Investigation on contribution of conductivity and permeability on electrical runout problem of eddy current displacement sensor

The coil impedance of eddy current sensor depends on sample electromagnetic properties and the displacement between sample and coil, so eddy current sensor can be developed to be a kind of nondestructive evaluation sensor to characterize the integrity of sample as well as a displacement sensor to measure distance or coating thickness. A bottleneck problem for eddy current displacement sensor is the ERO problem, which is caused by the change or uneven distribution of sample conductivity and permeability. The aim of this paper is to investigate the contributions of sample conductivity and relative permeability on ERO problem and present a way to solve the ERO problem in eddy current displacement measurement. Therefore, the relationship of the real and imaginary part of coil impedance with the varied sample conductivity and relative permeability are studied under the different inspections, such as the configuration of coil probe and excitation frequency. According to the investigation, the ellipse distribution of the real and imaginary part of coil impedance with the sample conductivity is proposed to remove the ERO problem after the sample is magnetized to be in saturation.