Adapted eddy current excitation using a dual gradiometric coil system

Due to the skin effect, the induced eddy current (EC) density at the sample's surface is higher than at deeper levels. Thus currents flowing close to the surface generate most of the background signals caused by the sample's own components. One way of minimizing those EC interference is to reduce the surface currents and enhance the current density at the depth in which defects can occur. Unfortunately, magnetic fields cannot be focused to one point in the way rays of light can be concentrated in optics. Magnetic fields are always divergent – as quantified by Maxwell's Equations. To tackle this problem we superimposed two different excitation fields generated by a dual gradiometric coil excitation scheme (DGC). It consists of two double-D coils. Both double-D coils are applied with the same frequency but driven with two different excitation currents ( J_1 , J_2 ) and phase shifts (φ _1 , φ _2 ). In addition we implemented a sensitive magnetometer in our eddy current testing system to measure very thick structures of the Airbus A-380. We report on FEM simulations and EC-measurements using the dual gradiometric coil excitation scheme to detect deep lying defects.