Abstract
A limited ability to discriminate between different materials is the fundamental problem with all conventional
eddy-current-based metal detectors. This paper presents the use, evaluation and classification of nontraditional
excitation signals for eddy-current metal detectors to improve their detection and discrimination
ability. The presented multi-frequency excitation signals are as follows: a step sweep sine wave, a linear
frequency sweep and sin(x)/x signals. All signals are evaluated in the frequency domain. Amplitude and
phase spectra and polar graphs of the detector output signal are used for classification and discrimination
of the tested objects. Four different classifiers are presented. The classification results obtained with the
use of poly-harmonic signals are compared with those obtained with a classical single-tone method. Multifrequency
signals provide more detailed information, due to the response function – the frequency characteristic
of a detected object, than standard single-tone methods. Based on the measurements and analysis, a
metal object can be better distinguished than when using a single-tone method.
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