Abstract
Computationally fast electromagnetic models of eddy current sensors are required in model-based measurements, machine interpretation approaches or in the sensor design phase. If a sensor geometry allows it, the analytical approach to the modeling has significant advantages in comparison to numerical methods, most notably less demanding implementation and faster computation. In this paper, we studied an eddy current sensor consisting of a transmitter coil with a finitely long I ferrite core, which was screened with a finitely thick magnetic shield. The sensor was placed above a conductive and magnetic half-layer. We used vector magnetic potential formulation of the problem with a truncated region eigenfunction expansion, and obtained expressions for the transmitter coil impedance and magnetic potential in all subdomains. The modeling results are in excellent agreement with the results using the finite element method. The model was also compared with the impedance measurement in the frequency range from 5 kHz to 100 kHz and the agreement is within for the resistance change due to the presence of the half-layer and for the inductance change. The presented model can be used for measurement of properties of metallic objects, sensor lift-off or nonconductive coating thickness.
Highlights
Eddy current sensors respond to changes in the material properties, geometry or position of the nearby electrically conductive, usually metallic, objects [1,2]
An eddy current sensor consists of a transmitter coil that generates a magnetic field and induces the eddy currents in the conductive material [1]
The overall characteristics of the eddy current sensors are primarily determined by the excitation, geometry and construction of the transmitter coil
Summary
Eddy current sensors (probes) respond to changes in the material properties (electrical conductivity, magnetic permeability), geometry or position of the nearby electrically conductive, usually metallic, objects [1,2]. Fast electromagnetic models of transmitter coils are required in model-based measurements, machine interpretation approach or in parametric and sensitivity analyses in the sensor design phase When it comes to more complicated sensor geometries, analytical electromagnetic models are not as versatile as numerical ones, e.g., finite-element or boundary-element models. Since the seminal work by Dodd and Deeds [16], who studied air-cored transmitter coils in planar and cylindrical geometries (layers, rods, tubes), a number of more complicated geometries have been analyzed using the truncated region eigenfunction expansion approach (TREE) [17]. In a paper that laid out the modeling path followed by other authors, Theodoulidis analyzed the coil with the cylindrical finite-length ferrite core that was positioned above a half space made of two nonmagnetic and conductive layers [18].
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