Surface Flaws Detection Using AC Magnetic Field Sensing by a Thin Film Inductive Microsensor

Alternating magnetic field was used for detection of surface flaws on conductive metallic specimens. The nondestructive sensor probe was composed of the planar coil with inductive magnetic thin film yoke as a sensing component and a single straight typed exciting coil. The planar inductive coil sensor with magnetic yoke was fabricated by sputtering, electroplating, dry etching, and photolithography process. The exciting coil was generated by the alternative current with the range of 0.1-1.2 A with frequency rage of 0.6-1.8 MHz, respectively, which alternating current (ac) magnetic fields by exciting coil were applied to the specimen with artificial flaws. The specimens were prepared with the slit shaped artificial surface flaws (minimum depth and width; 0.5 mm) on metallic plate (Al; nonmagnetic metal and FeC; magnetic metal). The detected signal for the positions and shapes of surface flaws on specimens were obtained with high sensitivity and high signal to ratio by the planar inductive microsensor probe. The measured output signals by the noncontacted scanning on surface of coins (one cent and two euros) were converted to the images of the flaws. These results were compared with the optical images of real coins, respectively.