Accuracy of coded excitation methods for measuring the time of flight: Application to ultrasonic characterization of wood samples

Abstract

Ultrasound computed tomography (USCT) using the transmission mode is a way to detect and assess the extent of decay in wood structures. The resolution of the ultrasonic image is closely related to the different anatomical features of wood. The complexity of the wave propagation process generates complex signals consisting of several wave packets with different signatures. Wave paths, depth dependencies, wave velocities or attenuations are often difficult to interpret. For this kind of assessment, the focus is generally on signal pre-processing. Several approaches have been used so far including filtering, spectrum analysis and a method involving deconvolution using a characteristic transfer function of the experimental device. However, all these approaches may be too sophisticated and/or unstable. The alternative methods proposed in this work are based on coded excitation, which makes it possible to process both local and general information available such as frequency and time parameters. Coded excitation is based on the filtering of the transmitted signal using a suitable electric input signal. The aim of the present study was to compare two coded-excitation methods, a chirp- and a wavelet-coded excitation method, to determine the time of flight of the ultrasonic wave, and to investigate the feasibility, the robustness and the precision of the measurement of geometrical and acoustical properties in laboratory conditions. To obtain control experimental data, the two methods were compared with the conventional ultrasonic pulse method. Experiments were conducted on a polyurethane resin sample and two samples of different wood species using two 500 kHz-transducers. The relative errors in the measurement of thickness compared with the results of caliper measurements ranged from 0.13% minimum for the wavelet-coded excitation method to 2.3% maximum for the chirp-coded excitation method. For the relative errors in the measurement of ultrasonic wave velocity, the coded excitation methods showed differences ranging from 0.24% minimum for the wavelet-coded excitation method to 2.62% maximum for the chirp-coded excitation method. Methods based on coded excitation algorithms thus enable accurate measurements of thickness and ultrasonic wave velocity in samples of wood species.

Reflection-Mode Ultrasound Computed Tomography Based on Wavelet Processing for High-Contrast Anatomical and Morphometric Imaging

“…But the spatiotemporal resolution of the image is reduced. The "lost" resolution can be enhanced using efficient signal processing methods, such as signal deconvolution [22,23] or pulse compression [24,25]. Our group has developed a procedure for multiscale wavelet decomposition of the signals allowing all available wave packets to be processed in terms of frequency and time [26].…”

“…This method, called "wavelet-based coded excitation" (WCE), can be used to determine, independently, the velocity of the ultrasonic wave and the path of the wave through the thickness of the cortical zone of a bone, geometrically assimilated to a succession of contiguous parallelepiped plates [20]. The method can also be adapted to the measurement of the apparent density of the bone mass [25,27]. When adapted to a circular antenna and to canonical targets, the method improves the contrast by at least 11% [28].…”