Simultaneous time-domain and frequency-domain metal detector

• US 7,078,906 B2
• Filed: 09/16/2004
• Issued: 07/18/2006
• Est. Priority Date: 09/23/2003
• Status: Active Grant

• Current Assignee: Johns Hopkins University
• Original Assignee: Johns Hopkins University
• Inventors: Nelson, Carl V.
• Primary Examiner(s): Lefkowitz, Edward
• Assistant Examiner(s): Whittington, Kenneth J.
• Application Number:
  US10/942,146
• Publication Number:
  US 20050062477A1
• Time in Patent Office: 670 Days
• Field of Search: 324/207.26, 324326-329, 324/228, 324/234, 324/236, 324/336, 324/323, 324/239, 324/243, 324/207.17, 324/327, 340/568.1, 340/540, 340/686.6, 340/551
• US Class Current: 324/327
• CPC Class Codes: G01V 3/15 specially adapted for use during transport, e.g. by a person, vehicle or boat

Abstract

A metal detector capable of operating in both a time-domain mode and a frequency-domain mode includes a transmitter component and a receiver component. The transmitter component includes a transmitter coil that can receive both an AC and DC current source for frequency-and time-domain operation respectively. The AC signal is impressed on the DC signal such that while the transmitter coil is charging, frequency-domain sensing is occurring and when the transmitter coil is switched off, time-domain sensing is occurring. The receiver component uses a primary field bucking method to ensure that both time-and frequency-domain modes can operate with the receiver coil.

Claims

1. A metal detector configured to operate in both a time-domain mode and a frequency-domain mode, the metal detector comprising:

a transmitter comprised of;

a transmitter coil;

a generator for generating an oscillatory wave current signal to be applied to the transmitter coil for operating in the frequency-domain mode;

a direct current source for generating a current signal to be applied to the transmitter coil for operating in the time-domain mode;

means for combining the oscillatory wave current and direct current signals into a single composite transmitter current signal; and

a transmitter coil switch for switching the single transmitter current signal on and off to the transmitter coil, and

a receiver comprised of;

a magnetic field detector for sensing eddy currents in nearby metal targets;

first receiver components configured to operate in the time-domain mode to detect metal targets; and

second receiver components configured to operate in the frequency-domain mode to detect metal targets,

wherein the metal detector operates in

(i) the frequency-domain mode while the transmitter coil switch is on to charge the transmitter coil with the direct current signal, and

(ii) the time-domain mode while the transmitter coil switch is switched off.

2. The metal detector of claim 1, wherein the second receiver components include a synchronous detector.

3. The metal detector of claim 1 wherein the means for combining the oscillatory wave current and direct current signals into a single transmitter current signal is comprised of a summing amplifier.

4. The metal detector of claim 1 wherein the magnetic field detector for sensing eddy currents in nearby metal targets is comprised of a receiver coil in a balanced configuration with respect to the transmitter coil.

5. The metal detector of claim 1, wherein the first receiver components include:

an analog-to-digital converter (ADC) coupled to an output of the magnetic field detector;

a micro-computer coupled to an output of the ADC; and

an output indicator coupled to the micro-computer.

6. A method of operating a metal detector including a transmitter coil for inducing a detectable magnetic field indicative of a nearby metal target, comprising:

during a first time period

(a) applying to the transmitter coil a composite current signal including an oscillatory current signal and a DC current signal;

concurrently

(b) performing frequency domain metal detection based on a first magnetic field induced as a result of step (a); and

during a second time period following the first time period

(c) removing the composite current signal from the transmitter coil; and

then

(d) performing time domain metal detection based on a second magnetic field induced as a result of step (c).

7. The method of claim 6, further comprising repeating steps (a) through (d).