Traditional Time Domain Research Articles

Application of wavelet transform for the impulse response of pile

The purpose of this paper is to study the capabilities of the impulse response method in length and flaw detecting for concrete piles and provide a suggested method to find small-size flaws in piles. In this work, wavelet transform is used to decompose the recorded time domain signal into a series of levels. These levels are narrowband, so the mix of different dominant bandwidths can be avoided. In this study, the impulse response method is used to analyze the signal obtained from the wavelet transform to improve the judgment of the flaw signal so as to detect the flaw location. This study provides a new way of thinking in non-destructive testing detection. The results show that the length of a pile is easy to be detected in the traditional reflection time or frequency domain method. However, the small flaws within pile are difficult to be found using these methods. The proposed approach in this paper is able to greatly improve the results of small-size flaw detection within piles by reducing the effects of any noise and clarifying the signal in the frequency domains.

Generalized finite difference time domain simulation on acoustical moving boundary problems

Mesh-free methods have the potential to solve acoustic problems with moving boundary, complex geometries and in flowing fluids. In this paper, generalized finite difference time domain (GFDTD), a time-domain representation of the generalized finite different method (GFDM), is extended to solve acoustic wave problems with moving boundary. To explore the numerical performance of GFDTD, its stability and phase velocity for wave propagations are firstly analyzed by von Neumann method. Compared with the other two widely used mesh-free methods, smoothed particle hydrodynamics (SPH) and modified smoothed particle hydrodynamics (MSPH), theoretical analysis shows that GFDTD has less dispersion, which is also well verified by 1D numerical tests. The GFDTD method is then applied to simulate wave propagation inside a 2D cylinder and the results are compared with those obtained by traditional finite difference time domain (FDTD) method. Finally, a moving boundary condition is introduced into the problem domain and the corresponding input impedance is analyzed.

Cerebrovascular and cardiovascular variability interactions investigated through conditional joint transfer entropy in subjects prone to postural syncope

Objective: A model-based conditional transfer entropy approach was exploited to quantify the information transfer in cerebrovascular (CBV) and cardiovascular (CV) systems in subjects prone to develop postural syncope. Approach: Spontaneous beat-to-beat variations of mean cerebral blood flow velocity (MCBFV) derived from a transcranial Doppler device, heart period (HP) derived from surface electrocardiogram, mean arterial pressure (MAP) and systolic arterial pressure (SAP) derived from finger plethysmographic arterial pressure device were monitored at rest in supine position (REST) and during 60° head-up tilt (TILT) in 13 individuals (age mean ± standard deviation: 28 ± 9 years, min–max range: 18–44 years, 5 males) with a history of recurrent episodes of syncope (SYNC) and in 13 age- and gender-matched controls (NonSYNC). Respiration (R) obtained from a thoracic belt was acquired as well and considered as a conditioning signal in transfer entropy assessment. Synchronous sequences of 250 consecutive MCBFV, HP, MAP, SAP and R values were utilized to estimate the information genuinely transferred from MAP to MCBFV (i.e. disambiguated from R influences) and vice versa. Analogous indexes were computed from SAP to HP and vice versa. Traditional time and frequency domain analyses were carried out as well.Main results: SYNC subjects showed an increased genuine information transfer from MAP to MCBFV during TILT, while they did not exhibit the expected rise of the genuine information transfer from SAP to HP. Significance: We conclude that SYNC individuals featured an impaired cerebral autoregulation visible during TILT and were unable to activate cardiac baroreflex to cope with the postural challenge. Traditional frequency domain markers based on transfer function modulus, phase and coherence functions were less powerful or less specific in typifying the CBV and CV controls of SYNC individuals. Conditional transfer entropy approach can identify the impairment of CBV and CV controls and provide specific clues to identify subjects prone to develop postural syncope.

Zależność między entropią aproksymacji rytmu serca a strukturalną i funkcjonalną przebudową przedsionków oraz przebiegiem klinicznym napadowego i przetrwałego migotania przedsionków

Introduction. Atrial remodeling due to atrial fibrillation (AF) is characterized by electrical and structural changes of cardiomyocytes that are in response for arrhythmia self-perpetuation and resistance to sinus rhythm conversion. This project was designed to study nonlinear methods of heart rate variability (HRV) (such as approximate entropy of heart rate [ApEn]) that can best characterize LA structural and functional remodeling and its association with clinical course in patients with paroxysmal and persistent AF. Material and methods. Traditional time and frequency domain HRV indices were analyzed in 75 patients (mean age 55 [49–62] years, 79% male) with paroxysmal or persistent AF on a background of ischemic heart disease (IHD) and/or hypertension without significant structural myocardial damage and in 19 control patients without AF (mean age 56 [49–61] years, 63% male). Echocardiography was performed to assess size and function of left atrium (LA). Results. In patients with paroxysmal or persistent AF the ApEn value was significantly lower than in the control patients group. Echo-parameters of LA, characterizing its structure and function, were correlated with ApEn value. The AF duration was negatively correlated with the ApEn value. Its values less than 0.93 was associated with a larger LA size (> 39 mm) and more frequent AF. Conclusions. There is a strong correlation of ApEn value and atrial remodeling. ApEn values less than 0.93 are related to AF recurrence and can be considered as prognostic risk factor of structural and functional remodeling of the heart muscle in patients with AF.

Sea Clutter Suppression Method for Over-the-horizon Radar with Short Coherent Integration Time Based on Compressed Sensing

To deal with the drawbacks of low reconstruction accuracy and poor suppression performance of clutter with short Coherent Integration Time (CIT) for traditional time domain cancellation method, a method of sea clutter reconstruction and suppression based on Compressed Sensing (CS) is proposed. Firstly, echoes model of Over-The-Horizon Radar (OTHR) with short CIT is established, and the rationality using CS to reconstruct sea clutter is analyzed. Secondly, the proposed method is elaborated in detail from sparse representation of echoes model based on redundant sinusoidal?dictionary, representation of sensing matrix based on dimensionality reduction dictionary and sea clutter reconstruction and suppression based on the modified Orthogonal Matching Pursuit (OMP) algorithm respectively. Finally, the computer simulation analysis and measured data verification are accomplished. The results indicate that the clutter suppression performance and engineering application value of the proposed method are better than the traditional time domain cancellation method and subspace methods in the condition of short CIT.

Reverse Time Migration of Elastic Waves Using the Pseudospectral Time-Domain Method

Reverse time migration (RTM), especially that for elastic waves, consumes massive computation resources which limit its wide applications in industry. We suggest to use the pseudospectral time-domain (PSTD) method in elastic wave RTM. RTM using PSTD can significantly reduce the computational requirements compared with RTM using the traditional finite difference time domain method (FDTD). In addition to the advantage of low sampling rate with high accuracy, the PSTD method also eliminates the periodicity (or wraparround) limitation caused by fast Fourier transform in the conventional pseudospectral method. To achieve accurate results, the PSTD method needs only about half the spatial sampling rate of the twelfth-order FDTD method. Thus, the PSTD method can save up to 87.5% storage memory and 90% computation time over the twelfth-order FDTD method. We implement RTM using PSTD for elastic wave equations and accelerate it by Open Multi-Processing technology. To keep the computational load balance in parallel ...

Microwave-assisted frequency domain measurement of fiber-loop ring-down system.

A new frequency domain measurement method for a fiber-loop ring-down system is proposed in this Letter. Compared to traditional time domain measurement, this method uses a microwave modulated continuous wave (CW) laser as a light source, making full use of the duty cycle to achieve enhanced measurement efficiency. By measuring the amplitude modulation over a frequency span, this technique can be used to determine the ring-down time in the frequency domain, which will then be used to calculate the loss in the ring.

Lift-off effect reduction based on the dynamic trajectories of the received-signal fast Fourier transform in pulsed eddy current testing

Reduction of interferences such as the lift-off effect in pulsed eddy current (PEC) testing has been difficult. A new approach based on the dynamic trajectories of the fast Fourier transform (FFT) of the received signals is presented in this paper for reducing the lift-off effect in PEC. The PEC probe scans the surface of the test sample and a set of received signals for a corresponding set of scanning points is stored. FFT is performed on all these stored signals, and the real and imaginary parts for a particular FFT harmonic of all the stored scanning points are plotted in a complex plane to form a dynamic trajectory. The defect trajectory can be recognized from the lift-off trajectory by observing the angle between them, as in conventional eddy current testing. By moving the defect trajectory in the complex plane for each harmonic to a zero-lift-off point and performing an inverse fast Fourier transform (IFFT) based on the new trajectories for all the harmonics, the traditional PEC time domain differential signals for a defect can be reconstructed. Then, the acquired results are amplified with a coefficient related to the inspection settings and final differential signals closely approximating the original defect differential signals without lift-off are obtained. Experimental results demonstrate that the proposed method is highly effective, particularly for the simultaneous inspection of internal and surface defects, although the location of the defect in the depth direction is unknown before inspection.

Time-Domain Analysis of Noise Coupling Between Package and PCB Power/Ground Planes Based on WLP-FDTD

In this paper, a numerical approach based on weighted Laguerre polynomials finite-difference time-domain (WLP-FDTD) method is proposed to analyze noise coupling in power/ground planes between package and printed circuit board (PCB). Compared to the traditional finite-difference time-domain (FDTD) method, which is limited by Courant-Friedrich-Lecy stability condition, the proposed method has the unconditionally stable characteristic. Examples of a ball grid array package mounted on a PCB were simulated, and the numerical results show that the computation time of the proposed WLP-FDTD method can be 4.4% of that of the FDTD method at the same accuracy condition. Therefore, the proposed method has higher efficiency than the conventional FDTD method without sacrificing precision.

A Hybrid Domain Degradation Feature Extraction Method for Motor Bearing Based on Distance Evaluation Technique

The vibration signal of the motor bearing has strong nonstationary and nonlinear characteristics, and it is arduous to accurately recognize the degradation state of the motor bearing with traditional single time or frequency domain indexes. A hybrid domain feature extraction method based on distance evaluation technique (DET) is proposed to solve this problem. Firstly, the vibration signal of the motor bearing is decomposed by ensemble empirical mode decomposition (EEMD). The proper intrinsic mode function (IMF) component that is the most sensitive to the degradation of the motor bearing is selected according to the sensitive IMF selection algorithm based on the similarity evaluation. Then the distance evaluation factor of each characteristic parameter is calculated by the DET method. The differential method is used to extract sensitive characteristic parameters which compose the characteristic matrix. And then the extracted degradation characteristic matrix is used as the input of support vector machine (SVM) to identify the degradation state. Finally, It is demonstrated that the proposed hybrid domain feature extraction method has higher recognition accuracy and shorter recognition time by comparative analysis. The positive performance of the method is verified.

A Krylov Space-Based Finite Element Time Domain Method for Broadband Frequency Domain Solutions

A Krylov space based time domain method for wave propagation problems is introduced. The proposed method uses the Arnoldi algorithm to obtain broad-band frequency domain solutions. This method is especially advantageous in cases where slow convergence is observed when using traditional time domain methods. The efficiency of the method is examined in several test cases to show its fast convergence in such problems.

Weight-Adjusted Discontinuous Galerkin Methods: Curvilinear Meshes

Traditional time-domain discontinuous Galerkin (DG) methods result in large storage costs at high orders of approximation due to the storage of dense elemental matrices. In this companion paper to [J. Chan, R. J. Hewett, and T. Warburton, Weight-adjusted discontinuous Galerkin methods: wave propagation in heterogeneous media, preprint, https://arxiv.org/abs/1608.01944, 2016], we propose weight-adjusted DG (WADG) methods for curvilinear meshes which reduce storage costs while retaining energy stability. A priori error estimates show that high order accuracy is preserved under sufficient conditions on the mesh, which are illustrated through convergence tests with different sequences of meshes. Numerical and computational experiments verify the accuracy and performance of WADG for a model problem on curved domains.

Extracting Low-Frequency Information from Time Attenuation in Elastic Waveform Inversion

Low-frequency information is crucial for recovering background velocity, but the lack of low-frequency information in field data makes inversion impractical without accurate initial models. Laplace–Fourier domain waveform inversion can recover a smooth model from real data without low-frequency information, which can be used for subsequent inversion as an ideal starting model. In general, it also starts with low frequencies and includes higher frequencies at later inversion stages, while the difference is that its ultralow frequency information comes from the Laplace–Fourier domain. Meanwhile, a direct implementation of the Laplace-transformed wavefield using frequency domain inversion is also very convenient. However, because broad frequency bands are often used in the pure time domain waveform inversion, it is difficult to extract the wavefields dominated by low frequencies in this case. In this paper, low-frequency components are constructed by introducing time attenuation into the recorded residuals, and the rest of the method is identical to the traditional time domain inversion. Time windowing and frequency filtering are also applied to mitigate the ambiguity of the inverse problem. Therefore, we can start at low frequencies and to move to higher frequencies. The experiment shows that the proposed method can achieve a good inversion result in the presence of a linear initial model and records without low-frequency information.

청음용 자동차 로드노이즈 추출 방법 연구

This study pertains to the extraction of the road noise component of signals from a vehicle's interior noise via the traditional frequency domain and time domain system identification methods. For road noise extraction based on the frequency domain system identification method, the appropriate matrix inversion strategy is investigated and causal and non-causal impulse response filters are compared. Furthermore, appropriate data lengths for the frequency domain system identification method are investigated. In addition to the traditional road noise extraction methods based on frequency domain system identification, a new approach to extract road noise via the time domain system identification method based on a parametric input-output model is proposed and investigated in the present study. In this approach, instead of constructing a higher order model for the full-band road noise, input and output signals are processed in the subband domain and lower order parametric models optimal to each subband are determined. These parametric models are used to extract road noises in each subband; the full band road noise is then reconstructed from the subband road noises. This study shows that both the methods in the frequency domain and the time domain successfully extract the road noise from the vehicle's interior noise.

Frequency Domain Probe Design for High Frequency Sensing of Soil Moisture

Accurate moisture sensing is an important need for many research programs as well as in control of industrial processes. This paper describes the development of a high accuracy frequency domain sensing probe for use in obtaining dielectric measurements of materials suitable for work ranging from 300 MHz to 1 GHz. The probe was developed to accommodate a wide range of permittivity’s ranging from εr = 2.5 to elevated permittivity’s as high as εr = 40. The design provides a well-matched interface between the soil and the interconnecting cables. A key advantage of the frequency domain approach is that a change of salt concentration has a significantly reduced effect on ε′, versus the traditional time-domain reflectometry, TDR, measured apparent permittivity, Ka.

Simple and Accurate Fluorescence Lifetime Measurement Scheme Using Traditional Time-Domain Spectroscopy and Modern Digital Signal Processing

Fluorescence lifetime measurement is an essential process for evaluation and application of optical gain media. An accurate and convenient scheme for fluorescence lifetime measurement of complex emission processes with either single or multiple fluorescence lifetimes is essential. We developed a new convolution-based scheme for fluorescence lifetime measurement. The innovation of the scheme lies in a combination of convolution-based digital signal processing algorithms with a low cost and simple time-domain spectroscopic setup. The convolution-based scheme is able to achieve accurate microsecond fluorescence lifetime resolution using the spectroscopic setup with only millisecond fluorescence lifetime resolution, for both single or multiple fluorescence lifetimes. We demonstrated the performance of the scheme, both theoretically and experimentally, by measuring fluorescence lifetimes associated with complicated emission processes in bismuth–erbium-doped fibers and erbium-doped fiber.

Multi-frequency finite-difference frequency-domain algorithm for active nanophotonic device simulations

First-principles simulation of active nanophotonic devices is indispensable to optical engineering. However, direct simulation of active devices with traditional time-domain methods have been prohibitively expensive due to the inherently large time-scale difference between optical and modulation frequencies. To overcome this challenge, we present a multi-frequency finite-difference frequency-domain algorithm that efficiently performs first-principles steady-state simulations in active devices. We validate our algorithm by simulating a modulated waveguide device and find that the result of the simulation is in excellent agreement with that of coupled mode theory, while also revealing features that are neglected in typical coupled mode theory treatments. We further demonstrate that this algorithm makes it possible to effectively simulate realistic active optical devices. Our algorithm should facilitate and expedite the design and analysis of active nanophotonic components.

Speckle Analysis Method for Distributed Detection of Temperature Gradients With <inline-formula> <tex-math notation="LaTeX">$\Phi $ </tex-math> </inline-formula>OTDR

A method to extend the operation of traditional single-frequency phase-sensitive optical time-domain reflectometry (ΦOTDR) to the monitoring of distributed temperature gradients along an optical fiber is proposed and experimentally validated. The measurement principle is derived from the perturbation response of a single-wavelength ΦOTDR signal, which is analyzed as a unidimensional speckle pattern. The method could be implemented in parallel to standard ΦOTDR systems used for distributed vibration sensing with a close to zero cost and without affecting its operation, as it only requires a low computational cost post-processing of the traces which are already acquired. Frequency scanning of the laser, heterodyning or additional hardware are not required. The distributed detection of a temperature gradient of 2.5 °C over 10 min is demonstrated.

Compressed sensing: Radar signal detection and parameter measurement for EW applications

AbstractState of the art system development is very much required for UAVs (Unmanned Aerial Vehicle) and other airborne applications, where miniature, lightweight and low‐power specifications are essential. Currently, the airborne Electronic Warfare (EW) systems are developed with digital receiver technology using Nyquist sampling. The detection of radar signals and parameter measurement is a necessary requirement in EW digital receivers. The Random Modulator Pre‐Integrator (RMPI) can be used for matched detection of signals using smashed filter. RMPI hardware eliminates the high sampling rate analog to digital computer and reduces the number of samples using random sampling and detection of sparse orthonormal basis vectors. RMPI explore the structural and geometrical properties of the signal apart from traditional time and frequency domain analysis for improved detection. The concept has been proved with the help of MATLAB and LabVIEW simulations.

Assessment of fetal maturation age by heart rate variability measures using random forest methodology

Fetal maturation age assessment based on heart rate variability (HRV) is a predestinated tool in prenatal diagnosis. To date, almost linear maturation characteristic curves are used in univariate and multivariate models. Models using complex multivariate maturation characteristic curves are pending.To address this problem, we use Random Forest (RF) to assess fetal maturation age and compare RF with linear, multivariate age regression. We include previously developed HRV indices such as traditional time and frequency domain indices and complexity indices of multiple scales.We found that fetal maturation was best assessed by complexity indices of short scales and skewness in state-dependent datasets (quiet sleep, active sleep) as well as in state-independent recordings. Additionally, increasing fluctuation amplitude contributed to the model in the active sleep state. None of the traditional linear HRV parameters contributed to the RF models. Compared to linear, multivariate regression, the mean prediction of gestational age (GA) is more accurate with RF than in linear, multivariate regression (quiet state: R2=0,617 vs. R2=0,461, active state: R2=0,521 vs. R2=0,436, state independent: R2=0,583 vs. R2=0,548).We conclude that classification and regression tree models such as RF methodology are appropriate for the evaluation of fetal maturation age. The decisive role of adjustments between different time scales of complexity may essentially extend previous analysis concepts mainly based on rhythms and univariate complexity indices. Those system characteristics may have implication for better understanding and accessibility of the maturating complex autonomic control and its disturbance.