Electromagnetic gradiometer

Claim 2

Apparatus for detecting and distinguishing between ferrous and electrically conductive bodies comprising coil means, a constant frequency a.c. source connected to said coil means to excite said coil means so that the coil means generates an a.c. magnetic field, said coil means being arranged so that in response to: (a) neither a ferrous body nor a conductive body being in the field there is generated by the coil means a substantially zero a.c. voltage, (b) a ferrous body being in the field there is generated by the coil means a non-zero a.c. voltage having a predetermined phase relative to the phase of the a.c. excitation voltage for the coil means, and (c) a conductive body being in the field there is generated by the coil means a non-zero a.c. voltage having a phase displaced from said predetermined phase relative to the phase of the a.c. excitation voltage; detector means responsive to the magnitude and phase of the voltage generated by the coil means for generating a first, variable amplitude aural signal in response to the coil means generating non-zero a.c. voltage having the predetermined phase with respect to the a.c. excitation voltage and for generating a second aural signal in response to the coil means generating a non-zero a.c. voltage having a phase displaced from said predetermined phase, said first and second aural signals being at differing frequencies to produce tones aurally distinguishable from each other, said variable amplitude being responsive to the amplitude of the a.c. voltage.

Claim 3

Apparatus for detecting and distinguishing between ferrous and electrically conductive bodies comprising coil means, a constant frequency a.c. source connected to said coil means to excite said coil means so that the coil means generates an a.c. magnetic field, said coil means being arranged so that in response to: (a) neither a ferrous body nor a conductive body being in the field there is generated by the coil means a substantially zero a.c. voltage, (b) a ferrous body being in the field there is generated by the coil means a non-zero a.c. voltage having a predetermined phase relative to the phase of the a.c. excitation voltage for the coil means, and (c) a conductive body being in the field there is generated by the coil means a non-zero a.c. voltage having a phase displaced from said predetermined phase relative to the phase of the a.c. excitation voltage; means for selectively generating an output signal selectively having first and second different distinguishable frequency components, phase detector means connected to be responsive to the coil means generating a non-zero a.c. voltage having a phase differing by at least a preselected value from the predetermined phase for activating the means for generating the signal output so only the first component is derived thereby, said means for generating the signal output generating only the second component while the coil means derives an a.c. voltage having the predetermined phase, means for varying the amplitude of the second component in response to the amplitude of the voltage generated by the coil means, and output means responsive to the first component and the second component with the variable amplitude.

Claim 4

Apparatus for detecting and distinguishing between ferrous and electrically conductive bodies comprising coil means, a constant frequency a.c. source connected to said coil means to excite said coil means so that the coil means generates an a.c. magnetic field, said coil means being arranged so that in response to: (a) neither a ferrous body nor a conductive body being in the field there is generated by the coil means a substantially zero a.c. voltage, (b) a ferrous body being in the field there is generated by the coil means a non-zero a.c. voltage having a predetermined phase relative to the phase of the a.c. excitation voltage for the coil means, and (c) a conductive body being in the field there is generated by the coil means a non-zero a.c. voltage having a phase displaced from said predetermined phase relative to the phase of the a.c. excitation voltage; means for selectively generating an output signal selectively having first and second different distinguishable frequency components, phase detector means connected to be responsive to the coil means generating a non-zero a.c. voltage having a phase differing by at least a preselected value from the predetermined phase for activating the means for generating the signal output so only the first component is derived thereby, said means for generating the signal output generating only the second component while the coil means derives an a.c. voltage having the predetermined phase, means for varying the amplitude of the second component in response to the amplitude of the voltage generated by the coil means, and output means responsive to the first component and the second component with the variable amplitude.

Claim 5

The apparatus of claim 2 wherein said frequencies are audio frequencies and said output means comprises means for generating distinguishable aural signals having tonal frequencies respectively corresponding to said first and second audio frequency components.

Claim 6

The apparatus of claim 3 wherein the phase detector means includes means for generating a voltage amplitude indicative of the phase difference of the voltage generated by the coil means relative to the predetermined phase, and further output means for visually indIcating the phase indicating voltage amplitude.

Claim 7

The apparatus of claim 2 further including variable gain means connected between the winding means and the phase detector means, as well as the means for varying the amplitude.

Claim 8

The apparatus of claim 2 wherein the coil means includes an excitation coil means and first and second differential windings respectively disposed on opposite sides of said first coil means, a common terminal for one end of each of said differential windings; a nulling circuit including first and second terminals respectively connected to second ends of said differential windings, voltage divider means connected between said first and second terminals, said voltage divider means including a pair of taps, means for connecting one of said taps to said common terminal without introducing any substantial phase shift, and means for connecting the other of said taps to said common terminal with the introduction of approximately 90* phase shift.

Claim 9

Apparatus for detecting and distinguishing between ferrous and electrically conductive bodies comprising coil means, a constant frequency a.c. source connected to said coil means to excite said coil means so that the coil means generates an a.c. magnetic field, said coil means being arranged so that in response to: (a) a ferrous body being in the field there is generated by the coil means a non-zero a.c. voltage having a predetermined phase relative to the phase of the a.c. excitation voltage for the coil means, and (b) a conductive body being in the field there is generated by the coil means a non-zero a.c. voltage having a phase of the a.c. excitation voltage; phase detector means responsive to the phase of the a.c. voltage generated by the coil means, a voltage controlled oscillator for generating a signal having a frequency controlled by the phase indication generated by the phase detector, and means for controlling the amplitude of the signal frequency generated by the oscillator in response to the amplitude of the a.c. voltage generated by the coil means.

Claim 10

Apparatus for detecting and distinguishing between ferrous and electrically conductive bodies comprising coil means, a constant frequency a.c. source connected to said coil means to excite said coil means so that the coil means generates an a.c. magnetic field, said coil means being arranged so that in response to: (a) a ferrous body being in the field there is generated by the coil means a non-zero a.c. voltage having a predetermined phase relative to the phase of the a.c. excitation voltage for the coil means, and (b) a conductive body being in the field there is generated by the coil means a non-zero a.c. voltage having a phase of the a.c. excitation voltage; phase detector means responsive to the phase of the a.c. voltage generated by the coil means, a voltage controlled oscillator for generating a signal having a frequency controlled by the phase indication generated by the phase detector, and means for controlling the amplitude of the signal frequency generated by the oscillator in response to the amplitude of the a.c. voltage generated by the coil means.

Claim 11

The apparatus of claim 7 wherein the phase detector means includes means for generating a bilevel signal voltage indicative of the phase of the a.c. voltage deviating by at least a predetermined amount from the predetermined phase, said voltage controlled oscillator being responsive to the bilevel signal voltage.

Claim 12

The apparatus of claim 8 further including output means, and wherein the coil means is arranged so that in response to neither a ferrous body nor a conductive body being in the field there is generated by the coil means a substantially zero voltage, the amplitude control means including means responsive to the bilevel signal for feeding the frequency generated by the oscillator to the output means with substantially constant amplitude while one of the levels is derived, and means for feeding the frequency generated by the oscillator to the output means at an amplitude that tends to follow the amplitude of the a.c. voltage generated by the coil means while the other level is derived.

Claim 13

The apparatus of claim 8 further including means for at will controlling the predetermined phase.

Claim 14

The apparatus of claim 7 wherein the coil means is arranged so that in response to neither a ferrous body nor a conductive body being in the field there is generated by the coil means a substantially zero voltage, and means for feeding the frequency generated by the oscillator to the output means at an amplitude that tends to follow the amplitude of the a.c. voltage generated by the coil means while the other level is derived.

Claim 15

Apparatus for detecting and distinguishing between ferrous and electrically conductive bodies comprising coil means, a constant frequency a.c. source connected to said coil means to excite said coil means so that the coil means generates an a.c. magnetic field, said coil means being arranged so that in response to: (a) a ferrous body being in the field there is generated by the coil means a non-zEro a.c. voltage having a predetermined phase relative to the phase of the a.c. excitation voltage for the coil means, and (b) a conductive body being in the field there is generated by the coil means a non-zero a.c. voltage having a phase displaced from said predetermined phase relative to the phase of the a.c. excitation voltage; phase detector means responsive to the phase of the a.c. voltage derived from the coil means for generating a bilevel signal voltage indicative of the phase of the a.c. voltage deviating by at least a predetermined amount from the predetermined phase, a voltage controlled oscillator for generating a signal having audio frequencies F1 and F2 in response to the phase indication generated by the phase detector, said frequencies F1 and F2 being respectively generated in response to the first and second levels of the bilevel signal voltage, means for controlling the amplitude of the frequency F1 signal generated by the oscillator in response to the amplitude of the a.c. voltage generated by the coil means while the first level is generated and for maintaining the frequency F2 signal amplitude constant while the second level is generated, and aural output means responsive to the variable and constant amplitude signals of frequencies F1 and F2, said F1 and F2 frequencies being aurally distinguishable from each other.

Claim 16

Apparatus for detecting and distinguishing between ferrous and electrically conductive bodies comprising coil means, a constant frequency a.c. source connected to said coil means to excite said coil means so that the coil means generates an a.c. magnetic field, said coil means being arranged so that in response to: (a) a ferrous body being in the field there is generated by the coil means a non-zEro a.c. voltage having a predetermined phase relative to the phase of the a.c. excitation voltage for the coil means, and (b) a conductive body being in the field there is generated by the coil means a non-zero a.c. voltage having a phase displaced from said predetermined phase relative to the phase of the a.c. excitation voltage; phase detector means responsive to the phase of the a.c. voltage derived from the coil means for generating a bilevel signal voltage indicative of the phase of the a.c. voltage deviating by at least a predetermined amount from the predetermined phase, a voltage controlled oscillator for generating a signal having audio frequencies F1 and F2 in response to the phase indication generated by the phase detector, said frequencies F1 and F2 being respectively generated in response to the first and second levels of the bilevel signal voltage, means for controlling the amplitude of the frequency F1 signal generated by the oscillator in response to the amplitude of the a.c. voltage generated by the coil means while the first level is generated and for maintaining the frequency F2 signal amplitude constant while the second level is generated, and aural output means responsive to the variable and constant amplitude signals of frequencies F1 and F2, said F1 and F2 frequencies being aurally distinguishable from each other.

Claim 17

The apparatus of claim 12 wherein the phase detector means includes means for generating a d.c. output voltage approximately proportional to the phase of the a.c. voltage generated by the coil means, and a d.c. voltmeter connected to be responsive to the d.c. voltage for visually displaying an indication of the d.c. voltage.

Claim 18

The apparatus of claim 12 further including means for varying the phase deviation required to change the level of the bilevel signal voltage.

Claim 19

The apparatus of claim 12 further including variable gain means connected between the coil means and the phase detector and the means for controlling the amplitude of the F1 frequency signal.