Reconstruction of the images of reflectors from ultrasonic echo signals using the maximum-entropy method

It is proposed to use the maximum-entropy method (MEM) for processing ultrasonic echo signals for reconstructing images of reflectors with a high signal-to-noise ratio and a low level of “side lobes” of the point-scattering function. When processing echo signals, the pulse-propagation paths can be considered taking reflections from irregular boundaries of a tested object with the wave-type transformation into account. In model experiments, images of reflectors were obtained taking the refractions of rays at the rough surface into account, when echo signals were recorded both using an ordinary single-element transducer in the transceiver mode and an antenna array that recorded echo signals in the double- and triple-scanning modes. The reconstructed images have a resolution that exceeds the resolution according to the Rayleigh criterion. The MEM makes it possible to obtain images of flaws with low-level side lobes, when less than 10% of the complete set of echo signals are used.     

The possibility of using the maximum-entropy method in ultrasonic nondestructive testing for increasing the resolution of echo signals

The problem of deconvoluting echo signals via regularization according to the maximum entropy method followed by the reconstruction of 2D images via the Fourier transformation synthetic aperture focusing technique (FT-SAFT) is considered. Numerical and experimental simulations have been performed. The possibility of improving the ray resolution of echo signals and reducing the speckle-noise level is demonstrated. The results obtained are compared to the results of constructing an AR model of the spectrum of echo signals.     

金属材料小缺陷超声反射信号建模及识别

为了能从含噪声金属材料超声检测信号中有效识别出微小缺陷回波,建立了金属材料超声反射信号模型并提出了基于相关系数的微小缺陷回波识别方法。对含微小缺陷金属材料超声脉冲反射信号的成分进行分析,建立了基于散射声场与高斯回波理论的优化超声回波模型。设计了超声缺陷回波位置识别方法。该方法对超声脉冲反射信号去噪后,取探头发射脉冲信号为参考信号;然后与去噪后的信号逐段求解相关系数;最后对该相关系数序列进行阈值化处理,获得缺陷回波在超声回波信号中的位置。将利用上述优化超声回波模型生成的超声反射信号及其频谱与实验获得的金属材料超声反射信号及其频谱进行了对比,结果表明:两者的时频域特征具有一致性。当将阈值设定为相关系数序列最大值的60%时,能够有效从超声背散射信号中识别出金属材料微小缺陷回波。     

Determination of the reflector type from an image reconstructed using echo signals measured with ultrasonic antenna arrays

The application of the digital image focusing (DFA) method to the determination of the types of detected reflectors is considered. For this purpose two antenna arrays (AAs) are used, which are placed on opposite sides of the reflector and using which echo signals are recorded in three acoustic channels in the double-scanning mode. The first and second acoustic channels transmit and receive echo signals using the first and second AA, respectively, and the third channel is tuned so that the first AA transmits pulses and the second AA receives echo signals. Using signals in each channel, many partial images can be reconstructed in a common coordinate system according to different acoustic schemes with allowance for both multiple reflections from irregular boundaries of a tested object and effects of transformation of the wave types. Combining partial images makes it possible to obtain a high-quality image, in which the entire boundary of the reflector is seen and using which an attempt to automate the procedure of evaluating the reflector size and determining its type was made. Such an approach allows one to reduce the subjective influence of an operator on the testing results. It is shown that in order to increase the image resolution, the spectrum of echo signals can be extrapolated by the spectrum splitting method jointly with the construction of an AR model of their spectrum. The results of model experiments that confirm the possibility of determining the reflector type are presented.     

基于经验与变分混合分解的超声回波信号噪声消除方法

超声缺陷检测结果易受超声回波信号中复杂噪声的干扰,为了提高超声缺陷检测的准确度,提出一种基于混合分解的 超声回波信号噪声消除方法。 采用经验模态分解算法结合相关系数指标对超声回波信号进行预处理,得到消除低频噪声分量 的超声回波预处理信号。 基于变分模态分解将该预处理信号分解为一系列窄带本征模态函数,引入互信息指标估计变分模态 分解的最优模态数量,并根据窄带本征模态函数与预处理信号的相关系数提取有用的模态分量,实现对超声回波信号去噪结果 的重构。 通过仿真和实测超声回波信号验证了本文方法的去噪性能,并与现有方法进行了对比。 结果表明,本文方法可同时消 除超声回波信号中的高频和低频噪声,在不同信噪比条件下 EMD、VMD 和本文方法去噪结果的 SNR 均值分别为 10. 01、9. 48 和 16. 09 dB,验证了本文方法对于超声回波信号噪声消除的优越性。     

Pattern recognition and signal processing of ultrasonic echo signal on two-phase flow

This paper presents a novel measurement method using ultrasonic echo signals on the flow of air–water mixtures. This method has the capability of measuring an instantaneous echo intensity profile along an ultrasonic beam, so it is expected to apply to pattern recognition of two-phase flow. Additionally, this method has an advantage compared with conventional techniques because of the clump-on type. The principle of the flow pattern recognition is based on the delay time and strength of the pulse echo. In this paper, first of all, the transmission of ultrasound through solid plates, which are made of plexiglass and carbon steel, has been investigated and the effective incidence angles for these materials were found. Then, echo signals reflected off a boundary between water and air in a vertical pipe, having a diameter of 50 mm, were obtained using an ultrasound system, and the effects estimated of a two-phase pattern, from bubbly to slug flow, on the signals. In addition, water flows down the inner surface of a pipe as annular flow, and the echo signals then also investigated.     

Ultrasonic defect detection of a petroleum pipeline in a viscoelastic medium

During detection of the defects in the inner wall of a petroleum pipeline via an ultrasonic nondestructive evaluation method, the defect echo and the inner-wall echo often overlap. It is difficult to identify the arriving time of the defect echo. When an ultrasonic wave propagates in a viscoelastic medium, the stress relaxation and creep deformation result in a signal characterized by frequency. The wave speed and attenuation rate are dependent on the frequency; thus, the ultrasonic signal has different shapes along the wave propagation path. Sometimes the wave shape is wry. The empirical mode decomposition technology used for separating the overlapping echo signals has been presented in this paper. With such technology, the original ultrasonic signal can be decomposed and then some intrinsic mode functions (IMFs) and a residue can be obtained. Different IMFs contain different echo signals. Some useful IMFs are selected to reconstruct the ultrasonic signal. The peaks of the reconstructed ultrasonic signal envelope indicate the arriving time of the echo signals. The experimental results show that this method is effective for detection of the defects in the inner wall of a petroleum pipeline. The text was submitted by the authors in English.     

Application of window functions in problems of pattern recognition of ultrasonic signals

A method for identifying ultrasonic pulses using window functions is discussed. The theoretical fundamentals of signal processing are considered. Experiments on the detection of echo signals in the presence of noise and interference and separation of closely positioned echo signals are described and the possibility of recognizing their combinations is considered.     

Coherent Reconstruction of Flaw Images during Detection of Echo Signals in a Separate Mode

The possibility of using a scheme for separate detection of echo signals for obtaining high-quality flaw images is substantiated. A conventional algorithm for obtaining images by the projection method in the spectral space and an algorithm for obtaining images that is not limited by the approximation of a plane incident wave are considered. Coherent storage of images reconstructed from echo signals, which were detected in a separate scheme but at different positions of the radiator (transmitter), reduces the speckle-noise level and increases the resolution of flaw images. Positive and negative features of these algorithms are analyzed. Numerical and model experiments have demonstrated the feasibilities of different schemes of detecting echo signals for obtaining flaw images.     

基于小波变换模极大值法的薄层材料超声测厚研究

薄层材料超声测厚过程中,薄层上下界面回波叠加导致信号难以区分。针对这一难点,利用小波变换模极大值法(Wavelet Transform Modulus Maxima-WTMM)对超声回波信号奇异点进行检测,克服了传统回波叠加无法分离的缺陷,得到了薄层厚度测量方法。     

A Pulse Coherent Spectrometer of Nuclear Magnetic Resonance for Magnetic Materials

A pulse coherent automated nuclear magnetic resonance spectrometer designed to study samples of magnetically ordered materials in the frequency range 20–200 MHz is described. The spectrometer provides the programming of time and frequency parameters of pulse sequences and allows phase-sensitive detection of echo signals with their subsequent representation in digital form. Experimentally observed nuclear spin echo signals in an isotope-enriched YIG film are presented.     

Compression of ultrasonic signals with the lifting scheme wavelet transform

The wavelet transform based on a lifting scheme was applied to compress the ultrasonic echo signals of pipeline defects. The advantages of lifting are the custom-design property, the speedup of the fast wavelet transform, and the promotion of fully in-place calculation of the wavelet transform. The optimum wavelet base for decomposition can be decided according to the wavelet coefficient energy ratio. The optimum number of decomposition levels can be chosen via considering the percent root-mean-square difference and the correlation coefficients. The optimum compression ratio of ultrasonic echo signals can be obtained as well via considering the percent root-mean-square difference and the correlation coefficients. The text was submitted by the authors in English.     

Analyzing the Phase of DFA Image for Determining the Type of Detected Reflector

Russian Journal of Nondestructive Testing - Methods have been developed in ultrasonic flaw detection for recording and analyzing echo signals with the aim to determine the type of a reflector and...     

Ultrasonic diagnostics of a metal structure on the basis of the method of time decorrelation of signals of acoustic emission resulting from kinetic indentation

It is shown that structural noise (SN) and informative echo signals can be decorrelated not only during ultrasonic flaw detection for extracting signals from SN but also in ultrasonic structuroscopy, in which reflections from a structure are not a disturbance but yield information on the structure of the tested material. Using the technique of wavelet processing of SN signals proposed in this study, signals of acoustic emission that appear during kinetic indentation of metals are analyzed in order to determine physicomechanical characteristics of metals.     

Changes in echo-pulse instantaneous frequency when scanning the transducer

A theoretical model has been proposed that makes it possible to explain peculiarities in the frequency characteristics of echo signals at the output of a receiving transducer depending on the geometrical parameters of a reflecting spherical surface when the transducer axis is displaced with respect to the surface’s center of curvature, that is, during scanning. It has been demonstrated that both with and without displacement of the transducer’s geometrical axis with respect to the center of curvature of the reflecting surface, the type of changes in the temporal dependences of the instantaneous frequency of signals at the output of the receiving transducer essentially depends on the geometrical parameters of a wavefront reflected from the surface: the smaller the radius of wavefront curvature at the aperture of the receiving transducer, the stronger the changes of the instantaneous frequency in the echo pulse. It has also been shown that the larger the curvature of this surface, the stronger the changes in the frequency characteristics of the echo pulse as the transducer axis is displaced with respect to the center of curvature of the reflecting spherical surface.     

超声波流量计中相关时差法的应用

介绍了一种基于互相关理论的时差法超声波流量测量算法.系统采用时差法声道布置,采集流体静止和有流速状态下的两组回波信号,通过计算相关函数的峰值得到两回波信号时间差,间接得出流量,提高了时差法渡越时间获取的精度.极性相关算法的引入,大大提高了系统运算速度,提高了实时性.采用伪随机信号作为超声激励信号,克服了算法测量范围小的缺陷,更可提高系统精度.实验表明该系统测量精度高,实时性好.     

The calibration of an ultrasonic antenna array installed on a wedge

A method that is proposed for calibrating an antenna array that is installed on a wedge provides refinement of the coordinates of the centers of piezoelectric elements. Precise knowledge of the coordinates will make it possible to obtain images in which reflections from reflectors will coincide with their actual positions with an accuracy no worse than a half wavelength. The calibration principle involves the minimization of the objective function that describes the difference between the antenna-array-measured echo signals from a 2-mm-diameter side drilled hole (SDH) at a depth of 12 mm and the calculated echo signals from the same hole. The algorithm operation was tested for echo signals calculated in the CIVA program. The calibration results for several antenna arrays on wedges and images of an SDH are presented, which were obtained using the method for digital antenna-array focusing (DAAF) for various acoustic systems.     

Excitation of a circular-aperture transducer with an echo pulse reflected from a spherical surface

A theoretical model is suggested that allows one to explain peculiarities of the frequency response of echo signals at the output of a receiving transducer versus geometrical parameters of reflecting surfaces. It is shown that the way the instantaneous frequency of signals at the output of the receiving transducer changes depends on the curvature radius of reflected wavefront at the receivingtransducer aperture. It has been established that the frequency modulation of the pulsed echo signal is due to nonuniformity of the distribution of the phase of acoustic signal at the receiving-transducer aperture; this can be characterized by the radii of Fresnel zones that correspond to the reflected wavefront in relationship to the aperture radius of the circular transducer. Data on the frequency modulation of pulsed echo signals expressed in terms of dimensionless parameters (normalized frequency deviations) that were obtained experimentally and calculated by the suggested theoretical model demonstrate fair qualitative and quantitative agreement.     

The fundamentals of the theory of spatiotemporal signal processing as applied to problems of ultrasonic flaw detection of articles from complexly structured materials

The basic propositions of spatiotemporal signal processing as applied to the problems of ultrasonic testing of complex-structure articles with a high attenuation of acoustic signals are considered. Different algorithms of extracting informational echo signals from white and structural noise during multichannel spatiotemporal signal processing are studied.     

Estimating the characteristics of piezoelectric transducers

A method for estimating the sensitivity of piezoelectric transducers (PETs) is proposed. It is based on normalizing the amplitude of the PET-exciting voltage and calculating the sensitivity with respect to a conventional unit level. The results obtained from measuring PETs produced by well-known foreign manufacturers and at our factory are presented. Attention is also given to measuring the duration of echo signals. The described technique for evaluating the characteristics of echo signals can also be applied to contact angle PETs. The results obtained and proposals put forward in this study can be considered when correcting the Russian Standard for measuring the PET characteristics.Translated from Defektoskopiya, Vol. 41, No. 1, 2005, pp. 20–28.Original Russian Text Copyright © 2005 by Senyutkin, Chineikina.