Abstract
Electromagnetic coupling and lift-off variation are obstacles in the eddy current method. In this paper, we proposed a sweep frequency eddy current method to achieve conductivity and permeability simultaneous measurements immune to lift-off. 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 frequency. In high frequency, the zero crossover-frequency is the 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 lift-off, a sensor structure with one excitation coil and two receiving coils is designed to compensate the mutual inductance changes caused by lift-off. Experiments and FEM simulations are also carried out to verify the proposed method. The results show the errors of conductivity and permeability are 1.3% and 3.9% respectively, and the error caused by reasonable variation range of lift-off can be significantly reduced.
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