ANALYSIS OF THE MAGNETIC CIRCUIT OF AN EDDY-CURRENT TRANSDUCER OF A DIFFERENTIAL DESIGN BASED ON THE THEORY OF CIRCUITS WITH DISTRIBUTED PARAMETERS

Abstract

The parameters of conductive products are monitored during their manufacture or use (thickness, electrical conductivity, magnetic permeability, rolling speed, the surface-variation coefficient of etched foil, and so on) by means of a transformer-type eddy-current transducer (ECT). A number of methods for calculating ECTs [1…4] are known; they are used to determine calculated quantities for specific designs. These methods include:

• a method for calculating the electromagnetic field in the working air gap of the magnetic circuit when a nonmagnetic conductive sheet is present;
• a method for calculating eddy currents in a stationary nonmagnetic conductive sheet located in the gap of a differential-design ECT and for determining the output EMF with allowance for the demagnetizing effect of those currents;
• a method for determining the pattern of magnetic-flux distribution in the working gap when a conductive nonmagnetic sheet moves through it, and for calculating the ECT with the sheet velocity taken into account;
• a procedure for determining the optimum ratios of the ECT magnetic-circuit parameters, taking maximum power transfer into account when measuring a conductive sheet of a specified thickness;
• a method for determining the densities of motional and transformer currents in a conductive sheet moving at velocity V in the uniform magnetic field of the ECT air The MMFs of these currents and the magnetic fluxes they produce are determined, and the resulting data are used to construct the magnetic-field distribution in the ECT gap;
• a method based on a mesh technique for determining currents in a conductive nonmagnetic sheet as it moves through the ECT gap. The current-closure paths and the laws governing current-density distribution over the sheet cross-section are determined for both the transformer and velocity components.

However, these known methods are used for specialized problems in engineering practice.

A simple method for calculating an ECT based on the theory of magnetic circuits with distributed parameters is proposed below.