Measurement of Magnetic Plates at a Few Hertz with Two Concentric Coils and Thickness Estimation Using Mutual Inductance

We propose a low-frequency eddy current measurement methodology for the thickness of ferromagnetic materials. The instrumentation is built with two concentric coils passing with up to a few hertz alternating currents. The magnetic plate under examination is regarded as the core of the coils that accounts for the mutual inductance. The mutual inductance available by taking the difference between the reactance of the two coils connected in series, cumulatively and differentially coupled, respectively, is a quantity that almost does not vary with the material's electrical conductivity but the magnetic permeability and is not liable to change with frequency. The mutual inductance can be used to estimate a magnetic plate's thickness. Furthermore, we use a set of test pieces (TPs) of the same magnetic property to establish a relationship between the thickness variation and the mutual inductance variation. This relationship is ideally permeability-invariant and applicable to other sets of TPs. The amount of thickness variation can be estimated, despite the difference on permeability.