Small Number Of Transducers Research Articles

A single-channel virtual receiving array using a time-reversal chaotic cavity.

Chaotic reverberation in a cavity, when coupled with time reversal acoustics, can be harnessed to build a perfect time-reversal mirror for transmitting and receiving highly focused sounds with a small number of transducers. In this article, a virtual receiving array, comprised of a single receiving transducer and a chaotic cavity, is developed based on time reversal processing of the reverberation inside the cavity. A prototype array, having 10 × 10 virtual receiving elements, is built and evaluated against a comparable physical array in terms of its localization and waveform reproduction capabilities. It turns out that the most crucial factor in the success of a virtual array is the ergodicity of its chaotic cavity, the exact mathematical expression for which is also derived. The virtual receiving array presented here may find some niche applications in reverberant environments, where a physical array turns out to be too costly or cumbersome to operate.

Benefits of software beamforming for ultrasonic inspections

Nondestructive testing (NDT) is used in more and more industries and inspection techniques must constantly adapt to complex materials (coarse grained materials, composites, etc.), defects (corrosion, fatigue crack, HTHA, etc.) or new inspection standards. Ultrasonic imaging methods have long been limited by low data throughput and a small number of transducers, but this is no longer the case with nowadays electronic devices and computers which enable large amounts of raw data to be acquired and processed in real time. The Delay-and-Sum (DAS) algorithm, which has been used almost automatically in many applications over the past few years, is now outperformed by advanced processing methods. In this context, this article presents few real-time advanced reconstruction techniques that enhance the quality of ultrasonic images on typical industrial applications. The first example concerns coherence beamformers such as Phase Coherence Imaging (PCI) which exploit the phase information of the signals. This type of method proves to be highly effective in detecting diffraction echoes from crack-like defects or in improving contrast and resolution of images from corroded samples inspections. The second example uses cross-correlation techniques that have been known for years to improve detection in various fields. Correlating the data with the transmitted signal reduces electronic noise and improves signal-to-noise ratio. This application also benefits from recent progress of ultrasonic devices that are able to modulate complex excitation signals in emission. This type of method is very promising for the inspection of attenuative materials and coarse-grained steels.

A Novel Baseline-Free Method for Damage Localization Using Guided Waves Based on Hyperbola Imaging Algorithm.

Most imaging methods based on ultrasonic Lamb waves in structural health monitoring requires reference signals, recorded in the intact state. This paper focuses on a novel baseline-free method for damage localization using Lamb waves based on a hyperbolic algorithm. This method employs a special array with a relatively small number of transducers and only one branch of the hyperbola. The novel symmetrical array was arranged on plate structures to eliminate the direct waves. The time difference between the received signals at symmetrical sensors was obtained from the damage-scattered waves. The sequence of time difference for constructing the hyperbolic trajectory was calculated by the cross-correlation method. Numerical simulation and experimental measurements were implemented on an aluminum plate with a through-thickness hole in the current state. The imaging results show that both the damages outside and inside the diamond-shaped arrays can be localized, and the positioning error reaches the maximum for the diamond-shaped array with the minimum size. The results indicate that the position of the through-hole in the aluminum plate can be identified and localized by the proposed baseline-free method.

Sparse ultrasonic guided wave imaging with compressive sensing and deep learning

A sparse UGW imaging algorithm based on compressive sensing and deep learning models (CSDL) is proposed in this paper to solve the problem that the imaging quality is restricted by the number of transducers in service. The sparse detection signals acquired by a small number of transducers are roughly and finely reconstructed by compressive sensing (CS) and deep learning models. Finally, the reconstructed detection signals are input to the imaging algorithm for sparse imaging. After CSDL, the average correlation coefficients in simulation are respectively improved from 0.9218 and 0.8896 to 0.9626 and 0.9441 with 32 and 16 transducers, which are close to the average correlation coefficient 0.9682 when all 64 transducers are in service. Experimental results also verify the feasibility of CSDL and the average correlation coefficients rise from 0.6091 and 0.6530 to 0.9058 and 0.8357 with 32 transducers and 16 transducers. The proposed method is proven to be effective with only a quarter of transducers in sparse imaging with almost no sacrifice in image quality.

A Novel Sensitivity Matrix Construction Method for Ultrasonic Tomography Based on Simulation Studies

Ultrasonic tomography (UT) is a nondestructive technique that is widely applied in industrial process imaging. In this paper, a novel sector diffusion method for construction of a sensitivity matrix is proposed to improve the performance of a linear back-projection (LBP) algorithm. The so-called sectordiffusion-matrix-based LBP (SLBP) was applied to reconstruct gas/liquid two-phase flow. A series of models has been established to evaluate the effectiveness of the method. The images reconstructed by traditional LBP and SLBP are presented and compared. Two quantitative evaluation criteria including position error and image correlation coefficient (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cr</sub> ) were defined and used to evaluate the quality of the reconstructed images. The imaging results indicate that the SLBP method is valid and feasible to improve the quality of reconstructed images, especially for a UT system with a small number of transducers. The evaluation criterion I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cr</sub> is improved by 26%, 20%, 9%, and 4% for four different flow patterns with an eight-transducer UT system in 128 × 128 pixels and improved by 26%, 20%, 10%, and 1% in 40 × 40 resolution.

Coding of Probe Signals in the Tomography of Acoustic Nonlinear Parameters

Different ways of coding radiated signals that probe a tomographized object such as a liquid medium are compared in order to reconstruct the spatial distribution of acoustic nonlinear parameters using a small number of transducers. The results from reconstructing model nonlinear scatterers are discussed. The region of spatial frequencies available for reconstruction is analyzed.

Defect position and size estimation in billet by ultrasonic transmission method using a small number of transducers

For a steel billets inspection, we have proposed defect detection and size estimation from time-of-flight (TOF) profile by ultrasonic transmission method with linear scanning. In this method, a defect is detected by deviation of TOF of transmitted wave caused by diffraction at the defect. In addition to defect detection, defect size can be estimated from deviation of TOF because TOF is affected by defect size. However, deviation of TOF is also affected by the depth of the defect, which cannot be precisely estimated only by linear scanning of a pair of transducers. In this study, we propose defect position and size estimation using multiple pairs of transducers and validity of the method was evaluated. By using multiple pairs of transducers, TOF of inclined paths, where transducers is not facing each other are also obtained as well as parallel paths, so that the position of defect can be estimated. As a result, the defect position could be estimated by using two pairs of transducers and defect size could also be estimated correctly compared with conventional linear scanning method.

Block-sparse reconstruction and imaging for Lamb wave structural health monitoring.

A frequently investigated paradigm for monitoring the integrity of plate-like structures is a spatially-distributed array of piezoelectric transducers, with each array element capable of both transmitting and receiving ultrasonic guided waves. This configuration is relatively inexpensive and allows interrogation of defects from multiple directions over a relatively large area. Typically, full sets of pairwise transducer signals are acquired by exciting one transducer at a time in a round-robin fashion. Many algorithms that operate on such data use differential signals that are created by subtracting prerecorded baseline signals, leaving only signal differences introduced by scatterers. Analysis methods such as delay-and-sum imaging operate on these signals to detect and locate point-like defects, but such algorithms have limited performance and suffer when potential scatterers have high directionality or unknown phase-shifting behavior. Signal envelopes are commonly used to mitigate the effects of unknown phase shifts, but this further reduces performance. The blocksparse technique presented here uses a different principle to locate damage: each pixel is assumed to have a corresponding multidimensional linear scattering model, allowing any possible amplitude and phase shift for each transducer pair should a scatterer be present. By assuming that the differential signals are linear combinations of a sparse subset of these models, it is possible to split such signals into location-based components. Results are presented here for three experiments using aluminum and composite plates, each with a different type of scatterer. The scatterers in these images have smaller spot sizes than delay-and-sum imaging, and the images themselves have fewer artifacts. Although a propagation model is required, block-sparse imaging performs well even with a small number of transducers or without access to dispersion curves.

A prototype for a tomography system using third-order acoustic nonlinear effects

A prototype of a tomography system for reconstructing the distributions of acoustic nonlinear parameters is developed and manufactured on the basis of the effect of nonlinear noncollinear interaction of three primary waves. Application of coded primary signals with further correlation processing of a detected combination signal makes it possible to reconstruct the complete image of an object as a result of a single experiment using a small number of transducers, i.e., three radiators and one receiver. A mirror system is proposed, consisting of two coaxial conical acoustic mirrors that make it possible to transform the front of a wave from a cylindrical transducer into a homogeneous quasi-plane beam with a large width close to the real medical diagnostics requirements. Results of physical experiments are given.

A spatial correlation method for reconstructing blood flow velocity vector in the nonlinear tomography

A method for reconstructing the total blood flow velocity vector distribution is proposed. Experimental data are obtained with the help of the second-order nonlinearity parameter tomography scheme that employs wideband coded signals and a small number of transducers. The data are processed using the selective matched filtering technique in combination with spatial correlation of fragments of the speckle structure that appears in the process of reconstruction of the spatial nonlinearity parameter distribution. Results of numerical simulations are presented.

Time reversal acoustic receiver.

The time reversal acoustics (TRA) principle has been employed for focusing ultrasonic waves in various biomedical and industrial applications. TRA‐based ultrasonic transmitters have numerous advantages over conventional methods of ultrasound focusing such as an ability to effectively focus and steer focal spot with a transmitter comprising even a single piezoelectric transducer. In this study it is shown that such TRA focusing systems with a small number of transducers can also work in the receiving mode and detect both active and passive targets with a spatial resolution close to a half wavelength. The feasibility tests were conducted using two types of TRA receivers. The first TRA receiver comprising a water‐filled reverberator with a few 600 kHz piezoceramic elements attached was used for the detection of the signal radiated by a miniature (1 mm) emitter in a water tank. It was shown that the TRA receiver can detect the position of this point emitter with accuracy better than 1 mm. The second test was conducted with an acoustic reverberator made of series of thin randomly shaped membranes and a single disk transducer with a frequency of 3.3 MHz. This TRA receiver allowed the localization of a reflecting target illuminated by a plane acoustic wave with resolution also better than 1 mm.

Time-Reversal Acoustics in Biomedical Engineering

Time-reversal is a very powerful method for focusing through complex and heterogeneous media and shows very promising results in biomedical applications. In this presentation, we review some of the main applications investigated during the past decade. An iterative implementation of the time-reversal process allows tracking gallstones in real time during lithotripsy treatments. In this application domain, a smart exploitation of the reverberations in solid waveguides permits the focusing of high amplitude ultrasonic shock waves with a small number of transducers. Because time reversal is able to correct the strong distortions induced by the skull bone on ultrasonic propagation, this adaptive focusing technique allows brain ultrasonic therapy through the skull bone. Finally, time-reversal focusing guided by speckle noise can be implemented to correct distortion in medical ultrasonic imaging and recent results will be presented.

Acoustic tomography of the nonlinear parameter by a small number of transducers

A wave approach to solving the problem of reconstructing the acoustic nonlinear parameter distribution by a simple transmitting-receiving system with broadband modulation of primary waves is proposed. The approach uses the effect of sound scattering by sound, which makes it possible to employ a complicated time processing of signals instead of spatial processing and, consequently, to reduce the number of transmitting and receiving transducers. The possibility of practical implementation of a two-dimensional tomographic scheme based on coherent correlation processing of broadband quasi-random signals at combination frequencies is analyzed. Results of model numerical experiments on reconstructing complicated spatial distributions of the nonlinear parameter are presented.

A design of fast steering filters based on the adaptive fusion of predesigned finite impulse response filters for microphone array

With a microphone array, a desired distant‐talking speech signal can be acquired selectively by steering the directivity in noisy environments. To form directivity, adaptive beamformers have been proposed as conventional beamformers. They can form null directivity with a small number of transducers. However, adaptive beamformers have a serious drawback: it can’t realize the noise‐robust adaptation in variable noisy environments, because it has necessary heavy computational costs to train multi‐channel adaptive steering filters. To solve this problem, we propose to design the fast steering filters based on the adaptive fusion of predesigned FIR filters in variable noisy environments. The adaptive fusion of predesigned FIR filters is realized by adding (or subtracting) various predesigned FIR filters immediately. Therefore, the proposed method can quickly design the steering filters even in variable noisy environments. In addition, microphone calibration is indispensable for applying the proposed method. Th...

A super-element approach for structural identification in time domain

For most time-domain identification methods, a complete measurement for unique identification results is required for structural responses. However, the number of transducers is commonly far less than the number of structural degrees of freedom (DOFs) in practical applications, and thus make the time-domain identification methods rarely feasible for practical systems. A super-element approach is proposed in this study to identify the structural parameters of a large-scale structure in the time domain. The most interesting feature of the proposed super-element approach is its divide-and-conquer ability, which can be applied to identify large-scale structures using a relatively small number of transducers. The super-element model used for time domain identification is first discussed in this study. Then a parameterization procedure based on the sensitivities of response forces is introduced to establish the identification equations of super-elements. Some principles are suggested on effective decomposing of the whole structure into super-elements for identification purposes. Numerical simulations are conducted at the end of this study. The numerical results show that all structural parameters can be identified using a relatively small number of transducers, and the computational time can also be greatly shortened.

Ultrasonic/Guided Waves for Structural Health Monitoring

Structural damage detection and monitoring is one of the major maintenance activities in transportation, processing and civil engineering. Current procedures are based on scheduled inspections which are often time/labour consuming and expensive. Guided ultrasonic waves offer the ability of inspecting large structures with a small number of transducers. Recent developments in smart sensor technologies allow for integration of these transducers with monitored structures. This is associated with a new design philosophy leading to more efficient and economically attractive structures. The paper briefly discusses various damage detection methods based on structural, ultrasonic and guided ultrasonic waves. The focus is on recent research advances in damage monitoring techniques, smart sensor technologies and signal processing.

3D Ultrasonic Imaging for Sheet Metal Hydroforming

With fluid forming processes getting more and more common in industrial application a lot of research is carried out to analyze the forming behavior of sheet metal within these processes. In order to gather experimental information about the forming behavior of the workpiece, an imaging system is presented, that will allow determining the actual shape within the forming process. The system has to be functional in liquid media as well as in a high pressure environment. Therefore an ultrasonic based system has been chosen, consisting of a small number of transducers which are alternately used as transmitter and receiver. It is possible to cover 100 by 100 mm² with only 10 by 10 transducers by the use of special algorithms. The reconstruction of the echographic images from the recorded data is done by a SAFTalgorithm (synthetic aperture focusing technique) followed by an analysis with special contourdetection algorithms, which are able to scan the image for contour data. It can be shown that the accuracy of the reconstruction is quite good in the 2.5-dimensional domain, if appropriate contour models for the description are used. Because of the small number of transducers and the specular reflection of the signals, the quality of the image can be improved significantly by the extension of the SAFT algorithm with an angle-weighted factor. The three dimensional reconstruction is also possible and will be demonstrated for simple geometries. The ability for sampling more complex geometries and enhancing the accuracy will be achieved by the integration of three dimensional contour models and three dimensional angle-weighting.

Time reversal kaleidoscope: A smart transducer for three-dimensional ultrasonic imaging

The design of two dimensional (2D) arrays for three dimensional (3D) ultrasonic imaging is a major challenge in medical and nondestructive applications. Thousands of transducers are typically needed for beam focusing and steering in 3D volumes. Here, we report a promising approach for producing 3D images with a small number of transducers using the combined concepts of time reversal mirrors and chaotic reverberating cavities. Due to multiple reverberations inside the cavity, a “kaleidoscopic” transducer array is created with thousands of virtual transducers equivalent to 2D matrices. Beyond the scope of 3D medical imaging, this work leads to the concept of “smart” transducer.

Time-reversal acoustics in biomedical engineering.

Time reversal is a very powerful method for focusing through complex and heterogeneous media and shows very promising results in biomedical applications. In this paper, we review some of the main applications investigated during the past decade. An iterative implementation of the time-reversal process allows tracking gallstones in real time during lithotripsy treatments. In this application domain, a smart exploitation of the reverberations in solid waveguides permits the focusing of high-amplitude ultrasonic shock waves with a small number of transducers. Finally, because time reversal is able to correct the strong distortions induced by the skull bone on ultrasonic propagation, this adaptive focusing technique is very promising for ultrasonic hyperthermia brain therapy.

Temperature Distribution in a Rectangular Space Measured by a Small Number of Transducers and Reconstructed from Reflected Sounds

In this paper, we describe a method for reconstruction of the temperature distribution in a rectangular space using a small number of acoustic transducers. The transducers were decreased in number by utilizing the reflected waves from wall surfaces. Spatial resolution of five divisions has been realized by installing only two speakers for a wall surface and two microphones for the counter wall surface. Additionally, the partition pattern in the resolution was switched depending on the situation that required close distribution without displacing the acoustic transducers. In this study, simulation and experiment were deployed for testing the adequacy of this method. The possibility of switching the partition pattern in the resolution was realized by numerical simulation and the result was compared to temperature distribution that was measured in real space. The experiment was performed in real space which has a significant temperature distribution with heated air. The temperature distribution reconstructed from the measurement results was similar to the distribution measured by thermocouples.