Decoupling Conductivity and Permeability Using Sweep-Frequency Eddy Current Method

Electromagnetic coupling and liftoff variation are obstacles in the eddy current method. In this article, we proposed a sweep-frequency eddy current method to achieve conductivity and permeability simultaneous measurements immune to liftoff. The relationship between eddy current detection signals of different frequencies and electromagnetic parameters of magnetic materials has been found, that is, the real part of mutual inductance is mainly related to permeability at low frequencies. In high frequency, the zero crossover frequency is proportional to the ratio of permeability and conductivity. By combining the analysis mentioned above, conductivity and permeability can be estimated simultaneously. In order to eliminate the interference of liftoff, a sensor structure with one excitation coil and two receiving coils is designed to compensate for the mutual inductance changes caused by liftoff. Experiments and finite-element method (FEM) simulations are also carried out to verify the proposed method. The results show that the errors of conductivity and permeability are 1.3% and 3.9%, respectively, and the error caused by a reasonable variation range of liftoff can be significantly reduced.