Apparatus for electromagnetically detecting flaws in metallic objects

Claim 1

In an apparatus for electromagnetically detecting flaws in metallic objects of the type employing an electromagnetic coupling coil and an electrically conductive sensor including a sensor ring disposed inside said coupling coil, said coupling coil being connected to an external source of a high frequency current through a bridge circuit such that a first high frequency current excites said coupling coil to produce a first high frequency magnetic field, said first high frequency magnetic field inducing a second high frequency current to flow in said sensor ring, said second high frequency current flowing in said sensor ring producing a second high frequency magnetic field which inturn induces a third high frequency current to flow in said object under test, where during relative movement between said sensor ring and said object under test, a flaw in said object under test will cause a change in the distribution and amount of said third high frequency current induced in said object under test as to cause a change in the amplitude and phase of said second high frequency current flowing in said sensor ring resulting in a corresponding change in the impedance of said coupling coil as seen by said external source of high frequency current through said bridge circuit and producing a corresponding change in the amplitude and phase of said first high frequency current flowing in said coupling coil, whereby a signal corresponding to said change in the amplitude and phase of said first high frequency current flowing in said coupling coil is produced in said bridge circuit, the improvement comprising: a portion of said sensor formed as a sensor head extending outwardly from said sensor ring and positioned adjacently the surface of said object under test such that said third high frequency current is induced to flow locally in an area of said object under test immediately adjacent said sensing head such that a variation in said third high frequency current caused by a flaw in said object under test located in said area immediately adjacent said sensing head is electromagnetically coupled to said sensing head to cause a chane in the amplitude and phase of said second high frequency current flowing in said sensor ring.