Image Processing Reference
In-Depth Information
beams overlapping, were reported (Meyer and Candy 2002, Rodríguez et al 2004). But an
extensive research in order to find simple and complete solutions to these problems is still
needed. In particular, the authors are currently investigating techniques for ultrasonic
radiation from perpendicular planes using arrays of few radiators working in near field
conditions. In parallel, we are developing digital signal processing tools for improving the
signal to noise ratio (SNR) in the echoes acquired in NDE of media with complex internal
structure (Lázaro et al 2002, Rodríguez et al 2004a, Pardo et al 2008).
In this technological context, a set of novel ultrasonic signal combination techniques have
been developed to be applied in flaw detection ultrasonic systems based on multiple
transducers. These combination techniques use a spatial-combination approach from the
echographic traces acquired by several transducers located at different external planes of the
piece under testing. In all these alternative techniques, the A-scan echo-information,
received from the different transducers involved, is fused in a common integrated two-
dimensional (2D) pattern, in which, each spot displayed incorporates distinct grades of SNR
improvement, depending on particular processing parameters.
In this chapter, some linear and non-linear digital processing techniques to fuse echo-traces
coming from several NDE ultrasonic transducers distributed on two perpendicular scanning
planes are described. These techniques are also applied to the flaw detection by using a 2D
combination of the ultrasonic traces acquired from the different transducers. The final
objective is to increase the detection capabilities of unfavorable-orientation flaws and also to
achieve a good 2D spatial location of them.
Individual ultrasonic echo-signals are measured by sucesively exciting several transducers
located at two perpendicular planes with electrical short-time pulses. Each transducer
acquires a one-dimensional (1D) trace, thus it becomes necessary to fuse all the measured 1D
signals with the purpose of obtaining an useful 2D representation of the material under
inspection. Three combination techniques will be presented in this chapter; they are based
on different processing tools: Hilbert, Wavelets and Wigner-Vile transforms. For each case,
the algorithms are presented and the mathematical expressions of the resulting 2D SNRs are
deduced and evaluated by means of controlled experiments.
Simulated and experimental results show certain combinations of simple A-scans registers
providing relatively high detection capacities for single flaws. These good results are
obtained in spite that the very-reduced number of ultrasonic channels involved and confirm
the accuracy of the theoretical expressions deduced for 2D-SNR of the combined registers.
2. Some antecedents of ultrasonic evaluation from perpendicular planes
Techniques for combining ultrasonic signal traces coming from perpendicular planes have
few antecedents. As a precedent of this type of scanning performed from two distinct
planes, the inspection of a high-power laser with critical optic components using ultrasonic
transducers situated in perpendicular planes is mentioned in (Meyer and Candy 2002). In
this particular case, the backscattering noise is valueless and the method seems centred in
the combination from the arrival time of the ultrasonic echoes, and thus the combination is
made with a time domain technique.
In (Rodríguez et al 2004), a testing piece containing a flaw was evaluated by using
transducers located at two scanning planes. In this case, the receiving ultrasonic traces
contain backscattering noise and the combination was performed in the time domain. Two
combination options were there presented: one based on a 2D sum operator and the other
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