Patent ID: 7423424
Filing Date: 2008-09-09
Classification: G01N

Abstract:
1. A method for the nondestructive and contact-free detection of faults, in a test specimen which is moved relative to a probe having an effective width, comprising the steps of: using a transmitter to apply periodic electromagnetic alternating fields to the test specimen and using the probe to detect a periodic electrical signal which has a carrier oscillation of which at least one of its amplitude and phase is modulated as a result of a fault in the test specimen if the fault reaches the effective width of the probe, filtering a signal from the probe using a frequency-selective first filter unit, sampling the signal which has been filtered using the first filter unit by means of a triggerable A/D converter stage to obtain a demodulated digital measurement signal, filtering the digital measurement signal using a digital frequency-selective adjustable second filter unit to obtain a useful signal, and evaluating the useful signal to detect a fault in the test specimen, wherein said sampling by the A/D converter stage is triggered at an nth integer fraction of the frequency of the carrier oscillation, n being selected as a function of the fault frequency which is obtained as the quotient of the relative speed between the test specimen and the probe and the effective width of the probe, and wherein the frequency-selective adjustable second filter unit is adjusted as a function of the fault frequency.