said metal objects being subjected to said oscillating magnetic fields, said oscillating magnetic fields including at least one low frequency magnetic field and at least one high frequency magnetic field, detecting means for monitoring the induced magnetic field developed by said metal objects being subjected to said oscillating magnetic fields and developing output signals corresponding to said low and high frequency magnetic fields, and signal measuring and processing circuit means including first circuit means responding to said output signals of said detecting means produced by said low frequency oscillating magnetic field by developing a first output signal which is out-of-phase with said excitation current signals, second and third circuit means for developing second and third output signals indicative of components of said output signals of said detecting means which are produced by said low frequency and said high frequency oscillating magnetic fields respectively, said second and third output signals being in phase with said excitation current signals, said first output signal being an indication of whether said metal objects are ferromagnetic or nonferromagnetic, said second and third output signals being a measurement of the energy loss attributable to said metal objects at said low frequency and high frequency oscillating magnetic fields respectively, fourth circuit means responsive to said second and third output signals for determining the relative magnitudes of said second and third output signals and developing a fourth output signal indicative of this relationship, and object classification circuit means responsive to said first output signal and said fourth output signal for generating an object classification output signal indicative of a specific class of metal objects.

Claim 16

Apparatus as claimed in claim 13 wherein said specified class of metal objects is small arms, such as pistols, the frequency of said low frequency magnetic field being approximately 100 hertz, and the frequency of said high frequency magnetic field being approximately 1,000 hertz.

Claim 17

Apparatus as claimed in claim 13 wherein said fourth output signal is indicative of a ratio between the energy loss at the high frequency oscillating magnetic field and the energy loss at the low frequency oscillating magnetic field.

Claim 18

Apparatus as claimed in claim 13 wherein said second and third circuit means include a second and tHird phase lock amplifier circuit respectively, each having a first and second input and an output, said output signals from said detector means being supplied to said first inputs and a reference signal comprised of the low frequency and high frequency signals developed by said multi-frequency excitation circuit being supplied to said second inputs, said second phase lock amplifier circuit developing said second output signal, said third phase lock amplifier circuit developing said third output signal, said second and third output signals corresponding to the energy loss attributable to said metal objects at said low and high frequencies respectively.

Claim 19

Apparatus as claimed in claim 13 wherein said first circuit means further includes a first phase lock amplifier circuit having a first and second input and an output, said output signals from said detector means being supplied to said first input and said reference signal being supplied to said second input, said first phase lock amplifier circuit developing said first output signal.

Claim 20

Apparatus as claimed in claim 15 wherein said ratio for a class of metal objects corresponding to small arms is approximately 10:1.