Metal detector and discriminator using differentiation for background signal suppression

• US 4,514,692 A
• Filed: 05/03/1982
• Issued: 04/30/1985
• Est. Priority Date: 05/03/1982
• Status: Expired due to Term

• Current Assignee: Fisher Research Labs, Inc.
• Original Assignee: FRL Incorporated (First Texas Holding Corporation)
• Inventors: Johnson, David E., Williams, Richard W.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US06/373,900
• Time in Patent Office: 1,093 Days
• Field of Search: 324/233, 324/326, 324/329, 340/38 L, 340/551, 340/567, 340/941, 328/165, 328/132, 328/142
• US Class Current: 324/329
• CPC Class Codes: G01V 3/105 forming directly coupled primary and secondary coils or loops

Abstract

Eddy current type metal detector for locating and distinguishing between metal objects of different types in the ground or other surroundings. Differentiation is employed to eliminate signals from the ground soil or other material in which the object is located, and objects of a particular type are detected from the phases of signals which they produce.

Claims

1. In apparatus for locating a metal object:

transmitting means and receiving means adapted for movement relative to the metal object, means for energizing the transmitting means to produce a signal in the receiving means which changes as the receiving means is moved in proximity to the metal object, means for demodulating the received signal to provide a signal which varies in accordance with the received signal, differentiator means responsive to the demodulated signal for providing a signal which corresponds to the second derivative of the demodulated signal and has a single peak which occurs while the receiving means is passing over the metal object, and means responsive to the second derivative signal for providing an output indication when the receiving means is positioned directly over the metal object.

2. The apparatus of claim 1 wherein the means responsive to the second derivative signal includes an audio signal source and means for modulating the audio signal in accordance with the second derivative signal.

3. In apparatus for locating a metal object:

transmitting means and receiving means adapted for movement relative to the metal object, means for energizing the transmitting means to produce a signal in the receiving means which changes in a predetermined manner as the receiving means is moved in proximity to the metal object, means for demodulating the received signal to provide a signal which varies in accordance with the received signal and has first and second components of different phase, differentiator means responsive to the demodulated signal for providing signals corresponding to the second derivatives of the two components, and means responsive to the second derivative signals for providing an output indication when the received signal changes in the predetermined manner.

4. In apparatus for locating a metal object:

transmitting means and receiving means adapted for movement relative to the metal object, means for energizing the transmitting means to produce a signal in the receiving means which changes in a predetermined manner as the receiving means is moved in proximity to the metal object, means for demodulating the received signal to provide a signal which varies in accordance with the received signal and has first and second components of different phase, summation means for providing a signal comprising a predetermined combination of the demodulated components, differentiator means for providing a signal corresponding to the second derivative of the signal from the summation means, and means responsive to the second derivative signal for providing an output indication when the received signal changes in the predetermined manner.

5. In apparatus for locating a metal object:

transmitting means and receiving means adapted for movement relative to the metal object, means for energizing the transmitting means to produce a signal in the receiving means which changes in a predetermined manner as the receiving means is moved in proximity to the metal object, means for demodulating the received signal to provide a signal which varies in accordance with the received signal and has first and second components of different phase, differentiator means responsive to the demodulated signal for providing signals corresponding to the second derivatives of the components of the demodulated signal, the first and second components being demodulated at predetermined phases relative to the transmitted signal and combined in such manner that one of the second derivative signals corresponds to the phase of the object to be located and the other second derivative signal is in phase quadrature with the signal corresponding to the background material in which the object is located, and an audio signal source, means for modulating the audio signal in accordance with one of the second derivative signals, and gating means controlled by the other second derivative signal for passing the modulated signal in accordance with said other signal to provide an output indication when the received signal changes in the predetermined manner.

6. In apparatus for locating a metal object:

means including a search coil and a transmitter circuit for energizing the search coil to transmit a magnetic field and thereby induce eddy currents to flow in nearby metal objects, a demodulator circuit responsive to signals induced in the search coil by eddy currents flowing in the metal objects, a first derivative circuit responsive to the rate of change of the signal from the demodulator circuit, a second derivative circuit responsive to the rate of change of the signal from the first derivative circuit for providing a signal which peaks in magnitude when the search coil passes over the metal object, an audio oscillator circuit, an audio modulator circuit which modulates the signal from the audio oscillator circuit in accordance with the magnitude of the signal from the second derivative circuit, and an audio transducer responsive to signals from the audio modulator circuit which provides an audible indication of the presence of a metal object when the search coil is positioned directly over the object.

7. In apparatus for locating an object within a predetermined range of objects in the ground:

transmitting and receiving means movable relative to the object, means for energizing the transmitting means to produce an alternating magnetic field which produces a signal in the receiving means, synchronous demodulation means responsive to the received signal for providing first and second components of said signal at different phases, means for combining said components to provide a quadrature phase signal and a maximum discrimination phase signal, said quadrature phase signal differing in phase by 90°

from a signal characteristic of the ground soil, said maximum discrimination phase signal having the phase of a signal characteristic of an object within the predetermined range, means for differentiating the quadrature phase signal and the maximum discrimination phase signal, means for combining the differentiated signals to provide a discrimination phase signal corresponding to the range of objects to be located, means for differentiating the differentiated quadrature phase signal and the discrimination phase signal, and output indicator means responsive to the second derivative of the quadrature phase signal and the derivative of the discrimination phase signal for indicating the presence of an object within the predetermined range.

8. The apparatus of claim 7 wherein the output indicator means comprises a source of audio signal, means for modulating the audio signal in accordance with the second derivative of the quadrature phase signal, an output transducer, and means responsive to the derivative of the discrimination phase signal for applying the modulated audio signal to the output tranducer.

9. The apparatus of claim 8 wherein the means for modulating the audio signal comprises a pulse width modulator.

10. In apparatus for locating a metal object in a background medium:

transmitting and receiving means movable relative to the background medium and the object to be detected, means for energizing the transmitting means to produce a signal in the receiving means corresponding to the magnetic and/or electrical properties of material in proximity to the transmitting and receiving means, synchronous demodulation means for providing first and second components of said signal at different phases, means for combining the first and second demodulated components to provide a quadrature phase signal which differs in phase by 90°

from a signal characteristic of the background medium, means responsive to the first and second demodulated components for providing a target signal at a phase corresponding to the object to be located, means for differentiating the phase quadrature signal, a zero crossing detector for monitoring the differentiated signal and providing an output signal when the differentiated signal is crossing zero, output indicating means, and means responsive to the output signal from the zero crossing detector for sampling the target signal when the differentiated signal is crossing zero and applying the sampled signal to the output indicating means.

11. The apparatus of claim 10 wherein the means for providing a target signal includes differentiators, and the target signal corresponds to the second derivatives of the demodulated components.

12. The apparatus of claim 11 wherein the means for providing a target signal includes ratio computing means for combining the differentiated signals whereby the target signal corresponds to the ratio of the differentiated signals.