Transient eddy current method for the characterization of magnetic permeability and conductivity

Recent analytical solutions, that correctly describe transient eddy current signals in voltage-controlled driver-pickup circuits, are applied for the case of a coaxial probe encircling a long ferromagnetic conducting tube. Experimental results, obtained for the case of a square wave excitation, are in excellent agreement with the predicted driver and pickup responses. Using the forward solutions, a novel inverse method, that enables simultaneous and accurate characterization of magnetic permeability and electrical conductivity, has been developed. Specifically, the method considers computed areas under scaled transient eddy current signal curves. In the generalized case, multiple parameters of interest can be extracted from a single transient signal by taking advantage of the frequency domain differentiation property of the Laplace transform. Preliminary experiments show that permeability and conductivity values, calculated for a variety of ferromagnetic and non-ferromagnetic tubes, agree well with published values (permeability) and with values obtained by four point measurement (conductivity). The inverse method introduced in this work may be straightforwardly extended to consider other parameters, such as lift-off and material thickness, and to consider other geometries, such as conducting plates.