Wavelet analysis: application to the magneto-inductive testing

摘要

The filtering of signals detected in a non-destructive testing can be effective in order to automatically delete the noise and extract a clear signal that contains the most significant information. Regarding filtering technique, the Fourier transform is one of the most known and used techniques although often, in the presence of non-stationary signals or with transient behavior, are more effective techniques such as wavelet transformation. In the present work the wavelet analysis is applied to the magneto-inductive control and in particular to signal called LF (Localised Fault) which by its nature contains information that are well suited to be highlighted with this technique. The LF signal shows to the operator a value of electric voltage generated by a magnetic field sensor: coils or Hall effect sensors. When an external or internal fault happens, eg. broken wire in the case of wire ropes or lack of material for corrosion in the pipes, an instantaneous variation of the magnetic field with respect to a background value occurs. Wavelet analysis allows highlighting these discontinuities of the signal even when the background noise tends to hide them. After validating the efficacy and safety offered by wavelet filtering it is also possible to adopt this technique during the certification of a system for controlling magneto- inductive. The conventional test for the certification of magneto-inductive device is the EN 12927-8:2004 Safety Requirements For Cableway Installations Designed To Carry Persons - Ropes - Part 8: Magnetic Rope Testing. In the paper the main results that have been obtained by applying the wavelet transform are presented. The presented examples regards different control systems primarily in the field of magneto-inductive wire ropes but also in the control of steel pipelines.