Pulse induction time domain metal detector
First Claim
Patent Images
1. A conducting metal detection apparatus comprising:
- transmission means including a transmit coil for transmitting a discontinuous pulse voltage waveform in order to transmit a magnetic field to a target volume, the discontinuous pulse voltage waveform providing periods of non-transmission of the magnetic field;
a detector coil for producing a detected signal by detecting changes in magnetic fields;
measurement means for measuring the detected signal during at least a first and second pre-determined period which is separate from that period which has significant signal resulting from decay of ground eddy currents, the measuring occurring during the periods of non-transmission of the magnetic field, the measurement means producing at least a first and a second measurement respectively; and
processing means for processing at least two of the measurements to provide an output signal derived by substantially removing that component of the detected signal due to electrically non-conducting ferrite within the target volume from the said detected signal by forming a combination of the at least two measurements, said combination being dependent upon the first time derivative of the magnetic field due to the electrically non-conducting ferrite during the periods of non-transmission, wherein the first time derivative of the magnetic field is dependent upon the duration and temporal evolution of the transmitted magnetic field, the output signal thus being useful to indicate the presence of a metallic object within the target volume.
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Abstract
This invention is for a pulse induction metal detector which uses combinations of the signal detected during non-transmission of the magnetic field to eliminate those contributions to the signal due to the remanent magnetization of ferrites and asynchronous magnetic fields within the target volume.
62 Citations
37 Claims
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1. A conducting metal detection apparatus comprising:
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transmission means including a transmit coil for transmitting a discontinuous pulse voltage waveform in order to transmit a magnetic field to a target volume, the discontinuous pulse voltage waveform providing periods of non-transmission of the magnetic field; a detector coil for producing a detected signal by detecting changes in magnetic fields; measurement means for measuring the detected signal during at least a first and second pre-determined period which is separate from that period which has significant signal resulting from decay of ground eddy currents, the measuring occurring during the periods of non-transmission of the magnetic field, the measurement means producing at least a first and a second measurement respectively; and processing means for processing at least two of the measurements to provide an output signal derived by substantially removing that component of the detected signal due to electrically non-conducting ferrite within the target volume from the said detected signal by forming a combination of the at least two measurements, said combination being dependent upon the first time derivative of the magnetic field due to the electrically non-conducting ferrite during the periods of non-transmission, wherein the first time derivative of the magnetic field is dependent upon the duration and temporal evolution of the transmitted magnetic field, the output signal thus being useful to indicate the presence of a metallic object within the target volume. - View Dependent Claims (2, 3, 4, 5, 6, 7, 33)
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8. A conducting metal detection apparatus comprising:
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transmission means for transmitting a discontinuous pulse voltage waveform to provide a magnetic field in a target volume, the discontinuous pulse voltage waveform providing periods of non-transmission of the magnetic field; a detector coil for producing a detected signal by detecting changes in the magnetic field; measurement means for measuring the detected signal within a time interval which is separate from a period having a significant signal resulting from decay of ground eddy currents, wherein the measuring occurs during the periods of non-transmission of the magnetic field; means for synchronously demodulating the detected signals such that a first demodulated signal is derived from a first period following a transition in the transmission of the magnetic field and a second demodulated signal is derived from a second period following a transition in the transmission of the magnetic field, both first and second periods being subsequent to a time necessary for substantial decay of ground eddy currents, the demodulation occurring during the period of non-transmission; and processing means for processing the demodulated signals by comparing respective magnitudes of the demodulated signals to provide an output signal substantially independent of a background environment having a substantial quantity of material with a substantial magnetic effect and a reactive to resistive response ratio which is substantially independent of a transmitted frequency below 100 kHz, wherein the transmission means provides a transmit signal which consists of a sequence of pulses repeated continuously for a selected time in which at least one of the pulses in the sequence is of a different period than that of least one other pulses in the sequence and the detected signals being synchronously demodulated with respect to the transmitted repetitively pulse magnetic field, said output signal being dependent upon the first time derivative of the electrically non-conducting ferrite during periods of non-transmission, the first time derivative of the magnetic field being dependent upon the duration and temporal evolution of the magnetic field, the output signal thus being useful to indicate the presence of a metal object within the target volume. - View Dependent Claims (9, 10, 11, 12, 34)
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13. A method of conducting metal detection comprising the steps of:
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transmitting a discontinuous pulse voltage waveform to form a magnetic field in a target volume, the discontinuous pulse voltage waveform providing periods of non-transmission of the magnetic field; detecting changes in magnetic fields, the changes being dependent upon the time duration of the transmitted pulse voltage waveform; providing a detected signal indicative of the changes; measuring the detected signal within at least a first and a second time interval which is separate from a period having a significant signal resulting from decay of ground eddy currents wherein the measuring occurs during the periods of non-transmission of the magnetic field the measuring being further adapted to produce a first and a second measurement respectively; and processing at least two of the measurements to provide an output signal derived by substantially removing that component of the detected signal due to the electrically non-conducting ferrite constituents in the target volume from the said detected signal by forming a combination of the at least two measurements, said combination being dependent upon the first time derivative of the electrically non-conducting ferrite during periods of non-transmission, wherein the first time derivative of the magnetic field is dependent upon the duration and temporal evolution of the magnetic field, the output signal thus being useful to indicate the presence of a metal object within the target volume. - View Dependent Claims (14, 15, 16, 35)
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17. A method of conducting metal detection comprising the steps of:
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transmitting a discontinuous pulse voltage waveform having pulses of both positive and negative magnitudes for predetermined durations such that a magnetic field is produced in a target volume and has a net time average value of substantially zero; detecting changes in magnetic fields, the changes being dependent upon the time duration of the transmitted pulse voltage waveform; providing a detected signal indicative of the changes; measuring average magnitudes of the detected signal during at least a first and a second selected period of time and within a time interval separate from a period having a significant signal resulting from decay of ground eddy currents; and processing a plurality of the measurements by multiplying the magnitude of at least one of the measurements and subtracting the so multiplied magnitude from the magnitude of at least one of the other measurements to provide an output signal derived by substantially removing that component of the detected signal due to the electrically non-conducting ferrite constituents in the target volume from the said detected signal by forming a linear combination of the at least two measurements, said combination being dependent on the first time derivative of the electrically non-conducting ferrite constituents, wherein the first time derivative of the magnetic field is dependent on the duration and temporal evolution of the magnetic field, the output signal thus being useful to indicate the presence of a metal object within the target volume. - View Dependent Claims (18, 27, 28, 29, 31, 36)
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19. A method of conducting metal detection comprising the steps of:
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generating and applying to a transmit coil a discontinuous pulse voltage waveform to form a magnetic field in a target volume, whether the discontinuous pulse voltage waveform provides periods of non-transmission to the magnetic field; detecting changes in magnetic fields, the changes being dependent upon the duration of the transmitted pulse voltage waveform; providing a detected signal indicative of the changes; synchronously demodulating the detected signals during non-transmission periods of the magnetic field to provide a first demodulated signal derived from a first period during one non-transmission period and a second demodulated signal derived from a second period during same or another non-transmission period such that both first and second periods begin a delay time after the start of the respective non-transmission periods, the delay time being of a duration necessary for substantial decay of ground eddy currents; and processing the demodulated signals by comparing the demodulated signals to provide an output signal substantially free of that component of the detected signal due to electrically non-conducting ferrite constituents in the target volume with a substantial effect and a reactive to resistive response ratio which is substantially independent of an interrogating frequency below 100 kHz from the said detected signal by forming a linear combination of the demodulated signals, said combination being dependent on the first time derivative of the electrically non-conducting ferrite constituents, wherein the first time derivative of the magnetic field is dependent on the duration and temporal evolution of the magnetic field, the output signal thus being useful to indicate the presence of a metal object within the target volume. - View Dependent Claims (20, 21, 22, 23, 24, 25, 26, 30, 32, 37)
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Specification