A simple conductivity measurement method using a peak-frequency feature of ferrite-cored eddy current sensor

Abstract

Eddy current sensing (ECS) is advantageous in metallic measurements, but the existing ECS-based conductivity measurement methods are limited in terms of complexity, accuracy, or measurement range. Based on a peak-frequency feature of sweep-frequency eddy current sensor, a conductivity measurement method for non-magnetic plates is proposed. Deriving from the analytic solution of Dodd-Deeds, it is found that the peak-frequency, which is the peak frequency of the imaginary component of the induction spectrum change, is inversely proportional to the specimen's conductivity; the product of the conductivity and the peak-frequency is solely determined by a geometrical coupled parameter called spatial-frequency, which is independent of the conductivity of the specimen and is specified by the lift-off and the dimensions of sensor coils; the lift-off and the sensor coils dimensions have no impact on the inverse linearity between the conductivity and the peak-frequency, although produce different scales. The findings are also confirmed with finite element method (FEM) simulations and experimental results. Based on the findings, we propose to measure the conductivity using the inverse proportionality between the peak-frequency and the conductivity, and to eliminate the lift-off effect by calibrating the sensor with a known conductivity.In addition, we propose to use ferrite-cored sensor in experimental measurements, to improve the induction intensity in low frequency range in practical. The induction spectra characteristics of eddy current sensors with and without a ferrite core by FEM (finite element method) are compared, and the results show that the ferrite-cored eddy current sensor features the same peak-frequency as the air-cored eddy current sensor, but exhibits a stronger inductive impedance variation.Experiments were carried out on six samples with conductivities from 16.09 MS/m to 57.29 MS/m at different lift-offs. The relative conductivity measurement errors are within 1.26 % and the maximum absolute error is 0.43 MS/m. We analyze the measurement errors in terms of frequency resolution and lift-off, and propose methods for improvement. The conductivity measurement method based on the peak-frequency of ferrite-cored eddy current sensor shows its advantages in terms of simplicity, accuracy, immunity to the lift-off, and wide measuring range in non-magnetic metal plate conductivity measurements, because of the rarely mathematically approximation induced error and the determined inverse ratio between the conductivity and the peak-frequency.