Characterization of coatings on magnetic metal using the swept-frequency eddy current method

Abstract

A measurement technique using the swept-frequency eddy current ͑SFEC͒ method for determining the thickness, conductivity, and permeability of metallic coatings on metal substrates for the case when either coating, substrate, or both are magnetic was developed. This technique involved using the empirically determined permeability of the metals as input to the model calculation. This technique is demonstrated for nickel layers ͑25-200 m͒ over copper substrates, copper layers ͑25-200 m͒ over nickel substrates, and zinc layers ͑50-400 m͒ over steel substrates. The electrical impedance was measured for air-core coils in the presence and absence of the layer using a 4194A Hewlett-Packard impedance analyzer. An analytic closed-form solution for calculating the impedance of a cylindrical air-core probe over a layered metallic half-space is presented. The determination of coating thickness and conductivity and permeability of the metals is based on the comparison of the data taken with air-core coils and theoretical calculation that using the closed-form solution developed. Most of the cases studied show experiment and theory agreeing fairly well, within 10%, with no adjustable parameters. The physical phenomena of eddy currents interaction with the coated magnetic metals are also discussed.

A Novel Eddy Current Method for Magnetic Plate Identification with Elimination of Lift-off Effect

“…However, the effect of nickel plating on the differences in cross frequency was negligible. It should be mentioned that the nonmagnetic coatings on magnetic base metal and the magnetic coatings on nonmagnetic base metal were demonstrated by Cheng-Chi Tai in 2000 [4]. The experimental results were good in agree with the analytical results based on the analytical solution given by Dodd and Deeds.…”

“…This measurement technique is known as the swept-frequency eddy current method. In terms of cross frequency, the relationship between the both the conductivity and permeability was discussed with the analytical solutions by Cheng-Chi Tai in 2000 [4]. It should be noted that the conventional consideration usually focused on the constant magnetomotive force F = ni given by the product of the number of winding and current.…”

Elimination of liftoff effect using a model-based method for eddy current characterization of a plate

“…Similarly, this approach was utilized to determine the thickness and conductivity of three-layered plane conductors using an eddy current sweep technique [8]. In order to be more efficient, novel look-up-table and feature-based methods, which avoid iterative computations of a forward model, were proposed [9][10][11][12][13][14]. In [15], the investigation reveals that conductivity effects are prominent in the rising edge of the transient response, while permeability effects dominate in the stable phase, which is valuable for simultaneous measurement of permeability and conductivity using pulsed eddy current method with consideration of normalization techniques to remove influence of liftoff.…”

Thickness measurement of nano-metallic film with electromagnetic sensor under large sensor-sample distance

“…have been developed. Usually, the resolution and precision of the eddy current methods are mainly in the order of the micrometers [8][9][10] and few studies to demonstrate the nanometer resolution of metallic film thickness measurement have been reported. Fortunately, Sakran et al [1] proposed a microwave eddy-current technique operating in the 100 MHz to 5 GHz, which can measure the thickness of conducting layers of 0.1 ȝm using the planar inductor sensor.…”