Method for detecting faults in a test piece moved relative to a probe in a non-destructive and non-contact manner using a filter whose frequency is set depending on the fault frequency

摘要

The method of detecting faults in a test piece (13) involves applying a periodic electromagnetic alternating field to the test piece by means of a transmitter (12) and detecting a periodic electric signal by means of the probe (14). The signal detected by the probe has a carrier oscillation whose amplitude and/or phase is modulated by a fault (15) in the test piece when the fault occurs in the operative range of the probe. The probe signal is filtered by means of a frequency selective first filter unit (18). The filtered signal is sampled by means of a triggerable A/D converter stage (35) to provide a demodulated digital measuring signal. This signal is filtered by means of a digital frequency selective adjustable second filter unit (52) to obtain a useful signal. This signal is analysed to identify a fault in the test piece. The A/D converter is triggered at an nth, integer fraction of the frequency of the carrier. The value n is selected depending on the fault frequency which is the ratio between the relative speed between the test piece and the probe and the effective widths of the probe. The frequency selective second filter is adjusted in dependence on the fault frequency. Independent claims also cover apparatus for carrying out the method.