Eddy current testing (ECT) is an effective technique for measuring electromagnetic parameters but it is affected by variations in probe lift-off. In order to suppress the effect of lift-off, a lift-off invariable feature is investigated for measuring the properties of magnetic rods. Specifically, the impedance variation of the coil surrounding the metallic rod is not disturbed by the lift-off distance at a specific excitation frequency, ie the spacing between the coil and the metallic rod. Numerous theoretical calculations and experiments have verified that the eddy current signal at this frequency is mainly determined by the electromagnetic properties of the metallic rod. Combining the lift-off invariable feature and the Newton inverse algorithm, one of the electromagnetic parameters (conductivity or relative permeability) can be accurately inverted. The experimental results indicate that the maximum measurement error is only 0.91%.
Huang et al. (Wed,) studied this question.
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