Fourier Spectrum Of Signal Research Articles

Prediction of Fetal Growth Restriction for Fetal Umbilical Arterial/Venous Blood Flow Index Evaluated by Ultrasonic Doppler under Intelligent Algorithm

The empirical wavelet transform (EWT) algorithm was applied in ultrasound to explore the predictive value for fetal growth restriction (FGR) in fetal arteriovenous indexes. 142 pregnant women who received prenatal ultrasonic examination and delivered were selected. They were classified into control group and FGR group. There were 102 patients with normal pregnancy in the control group, and 40 patients with delayed fetal growth in the FGR group. The extended triple collocation (ETC) algorithm was employed to divide the Fourier spectrum of signals adaptively, and the constructed small filter banks were classified into corresponding intervals. The instantaneous frequency was analyzed, and the arterial blood flow indexes of the two groups were compared. The results showed that the time-frequency analysis method under EWT had lower normalization error and higher accuracy. The inner diameter and cross-sectional area of FGR were remarkably smaller than those of the control group, and the differences were statistically significant (P < 0.05). There were no significant differences in mean blood flow and mean blood velocity between the control group and FGR group (P > 0.05). The arterial blood flow parameters of the systolic flow velocity (VS) and the diastolic flow velocity (VD) in the FGR group were notably lower than those in the control group, and the differences were significant (P < 0.05). In conclusion, the frequency principal component extracted by EWT algorithm was less disturbed by noise, which could accurately and effectively evaluate fetal arteriovenous blood flow indexes and predict FGR.

Assessment of Plain and Glass Fiber-Reinforced Concrete Under Impact Loading: A New Approach via Ultrasound Evaluation

Impact loading leads to micro-crack formation that can compromise the performance of the concrete. The purpose of this paper is to evaluate plain concrete and fiber-reinforced concrete specimens using ultrasound methods under impact loading. These specimens were prepared and subjected to impact loading. Ultrasound tests were performed at different stages of impact loading on each specimen. The loading continued until cracks on the surface of the specimens were observed. Investigations were performed for both plain concrete and fiber-reinforced concrete to establish a correlation between ultrasound response characteristics, and the damage caused by impact loading due to the energy of blows transferred to the specimens. Analysis of signal records was based on the Fourier spectrum of signals and higher order harmonics. These investigations on the records improved the detection of damage on concrete, specifically micro-cracks. It was indicated that damage caused by impact loading deviates the predominant frequencies of diffused ultrasound waves from the excitation frequency. Finally, a new method was developed for detecting damage such as micro-cracks in concrete. The method was verified by using another impact mechanism for other specimens.

Improved empirical wavelet transform method based on spectrum trend for gearbox fault signal processing

The empirical wavelet transform decomposes the signal by segmenting the Fourier spectrum and constructing an adaptive filter bank and then segments the spectrum by detecting the maximum value in the spectrum. However, the number of mono-components is unknown or difficult to confirm when processing actual gearbox fault signals, the result of decomposition will be neither reasonable nor conducive to demodulation analysis if the maximum value is concentrated in one component. To solve this problem, a concept of an amplitude modulation–frequency modulation signal in the form of a wave crest is proposed. The spectrum trend of the Fourier spectrum of signal is extracted by the empirical mode decomposition method, and then the number of mode components is presupposed and the spectrum is segmented according to the extracted spectrum trend. The extraction method of the spectrum trend based on improved empirical wavelet transform based on spectrum trend method is proposed for gearbox fault diagnosis. The experiment, including the vibration test platform of horizontal driving machine and the processing of experimental signals based on the improved empirical wavelet transform based on spectrum trend method are also given. According to the simulation and experiment results, it can be clearly found that the spectrum trend can describe the distribution of signal spectrum well, and the components obtained by the improved empirical wavelet transform based on spectrum trend method are more suitable than original empirical wavelet transform method for demodulation analysis.

Dynamic unbalance detection of Cardan shaft in high-speed train applying double decomposition and double reconstruction method

The unbalance of Cardan shaft compromises operations of high-speed train. A new method is proposed to detect the unbalance by applying DDDR (double decomposition and double reconstruction method). The vibration acceleration of gearbox was decomposed into eight scale wavelet coefficients through wavelet packet decomposition. The eight single scale vibration signals were reconstructed by the corresponding scale wavelet coefficients. Hankel matrices in different scales were constructed through the reconstructed vibration signals in wavelet domain. SVD (singular value decomposition) of Hankel matrices was executed, and critical singular values were selected based on the maximum change of singular values. Those selected singular values were used to reconstruct the single scale vibration signal. So far, DDDR processing of signal has been completed. Fourier spectrum of signal acquired by DDDR processing was used to detect dynamic unbalance of high-speed train Cardan shaft. The validity of this method is supported by experimental data collected on dynamic unbalance experiments. The results show that this method can effectively extract the vibration characteristics of fundamental, multiplier, and divider frequencies. With comparison to the traditional wavelet decomposition, wavelet singularity value decomposition, the clarity and sensitive force have been significantly improved.

Инвариантность текущего энергетического Фурье-спектра действительных дискретных сигналов на конечных интервалах

Digital spectral analysis of signals based on DFT has a number of advantages. However, the transition from analog to digital methods and techniques is accompanied by a number of undesirable effects. Signals in each subject area usually have their own specifics. Therefore, it is necessary to study these effects in applications of spectral Fourier analysis. Such research is important for three reasons. Firstly, DFT properties are accurate, have their own specificity and significantly differ from the properties of the Fourier transform of continuous signals. Secondly, signals in each subject area have their own specificity. Thirdly, researchers often have prevailing knowledge in some particular domain, rather than in the field of digital signal processing techniques. As a result, in practice, some of the processes and effects arising in applications of digital spectral analysis, unfortunately, escape the attention of researchers which can result in erroneous conclusions. The paper deals with the problems of measuring Fourier spectrum of signals in the base of discrete exponential functions. Methods and algorithms of sliding measurements of energy Fourier spectrum of signals on finite intervals were described. The invariance of current energy Fourier spectrum to moving discrete real signals (which are not periodic) were investigated. The authors identify a new effect of digital spectral analysis — the effect of non-invariance of the current energy Fourier spectrum. Theoretical and practical results of analysis of invariance of current energy Fourier spectrum of tonal components are shown. The conducted studies allow us: — to see in a new light the measurement results on finite intervals of current Fourier spectrum and the current energy Fourier spectra of signals; give a numerical estimate of the non-invariance of the current energy Fourier spectrum of real tonal components. — to increase the effectiveness of digital spectral analysis in its many applications, in particular, for solving the problems on detection and identification of hidden periodicities in such subject areas as radar, vibroacoustic diagnostics, passive sonar, biomedicine, etc.

Application of the Mittag–Leffler expansion to sampling discontinuous signals

In applying Shannon's sampling theorem, evaluation of the sampled signal Fourier spectrum is based on the fact that sampling the continuous-time signal is the result of multiplying the signal by distributions. If the signal has discontinuities, a multiplication of distributions - an undefined operation - is encountered. Such undefined operation has led to errors in the literature which to date accompany the formulation of sampling of signals containing discontinuities. This paper presents an approach to evaluating the product of distributions as a means of sampling discontinuous signals, eliminating such errors. It is shown that the value of the product of distributions may be found by invoking the Mittag-Leffler expansion. As an illustration of errors that have existed for decades and still exist in the digital signal processing literature whenever discontinuous signals are sampled the approach of impulse invariance provides a case in point. It was already noted that this approach has an inherent error. Yet, impulse invariance is still considered as one of the two main approaches for converting analogue to digital filters. In this study, the true spectra of sampled discontinuous signals are evaluated, and a new approach to the transformations between continuous-time and discrete-time systems eliminating the error, is proposed.

Acoustic characteristics of a barrier-discharge XeCl excilamp

The generation of an acoustic signal by means of voltage pulses (f = 15 kHz) applied to the electrodes of a barrier-discharge excilamp based on a Xe/Cl2 = (50–500)/1 mixture kept at a pressure of 5–500 Torr is studied. It is shown that, from the time variation of the acoustic signal intensity, one can judge the time instant the excilamp starts operating in a steady mode. Optimal (in power and efficiency) operating conditions of the excilamp are found (Xe/Cl2 = 240/1, p = 98 Torr, η ≈ 9.5%). It is experimentally demonstrated that the discharge energy at a low pressure is spent largely on heating the gas. This is indicative of the volume heat release and volume glow discharge (as the pressure grows, the efficiency of this source of energy consumption drops and more and more energy is spent on acoustic vibration excitation). Under higher pressures, the Fourier spectrum of the acoustic signal becomes richer, the intensity of the spectrum rises, and the dispersion of the signal grows. At very high pressures, the intensity of the acoustic signal drops to a level corresponding to the natural vibrations of the excilamp envelope without the discharge (when the discharge is quenched, the Fourier spectrum of the signal becomes depleted and contains only harmonics corresponding to the carrier frequency of voltage pulses from the power source).

Signal formation analysis of the electron beam current distribution measurements

The Fourier spectra of signals and the modulation transfer functions are chosen as main characteristics of the electron beam (EB) analyzers. This approach can be used for EB analyzer design and optimization. It is concluded that the space–frequency characteristics of the scanning (modulation) system are the limiting factors for the signal distortion. A matrix of more than 32×32 sufficiently short impulses and at a transfer rate twice higher than the maximum spectrum frequency can create an adequate image of the beam current radial or angular distribution. At the data processing of the rotating probe or of the moving slit measurements an inverse transformation is needed and a false minimum can appear in the calculated current distributions.