Metal detector system having identical balanced field coil system on opposite sides of a detection zone

Claim 2

A metal detector system comprising a coil assembly including a field coil and a sensor coil disposed in partially overlapping substantially minimum mutual inductive coupling relation so that the sensor coil provides a zero or small residual output signal under an energized condition of the field coil in the absence of extraneous metal objects within the magnetic field produced by the energized field coil, a source of alternating current electrical excitation coupled to the field coil, a receiver tuned to a frequency of the electrical excitation applied to the coil assembly and including input summing means coupled to receive the sensor coil output signal, nulling means coupled to the input summing means for effectively cancelling residual output signal of the sensor coil, said receiver further including detector means coupled to the output of the summing means for detecting the envelope of the signal derived from the output of the summing means, and drift compensator means coupled to the output of the detector means for cancelling long term drift of the sensor coil output signal, said drift compensator means including amplifier means having a differential amplifier input stage, a resistor network coupling the output of the detector means to one input terminal of the differential amplifier input stage, and an integrator network coupling the output of the detector means to the other input terminal of the differential amplifier input stage.

Claim 3

A metal detector system comprising first and second generally planar coIl assemblies disposed respectively on opposite sides of a passageway detection zone in parallel relation to one another, each coil assembly including a loop configured field coil and a loop configured sensor coil positioned in substantially vertical orientation and fixedly secured relative to one another in partially overlapping substantially minimum mutual inductive coupling relation whereby the sensor coil of each coil assembly provides a zero or small residual output signal under an alternating current electrically energized condition of the field coils in the absence of extraneous metal objects within the detection zone, and the field coils being connected in parallel and each having a grounded center tap and a pair of taps symmetrically disposed about the center tap, a source of alternating current electrical excitation coupled across the symmetrically disposed taps of the field coil of the first coil assembly, a capacitor connected across the field coils to form a resonant circuit tuned to a frequency of the electrical excitation provided by the alternating current source, a first receiver tuned to the resonant frequency of the coil assemblies and including input summing means coupled to the sensor coil of the first coil assembly, a second receiver tuned to the resonant frequency of the coil assemblies and including input summing means coupled to the sensor coil of the second coil assembly, and first and second null adjust circuits coupled respectively to the input summing means of the first and second receivers for cancelling residual signal produced at the output of the related sensor coil, each null adjust circuit including two series connected potentiometers having the remote ends thereof coupled to one of the symmetrically disposed taps of the field coil of the second coil assembly and the junction therebetween coupled to the other of the symmetrically disposed taps of the field coil of the second coil assembly with the slider contact of one potentiometer of each null adjust circuit coupled through a capacitor and the slider contact of the other potentiometer of each null adjust circuit coupled through a resistor to the input summing means of the associated receiver whereby residual signal of each sensor coil may be cancelled by appropriate setting of the slider contacts of the potentiometers of the related null adjust circuit.

Claim 4

A metal detector system comprising first and second generally planar coIl assemblies disposed respectively on opposite sides of a passageway detection zone in parallel relation to one another, each coil assembly including a loop configured field coil and a loop configured sensor coil positioned in substantially vertical orientation and fixedly secured relative to one another in partially overlapping substantially minimum mutual inductive coupling relation whereby the sensor coil of each coil assembly provides a zero or small residual output signal under an alternating current electrically energized condition of the field coils in the absence of extraneous metal objects within the detection zone, and the field coils being connected in parallel and each having a grounded center tap and a pair of taps symmetrically disposed about the center tap, a source of alternating current electrical excitation coupled across the symmetrically disposed taps of the field coil of the first coil assembly, a capacitor connected across the field coils to form a resonant circuit tuned to a frequency of the electrical excitation provided by the alternating current source, a first receiver tuned to the resonant frequency of the coil assemblies and including input summing means coupled to the sensor coil of the first coil assembly, a second receiver tuned to the resonant frequency of the coil assemblies and including input summing means coupled to the sensor coil of the second coil assembly, and first and second null adjust circuits coupled respectively to the input summing means of the first and second receivers for cancelling residual signal produced at the output of the related sensor coil, each null adjust circuit including two series connected potentiometers having the remote ends thereof coupled to one of the symmetrically disposed taps of the field coil of the second coil assembly and the junction therebetween coupled to the other of the symmetrically disposed taps of the field coil of the second coil assembly with the slider contact of one potentiometer of each null adjust circuit coupled through a capacitor and the slider contact of the other potentiometer of each null adjust circuit coupled through a resistor to the input summing means of the associated receiver whereby residual signal of each sensor coil may be cancelled by appropriate setting of the slider contacts of the potentiometers of the related null adjust circuit.

Claim 5

The system of claim 2 wherein each receiver includes a logarithmic amplifier coupled to the output of the input summing means for compressing the amplitude of the summing means output signal for application to an indicator meter, whereby the null adjust procedure may be accomplished with minimum need for changes of the meter scale sensitivity and long term drift of the summing means output may be monitored with a selected meter scale sensitivity.

Claim 6

The system of claim 2 wherein each receiver further includes drift compensator means coupled to the output of the input summing means for cancelling long term drift of the summing means output signal, and an alarm coupled to the output of the drift compensator means for indicating when the drift compensator output signal of either receiver exceeds a predetermined threshold so that a determination may be made as to whether a detected object passed through the detection zone to the left or right of the center of the passageway.

Claim 7

A metal detector system comprising first and second generally planar coil assemblies disposed respectively on opposite sides of a passageway detection zone in parallel relation to one another, each coil assembly including a loop configured field coil and a loop configured sensor coil positioned in substantially vertical orientation and fixedly secured relative to one another in partially overlapping substantially minimum mutual inductive coupling relation whereby the sensor coil of each coil assembly provides a zero or small residual output signal under an alternating current electrically energized condition of the field coils and thE absence of extraneous metal objects within the detection zone, a source of alternating current electrical excitation coupled to the field coils, a first receiver coupled to the sensor coil of the first coil assembly tuned to a frequency of the electrical excitation applied to the coil assemblies, a second receiver coupled to the sensor coil of the second coil assembly tuned to a frequency of the electrical excitation applied to the coil assemblies, means coupled to the first and second receivers respectively for cancelling residual signal produced at the output of the related sensor coil, means coupled to the output of the first receiver for indicating when the first receiver output signal exceeds a predetermined threshold set to enable detection of a prescribed metallic object in the half of the passageway adjacent the coil assembly connected to the first receiver, means coupled to the output of the second receiver for indicating when the second receiver output signal exceeds the predetermined threshold to enable detection of the prescribed metallic object in the half of the passageway adjacent the coil assembly connected to the second receiver, and means coupled to the output of the first and second receivers for indicating when both the first and second receiver output signals exceed a threshold set to enable detection of the prescribed metallic object in the center region of the passageway.

Claim 8

A metal detector system comprising first and second generally planar coil assemblies disposed respectively on opposite sides of a passageway detection zone in parallel relation to one another, each coil assembly including a loop configured field coil and a loop configured sensor coil positioned in substantially vertical orientation and fixedly secured relative to one another in partially overlapping substantially minimum mutual inductive coupling relation whereby the sensor coil of each coil assembly provides a zero or small residual output signal under an alternating current electrically energized condition of the field coils and thE absence of extraneous metal objects within the detection zone, a source of alternating current electrical excitation coupled to the field coils, a first receiver coupled to the sensor coil of the first coil assembly tuned to a frequency of the electrical excitation applied to the coil assemblies, a second receiver coupled to the sensor coil of the second coil assembly tuned to a frequency of the electrical excitation applied to the coil assemblies, means coupled to the first and second receivers respectively for cancelling residual signal produced at the output of the related sensor coil, means coupled to the output of the first receiver for indicating when the first receiver output signal exceeds a predetermined threshold set to enable detection of a prescribed metallic object in the half of the passageway adjacent the coil assembly connected to the first receiver, means coupled to the output of the second receiver for indicating when the second receiver output signal exceeds the predetermined threshold to enable detection of the prescribed metallic object in the half of the passageway adjacent the coil assembly connected to the second receiver, and means coupled to the output of the first and second receivers for indicating when both the first and second receiver output signals exceed a threshold set to enable detection of the prescribed metallic object in the center region of the passageway.