Metal detector having digital signal processing

• US 4,709,213 A
• Filed: 07/08/1985
• Issued: 11/24/1987
• Est. Priority Date: 07/23/1982
• Status: Expired due to Term

• Current Assignee: Garrett Electronics Incorporated
• Original Assignee: Garrett Electronics Incorporated
• Inventors: Podhrasky, Robert J.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US06/752,018
• Time in Patent Office: 869 Days
• Field of Search: 324/225, 324/233, 324/239, 324/326-329, 324/339, 324/115, 324/131, 340/347 CC, 340/347 M
• US Class Current: 324/329
• CPC Class Codes: G01V 3/105 forming directly coupled primary and secondary coils or loops
• CPC Class Codes: H03K 17/9512 using digital techniques
• CPC Class Codes: H03K 17/9525 controlled by an oscillatory signal H03K17/9537 takes precedence

Abstract

A metal detector circuit includes a transmit coil (12) and a receive coil (56) arranged in a balanced induction configuration in an electromagnetic field. The receive signal from the receive coil (56) is input to electronic switches (146, 148) which receive quadrature reference inputs from a phase shift circuit (112). The phase demodulated outputs of the switches (146, 148) are passed through amplifiers and input to an analog-to-digital converter (324) to produce digital signal samples which are transmitted through a bus (326). The bus (326) is connected to random access memory (414, 416) and a read only memory (418) which includes a stored signal processing program. A microprocessor (374) is connected to the bus (376) for receiving the digital signal samples and the stored program from memory (418). The microprocessor (374) executes the stored signal processing program to produce a digital output signal which is transmitted through the bus (326) to a digital-to-analog converter (360). The converter (360) produces an analog output signal which is passed to an output driver circuit which produces a target indication signal at a speaker (284). The digital signal processing provided by the microprocessor (374) includes concurrent ground cancellation and discrimination without the need for operator selection of these functions. The digital output produced by the microprocessor (374), upon detection of an object in the electromagnetic field, can be displayed in digital readouts (474-482).

Claims

1. A metal detector circuit, comprising:

means for generating an oscillatory signal;

a transmit coil connected to receive said oscillatory signal and generate an electromagnetic field therefrom;

a receive coil located in said electromagnetic field for producing a receive signal due to said electromagnetic field;

means for phase demodulating said receive signal with respect to the phase of said oscillatory signal to produce a phase demodulated signal;

means for digitizing said phase demodulated signal to produce digital signal samples, said means for digitizing having a predetermined amplitude input range for signals received thereby;

digital signal processing means connected to receive said digital signal samples and produce therefrom an output signal in accordance with a stored signal processing program to indicate detection of an object;

means responsive to a digital offset signal produced by said digital signal processing means for generating a corresponding analog offset signal;

means for combining said analog offset signal with said phase demodulated signal;

said digital signal processing means producing said digital offset signal as a fixed amplitude signal when the amplitude of said phase demodulated signal goes beyond said predetermined input range for said means for digitizing as indicated by said digital signal samples received from said means for digitizing, said analog signal driving the signal provided to said means for digitizing to within said predetermined input range; and

means for generating a target indication signal in response to said output signal.

2. A metal detector circuit as recited in claim 1 wherein said digital signal processing means comprises:

a read only memory having said stored signal processing program therein; and

a microprocessor connected to receive said digital signal samples and connected to said read only memory for executing said stored signal processing program.

3. A metal detector circuit as recited in claim 2 wherein said digital signal processing means includes:

a random access memory connected to said microprocessor; and

a digital-to-analog converter connected to said microprocessor for producing said output signal.

4. A metal detector circuit as recited in claim 1 wherein said means for generating a target identification signal comprises a tone generator circuit which is modulated by said output signal.

5. A metal detector circuit as recited in claim 1 wherein said means for generating a target indication signal comprises a digital display driven by said output signal.

6. A metal detector circuit as recited in claim 1 including means for storing an offset voltage to offset the amplitude of said digital signal samples.

7. A metal detector circuit, comprising:

means for generating an oscillatory signal;

a transmit coil connected to receive said oscillatory signal and generate an electromagnetic field therefrom;

a receive coil located in said electromagnetic field for producing a receive signal due to said electromagnetic field;

first and second phase demodulators having said receive signal and said oscillatory signal provided thereto for producing respective first and second phase demodulated signals;

means for providing first and second relative phase relations between the receive signal and the oscillatory signal provided respectively to said first and second phase demodulators;

means for digitizing said phase demodulated signals to produce digital signal samples, said means for digitizing having a predetermined amplitude input range for signals received thereby;

digital signal processing means for receiving and processing said digital signal samples in accordance with a stored signal processing program to produce a digital output signal;

means responsive to a digital offset signal produced by said digital signal processing means for generating a corresponding analog offset signal;

means for combining said analog offset signal with either of said phase demodulated signals;

said digital signal processing means producing said digital offset signal as a fixed amplitude signal for combining with one of said phase demodulated signals when the amplitude of said one phase demodulated signal goes beyond said predetermined input range for said means by digitizing as indicated by said digital signal samples received from said means for digitizing, said analog offset signal driving the signal provided to said means for digitizing to within said predetermined input range; and

means for producing a digital display output in response to said digital output signal, said response for indicating detection of an object in said electromagnetic field.

8. A metal detector circuit as recited in claim 7 including:

means for converting said digital output signal into an analog output signal; and

tone generator means responsive to said analog output signal for generating an audio response to indicate detection of an object in said electromagnetic field.

9. A metal detector circuit as recited in claim 7 including first and second amplifiers connected respectively to the outputs of said phase demodulators for producing third and fourth phase demodulated signals which are input to said means for digitizing.

10. A metal detector circuit as recited in claim 7 including a keyboard for entering parameters into said digital signal processing means.

11. A metal detector circuit as recited in claim 7 wherein said digital signal processing means includes:

a read only memory for storing said signal processing program;

a microprocessor for executing said signal processing program; and

a random access memory for storing information for use by said microprocessor.

12. A metal detector circuit as recited in claim 7 including means for storing offset voltages to offset the amplitudes of said first and said second phase demodulated signals.

13. A metal detector circuit, comprising:

an oscillator for generating a transmit signal,

a transmit coil connected to receive said transmit signal for generating an electromagnetic field,

a receive coil located in said electromagnetic field for producing a receive signal through inductive coupling from said transmit coil,

means connected to receive said transmit signal and said receive signal for phase demodulating said receive signal to produce first and second phase demodulated signals,

means for digitizing said first and second phase demodulated signals to produce digital signal samples said means for digitizing having a predetermined amplitude input range for signals received thereby,

a digital communication bus connected to receive said digital signal samples from said means for digitizing;

a digital memory connected to said communication bus, said memory including a stored program therein,

a microprocessor connected to said communication bus, said microprocessor executing said stored program to process said digital signal samples and generate a digital output signal to indicate detection of an object in said electromagnetic field,

a digital-to-analog converter connected to receive digital offset signals from said microprocessor for producing respective analog offset signals,

means for combining said analog offset signals with said phase demodulation signals,

said microprocessor producing one of said digital offset signals as a fixed amplitude signal when one of said phase demodulated signals exceeds said predetermined input range for said means for digitizing as indicated by said digital signal samples received from said means for digitizing, said one analog offset signal driving the signal provided by said means for digitizing to within said predetermined input range; and

output means connected to said microprocessor for receiving said digital output signal and generating therefrom a target indication signal.

14. A metal detector circuit as recited in claim 13 wherein said first and second phase demodulated signals represent quadrature phase components of said receive signal.

15. A metal detector circuit, comprising:

an oscillator for generating a transmit signal,

a transmit coil for receiving said transmit signal to generate an electromagnetic field,

a receive coil in said electromagnetic field for producing a receive signal through inductive coupling from said transmit coil,

a first switch having a signal input connected to receive said receive signal and a control input connected to receive said transmit signal,

a second switch having a signal input connected to receive said receive signal and a control input connected to receive said transmit signal,

means for providing a first relative phase shift between said transmit signal and said receive signal provided to said first switch and for providing a second relative phase shift between said transmit signal and said receive signal provided to said second switch, wherein said receive signal is phase demodulated by operation of said switches to produce first and second demodulation signals respectively at the outputs of said first and said second switches,

means for digitizing said first and said second demodulation signals to produce digital signal samples, said means for digitizing having a predetermined amplitude input range for said signals received thereby,

a digital bus connected to said means for digitizing for conveying said digital samples,

a digital memory connected to said bus, said memory having a signal processing program stored therein,

a microprocessor connected to said bus for receiving said digital signal samples and executing said signal processing program to generate digital offset signals having fixed amplitudes and to generate a digital output upon detection of an object in said field,

a digital-to-analog converter connected to said bus for receiving said digital output for generating therefrom an analog output signal and for receiving said digital offset signals for generating therefrom respective analog offset signals,

means for combining said analog offset signals with said phase demodulation signals,

said microprocessor producing one of said digital offset signals when one of said phase demodulated signals exceeds said predetermined input range for said means for digitizing as indicated by said digital signal samples received from said means for digitizing, said analog offset signals driving the signals provided to said means for digitizing to within said predetermined input range, and

output means connected to said microprocessor for receiving said digital output signal and generating therefrom a target indication signal.

16. A method of operation for a metal detector comprising the steps of:

generating an electromagnetic field;

producing a receive signal derived from said electromagnetic field wherein said receive signal is variable dependent upon the presence of ferrous and/or conductive material in said electromagnetic field;

phase demodulating said receive signal with respect to the phase of said electromagnetic field to produce a phase demodulated signal;

digitizing said phase demodulated signal to produce digital signal samples, when said phase demodulated signal is within a predetermined input amplitude range;

processing said digital signal samples in accordance with a stored signal processing program to produce an output signal to indicate detection of an object in said electromagnetic field,

generating a digital offset signal in response to said processing of said digital signal samples in accordance with said stored signal processing program, when the amplitude of said phase demodulated signal exceeds said predetermined input amplitude range,

generating an analog offset signal in response to said digital offset signal,

combining said analog offset signal with said phase demodulated signal prior to said step of digitizing, said analog offset signal for driving said phase demodulated signal to within said predetermined input range; and

driving an output circuit with said output signals to produce a response for indicating detection of an object in said electromagnetic field.

17. A method as recited in claim 16 wherein said step of driving an output circuit to produce a response comprises producing a digital display and generating an audio tone.

18. A method as recited in claim 16 including a step of phase demodulating said receive signal with respect to a reference signal that is phase related to said electromagnetic field.

19. A method as recited in claim 16 including a step of generating an offset signal which is used to offset the amplitude of said digital signal samples.

20. A method as recited in claim 16 wherein the step of generating digital signal samples comprises phase demodulating said receive signal and digitizing said phase demodulated receive signal.

21. A method as recited in claim 16 wherein the step of processing said digital signal samples comprises measuring the amplitude of said digital signal samples and generating said output signal to be proportional to the amplitude of said digital signal samples.

22. A method of operation for a metal detector, comprising the steps of:

generating an oscillatory signal;

generating an alternating electromagnetic field by passing said oscillatory signal through a coil;

producing a receive signal derived from said electromagnetic field wherein said receive signal is variable dependent upon the presence of ferrous and/or conductive material in said electromagnetic field;

phase demodulating said receive signal with respect to a first reference signal derived from said oscillatory signal to produce a first phase demodulated signal;

phase demodulating said receive signal with respect to a second reference signal derived from said oscillatory signal to produce a second phase demodulated signal;

digitizing said first and second said phase demodulated signals to produce digital signal samples when said phase demodulated signals are within a predetermined amplitude input range;

processing said digital samples in accordance with a stored signal processing program to produce an output signal to indicate detection of an object;

generating first and second digital offset signals by operation of said signal processing program in response to said digital signal samples when the amplitude of said demodulated signals exceeds said predetermined amplitude input range,

generating analog offset signals in response to said digital offset signals,

combining respectively said analog offset signals with said phase demodulated signals prior to said step of digitizing;

said analog offset signals for driving said respective phase demodulated signals to within said predetermined input range; and

driving an output circuit with said output signal to produce a response, said response for indicating detection of an object in said electromagnetic field.

23. A method of operation for metal detector as recited in claim 22 wherein said first and said second reference signals are in phase quadrature.

24. A method of operation for a metal detector as recited in claim 22 wherein the step of processing said digital signal samples comprises measuring the amplitudes of said samples and generating said output signal as a function of the amplitude of said digital signal samples.

25. A method of operation for a metal detector as recited in claim 22 wherein the step of processing said digital signal samples comprises measuring the ratio of the amplitudes of said first and said second phase demodulated signals and generating said output signal when said measured ratio is within a preset range.

26. A method of operation for a metal detector as recited in claim 22 including the steps of:

determining a ground cancel coefficient by comparing a first reading of said phase demodulated signals taken when said field does not include mineralized ground to a second reading of said phase demodulated signals when said field includes mineralized ground; and

altering said digital signal samples by said ground cancel coefficient to eliminate mineralized ground effects from said output signal.

27. A method of operation for a metal detector as recited in claim 22 including the steps of:

receiving an auto-tune command; and

determining the amplitudes of a selected set of said digital signal samples for said first and said second phase demodulated signals and selecting offset quantities which when added to said selected set of digital signal samples produces a null condition for said output signal.