Detection of Non-Ferromagnetic Metal Balls Based on the Analytical Solution of Motion-Induced Eddy-Current Field

The rapid real-time detection and sorting of moving non-ferromagnetic metals in enclosed or hazardous environments enable the efficient reuse of metal resources, serving as a critical component in industrial production. In this article, copper, aluminum, and brass balls are selected as the typical non-ferromagnetic metal specimens, and a novel rapid detection and sorting method for moving metal objects is proposed based on the analytical solution of motion-induced eddy-current field. Firstly, the peak value of induced voltage with integral form is obtained, which indicates that the peak value is proportional to the product of conductivity and the square of moving speed. Subsequently, different non-ferromagnetic metal balls are adopted to move in the dc coil magnetic field, with the forms of single or discrete, tangent, and different radii. The experiments show that the moving metal balls can be effectively distinguished by the peak values of induced voltage, and the relative errors are within 15%. Finally, based on the approximate linear relationships between the peak voltage and the radii of moving metal balls when taking the double logarithm, the radii can be directly calculated by the peak values, slope, intercept, and without numerical inversion. Compared with the conventional eddy current methods for stationary conductors, the analytical method proposed in this article is intuitive in form, easy to operate, and without inversion calculation, which can be used for the online and real-time detection of non-ferromagnetic moving metal balls with different moving speeds, conductivity values, and radii.