Metallic waste classification benefits the environment, resource reuse and industrial economy. This paper provides a fast, non-contact and convenient method based on eddy current to classify metals. The characteristic phase to characterize different conductivity is introduced and extracted from mutual inductance in the form of amplitude and phase. This characteristic phase could offer great separation for non-tilting metals. Although it is hard to classify tilting metals by only using the characteristic phase, we propose the technique of phase compensation utilizing photoelectric sensors to obtain the rectified phase corresponding to the non-tilting situation. Finally, we construct a classification algorithm involving phase compensation. By conducting a test, a 95 % classification rate is achieved.

The classification of conductivity is significant for recycling metallic scraps. The eddy current sensor is distinct from other classification methods by its merits of non-contact, economical setup , fast measuring, and so forth. By introducing a setup that could ensure the single-valued mutual inductance trajectory on the complex plane, we propose the circle fitting method to extract the global features of different trajectories. It is subsequently observed that the fitting circle centers-one of the global featuresfor the same conductivity with various tilting angles are distributed longitudinally close to each other. The feature line, which is parallel to the abscissa axis, is introduced to represent this gathering distribution behavior. The feature lines for different conductivity are distinguishable. Thus, we construct a simple classification method with only two steps. The test results show great fidelity of the proposed classification technique which can successfully classify the tilting metallic samples within 11.3 • .

Eddy current sensors have been widely applied to various measurements, whereas it is still obscure if these measurement techniques are workable for sloping samples. We start from a modified Dodd and Deeds's analytical solution for finite-size samples and find that the pseudo-linearity exists in the magnitude-phase curve of the theoretical mutual inductance. The curves for different conductivities have no intersections. The experiments for verifying the pseudo-linearity are conducted at multiple frequencies from 20 kHz to 100 kHz. We subsequently involve the sloping samples in our simulations and experiments at 20 kHz. The pseudo-linearity preserves in both the simulated and experimental results. To characterize this pseudo-lineariy, we resort to the method of least squares. The obtained intercepts for the same conductivity at different tilting angles are almost the same. Hence the intercept is independent of the tilting angle. The intercepts for different conductivities are clearly separated. Thus, the intercepts for non-sloping samples can be directly utilized as the criterion to classify sloping samples.We then test the classification process at multiple frequencies, which works properly at all the frequencies. Our classification rates are advanced compared to those in the literature. This sloping-invariance (that is the tilting-angle-independent intercept) might make the eddy current sensors find wider applications.