DIGITAL NULLING PULSE INDUCTIVE METAL DETECTOR

• US 20050104595A1
• Filed: 09/16/2004
• Published: 05/19/2005
• Est. Priority Date: 09/23/2003
• Status: Active Grant

• Current Assignee: Johns Hopkins University
• Original Assignee: Johns Hopkins University
• Inventors: Nelson, Carl V.
• Granted Patent: US 6,927,577 B2
• Time in Patent Office: Days
• US Class Current: 324/327
• CPC Class Codes: G01V 13/00 Manufacturing, calibrating, cleaning, or repairing instruments or devices covered by groups G01V1/00 – G01V11/00

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.

Claims

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.

2. The method of claim 1 further comprising periodically repeating steps (a) through (c), thereby periodically determining the nulling signal.

3. The method of claim 1, wherein:

step (a) includes inducing multiple analog time-series signal responses of the receiver coil over an area known to be metal-free; and

step (c) comprises averaging the multiple digitized time series signal responses.

4. The method of claim 3, wherein said averaging comprises:

time-overlapping the multiple digitized time series signal responses; and

averaging magnitudes of time-aligned samples of the multiple digitized time series signal responses, to produce the digitized time-series nulling signal.

5. The method of claim 1 further comprising further amplifying the time-series response signal to assist in detecting small metal targets.

6. The method of claim 1, further comprising, between steps (c) and (d):

storing the digitized time-series nulling signal; and

then

accessing the stored digitized time-series nulling signal.

7. A pulse inductive metal detector including a transmitter coil configured to induce inductive target responses in a receiver coil, comprising:

(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.

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:

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.

9. The pulse inductive metal detector of claim 8 wherein the analog time-series nulling signal is periodically re-determined.

10. The pulse inductive metal detector of claim 8 further comprising additional gain amplifiers to amplify the time-series response signal to assist in detecting small metal targets.

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:

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.