Planar Electromagnetic Sensor for Inspecting Low Conductivity Material

G. Zysko, C. Ktistis, W. Yin, A.J. Peyton and R.Denno

School of Electrical and Electronic Engineering, University of Manchester, M60 1QD, UK Email: Grzegorz.Zysko@postgrad.manchester.ac.uk

ABSTRACT

Electromagnetic sensing using eddy current techniques is used for a range of technological applications. A recent area of interest is the application of these methods to inspect materials relatively low in conductivity in the range 0.1 S/m to 30 S/m. The motivation for this research is that materials with these conductivities are encountered in situations such as biological tissues for medical applications and ionized water for industrial process application. Within tomography, several studies have been reported, but in general circular arrays are used. Notable exceptions are the work published by Igney et al with a planar array for monitoring life signs in biomedicine; and Yin et al with a dual coil sensor for measuring the depth and conductivity of saline in a tank.

The aim of this study is to build on this previous research and to investigate the use of a planar coil array in obtaining cross-sectional images of the low conductive materials in the plane perpendicular to the sensor for industrial application.

The paper considers an array of 8 equally spaced coil elements. Sensitivity maps were calculated using both finite element and analytical approaches. Both sets of maps were compared and also validated against measurements. Measurements were taken from a saline tank using a commercial impedance analyser. Observations about the limits of detection are presented. Images were reconstructed from simulated data using iterative linear methods. In-homogeneities having higher conductivity than a base material were considered. The system resolution and sensitivity due to the sensor construction are discussed. Finally a comparison with the experimental results is presented.

The main conclusion is that although the research was conducted under simplified laboratory conditions, the results suggest that EM techniques can monitor conductivity using a planar array of coils, especially for variations near the coil array.

Keywords Mutual induction, electromagnetic, eddy current, planar, low conductivity