Detection of electrically conductive materials beneath surface coatings employing eddy currents

Claim 3

The method according to claim 1 wherein said inductance coil is energized with an alternating current frequency of approximately 4.5 KHz.

Claim 4

The method accorrding to claim 1 wherein detecting said change in impedance is accomplished by balancing the impedance of said inductance coil in a bridge circuit including a second inductance coil and detecting when bridge balance changes as a result of the impedance of the first mentioned inductance coil changing by an amount exceeding said threshold level.

Claim 7

Apparatus for testing for the presence of subsurface conductive metal material within a body, comprising: a first inductance coil adapted for positioning in close proximity to the surface of said body, a second inductance coil disposed in the same general environment as the first inductance coil but away from the surface of said body, means for completing a bridge circuit with said first and second inductance coils, alternating current oscllator means coupled to a pair of opposite terminals of said bridge circuit for applying an alternating current thereto at a frequency less than 10 KHz to induce eddy currents in said body while minimizing skin effects so that eddy currents are not induced only superficially in said body, bridge circuit output means connected to conjugate terminals of said bridge circuit for sensing the output of said bridge, said output increasing with increase in the depth of conductive metal material adjacent said first inductance coil due to the increase in eddy currents induced therein, means for establishing a threshold level relative to the output sensed by said bridge circuit output means wherein said threshold level is established to eliminate the effect of eddy currents induced only superficially of said body, and means responsive to said bridge circuit output means and said threshold level establishing means for providing an indication of the presence of said subsurface conductive metal material when output of said bridge exceeds said threshold level.

Claim 8

Apparatus for testing for the presence of subsurface conductive metal material within a body, comprising: a first inductance coil adapted for positioning in close proximity to the surface of said body, a second inductance coil disposed in the same general environment as the first inductance coil but away from the surface of said body, means for completing a bridge circuit with said first and second inductance coils, alternating current oscllator means coupled to a pair of opposite terminals of said bridge circuit for applying an alternating current thereto at a frequency less than 10 KHz to induce eddy currents in said body while minimizing skin effects so that eddy currents are not induced only superficially in said body, bridge circuit output means connected to conjugate terminals of said bridge circuit for sensing the output of said bridge, said output increasing with increase in the depth of conductive metal material adjacent said first inductance coil due to the increase in eddy currents induced therein, means for establishing a threshold level relative to the output sensed by said bridge circuit output means wherein said threshold level is established to eliminate the effect of eddy currents induced only superficially of said body, and means responsive to said bridge circuit output means and said threshold level establishing means for providing an indication of the presence of said subsurface conductive metal material when output of said bridge exceeds said threshold level.

Claim 9

The apparatus according to claim 5 wherein said bridge circuit includes said first and second inductance coils in first and second legs thereof and resistances in thrid and fourth legs thereof, said pair of opposite terminals having said first and second legs disposed in series therebetween, and said conjugate terminals being located respectively at the interconnection of said first and second legs, and at the interconnection between said third and fourth legs.

Claim 10

The apparatus according to claim 5 wherein said alternating current means operates at a frequency of approximately 4.5 KHz.

Claim 11

The apparatus according to claim 5 including a half pot core upon which said first inductance coil is wound, and means mounting said half pot core with an open end positionable toward said surface of said body.

Claim 12

The apparatus according to claim 8 including a case for housing said apparatus and having a lower panel of insulating material, said half pot core being mounted open end downward on said panel.

Claim 13

The apparatus according to claim 8 including a second half pot core upon which said second inductance coil is wound, said second half pot core being mounted back-to-back with the first mentioned half pot core.