Abstract A transmitting-receiving coil configuration for a metal detector. A transmitting coil is located in the same plane as two receiving coils one of which is of greater diameter than the receiving coil and the other of which is of smaller diameter. The receiving coil of smaller diameter is elliptically shaped to provide a gradual null in the direction of the major axis of the ellipse. The receiving coils are connected in series opposition and initial balance is obtained by varying the diameter of the transmitting coil and final balance is obtained by varying the number of turns on the inside receiving coil. When in the vicinity of a metal object a current 90* out of phase with the alternating current fed to the transmitting coil is induced in the receiving coils. When the center of the coils is moved directly over the metal object a null is obtained. The areas bounded by the coils are adjusted to compensate for the background effect.
In a metal detector of the type in which a transmitting coil is excited with alternating current of a given phase to generate electromagnetic waves and those of said waves which are reflected off metal induce a current in at least a receiving coil of said detector, which current has a component which is in phase quadrature with said given phase, the improvements comprising, said transmitting coil being of a first diameter, a first receiving coil being concentric and co-planar with said transmitting coil and being of a greater diameter than said transmitting coil, a second receiving coil being co-planar with said first receiving coil and being connected in series opposition therewith, said second receiving coil being of a smaller diameter than said transmitting coil and having a cross section of elliptical shape, each of said coils bounding an area, the difference between the area bounded by said first receiving coil and that bounded by said transmitting coil added to the area bounded by said second receiving coil being equal to the difference between the area bounded by said transmitting coil and that bounded by said second receiving coil, initial balance between said transmitting and receiving coils having been effected by adjusting the diameter of said transmitting coil and final balance having been effected by varying the number of turns on said second receiving coil.
Claim 2
Dependent Claim
225 words
In a metal detector of the type in which a transmitting coil is excited with alternating current of a given phase to generate electromagnetic waves and those of said waves which are reflected off metal induce a current in at least a receiving coil of said detector, which current has a component which is in phase quadrature with said given phase, the improvements comprising, said transmitting coil being of a first diameter, a first receiving coil being concentric and co-planar with said transmitting coil and being of a greater diameter than said transmitting coil, a second receiving coil being co-planar with said first receiving coil and being connected in series opposition therewith, said second receiving coil being of a smaller diameter than said transmitting coil and having a cross section of elliptical shape, each of said coils bounding an area, the difference between the area bounded by said first receiving coil and that bounded by said transmitting coil added to the area bounded by said second receiving coil being equal to the difference between the area bounded by said transmitting coil and that bounded by said second receiving coil, initial balance between said transmitting and receiving coils having been effected by adjusting the diameter of said transmitting coil and final balance having been effected by varying the number of turns on said second receiving coil.