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Distortion Effects (distortion + effects)
Selected AbstractsA method to determine direct- and quadrature-axis inductances of permanent magnet synchronous motorsELECTRICAL ENGINEERING IN JAPAN, Issue 3 2010Shu Yamamoto Abstract The equivalent circuit constants of permanent magnet synchronous motors are needed in the calculation of operation characteristics, construction of a control system, etc. These constants can be computed from the data on structural form and materials. However, measurements are necessary to obtain highly precise values. Methods for measurement of the d- and q-axis inductances can be roughly divided into rotational and standstill methods. The standstill methods have the advantage that they are easy to carry out. However, it is difficult to consider magnetic saturation and distortion of the change in the armature winding inductance. The accuracy of the standstill method can be improved if these effects can be readily taken into account. This paper describes a standstill method for measuring accurate d- and q-axis synchronous inductances of permanent magnet synchronous motors. By utilizing the fact that the EMF interference terms in the motor voltage equation considering the distortion of the inductance change are equal to zero when the rotor is in a specific position, the proposed method determines the inductances considering both magnetic saturation and inductance distortion effects from simple off-line standstill testing. In addition, this method is capable of taking cross-magnetic saturation into account when used with the necessary testing equipment. The proposed method was implemented on a 0.4-kW interior permanent magnet synchronous motor with concentrated stator winding. The validity of the proposed method was demonstrated by comparing the measured and calculated results of the no-load and on-load characteristics. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(3): 41,50, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20969 [source] Improving Kirchhoff migration with repeated local plane-wave imaging?GEOPHYSICAL PROSPECTING, Issue 6 2005A SAR-inspired signal-processing approach in prestack depth imaging ABSTRACT A local plane-wave approach of generalized diffraction tomography in heterogeneous backgrounds, equivalent to Kirchhoff summation techniques when applied in seismic reflection, is re-programmed to act as repeated synthetic aperture radar (SAR) imaging for seismic prestack depth migration. Spotlight-mode SAR imaging quickly provides good images of the electromagnetic reflectivity of the ground via fast Fourier transform (FFT)-based signal processing. By calculating only the Green's functions connecting the aircraft to the centre of the illuminated patch, scattering structures around that centre are also recovered. SAR technology requires us to examine seismic imaging from the local point of view, where the quantity and quality of the available information at each image point are what are important, regardless of the survey geometry. When adapted to seismics, a local image of arbitrary size and sampling is obtained by FFT of seismic energy maps in the scattering wavenumber domain around each node of a pre-calculated grid of Green's functions. These local images can be used to generate a classic prestack depth-migrated section by collecting only their centres. However, the local images also provide valuable information around the centre, as in SAR. They can therefore help to pre-analyse prestack depth migration efficiently, and to perform velocity analysis at a very low cost. The FFT-based signal-processing approach allows local, efficient and automatic control of anti-aliasing, noise and resolution, including optimized Jacobian weights. Repeated local imaging could also be used to speed up migration, with interpolation between local images associated with a coarse grid of Green's functions, as an alternative to interpolation of Green's functions. The local images may, however, show distortions due to the local plane-wave approximation, and the velocity variations across their frame. Such effects, which are not necessarily a problem in SAR, should be controlled and corrected to further enhance seismic imaging. Applications to realistic models and to real data show that, despite the distortion effects, the local images can yield similar information to prestack depth migration, including common-image-point gathers for velocity analyses and AVO/AVA effects, at a much lower cost when a small target is considered. [source] The 2dF Galaxy Redshift Survey: correlation functions, peculiar velocities and the matter density of the UniverseMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2003Ed Hawkins ABSTRACT We present a detailed analysis of the two-point correlation function, ,(,, ,), from the 2dF Galaxy Redshift Survey (2dFGRS). The large size of the catalogue, which contains ,220 000 redshifts, allows us to make high-precision measurements of various properties of the galaxy clustering pattern. The effective redshift at which our estimates are made is zs, 0.15, and similarly the effective luminosity, Ls, 1.4L*. We estimate the redshift-space correlation function, ,(s), from which we measure the redshift-space clustering length, s0= 6.82 ± 0.28 h,1 Mpc. We also estimate the projected correlation function, ,(,), and the real-space correlation function, ,(r), which can be fit by a power law (r/r0), with r0= 5.05 ± 0.26 h,1 Mpc, ,r= 1.67 ± 0.03. For r, 20 h,1 Mpc, , drops below a power law as, for instance, is expected in the popular , cold dark matter model. The ratio of amplitudes of the real- and redshift-space correlation functions on scales of 8,30 h,1 Mpc gives an estimate of the redshift-space distortion parameter ,. The quadrupole moment of ,(,, ,) on scales 30,40 h,1 Mpc provides another estimate of ,. We also estimate the distribution function of pairwise peculiar velocities, f(v), including rigorously the significant effect due to the infall velocities, and we find that the distribution is well fit by an exponential form. The accuracy of our ,(,, ,) measurement is sufficient to constrain a model, which simultaneously fits the shape and amplitude of ,(r) and the two redshift-space distortion effects parametrized by , and velocity dispersion, a. We find ,= 0.49 ± 0.09 and a= 506 ± 52 km s,1, although the best-fitting values are strongly correlated. We measure the variation of the peculiar velocity dispersion with projected separation, a(,), and find that the shape is consistent with models and simulations. This is the first time that , and f(v) have been estimated from a self-consistent model of galaxy velocities. Using the constraints on bias from recent estimates, and taking account of redshift evolution, we conclude that , (L=L*, z= 0) = 0.47 ± 0.08, and that the present-day matter density of the Universe, ,m, 0.3, consistent with other 2dFGRS estimates and independent analyses. [source] Advanced tilt correction from flow distortion effects on turbulent CO2 fluxes in complex environments using large eddy simulationTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 643 2009F. Griessbaum Abstract Measurement of the turbulent fluxes of gases, momentum and heat can be biased by obstacles such as buildings or instrument platforms distorting the flow of air to the flux instruments. Standard methods have long been used to account for non-horizontal mean flows. Here we demonstrate a novel approach to correct for the effects of flow distortion which combines numerical flow modelling with eddy covariance measurements of the fluxes. This approach applies a flow distortion correction to the data prior to the application of the standard planar-fit and double-rotation methods. This new direction-dependent flow correction allows the determination of the correct orthogonal wind vector components and hence the vertical turbulent fluxes. We applied the method to a 10 Hz dataset of 3D wind components, temperature, and the concentrations of carbon dioxide and water vapour, as measured on top of a military tower above the city of Münster in northwest Germany during spring and summer 2007. Significant differences appeared between the fluxes that were calculated with the standard rotation methods alone and those that underwent flow distortion correction prior to the application of the rotation methods. The highest deviations of 27% were obtained for the momentum flux. Pronounced differences of 15% and 8% were found for the diurnal net fluxes of carbon dioxide and water vapour, respectively. The flow distortion correction for the carbon dioxide fluxes yielded the same magnitude as the WPL (Webb,Pearman,Leuning) correction for density fluctuations. Copyright © 2009 Royal Meteorological Society [source] COVARIATE-ADJUSTED REGRESSION FOR LONGITUDINAL DATA INCORPORATING CORRELATION BETWEEN REPEATED MEASUREMENTSAUSTRALIAN & NEW ZEALAND JOURNAL OF STATISTICS, Issue 3 2009Danh V. Nguyen Summary We propose an estimation method that incorporates the correlation/covariance structure between repeated measurements in covariate-adjusted regression models for distorted longitudinal data. In this distorted data setting, neither the longitudinal response nor (possibly time-varying) predictors are directly observable. The unobserved response and predictors are assumed to be distorted/contaminated by unknown functions of a common observable confounder. The proposed estimation methodology adjusts for the distortion effects both in estimation of the covariance structure and in the regression parameters using generalized least squares. The finite-sample performance of the proposed estimators is studied numerically by means of simulations. The consistency and convergence rates of the proposed estimators are also established. The proposed method is illustrated with an application to data from a longitudinal study of cognitive and social development in children. 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