Metal detector employing static discrimination

• US 20030160617A1
• Filed: 02/14/2003
• Published: 08/28/2003
• Est. Priority Date: 02/11/2002
• Status: Active Grant

• Current Assignee: Garrett Electronics Incorporated
• Original Assignee: White's Electronics Incorporated
• Inventors: Rowan, Mark D.
• Granted Patent: US 6,911,823 B2
• Time in Patent Office: Days
• US Class Current: 324/329
• CPC Class Codes: G01V 3/104 using several coupled or uncoupled coils G01V3/101 takes precedence

Abstract

A metal detector employing static discrimination. Within the scope of the invention is a two-frequency method and apparatus for discriminating between metal objects and ground. Also within the scope of the invention is a three-frequency method and apparatus for discriminating between ferrous and non-ferrous metal objects. The methods and apparatus do not require movement of the search head of the metal detector.

Claims

1. A method for characterizing a metal object, comprising the steps of:

interrogating the metal object with a first sinusoidal frequency component at a first frequency and a second sinusoidal frequency component at a second frequency, said first and second frequencies being distinct from one another;

determining first and second response frequency components corresponding to said first and second frequency components; and

characterizing the metal object by comparing said first response frequency component with said second response frequency component.

2. The method of claim 1, wherein said comparing is by forming a ratio of said first and second response frequency components.

3. The method of claim 2, further comprising comparing said ratio to reference values for said ratio determined for a plurality of reference metal objects, to identify the metal object.

4. The method of claim 1, for further discriminating between the metal object and ground, the method further comprising:

interrogating the ground with a third sinusoidal frequency component at said first frequency and a fourth sinusoidal frequency component at said second frequency;

determining third and fourth response frequency components corresponding to said third and fourth sinusoidal frequency components; and

adjusting said first and second response frequency components with information obtained from said third and fourth response frequency components so as to cancel the effect of the ground.

5. The method of claim 4, wherein said response frequency components each have W and Z response frequency sub-components where W and Z define real and imaginary axes of a two-dimensional coordinate system, wherein the method further comprises:

determining an A factor for multiplying the W response sub-component for the second sinusoidal response frequency component so that it equals the W response sub-component for the first sinusoidal response frequency component;

determining a B factor for multiplying the Z response sub-component for the second sinusoidal response frequency component so that it equals the Z response sub-component for the first sinusoidal response frequency component;

determining a C factor for multiplying the W response sub-component for the fourth sinusoidal response frequency component so that it equals the W response sub-component for the third sinusoidal response frequency component;

determining a D factor for multiplying the Z response sub-component for the fourth sinusoidal response frequency component so that it equals the Z response sub-component for the third sinusoidal frequency response component; and

computing the arctangent of the ratio defined by the quantity (B-D) multiplied by the Z response sub-component for the second sinusoidal response frequency component divided by the quantity (A-C) multiplied by the W response sub-component for the second sinusoidal response frequency component.

6. A method for discriminating between ferrous and non-ferrous metal objects, comprising the steps of:

interrogating an unknown metal object with a first sinusoidal frequency component at a first frequency, a second sinusoidal frequency component at a second frequency, and a third sinusoidal frequency component at a third frequency, said first, second and third frequencies being distinct from one another;

determining first, second and third response frequency components corresponding to said first, second and third frequency components;

determining a first phase shift from said first and second response frequency components corresponding to changing interrogation from said first frequency to said second frequency;

determining a second phase shift from one of said first and second response frequency components and said third response frequency component corresponding, respectively, to changing interrogation from one of said first and second frequencies to said third frequency; and

characterizing the metal object as being ferrous or non-ferrous by use of the first phase shift and the second phase shift.

7. The method of claim 6, further comprising comparing the first and second phase shifts with corresponding reference phase shifts for known metal objects including at least one ferrous metal object and at least one non-ferrous metal object.

8. The method of claim 7, wherein, if the first phase shift matches a first corresponding reference phase shift for a non-ferrous metal object, the method further comprises checking whether the second phase shift also matches a second corresponding phase shift for the non-ferrous metal object and determining that the unknown metal object is non-ferrous if so.

9. The method of claim 7, wherein, if the first phase shift matches a first corresponding reference phase shift for a ferrous metal object, the method further comprises checking whether the second phase shift also matches a second corresponding phase shift for the ferrous-metal object and determining that the unknown metal object is ferrous if so.

10. The method of claim 7, wherein, if the first phase shift matches first corresponding reference phase shifts for both ferrous and non-ferrous metal objects, the method further comprises comparing the second phase shift with second corresponding reference phase shifts for the same ferrous and non-ferrous metal objects and thereby resolving the ambiguity.

11. The method of claim 7, wherein the unknown metal object is in proximity with ground, and the method further comprising:

interrogating the ground with a fourth sinusoidal frequency component at said first frequency and a fifth sinusoidal frequency component at said second frequency,

determining fourth and fifth response frequency components corresponding to said fourth and fifth frequency components; and

adjusting said first and second response frequency components with information obtained from said fourth and fifth response frequency components so as to cancel the effect of the ground on the first phase shift.

12. The method of claim 11, wherein said response frequency components each have W and Z response frequency sub-components where W and Z define real and imaginary axes of a two-dimensional coordinate system, wherein the method further comprises:

determining an A factor for multiplying the W response sub-component for the second sinusoidal response frequency component so that it equals the W response sub-component for the first sinusoidal response frequency component;

determining a B factor for multiplying the Z response sub-component for the second sinusoidal response frequency component so that it equals the Z response sub-component for the first sinusoidal response frequency component;

determining a C factor for multiplying the W response sub-component for the fourth sinusoidal response frequency component so that it equals the W response sub-component for the third sinusoidal response frequency component;

determining a D factor for multiplying the Z response sub-component for the fourth sinusoidal response frequency component so that it equals the Z response sub-component for the third sinusoidal frequency response component; and

computing the arctangent of the ratio defined by the quantity (B-D) multiplied by the Z response sub-component for the second sinusoidal response frequency component divided by the quantity (A-C) multiplied by the W response sub-component for the second sinusoidal response frequency component.

13. The method of claim 12, further comprising interrogating the ground with a sixth sinusoidal frequency component at said third frequency, determining a sixth response frequency component corresponding to said sixth frequency components, and adjusting said first, second and third response frequency components with information obtained from said fourth, fifth and sixth response frequency components so as to cancel the effect of the ground on the first and second phase shifts.

14. An apparatus for discriminating between a metal object and ground, comprising:

an interrogating portion for interrogating the metal object with a first sinusoidal frequency component at a first frequency and a second sinusoidal frequency component at a second frequency, said first and second frequencies being distinct from one another;

a sampling portion for determining first and second response frequency components corresponding to said first and second frequency components; and

a processing portion for characterizing the metal object by comparing said first response frequency component with said second response frequency component.

15. The apparatus of claim 14, wherein said processing portion is further adapted so that said comparing is by forming a ratio of said first and second response frequency components.

16. The apparatus of claim 15, wherein said processing portion is further adapted to compare said ratio to reference values for said ratio determined for a plurality of reference metal objects, to identify the metal object.

17. The apparatus of claim 14, for further discriminating between the metal object and ground, wherein said interrogating portion is further adapted to interrogate the ground with a third sinusoidal frequency component at said first frequency and a fourth sinusoidal frequency component at said second frequency, wherein said sampling portion is further adapted to determine third and fourth response frequency components corresponding to said third and fourth sinusoidal frequency components, and wherein said processing portion is further adapted to adjust said first and second response frequency components with information obtained from said third and fourth response frequency components so as to cancel the effect of the ground.

18. The apparatus of claim 17, wherein said response frequency components each have W and Z response frequency sub-components where W and Z define real and imaginary axes of a two-dimensional coordinate system, wherein said processing portion is further adapted to determine an A factor for multiplying the W response sub-component for the second sinusoidal response frequency component so that it equals the W response sub-component for the first sinusoidal response frequency component, determine a B factor for multiplying the Z response sub-component for the second sinusoidal response frequency component so that it equals the Z response sub-component for the first sinusoidal response frequency component, determine a C factor for multiplying the W response sub-component for the fourth sinusoidal response frequency component so that it equals the W response sub-component for the third sinusoidal response frequency component, determine a D factor for multiplying the Z response sub-component for the fourth sinusoidal response frequency component so that it equals the Z response sub-component for the third sinusoidal frequency response component, and compute the arctangent of the ratio defined by the quantity (B-D) multiplied by the Z response sub-component for the second sinusoidal response frequency component divided by the quantity (A-C) multiplied by the W response sub-component for the second sinusoidal response frequency component.

19. An apparatus for discriminating between ferrous and non-ferrous metal objects, comprising:

an interrogating portion for interrogating an unknown metal object with a first sinusoidal frequency component at a first frequency, a second sinusoidal frequency component at a second frequency, and a third sinusoidal frequency component at a third frequency, said first, second and third frequencies being distinct from one another;

a sampling portion for determining first, second and third response frequency components corresponding to said first, second and third frequency components; and

a processing portion for determining a first phase shift from said first and second response frequency components corresponding to changing interrogation from said first frequency to said second frequency, determining a second phase shift from one of said first and second response frequency components and said third response frequency component corresponding, respectively, to changing interrogation from one of said first and second frequencies to said third frequency, and characterizing the metal object as being ferrous or non-ferrous by use of the first phase shift and the second phase shift.

20. The apparatus of claim 19, wherein said processing portion is further adapted to compare the first and second phase shifts with corresponding reference phase shifts for known metal objects including at least one ferrous metal object and at least one non-ferrous metal object.

21. The apparatus of claim 20, wherein said processing portion is further adapted, if the first phase shift matches a first corresponding reference phase shift for a non-ferrous metal object, to check whether the second phase shift also matches a second corresponding phase shift for the non-ferrous metal object and determine that the unknown metal object is non-ferrous if so.

22. The apparatus of claim 20, wherein said processing portion is further adapted, if the first phase shift matches a first corresponding reference phase shift for a ferrous metal object, to check whether the second phase shift also matches a second corresponding phase shift for the ferrous-metal object and determine that the unknown metal object is ferrous if so.

23. The apparatus of claim 20, wherein said processing portion is further adapted, if the first phase shift matches first corresponding reference phase shifts for both ferrous and non-ferrous metal objects, to compare the second phase shift with second corresponding reference phase shifts for the same ferrous and non-ferrous metal objects and thereby resolve the ambiguity.

24. The apparatus of claim 20, wherein the unknown metal object is in proximity with ground, wherein said interrogating portion is further adapted to interrogate the ground with a fourth sinusoidal frequency component at said first frequency and a fifth sinusoidal frequency component at said second frequency, wherein said sampling portion is further adapted to determine fourth and fifth response frequency components corresponding to said fourth and fifth frequency components, and wherein said processing portion is further adapted to adjust said first and second response frequency components with information obtained from said fourth and fifth response frequency components so as to cancel the effect of the ground on the first phase shift.

25. The apparatus of claim 24, wherein said response frequency components each have W and Z response frequency sub-components where W and Z define real and imaginary axes of a two-dimensional coordinate system, wherein said processing portion is further adapted to determine an A factor for multiplying the W response sub-component for the second sinusoidal response frequency component so that it equals the W response sub-component for the first sinusoidal response frequency component, determine a B factor for multiplying the Z response sub-component for the second sinusoidal response frequency component so that it equals the Z response sub-component for the first sinusoidal response frequency component, determine a C factor for multiplying the W response sub-component for the fourth sinusoidal response frequency component so that it equals the W response sub-component for the third sinusoidal response frequency component, determine a D factor for multiplying the Z response sub-component for the fourth sinusoidal response frequency component so that it equals the Z response sub-component for the third sinusoidal frequency response component, and compute the arctangent of the ratio defined by the quantity (B-D) multiplied by the Z response sub-component for the second sinusoidal response frequency component divided by the quantity (A-C) multiplied by the W response sub-component for the second sinusoidal response frequency component.

26. The apparatus of claim 25, wherein said interrogating portion is further adapted to interrogate the ground with a sixth sinusoidal frequency component at said third frequency, wherein said sampling portion is further adapted to determine a sixth response frequency component corresponding to said sixth frequency components, and wherein said processing portion is further adapted to adjust said first, second and third response frequency components with information obtained from said fourth, fifth and sixth response frequency components so as to cancel the effect of the ground on the first and second phase shifts.