Power Spectral Density Curves Research Articles

Effects of ESD transients on noise in tunneling recording heads

Noise power spectral density (PSD) and transfer curves (resistance versus applied magnetic field) are collected and compared on two groups of tunneling magnetoresistive (TuMR) sensors. The first group was comprised of TuMR sensors that were normal, undamaged TuMR sensors. The second group included TuMR sensors that had been damaged by electrostatic discharge (ESD) transients while handling in the lab. Representative TuMR sensors from the second, ESD damaged group were observed to have abrupt shifts in the ferromagnetic resonance (FMR) peak position and shape with applied magnetic field when compared to similar data for undamaged TuMR sensors. These abrupt shifts in FMR peaks occur at applied magnetic fields that correspond to fields at which discontinuities in quasi-static transfer curves are observed.

Comparison of fast Fourier transformation and autoregressive modelling as a diagnostic tool in analysis of lower extremity venous signals

In this study, we have compared the efficacy of autoregressive modelling (ARM) and fast Fourier transformation (FFT) of Doppler signals from lower extremity veins of healthy volunteers in various physiologic situations. Compared to FFT, ARM produced smooth spectra and less spectral broadening both in sonograms and power spectra. However, faulty positioning of the peaks along the time axis in FFT-derived power spectral density curves show that FFT is not a suitable method if these graphs are to be used as a diagnostic tool. Analysis of ARM-based venous sonograms and power spectral density graphs revealed that FFT should not be used in signals with high power spectral density levels and low-frequency bandwidth within limited segments of time.

Maglev Vehicle/Guideway Vertical Random Response and Ride Quality

Summary The research and development (R & D) of maglev technology had made a great progress in China since the early 1980s. Especially, a 35 km-long Shanghai high-speed maglev railway employing the German Transrapid system began to be constructed on March 1, 2001. Based on the Transrapid system, the paper develops a 10-degree-of-freedom model of maglev vehicle running over three types of guideways with constant speed. Random guideway irregularities are discussed and taken into account for simulation of the vehicle response and for evaluation of the ride comfort. Using the direct time integration method and the discrete fast Fourier transform (DFFT), random responses of the maglev vehicle-guideway systems are obtained and analyzed. Numerical results show that the resonant frequency of car body acceleration response is 0.5–1 Hz, and there is a 2.2 Hz periodic vibration due to the periodic configuration of rigid piers when the maglev vehicle travels over the elevated-beam guideway. The car body acceleration power spectral density (PSD) curves meet well the ride quality criterion of the urban tracked aircushion vehicle (UTACV) and the maximum acceleration of car body is less than 0.05 g. Moreover, the Sperling ride index values are less than 2.5 as long as the operational speed is less than 450 km/h. It is concluded that the maglev vehicle ride quality is quite well.

Influence of coherent structures on mass transfer near stagnation region in agitated small tanks

Electrochemical method is applied to study coherent structures in a complex three-dimensional flow generated by a rotating magnetic rod. In order to detect these structures, the power spectral density (PSD) analysis of the limiting diffusion current fluctuations is provided. This analysis reveals the existence of resonance frequencies when the probe is located in the very vicinity of the stirrer. The main resonance is related to the mechanical excitation frequency. The detailed analysis of the PSD curves points out some mechanisms of development of turbulence as sub-harmonics and non-linear interactions. For different locations of the probe and different angular rates of the stirrer Ω, a similarity of the energy repartition is obtained when the PSD is plotted versus the adimensional frequency f/Ω. Flow visualization by laser tomography is also provided and the local secondary flow past the electrode probe is sketched. The laser Doppler velocimetry (LDV) at a given location in a horizontal z-plane indicates a tornado wise mean flow structure.

Surface characterization techniques for determining the root-mean-square roughness and power spectral densities of optical components

Surface topography and light scattering were measured on 15 samples ranging from those having smooth surfaces to others with ground surfaces. The measurement techniques included an atomic force microscope, mechanical and optical profilers, confocal laser scanning microscope, angle-resolved scattering, and total scattering. The samples included polished and ground fused silica, silicon carbide, sapphire, electroplated gold, and diamond-turned brass. The measurement instruments and techniques had different surface spatial wavelength band limits, so the measured roughnesses were not directly comparable. Two-dimensional power spectral density (PSD) functions were calculated from the digitized measurement data, and we obtained rms roughnesses by integrating areas under the PSD curves between fixed upper and lower band limits. In this way, roughnesses measured with different instruments and techniques could be directly compared. Although smaller differences between measurement techniques remained in the calculated roughnesses, these could be explained mostly by surface topographical features such as isolated particles that affected the instruments in different ways.

Vibration and ride comfort studies on a tracked vehicle

This paper describes a field test conducted on a military tracked vehicle. The accelerations experienced at selected locations on the vehicle for four different types of terrain were recorded and analysed in spectral and statistical modes. The first few natural frequencies of the vehicle were obtained using an in-plane N+2 degree-of-freedom linear dynamic model of the vehicle with two degrees of freedom corresponding to bounce and pitch modes of the hull and N degrees of freedom corresponding to the bounce modes of the N road wheel assemblies. The analytically obtained natural frequencies match with peaks in the experimentally obtained transfer function and acceleration power spectral density curves. From the point of view of crew comfort, acceleration levels encountered in the crew cabin have been compared with those specified in ISO 2631 standard. Ride quality assessment has also been made on the basis of absorbed power.

GEOSCOPE Station Noise Levels

The noise level at GEOSCOPE seismograph stations operating in 1995 has been studied in order to quantify the quality of stations for periods ranging from 0.2 to 8000 sec. The power spectral density curves presented in this article are a useful tool for selecting stations as a function of signal-to-noise ratio in the frequency band of interest. Seismic-noise level is the lowest for continental stations in the entire frequency band. It is similarly low at most coastal stations (stations located less than 150 km away from the coast). Finally, the noise level is low for island stations at long periods but increases significantly for periods smaller than 20 seconds, and in particular in the period range of the microseismic peak. The noise level on horizontal components varies, in most stations, as a function of local time for periods greater than 20 sec, being higher during the day than during the night. Only stations located in cold areas with little daily temperature variations and stations installed in a long tunnel do not display these daily variations. There is no seasonal variations of short-period noise (periods less than 5 sec). For some continental stations, we observe variations in the amplitude of the 7-sec microseismic peak during the year. For all three components, the peak amplitude is higher and shifted toward longer periods in fall and winter than in spring and summer. This phenomenon can be explained by the increase of the number and the size of oceanic storms in fall and winter. Long-period seismic noise (periods greater than 30 sec) also varies for some stations as a function of the season; however, no systematic characteristics have been observed.

Study of Stainless Steel Localized Corrosion in Sodium Hypochlorite Solution by Detection and Analysis of Electrochemical Noise

Different types of AISI 316 stainless steel were tested in a sodium hypochlorite solution (7% active chlorine). Specimens, obtained from a 3 mm thick rolled sheet, were made according to ASTM E8 and mounted in constant load tension machines. A fraction of the immersed surface was covered by an epoxy resin in order to limit a 1 cm 2 free surface. Applied load was calculated in order to reach in the specimen gauge length a stress equal to 70% of the material yield strength (σ y(0.2%) ). Potential noise was collected between two identical specimens with a sampling rate of 1 sample/s. For each run 1024, 2048 or 3072 points were sampled to obtain the spectra. For each sample potential monitoring was carried out up to 240 hours. Potential noise was then analyzed in the frequency domain and correlated to corrosion attack. Corrosion developed initially as crevice under epoxy coating. Based on stainless steel composition different corrosion morphology was evidenced: generalized corrosion, cavernous pits or SCC phenomena. The power spectral density (PSD) of the potential noise reduced with time increasing as a consequence of an increase of the number of corrosion events. The slopes of the PSD curve in the medium frequency range were correlated to the type of localized corrosion phenomena: higher values being connected to pitting corrosion. The presence of a significant peak in the power spectra was probably due to the occurrence of a steady state crevice corrosion, however it was not yet possible to establish if there was a typical frequency for such phenomenon.

Assessment of fatigue load spectrum from fracture surface topography

The possibility of deducing load spectrum parameters from fatigue failure surfaces is explored by applying innovative, three-dimensional topographic characterization and analysis techniques to failure surfaces in aluminum sheet. Precise, high-resolution elevation maps of fracture surfaces were obtained using confocal optics scanning laser microscopy. Elevation power spectral density curves resulting from a fast Fourier transform of the elevation data appear sensitive to stress intensity range and environment. A conjugate fracture surface matching procedure, FRASTA, can detect and may provide a way to quantify overloads.

Comparison of satellite-derived wind measurements with other wind measurement sensors

The purpose of this paper is to compare the good data from the Jimsphere launches with the data from the satellite system. By comparing the wind speeds from the Fixed Pedestal System 16 (FPS-16) Radar/Jimsphere Wind System and NASA's 50-MHz Radar Wind Profiler, the validation of winds from Geostationary Operational Environmental Satellite 7 (GOES-7) is performed. This study provides an in situ data quality check for the GOES-7 satellite winds. Comparison was made of the flowfields in the troposphere and the lower stratosphere of case studies of pairs of Jimsphere balloon releases and Radar Wind Profiler winds during Space Shuttle launches. The mean and standard deviation of the zonal component statistics, the meridional component statistics, and the power spectral density curves show good agreement between the two wind sensors. The standard deviation of the u and v components for the STS-37 launch (consisting of five Jimsphere/Radar Wind Profiler data sets) was 1.92 and 1.67 m/s, respectively; for the STS-43 launch (there were six Jimsphere/Wind Profiler data sets) it was 1.39 and 1.44 m/s, respectively. The overall standard deviation was 1.66 m/s for the u component and 1.55 m/s tor the v component, and a standard deviation of 2.27 m/s tor the vector wind difference. The global comparison of satellite with Jimsphere balloon vector winds shows a standard deviation of 3.15 m/s for STS-43 and 4.37 m/s for STS-37. The overall standard deviation of the vector wind was 3.76 m/s, with a root-mean-square vector difference of 4.43 m/s. These data have demonstrated that this unique comparison of the Jimsphere and satellite winds provides excellent ground truth and a frame of reference during testing and validation of satellite data

Evaluation of Coating Degradation with Electrochemical Impedance Spectroscopy and Electrochemical Noise Analysis

The performance of two coating systems on cold‐rolled steel has been evaluated with electrochemical impedance spectroscopy and electrochemical noise analysis during exposure to 0.5N for five months. The decrease of the pore resistance and the polarization resistance and the increase of the double layer capacitance show qualitatively that the disbonded area increased with time for the alkyd system (CR‐2). No deviation from capacitive behavior was observed for the epoxy polyamide coating (CR‐9) which showed excellent performance. Potential and current noise data have been collected simultaneously. This approach allows evaluation of the frequency dependence of the spectral noise response and determination of the spectral noise resistance . From the statistical analysis of the noise data the noise resistance has been obtained. Good agreement between these two parameters has been observed. and show the same time dependence as and . However, their numerical values are much lower. For CR‐2 a parallel shift of the potential power spectral density (PSD) curves to lower values and of the current PSD curves to higher values with a constant slope of −20 dB has been observed for CR‐2. No obvious time dependence was indicated for CR‐9.

Fractal dimensions of soil strengths for paddy fields in China

Spatial variation of soil strengths for paddy fields in China was described using the concept of fractals. Soil strength was recorded with a cone penetrometer, along a series of transects, at 10 cm intervals. Fractal dimensions were calculated both from semi-variograms and from power spectral density (PSD) curves of soil strength profiles. In paddy fields after harvesting, such data gave fractal dimensions very close to 2, reflecting the characteristic nature of such paddy soils. This value implies that they were almost totally random, irrespective of site, date, direction or location of measurements. The values of fractal dimensions close to 2 corresponds to poor vehicle performance.

Light scatter from polysilicon and aluminum surfaces and comparison with surface-roughness statistics by atomic force microscopy

Optical-scatter measurements from polysilicon and aluminum surfaces were performed by using 632.8-nm illumination at 45 deg and 488-nm illumination at 76.8 deg. Scatter was recorded up to 60 deg from the specular beam by using a concentric ring photodetector. The results are compared with surface statistics derived from atomic force microscopy. Quantitative predictions of the scatter were derived from power spectral density curves and angle-resolved-scattering theory. The agreement was fair for polysilicon samples with rms surface roughnesses of ~18 and 42 nm and aluminum with 17-nm rms roughness but poor for other samples. The discrepancy is attributed primarily to internal scatter within the measuring instrument.

Spectral Analysis of Highway Pavement Roughness

This paper reports new findings in the spectral analysis of pavement roughness. Elevation profiles of federal and interstate highways were measured with a profilometer, and spectral analysis was used to compute power spectral densities for the longitudinal elevation of pavements. The longitudinal profiles obtained were also used to compute the International Roughness Index (IRI) using a quarter‐car simulation method. An extensive review of the existing techniques for pavement spectral analysis was done from which it was found that previously developed models do not explain pavement spectral properties very well, especially for long wavelengths in the spectral decomposition. A new equation that fits the power spectral density curves for several pavement types was developed. A set of models was also developed to predict the power spectral density as a function of pavement type and age, and the IRI.

Thickness estimation of silicon-on-lnsulator by means of the fourier transform of bilinearly transformed infrared reflectance data

Knowledge of film thickness is essential for device design in silicon-on-insulator technology. A new thickness estimation technique, based on the calculation of the spatial frequencies of bilinearly transformed infrared reflectance data in a spectral window, is introduced. The assignment of dominant spectral peaks in the power spectral density curve to the optical thickness of the silicon, silicon dioxide and the combined layer, is also presented. Examples for silicon-on-silicon dioxide with the silicon layer ranging in thickness between 1000 nm and 50 nm, with fixed oxide thickness, are presented. Thickness values of both layers to better than a few percent accuracy, were obtained for silicon layers down to 100 nm and with reduced accuracy for layers as thin as 50 nm.

New statistical features sensitive to sodium boiling noise

New statistical features sensitive to Na boiling noise have been constructed. The features have been tested on Na boiling noise data of a BOR-60 experiment. It is found that the determinant of the covariance matrix of N judiciously selected segmented areas under the power spectral density curve, is the most sensitive feature for Na boiling noise.

Power spectral density analysis of lateral human standing sway.

A number of investigators have used power spectral density (PSD) analysis to try to identify and quantify various sensory systems which are involved in upright standing. Using PSD analysis, over a frequency range from 0.02-2.5 Hz, the present study examined two trials of lateral sway in each of 80 men. The results showed that little change occurred in PSD values between the first and second trials. Exponential curves were fit to the averaged trials data of each subject. The values of the parameter estimates obtained from the curve-fitting were regressed on the values of various direct, and derived anthropometric variables to try to explain the variance of the parameter estimates in terms of the anthropometrics. The subject's center of gravity location accounted for no more than 15.8% of the parameter variance, whereas the remaining anthropometrics explained even less. The overall averaged data suggest that three functions (related to visual, vestibular, and proprioceptive processes) may underlie the sway pattern over this frequency range. PSD curves of individuals with a history of high-level athletic activity clearly differed from those of sedentary subjects.

Spectrum analysis of road roughness for earthmoving machinery

The power spectral density functions of roughness of road surfaces for earthmoving machines such as tractors, scrapers and dumpers were determined by use of an accelerometer mounted on a towed fifth wheel of which the frequency-response function had previously been known. From obtained data, a range of power spectral density curves of the road roughness for these earthmoving machines was derived and expressed mathematically.

Class 1E Seismic Qualification by Test and Analysis of Generic Multi-Cabinet Electrical Equipment

A cost-effective procedure is developed for combining computer analyses with earlier shaking table tests of generic-type, multi-cabinet equipment to demonstrate Class 1E seismic qualification of new customer equipment, without the expense of additional shaking table tests. Comparisons of both shock response spectra and power spectral density curves are introduced to show that g-level strength and operability requirements, respectively, can be assured for each new application of the previously qualified generic-type equipment.

Computer optimization of trailer suspensions

This paper presents an optimization procedure which has been developed to select appropriate suspension design parameters for a simple two wheel trailer. A modern approach to the optimization problem has been used applying non-linear programming to the constrained search for the minimum of an objective function. The acceleration response to roadway input of selected points on the load of the trailer has been designated for minimization. This acceleration has an important significance to the trailer design because of its effect on the stable and smooth ride of the trailer. Two computational procedures have been used and the results compared. The first approach involves the linear superposition of responses to a series of discrete sinusoidal inputs while the second applies more contemporary random vibration theory using power spectral density curves to simulate the actual road surfaces. The results of the optimization procedure are then compared with the response characteristics of the original trailer design and demonstrate that a substantial improvement has been made.