INDUCTIVELY COUPLED METAL DETECTOR ARRANGEMENT

• US 3,742,341 A
• Filed: 08/16/1971
• Issued: 06/26/1973
• Est. Priority Date: 08/16/1971
• Status: Expired due to Term

• Current Assignee: Garth A. Clowes, Masamitsu Nagaoka
• Original Assignee: Garth A. Clowes, Masamitsu Nagaoka
• Inventors: Clowes, Garth A., Nagaoka, Masamitsu
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US05/172,200
• Time in Patent Office: 680 Days
• Field of Search: 324/3,6,41,67
• US Class Current: 324/329
• CPC Class Codes: G01V 3/101 by measuring the impedance of the search coil; by measuring features of a resonant circuit comprising the search coil measuring impedance or characteristics derived therefrom G01R27/00, e.g. quality factor G01R27/26

Abstract

A metal detector arrangement for detecting the presence of a metal object in proximity thereto. A case means is provided in which there is a cavity. An electrically operated detector circuit is positioned within the cavity and the detector circuit generally comprises a search coil connected to an oscillator. The oscillator has a tank circuit and the inductor of the tank circuit is connected to the primary of the transformer. The search coil is connected to the secondary of the transformer. The oscillator generates an output at a preset frequency. When a metal object is in proximity to the metal detector the impedance to the signal radiated by the search coil is changed thereby changing the inductance of the tank circuit. The change in inductance of the tank circuit in the presence of a metal object raises the frequency of the oscillator output signal. A receiver coil is positioned within the search coil and receives the radiated signal from the search coil. The receiver coil is connected to an amplifier and the response of the amplifier is such that the increase in frequency of the radiated oscillator output signal changes the magnitude of the amplifier output signal. An indicator, such as a light bulb, is coupled to the amplifier through a switch means and the light bulb changes in intensity in proportion to the change in the magnitude of the amplifier output signal. The switch means turns off the indicator for the amplifier output having a preselected magnitude. A sensitivity control means is also provided to increase the magnitude of the amplifier output signal for the condition of the presence of a given mass of metal at a given distance from the detector and, thus, provide a variable sensitivity to aid in the detection of very small metal objects.

Claims

1. A metal detector arrangement of the type adapted to detect the presence of a metal object and generate detectable signals in response thereto, and comprising, in combination;

a case means having peripheral walls, a top wall and a bottom wall transparent to electromagnitic radiation in a preselected bandwith and defining a cavity;

an electrically operated detector circuit mounted in said case means, and said electrically operated detector circuit comprising;

oscillator means for generating output signals having a frequency within said preselected bandwith, and said oscillator means having a metal responsive means comprising a tank circuit, and said tank circuit having an inductor and a capacitor, and said inductor comprising a primary of a transformer, and said metal responsive means varying said output signal frequency from a first frequency to other frequencies in response to the presence of the metal object in regions external said cavity;

search coil means comprising a loop antenna having a first axis and extending around said peripheral walls of said case means in said cavity and connected to said oscillator means for radiating said output signal to regions external said case means, and said search coil means coupled to the secondary of said transformer for varying the inductance of said inductor to vary the frequency of said oscillator output signal for the condition of the metal object in proximity thereto;

receiver coil means having a second axis substantially perpendicular to said first axis and said second axis substantially coplanar with said search coil, and said receiver coil positioned within said loop of said search coil means and spaced a preselected distance therefrom, said receiver coil means receiving said radiated output signal from said search coil mean;

tuned amplifier means connected to said receiver coil means for generating an information signal having a magnitude proportional to the difference between said first frequency and said other frequencies;

switch means connected to said tuned amplifier means for receiving said information signal and having an open condition for blocking said information signal for said information signal having a first magnitude, and a closed condition for transmitting said information signal for said information signal having a second magnitude different from said first magnitude;

indicator means connected to said switch means for generating the detectable signal in response to said information signal at said second magnitude;

sensitivity control means mounted on said case means in regions adjacent and spaced from said receiver coil means for varying the magnitude of said information signal; and

a source of electrical energy for powering said electrically operated detector circuit.

2. The arrangement defined in claim 1 wherein:

said receiver coil further comprises an electrically conductive core means having a first permeability for electromagnetic radiation in said preselected bandwidth, and positioned along said second axis, and having a first end and a second end, and said receiver coil is wound on said core means intermediate said first end and said second end;

said first axis is substantially equidistant from said first and said second ends of said core;

said sensitivity control means is free of contact with said receiver coil and said search coil;

said case means further comprises divider walls for separating said cavity into a coil portion and a circuit portion;

said search coil, said receiver coil, and said sensitivity control means in said coil portion at said cavity; and

said oscillator means, said tuned amplifier means, said switch means and said source of electrical energy in said circuit portion of said cavity.

3. The arrangement defined in claim 2 wherein:

the magnitude of said information signal increases in response to an increase in frequency of said output signal of said oscillator means.

4. The arrangement defined in claim 2 wherein:

the magnitude of said information signal decreases in response to an increase in frequency of said output signal of said oscillator means.

5. The arrangement defined in claim 2 wherein:

said search coil is a shielded electromagnetic radiation conductor;

said sensitivity control means further comprises;

a knob means mounted on said case means and manually operable from regions external said case means;

an electrically conductive member interior said case means and coupled to said knob means, and moveable by said knob with respect to said receiver coil in regions adjacent thereto;

said handle means further comprises;

a palm grip member coupled to said case means; and

an elongated rod-like member detachably coupled to said case means.

6. The arrangement defined in claim 5 wherein:

said electrically conductive member of said sensitivity control means comprises an elongated member having a second permeability greater than said first permeability and having a length substantially equal to the length of said core of said receiver coil and rotatably mounted in said case means with respect to said receiver coil about said first axis, whereby said electrically conductive member decreases the magnitude of said information signal for a first position substantially parallel to said core and increases the magnitude of said information signal for second positions substantially at right angles to said core.

7. The arrangement defined in claim 2 wherein:

said search coil is a shielded electromagnetic radiation conductor;

said sensitivity control means further comprises;

a knob means rotatably mounted on said case means for rotation about said first axis;

a dielectric arm means coupled at the midpoint thereof to said knob means in said cavity, and said dielectric arm means having a length greater than the length of said core of said receiver coil;

an electrically conductive member coupled to least one end of said arm means and moveably positionable in first positions comprising regions adjacent one of said first and said second ends of said core to second positions comprising regions spaced therefrom, whereby said first positions increase the magnitude of said information signal and said second positions decrease the magnitude of said information signal;

said handle means further comprises;

a palm grip member coupled to said case means; and

an elongated rod-like member detachably coupled to said case means.

8. The arrangement defined in claim 2 wherein:

said indicator means comprises a source of visual electromagnetic radiation, and the detectable signal comprises visual electromagnetic radiation;

means for changing the intensity of said visual electromagnetic radiation in response to changes in said magnitude of said information signal;

said handle means further comprises;

a palm grip member coupled to said case means; and

an elongated rod-like member detachably coupled to said case means.

9. The arrangement defined in claim 8 wherein:

said visual electromagnetic radiation increases in intensity in response to increases in said magnitude in said information signal.

10. The arrangement defined in claim 2 wherein:

said indicator means comprises a source of audio energy and the detectable characteristic comprises an audible signal;

means for changing the frequency of said audible signal in response to changes in said magnitude of said informaTion signal;

a handle means coupled to said case means and comprising;

a palm grip member coupled to said case means; and

an elongated rod-like member detachably coupled to said case means.

11. The arrangement defined in claim 10 wherein:

said audio signal increases in frequency in response to increases in said magnitude of said information signal.