A1.2 - Impedance Model of Eccentric Coil-Target Arrangements in Eddy Current Techniques

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
A1 - Impedance Spectroscopy
J. Weidenmüller, M. Heidary Dastjerdi, C. Knopf, J. Himmel - Hochschule Ruhr West, Mülheim an der Ruhr (Germany), O. Kanoun - Technische Universität Chemnitz (Germany)
28 - 33


Eddy current techniques are successfully applied over a wide application field in NDT (Non Destructive Testing), such as measurement of position, displacement, vibration, proximity and alignment. Nevertheless, this technology is not established for monitoring tasks in hot rolling mills.
A desired application is the mass flow control of the semi-finished product (rod) during the rolling process. Between the rolling stands, the cross-sectional geometry (x-y plane) of the rod and its speed (z-axis) are of interest. A difficulty for the measurement is the changing x-y position (lateral offset) of the rod due to vibrations and the loose guidance in hot rolling mills. This contribution describes an analytic eddy current model which considers rod offsets (in x-y plane) for a static encircling coil (eddy current sensor element). The model is based on the TREE method (truncated region eigenfunction expansion) and can be used for coil design, selection of test frequencies and interpretation of the inspection data.
The paper is subdivided into five parts. After a short introduction a simplified circuit diagram of the setup illustrates the approach. Subsequently, the analytic model is introduced. This model yields the coil impedance in air (Z0 gained from the well-established Dodd and Deeds models) and, in particular, the impedance change produced by the eddy currents in the rod. Then the analytical findings will be compared with the experimental results. A final discussion concludes the paper in the last section.