Abstract
This work presents in detail a new method for the optimization of rectangular spread spectrum excitation signals for its use in simple and low-cost ultrasonic pulsers, but that could be extended to other applications based on spread spectrum signals. Starting from rectangular linear frequency modulated (RLFM) chirps, it uses the transfer function of the transmitted signal and received echoes from reference specimens to iterate through a recursive algorithm to obtain arbitrary position and width pulse (APWP) signals with the desired bandwidth, maximizing the energy and the flatness of the spectrum, and enhancing the resolution and dynamic range of conventional chirp excitation signals. This optimization procedure can be repeated for any transducer and material, so that it achieves the best performance in each experimental environment. Such characteristics are ideal for Time-of-Flight estimation, imaging, and applications in which spectral regularity is needed, such as the Split Spectrum Processing (SSP) algorithm, which is used as example to test the performance of the proposed excitation signals. It also allows to specify and change the bandwidth reducing the need to change the transducers. The method is tested with different transducers (2 and 5 MHz focused transducers) and complex composite materials (aluminum-carbon aviation composite and high porosity GFRP composite) in immersion setup for imaging applications using SSP algorithm.
Highlights
U LTRASONIC nondestructive testing has been used in all areas of industry and research for many years, as it provides a wide range of applications and solutions for the characterization of the physical and mechanical properties of the materials, their inner composition, structural condition, aging behavior, etc [1], [2]
Regarding the methodologies used, despite we would need a full book only to list the number of processing algorithms, procedures and methods used in ultrasound technologies, they can be grouped in two sorts of methods, those based on time domain analysis and those based on frequency domain analysis, which combined lead to a third category for the methods that use both approximations combined [7], [8]
To overcome all the aforementioned limitations, we developed a new set of signals based on arbitrary position and width pulses (APWP) obtained from the conversion of NLFM-compensated signals to rectangular APWP prototypes followed by an iterative optimization process, which results in signals with flat spectrum, high signal-to-noise ratio (SNR) and programmable bandwidth
Summary
U LTRASONIC nondestructive testing has been used in all areas of industry and research for many years, as it provides a wide range of applications and solutions for the characterization of the physical and mechanical properties of the materials, their inner composition, structural condition, aging behavior, etc [1], [2]. The number of environments and materials in which it can be used is endless, from gases to biological tissues, new complex nano-doped composites or large vessel hulls. The range of applications is overwhelming, but almost all of them can be grouped, if attending to their purpose, in three general groups of application: defect detection, characterization and imaging [3]–[6].
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