DIGITAL NULLING PULSE INDUCTIVE METAL DETECTOR
First Claim
1. A method of improving metal detection sensitivity in a pulse inductive metal detector, the metal detector including a transmitter coil configured to induce inductive target responses in a receiver coil, the method comprising:
- During a calibration mode (a) inducing one or more analog time-series signal responses of the receiver coil over an area known to be metal-free;
(b) digitizing the one or more analog time-series signal responses; and
(c) determining a digitized time-series nulling signal based on the one or more digitized time series signal responses from step (b); and
then During a normal operating mode used to detect metal targets (d) inducing an analog instant time-series signal response of the receiver coil over a target search area;
(e) converting the digitized nulling signal to an analog nulling signal; and
(f) synchronously subtracting the analog nulling signal from the analog instant time-series signal response to produce a time-series signal response having reduced adverse ground or transmitter coil transient responses.
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Accused Products
Abstract
A method of improving pulse inductive metal detector sensitivity by minimizing adverse ground responses and transmitter coil transients is disclosed. The method provides a calibration mode for a typical pulsed EMI metal detector. The purpose of the calibration mode is to determine and record a nulling signal representative of the transmitter coil coupling to the receiver coil and a ground response that has no metal. The nulling signal is then used during normal operation of the metal detector by combining it with the instant receiver coil signal in a difference amplifier. The difference amplifier effectively subtracts the nulling signal from the instant signal yielding a response signal that has removed the ground response that may be present in the instant signal. The metal detector can be periodically re-calibrated. It may also be re-calibrated upon discovery of a metal target to provide the most up to date nulling signal for the ground around the metal target.
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Citations
11 Claims
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1. A method of improving metal detection sensitivity in a pulse inductive metal detector, the metal detector including a transmitter coil configured to induce inductive target responses in a receiver coil, the method comprising:
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During a calibration mode (a) inducing one or more analog time-series signal responses of the receiver coil over an area known to be metal-free;
(b) digitizing the one or more analog time-series signal responses; and
(c) determining a digitized time-series nulling signal based on the one or more digitized time series signal responses from step (b); and
thenDuring a normal operating mode used to detect metal targets (d) inducing an analog instant time-series signal response of the receiver coil over a target search area;
(e) converting the digitized nulling signal to an analog nulling signal; and
(f) synchronously subtracting the analog nulling signal from the analog instant time-series signal response to produce a time-series signal response having reduced adverse ground or transmitter coil transient responses. - View Dependent Claims (2, 3, 4, 5, 6)
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7. A pulse inductive metal detector including a transmitter coil configured to induce inductive target responses in a receiver coil, comprising:
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(a) means for inducing, in a calibration mode, one or more analog time-series signal responses of the receiver coil over an area known to be metal-free;
(b) means for digitizing, in the calibration mode, the one or more analog time-series signal responses; and
(c) means for determining, in the calibration mode, a digitized time-series nulling signal based on the one or more digitized time series signal responses from step (b);
(d) means for inducing, in a normal operating mode, an analog instant time-series signal response of the receiver coil over a target search area;
(e) means for converting, in the normal operating mode, the digitized nulling signal to an analog nulling signal; and
(f) means for synchronously subtracting, in the normal operating mode, the analog nulling signal from the analog instant time-series signal response to produce a time-series signal response having reduced adverse ground or transmitter coil transient responses.
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8. A pulse inductive metal detector capable of greater sensitivity and metal classification by minimizing adverse ground responses and transmitter coil transients, the pulse inductive metal detector comprising:
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a time-series nulling calibration circuit that operates on receiver coil signals when the pulse inductive metal detector is in a calibration mode, the time-series nulling calibration circuit comprising;
an analog to digital converter that receives a time-series receiver coil signal during calibration mode and converts it to a time-series digital signal; and
a digital-to-analog converter that is initially set to zero before receiving the calibration mode digitized receiver coil time-series signal converting it to an analog time-series nulling signal, wherein the pulse inductive metal detector stores the analog time-series nulling signal and applies it to a difference amplifier when the pulse inductive metal detector is operating in a normal mode such that the time-series nulling signal is synchronously subtracted from an instant time-series receiver coil signal yielding a time-series response signal that minimizes transmitter transient decay voltages and DC offset voltages. - View Dependent Claims (9, 10)
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11. A method of improving metal detection sensitivity in a pulse inductive metal detector, the metal detector including a transmitter coil configured to induce inductive target responses in a receiver coil, the method comprising:
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During a Coarse Calibration Mode (a) inducing multiple responses of the receiver coil in air;
(b) digitizing the multiple air responses;
(c) averaging the multiple digitized air responses into a digitized coarse nulling signal;
During a Fine Calibration Mode (d) inducing multiple responses of the receiver coil near a ground area absent a metal target;
(e) converting the digitized coarse nulling signal to an analog coarse nulling signal;
(f) subtracting the analog coarse nulling signal from each of the multiple ground responses from step (d), to produce multiple adjusted ground responses;
(g) digitizing each of the multiple adjusted ground responses; and
(h) averaging the multiple digitized adjusted ground responses into a digitized fine nulling signal;
During a Normal Operating Mode used to detect metal targets (i) inducing a signal response of the receiver coil over a target search area;
(j) converting the digitized coarse nulling signal and the digitized fine nulling signal respectively to an analog coarse nulling signal and an analog coarse nulling signal; and
(k) synchronously subtracting the analog coarse and fine nulling signals from the signal response from step (i) to reduce therein background noise and transmitter coil transient responses.
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Specification