Nonlinear clutter suppression using split spectrum processing andoptimal detection

Nonlinear processing of outputs from a filter bank to reduce ultrasonic clutter is often referred to as split spectrum processing (SSP). In this work, the optimal detector (OD) for a known transient in additive Gaussian noise is formulated as a filter bank followed by a nonlinear operation (decision). Then, the new formulation is used for several purposes. The first purpose is to present a simple framework for comparison of SSP based on the polarity thresholding (PT) method with SSP based on the OD. The second purpose is to point out that although the OD relies on a test statistic which is a linear function of the observed samples, it can be used for nonlinear clutter suppression. This is because the OD output is used to gate (mask) the original ultrasonic signal and the OD output is a nonlinear function of the filter bank outputs. The third purpose is to identify practical advantages of OD-based SSP, including fewer demands on the experimental data used for parameter tuning, a better understanding of the processing performed, improved resolution, and fewer parameters to tune. The fourth purpose is to discuss the difference between contrast enhancement in clutter suppression and receiver operating characteristics in detection theory. Statistical and geometrical interpretations of both PT and OD are also presented, as well as experimental results from simulated and real ultrasonic data     

航空发动机转静子碰摩故障信号用小波分析的方法检测和分析

针对试飞中发动机转静子碰摩故障信号,首先应用小波阈值方法对原始信号进行降噪处理,由此滤除高频噪声干扰,同时可保留含有重要故障特征的突变信息;对降噪后信号进行连续小波变换,时频分析结果具有很好的实时跟踪转速频率能力,且频带清晰,故障特征明显,为今后真实飞行载荷下的实时故障监测提供一种有效途径。     

Wigner functions from the two-dimensional wavelet group

Following a general procedure developed previously [Ann. Henri Poincaré 1, 685 (2000)], here we construct Wigner functions on a phase space related to the similitude group in two dimensions. Since the group space in this case is topologically homeomorphic to the phase space in question, the Wigner functions so constructed may also be considered as being functions on the group space itself. Previously the similitude group was used to construct wavelets for two-dimensional image analysis; we discuss here the connection between the wavelet transform and the Wigner function.     

Examination of singular scalar light fields using wavelet processing of fork fringes

A single frame fork fringe pattern automatic processing method for detecting optical vortices in coherent light fields using two-dimensional continuous wavelet transformation is proposed. When a vortex sign is of no importance, it is sufficient to calculate the fork interferogram modulation distribution and its normalized gradient map to establish vortex locations without resorting to complicated phase calculations. Normalization of modulation gradient maps enables unambiguous vortex discrimination from local modulation minima without phase singularity. The issue of vortex detection resolution versus carrier fringe frequency and orientation is discussed. Corroboration of simulation and experimental studies of integer and noninteger singular light beams as well as speckle fields reported in the literature and analyzed using different approaches is presented.     

Spatial fringe pattern analysis using the two-dimensional continuous wavelet transform employing a cost function

We present a novel ridge extraction algorithm for use with the two-dimensional continuous wavelet transform to extract the phase information from a fringe pattern. A cost function is employed for the detection of the ridge. The results of the proposed algorithm on simulated and real fringe patterns are illustrated. Moreover, the proposed algorithm outperforms the maximum ridge extraction algorithm and it is found to be robust and reliable.     

Ball grid array (BGA) substrate conduct paths inspection using two-dimensional wavelet transform

The objective of this paper is to implement two-dimensional wavelet transform (2-D WT) in the detection of mousebite, spur, open, and short defect candidates on ball grid array (BGA) substrate conduct paths. Once the defect candidates are located, traditional BGA substrate inspection algorithms can further detect true defects among these suspicious defects. Therefore, the scope and effort during the inspection stage can be significantly reduced. The binary BGA substrate image is processed that shows only conduct path boundaries, which are decomposed directly by 2-D WT. Then, the inter-scale ratio from the wavelet transform modulus sum (WTMS) across adjacent decomposition levels for the edge pixels on BGA substrate conduct path boundaries is calculated. Since irregular edges in a small domain can preserve much more wavelet energy, an edge pixel is considered as an abnormal one or a defect candidate if its inter-scale ratio is less than a predefined threshold. The proposed approach is template-free and easy to implement, so it is suitable for small batch production. Real BGA substrates with synthetic boundary defects are used as testing samples to evaluate the performance of the proposed approach. Experimental results show that the proposed method is able to capture all the true mousebite, spur, open, and short defects without any missing errors by appropriate wavelet basis, decomposition level, and image resolution.     

Continuous two-dimensional on-axis optical wavelet transformer and wavelet processor with white-light illumination

The wavelet transform can be expressed mathematically as a convolution between the input function and a continuous set of scaled wavelet mother functions. Optics has managed to implement only the hybrid wavelet transform in which the set of scaled wavelet mother functions is discrete but the shift is continuous. White-light illumination is used to obtain a two-dimensional, fully continuous, on-axis wavelet transformer. When the illumination source is also spatially incoherent, a complete wavelet processor may be constructed.     

Reliable one-dimensional CPWTR models for two-dimensional insulated polygonal ducts

The heat transfer characteristics of various insulated long polygonal ducts are analyzed using one-dimensional Plane Wedge Thermal Resistance (PWTR) and Plate Thermal Resistance (PTR) models. It is found that errors generated by the PWTR model are all positive and those generated by the PTR model are all negative. Thus, the Combined Plate Wedge Thermal Resistance (CPWTR) model generated by implementing PWTR and PTR models and equating the proportion factors can neutralize the positive and negative errors and generate reasonably accurate results when compared with the two-dimensional numerical analysis. Most errors generated by the one-dimensional CPWTR model are within ±1% for practical sizes and insulated thickness for air-conditioning and refrigeration applications. Meanwhile, the PTR model can be used to estimate accurately the maximum or minimum surface temperatures, respectively, with hot or cold fluids inside an insulated triangular, rectangular or pentagonal duct.     

Hydrothermal processing of dye-adsorbing one-dimensional hydrogen titanate

One-dimensional hydrogen titanate consisting of mixed nanobelts, nanotubes, and nanorods have been processed via hydrothermal followed by high temperature calcination. The processed products have been characterized via transmission electron microscope, X-ray diffraction, spectrophotometer, spectrofluorometer, and Brunauer-Emmett-Teller surface-area measurement techniques for analyzing their morphology, structure, specific surface-area, band-gap, and photoluminescence. The dye-adsorption (in the dark) and photocatalytic activity (under ultraviolet-radiation exposure) of hydrothermally processed one-dimensional hydrogen titanate have been examined using methylene blue as a model catalytic dye agent. Under the present processing and test conditions, it is demonstrated that one-dimensional hydrogen titanate is a better dye-adsorbent than a photocatalyst due to its higher specific surface-area and relatively lower crystallinity.     

Extracting one-dimensional wavelet features with a diffractive optical inner-product transform

A diffractive optical element (DOE) that performs a wavelet transform on one-dimensional signals is presented. The DOE performs an inner product between the input image and a set of wavelet basis functions. The input image is displayed in a spatial light modulator. It is constructed from a one-dimensional signal that is spread over the second dimension. Simulated results obtained by the use of elementary one-dimensional signals are presented.     

Phase retrieval of singular scalar light fields using a two-dimensional directional wavelet transform and a spatial carrier

We evaluate a method based on the two-dimensional directional wavelet transform and the introduction of a spatial carrier to retrieve optical phase distributions in singular scalar light fields. The performance of the proposed phase-retrieval method is compared with an approach based on Fourier transform. The advantages and limitations of the proposed method are discussed.     

Bistatic clutter analysis and range ambiguity resolving for space time adaptive processing

The clutter characteristics of bistatic airborne radar are more complex than those of monostatic airborne radar. The clutter spectra not only vary severely with range, but also vary with bistatic configuration. The problem of range dependence is more serious in monostatic airborne radar. In this paper, the geometry of arbitrary bistatic airborne radar configuration is firstly analysed, and a formula for Doppler frequency calculation with the variables of azimuth angle and bistatic range is deduced, which is an efficient tool for bistatic clutter analysis. Because of the severe clutter range dependence, the processing of compensation is indispensable in space time adaptive processing (STAP). However, when range ambiguity occurs, the compensation is difficult to be applied to each clutter range cell. To solve this problem, a range ambiguity resolving approach is further proposed by utilising azimuth elements in phased array. Because this approach will result in spatial degrees of freedom (DOF) loss, the overlapped subarray processing is introduced in order obtain enough spatial DOF for STAP. By doing so, the compensation for mitigating range dependence can be applied effectively to bistatic clutter.     

Perspectives on solution processing of two-dimensional MXenes

Since the discovery of two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, research on these wonder 2D inorganic compounds has become increasingly intensified with their members quickly expanding. MXenes' state-of-the-art applications are heavily dependent on their processing strategies and manufacturing methods. Nevertheless, the solution processing of MXenes has not been comprehensively reviewed. This review summarizes progress regarding solution processing of MXenes over the past decade as well as outlines key perspectives for future scalable manufacturing strategies. Etching of MAX phases and delamination of MXene are briefly introduced. Rheological properties of MXene dispersions and wetting of the MXene inks, which are crucial for the achievement of high-resolution printing and homogeneous coating, are discussed in detail. We have discussed the ink formulation strategies and fine-tuning of the ink properties to match with that of the targeted substrates to yield efficient yet high-quality printed/coated films/structures. As such, we demonstrate a “map of guidelines” for solution-based processing of MXenes toward high-performance applications, such as electrochemical energy storage, conductive electrodes, electromagnetic interference shielding, and so on.     

基于小波变换的激光超声信号处理研究

激光超声是利用激光照射在媒质中产生的一种超声波,与传统的压电技术相比,它具有非接触,宽带,能定量检测以及时空分辨率高等特点,现已广泛用于材料的无损检测[1,2,3],然而,在对超薄材料的声速的测量中,需要精确测量直达波和回波的时间间隔,但由于受激光超声本身,声传播介质,传感器和测量系统等多种因素的影响,所采集的信号往往是比较复杂叠加波形,如何从中提取有用的波形信息,是测量超薄材料声速的关键.     

基于小波变换的裂谱分析法

 为了提高超声无损检测（UNDT）和无损评价（UNDE）中基础数据的信噪比（SNR）,提出了一种基于小波变换多分辨率分析的裂谱分析新方法．该方法在分析传统裂谱分析（SSP）方法原理及其局限性的基础上,通过采用小波变换多分辨率分析能力将原始超声回波信号进行等Q子带分解,然后按照一定的信噪分离规则来消除噪声,达到提高信噪比的目的．实验结果表明,与传统裂谱分析方法相比,该方法提高了消噪性能的稳定性,增强了湮没晶粒(或其他散射体)散射中的缺陷回波信号能力．     

Automatic defect inspection of patterned thin film transistor-liquid crystal display (TFT-LCD) panels using one-dimensional Fourier reconstruction and wavelet decomposition

Large-sized flat-panel displays have become increasingly important for use in computer monitors and televisions. This paper has considered the problem of automatic visual inspection of micro-defects including pinholes, scratches and particles in patterned thin film transistor-liquid crystal display (TFT-LCD) panel surfaces. For large-sized TFT-LCD panel inspection, high-resolution line scan is demanded. We propose a global one-dimensional (1-D) Fourier-based image reconstruction scheme that directly works on the 1-D line images instead of the traditional two-dimensional area images. The proposed method fully uses the inherent geometric structure of a TFT-LCD panel. It first eliminates the frequency components that represent the periodic pattern of a TFT-LCD line image in the 1-D Fourier spectrum and then back-transforms the 1-D Fourier-domain image to the 1-D spatial domain image using the inverse Fourier transform. The Fourier reconstruction process can effectively remove the patterned background and distinctly preserve local anomalies in the resulting 1-D image. Wavelet decomposition is further applied to remove uneven illumination in the filtered image so that defects can be easily segmented with simple statistical control limits. Experimental results on a number of micro-defects embedded in TFT-LCD panels show that the proposed method can reliably detect various ill-defined defects without designing and measuring the quantitative features of individual defect types.     

Short-time Fourier transform and wavelet transform with Fourier-domain processing

We discuss the semicontinuous short-time Fourier transform (STFT) and the semicontinual wavelet transform (WT) with Fourier-domain processing, which is suitable for optical implementation. We also systematically analyze the selection of the window functions, especially those based on the biorthogonality and the orthogonality constraints for perfect signal reconstruction. We show that one of the best substitutions for the Gaussian function in the Fourier domain is a squared sinusoid function that can form a biorthogonal window function in the time domain. The merit of a biorthogonal window is that it could simplify the inverse STFT and the inverse WT. A couple of optical architectures based on Fourier-domain processing for the STFT and the WT, by which real-time signal processing can be realized, are proposed.     

Surface porosity during vacuum bag-only prepreg processing: Causes and mitigation strategies

In this study, we employ a parametric approach coupled with surface analysis to identify the source(s) of surface porosity and to develop effective mitigation strategies. Results confirmed that surface porosity was primarily associated with air that was trapped at the tool–prepreg interface during layup. The magnitude and distribution of surface porosity was affected by multiple parameters, including vacuum hold time, freezer and out time, and material and process modifications that affect air evacuation. The results indicated that prepreg out time (and thus tack) and vacuum quality were the primary drivers of surface porosity; for example, surface porosity decreased by 83% after just four days of out time and by 99% after 14 days of out time. These factors were used to formulate guidelines to mitigate surface porosity by (a) increasing the driving force for air evacuation and/or (b) increasing the permeability of the tool–prepreg interface.