Metallic intrusion detector system

Claim 1

In a detector system for detecting the intrusion of metallic objects into a given volumetric space into which metallic objects may be expected to intrude, the combination comprising: a transmitter circuit means energizable to generate in said given volumetric space an electromagnetic field having a predetermined energy level per volume; receiver circuit means coupled to said electromagnetic field and arranged to provide a signal voltage directly related to the energy level per unit volume of said electromagnetic field and an output voltage correlated in amplitude and polarity to said signal voltage whereby intrusion into said volumetric space of a ferrous or non-ferrous metallic object will effect variations in said signal voltage reflected as variations in said output voltage; and output circuit means connected to said receiver circuit means and including means to apply said variations in output voltage to indicate whether an intruding metallic object is a ferrous or non-ferrous object and to indicate whether such intruding metallic object possesses a surface area above or below a predetermined norm; said receiver circuit including: a. a sensor coil arranged to couple electromagnetic energy from said electromagnetic field to develop said signal voltage in said sensor coil; b. means for summing said signal voltage with a first balancing voltage of opposite phase to produce a residual voltage of relatively low amplitude constituting the difference in amplitude between the transmitted voltage and the signal voltage; c. a summing transformer connected to receive said residual voltage; d. a nulling circuit operatively interposed between said summing transformer and one of said amplifiers and adjustable to sum said residual voltage against a second balancing voltage to produce a null in the output of said summing transformer in the absence of a metallic object in said volumetric space defined between the transmitter and sensor coils; e. a band pass amplifier connected to the secondary of said summing transformer to receive a signal therefrom when a ferrous or non-ferrous metallic object intrudes into said electromagnetic field; f. a plurality of phase-sensitive detector circuits connected in parallel to the output of said band pass amplifier; and g. a plurality of phase shifting circuits corresponding in number to the number of said phase-sensitive detector circuits, one of said phase shifting circuits being operatively interposed between an associated phase-sensitive detector circuit and one of said amplifiers in the transmitter circuit means whereby the output from each phase detector is applicable to indicate by virtue of its polarity whether the intruding metallic object is ferrous or non-ferrous.