This study was carried out in order to investigate the effect of the specimen speed on the MFL signals and the induced field for developing the automatic inspection system of NMI. A slot-type defect with 30 mm length, 0.3 mm width and 0.15 mm depth was machined on 0.28 mm thick steel sheet by EDM. The movement of the steel sheet changes the distribution of flux density that was distorted to the moving direction of sheet between the poles of an electromagnet by superposition of stray field and induced field. Induced magnetic field was proportional to the speed of the sheet. A vertical induced field was generated with a uniform value in the center region of the yoke by the movement of the sheet and a horizontal induced field was increased as the magnetic sensor approached to the poles of the yoke. MFL was quite similar values, even though the speed of the sheet changed. This means that the magnetic properties of the sheet were not affected when the steel sheet was moving within the magnetic field. After all the magnetic flux density within the poles of electromagnet and above the moving sheet is the value of superposition of induced field that depends on the velocity of sheet and MFL that is independent of sheet velocity. The MFL values have maximum value at the center between the poles in all speed of the sheet.
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