| 摘要 |
An alternating magnetic field is generated to cause Foucault currents to circulate at the surface of the product (B) by relative motion with a detector, a signal representative of the variations of Foucault currents is collected by means of a differential sensor (11, 12) and is demodulated by a projection according to an adjustable phase angle to detect the perturbation introduced by the defects to be detected, said phase angle (a) being determined so that the projection of the collected signal has a minimum mean value in order to eliminate the perturbations of the collected signal due to repetitive surface irregularities; the component (Y') of the collected signal obtained by said projection is converted in digital form; the digital signal thus obtained is filtered by means of a digital filter of which the characteristics are predetermined as a function of a type of defect to be detected, and a defect detection signal is emitted when the output signal of the digital filter exceeds a predetermined threshold. |
