Patent ID: 7061236
Filing Date: 2006-06-13
Classification: G01V

Abstract:
1. A metal detector for detecting the presence of foreign ferromagnetic or electrically conductive articles in an object to be investigated, which compensates for the bulk effect which results from the size or shape of the object, comprising: an electromagnetic radiator formed as a loop antenna; an oscillator circuit having an output frequency which is dependent upon the inductive load presented by the electromagnetic radiator, the oscillator circuit being arranged to supply a first, ac electrical signal to the electromagnetic radiator such that the electromagnetic radiator generates an alternating magnetic field; first and second electromagnetic receiving antennae electrically connected to one another and each being arranged in proximity to the electromagnetic radiator such that lines of flux of the alternating magnetic field link the first and second receiving antennae; a receiving antenna signal detector electrically connected with the first and second electromagnetic receiving antennae and arranged to provide an output indicative of the amplitude of a second electrical signal induced in the first and second antennae; an oscillator circuit signal detector electrically connected with the oscillator circuit and arranged to provide an output representative of a parameter of the first ac electrical signal supplied thereby, wherein said parameter varies with the proximity of said object and said electromagnetic radiator; and processor means in communication with the receiving antenna signal detector and the oscillator circuit signal detector, the processor means being arranged to monitor the output of the oscillator circuit signal detector and to adjust the output of the receiving antenna signal detector in dependence upon the output of the oscillator circuit signal detector, in order to compensate for changes in the parameter of the first ac electrical signal caused by the presence in the proximity of the electromagnetic radiator of the object to be investigated.