Metallic fine contaminant detection technique using harmonic responses

There is a possibility that small metallic contaminants could be mixed with industrial products made of ceramics or plastic resin. If this event occurs in a battery production line, it may result in a recall because of the possibility of fire, in the worst case. Consequently, a detection system for fine metallic contaminants is required. Presently, there are conventional methods such as using an eddy current, a CCD camera and an X-ray. However, these methods are not enough to detect a contaminant of stainless steel. We have proposed a fine stainless steel contaminant detection technique for industrial products. An AC drive magnetic field with a frequency of 1 kHz and a DC bias field were applied. In each of the AC and DC fields differentially connected detection coils were placed and the output signals of the detection coils were recorded and analyzed. A segment of stainless steel (SUS304) wire with dimensions of ∅0.2 mm × L5 mm was prepared as a specimen. Since the signal is highly affected by the M-H characteristics of the specimen, the AC drive field and the DC bias field were varied and optimized so that the largest harmonic signals can be obtained. In the case of without specimen, neither a 2nd nor a 3rd harmonic can be seen. When a specimen was placed in the detection coil, both the 2nd and 3rd peaks dramatically increased due to the non-linearity of the M-H characteristics of the specimen. The peak signal of the 3rd harmonic was three times larger than that of the 2nd harmonic at the optimal condition for each harmonic signal. The reason for this result was investigated and it was found that minor loops arose in the conditions of smaller amplitudes of the AC drive field and the DC bias field as compared to the saturation point of the magnetization of the specimen and that accordingly, the 2nd second harmonic signal was weak.