Measuring the best linear approximation of systems suffering nonlinear distortions: an alternative method

When measuring the best linear approximation of systems suffering nonlinear distortions, a specific class of periodic multiharmonic signals is normally used. These are signals with uniformly distributed random phases, termed random phase multisines. In this paper, it is shown that measurements of the best linear approximation of nonlinear systems can also be obtained by using a special type of low crest factor multisines. These signals are compared to random phase multisines and their properties are analyzed in detail.     

The Diffraction Theory of the Aberrations of Stigmatic Orthomorphic Optical or Electron Optical Systems Containing Toric Lenses or Quadrupoles

The diffraction theory of the aberrations of optical systems with an axis of rotational symmetry is now thoroughly established, and the extension to kindred electron optical systems is straightforward. In electron optics, however, certain types of systems containing quadrupoles are of importance, and such systems suffer from aberrations which affect only very oblique rays in axially symmetrical systems. The distortions of the wave surface which correspond to these new aberrations are listed in the present note, and Nijboer's expressions for the consequent disturbances in the image plane of a rotationally symmetrical system are supplemented accordingly. Only the special case—which is, nevertheless, an important one—of stigmatic systems, producing an image free of first-order distortion?, is considered. The analysis is applicable to electron optical orthogonal systems containing quadrupoles and optical systems containing parallel toric lenses alike.     

Analysis of the impact of random summing on passive assay of pebble bed reactor fuel using gamma-ray spectrometry

Pebble bed reactors (PBR) are characterized by multi-pass fuel systems in which spherical fuel pebbles are circulated through the core until they reach a proposed burnup limit. The fuel is assayed on-line to ensure that the burnup limit is not breached. However, random summing effects can impact the response of the burnup measurement system and result in distortions that degrade the accuracy of the assay results. Monte Carlo analysis was performed to estimate the magnitude and effect of random summing on the absolute and relative indicators that have been identified as usable in on-line assay. For a throughput rate of 10⁵ counts/s and trapezoidal pulse shaping of the signals, the results show that absolute indicators suffer from severe distortions due to this effect. Relative indicators are found to be resistant to random summing with the deviation in the ratio of peak areas remaining less than 5–15% depending on pulse width.     

结构损伤与荷载共同识别的研究

针对结构中同时存在未知损伤和荷载的情况,基于虚拟变形法(VDM)发展一种两者共同识别的时域方法。VDM方法利用虚拟变形模拟结构损伤,可快速计算模型改变后的响应。该文首先结合有限元理论把VDM方法拓展到具有多个单元变形的结构中;然后考虑结构存在未知荷载时,利用未损伤理论模型同时识别荷载和虚拟变形,继而由虚拟变形和单元实际变形的关系来识别判断损伤类型和识别损伤大小;最后通过一个悬臂梁的试验进行方法验证,试验中未知荷载和损伤(包括其类型和大小)均能够被有效识别,并利用提出的移动时间窗和荷载形函数方法实现损伤与荷载的在线识别。     

有限元新型自然坐标方法研究进展

网格畸变敏感问题一直是当前有限元法难以解决的问题,而新型自然坐标方法的诞生可以在一定程度上对解决这个难题有所帮助。该文介绍了有限元新型自然坐标方法研究的新近进展。包括第一类四边形面积坐标及其应用(单元构造,解析刚度矩阵的建立,以及在几何非线性问题中的应用等);第二类四边形面积坐标及其应用;六面体体积坐标及其应用。数值算例表明:无论网格如何扭曲畸变,这些基于新型自然坐标方法的有限元模型仍然保持高精度,对网格畸变不敏感。这显示了新型自然坐标方法是构造高性能单元模型的有效工具。     

Interaction of multiple distortions in spatial light modulators

Spatial light modulators are the key components in real-time optical image-processing systems. The phase and the intensity of their outputs will often depart from ideal behavior. An experimental method is described that permits the effects of multiple distortions, present simultaneously, to be modeled. A computer simulation of a bismuth silicon oxide-based correlator is presented, with spatial light modulators subject to three types of distortion, including phase and amplitude. The experimental method permits both the main effects of the distortions and their interactions to be predicted. Combining all the distortions simultaneously gives a more accurate assessment of the suitability of a spatial light modulator for a given optical processing task. Images of 256 × 256 pixels were used, and the simulation took 15 min. with a Sun SPARCstation 2.     

Correlation processing for correction of phase distortions in subaperture imaging

Ultrasonic subaperture imaging combines synthetic aperture and phased array approaches and permits low-cost systems with improved image quality. In subaperture processing, a large array is synthesized using echo signals collected from a number of receive subapertures by multiple firings of a phased transmit subaperture. Tissue inhomogeneities and displacements in subaperture imaging may cause significant phase distortions on received echo signals. Correlation processing on reference echo signals can be used for correction of the phase distortions, for which the accuracy and robustness are critically limited by the signal correlation. In this study, we explore correlation processing techniques for adaptive subaperture imaging with phase correction for motion and tissue inhomogeneities. The proposed techniques use new subaperture data acquisition schemes to produce reference signal sets with improved signal correlation. The experimental test results were obtained using raw radio frequency (RF) data acquired from two different phantoms with 3.5 MHz, 128-element transducer array. The results show that phase distortions can effectively be compensated by the proposed techniques in real-time adaptive subaperture imaging.     

能动主镜的风载变形研究

风载引起的主镜变形是大口径望远镜必须考虑的问题.针对类似于Subaru望远镜的薄镜面能动光学系统,采用固定点力反馈控制方法对风载进行校正,本文引入了压强模式的概念,提取出风载产生的镜面变形的大部分信息,并据此对固定点力反馈控制方法进行了研究,得到如下结论:风载产生的镜面变形主要是平移、倾斜、像散和离焦等低阶像差;相同系...     

The Functionary,the Citizen,and the Engineer

Knowledge and action rest upon a background of shared expectations. ‘Technology’ refers to a particular background of expectations. Key words are: efficiency, productivity, utility, order, control, and objectivity. If, however, such expectations are carried too far, the behavior of mere functionaries becomes established: one becomes preoccupied with the completion of assignments with no time or inclination for the asking of troublesome questions. This paper examines one of the consequences of such behavior. A model is presented illustrating how organizational systems selectively distort assessments. Within such systems, favorable information is more likely to survive and thus a distorting tendency emerges. Such systemic distortions do not require individual fraud or deceit; the ‘normal’ behaviors of functionaries suffice. We claim that these functionary practices arise from taking the technological expectations too far. Examples are presented. Suggestions on avoiding functionary behavior and systemic distortions are discussed. We suggest that greater emphasis be given to the notion of ‘citizen engineers’.     

Ferroelectric order in individual nanometre-scale crystals

Ferroelectricity in finite-dimensional systems continues to arouse interest, motivated by predictions of vortex polarization states and the utility of ferroelectric nanomaterials in memory devices, actuators and other applications. Critical to these areas of research are the nanoscale polarization structure and scaling limit of ferroelectric order, which are determined here in individual nanocrystals comprising a single ferroelectric domain. Maps of ferroelectric structural distortions obtained from aberration-corrected transmission electron microscopy, combined with holographic polarization imaging, indicate the persistence of a linearly ordered and monodomain polarization state at nanometre dimensions. Room-temperature polarization switching is demonstrated down to ~5?nm dimensions. Ferroelectric coherence is facilitated in part by control of particle morphology, which along with electrostatic boundary conditions is found to determine the spatial extent of cooperative ferroelectric distortions. This work points the way to multi-Tbit/in(2) memories and provides a glimpse of the structural and electrical manifestations of ferroelectricity down to its ultimate limits.     

Frequency characteristics of loop oscillographic galvanometers

Conclusions An analysis of the above experiments shows that in concentrated liquid damping of loop oscillographic galvanometers there can be two kinds of distortions of the frequency characteristic, namely, distortions due directly to the presence of bounded damping sections (concentrated damping effect), and the residual distortions which have not been smoothed out and are due to the complex nature of the loop oscillatory system (string effect). On the basis of the above information on the nature and origin of distortions it is possible to find the number and position of dampers which will make distortions virtually unnoticeable. Moreover, the concentrated damping effect can be used, if necessary, to obtain a galvanometer with frequency characteristics which differ in the required manner from the theoretical characteristics of a system with a single degree of freedom.     

Image restoration from vibrating photographic systems

Many photographic systems are located on moving platforms. Because of mechanical vibrations that exist on those platforms, the photographed image is distorted. To avoid those distortions requires that very complicated mechanical attenuators be used. We suggest an optoelectronic image-processing algorithm to overcome the distortions caused by such vibrations. The algorithm for different types of vibration is reformulated in an exact manner, as opposed to the reformulations of previously known calculations. Furthermore, new types of vibration are explored. The algorithm is fully computerized and thus avoids the need for attenuating devices.     

Crystallite size distribution and lattice distortions in uniaxially drawn polyethylene

Crystallite size distributions and lattice distortions in the lateral direction as well as in the chain direction of uniaxially drawn polyethylene are analysed from 1 1 0 and 0 0 2 Debye-Scherrer line profiles by means of a Fourier technique. The size distribution in the [1 1 0] direction of the original (undrawn) sample consists of two well-defined components having maxima at 220 and 310 Å. In this lateral direction, crystallites break up into smaller units by drawing in such a way that each component in the original size distribution has its own scheme of degradation. In the chain direction [0 0 1] a similar mechanism of crystallite reconstitution is possibly operating, although the size distribution of the original sample in this direction could not be obtained. Lattice distortions in the drawn samples are of the strained-lattice type, except for the case of cold-drawing in the lateral direction [1 1 0]. [1 1 0] distortions in the cold-drawn specimen can be regarded as paracrystalline distortion. In this case, the strength of paracrystalline distortion is estimated as about one-half of the value given by Hosemann for similar polyethylene samples.     

Deformation behaviour of aluminium macrocrystals in the grain boundary region

Lattice distortions of deformed aluminium macrocrystals were observed by Electron-microprobe-Kossel techniques. Grain boundary effects due to deformation appeared in various ways such as extraordinary separation and branching of Kossel lines, and as intergranular rotation of the grains in pseudo-Kossel X-ray diffraction patterns obtained from the grain boundary regions of the specimens. Such distortions of the patterns peculiar to the grain boundary regions were attributed to the effect of local slipping induced by the active slip systems of the neighbouring crystals. By the interpretation of these patterns and the orientation relationships between neighbouring grains, it was found that the local slipping was induced towards the slip direction of the active slip plane in the neighbouring crystals, or induced by a type of local stress along the slip direction of the active slip planes in the neighbouring crystals. It was also recognised that at an early stage of stressing, deformation bands were produced in the grain boundary region as a result of a grain boundary effect.     

Generalization of equivalent crystal theory to include angular dependence

The original equivalent crystal theory is a semi-empirical method for calculating the configurational energy of atomic systems. Each atomic site in the real crystal with defects is assigned an equivalent lattice constant, in general different from the ground state value. This parameter corresponds to a local compression or expansion of the perfect lattice. The basic method considers these volumetric transformations and, in addition, introduces the possibility that the reference lattice is anisotropically distorted. These distortions however, were introduced ad-hoc. In this work, we generalize the original Equivalent Crystal Theory by introducing site-dependent directional distortions of the lattice which account for the dependence of the energy on anisotropic local density variations. This is done in the spirit of the original framework of ECT, but includes a gradient in the density. This approach is introduced to ECT and may apply to other semi-empirical methods by making use of readily available first-principles results to fix parameters. We develop here the basic framework, and apply it to the calculation of Fe(1 1 0) and Fe(1 1 1) surface energy formation, and to the vacancy energy curve. The results, compared with first-principles calculations, show an improvement over previous semi-empirical approaches.     

Design of test signals for identification of linear systems withnonlinear distortions

The design of nonbinary multifrequency excitation signals (multisines) for identifying linear systems in the presence of nonlinear distortions is examined. The use of such signals in parametric and nonparametric frequency-domain estimation is addressed, and a comparison with the maximum length binary sequence is made. The performance of a multisine composed of only prime harmonics is assessed for robustness to nonlinear effects. Practical results are presented, and guidelines for test signal design are outlined     

A Method for Crash Tests on Laminated Composite Scaled-Down Models

Based on the experimental studies carried out at the University of Valenciennes in close collaboration with ONERA, a study of GFRP profile crashworthiness has been carried out in order to find out a method permitting to scale down this type of structures. The relations obtained from Cauchy's similarity law have been found not to be directly applicable because of distortions due to the type of material used or the type of loading applied. An empirical model has been developed using data originating from experimental static tests in order to bring to the fore these distortions. Changing scale relations have been determined from this model then applied on the experimental data curves. The results obtained display a significant improvement as compared to the ones obtained with Cauchy's similarity law. Taking into account the distortions can permit an optimisation of the experimental conditions as well as the stratified materials stacking sequence. The aim is to limit their influence on the results obtained from scaled-down structures.     

Effects of natural images on the detectability of simple and compound wavelet subband quantization distortions

Quantization of the coefficients within a discrete wavelet transform subband gives rise to distortions in the reconstructed image that are localized in spatial frequency and orientation and are spatially correlated with the image. We investigated the detectability of these distortions: Contrast thresholds were measured for both simple and compound distortions presented in the unmasked paradigm and against two natural-image maskers. Simple and compound distortions were generated through uniform scalar quantization of one or two subbands. Unmasked detection thresholds for simple distortions yielded contrast sensitivity functions similar to those reported for 1-octave Gabor patches. Detection thresholds for simple distortions presented against two natural-image backgrounds revealed that thresholds were elevated across the frequency range of 1.15-18.4 cycles per degree with the greatest elevation for low-frequency distortions. Unmasked thresholds for compound distortions revealed relative sensitivities of 1.1-1.2, suggesting that summation of responses to wavelet distortions is similar to summation of responses to gratings. Masked thresholds for compound distortions revealed relative sensitivities of 1.5-1.7, suggesting greater summation when distortions are masked by natural images.     

Identification of linear systems in the presence of nonlineardistortions

This paper treats the identification of linear systems in the presence of nonlinear distortions. It extends the theory developed for measurement setups where the input is exactly known and the output is observed with errors (output error framework) to measurement setups where both the input and output are observed with errors (errors-in-variables framework). An appropriate measurement strategy and identification algorithm are presented     

A Filtering Technique for Three-Phase Power Systems

A novel filter for use in three-phase power systems is introduced. When the input to the filter is a balanced three-phase set of signals, the filter suppresses noise and distortions and extracts a smooth three-phase fundamental component. When the input signal to the filter is unbalanced, it extracts the fundamental positive-sequence component of the input signal. The filter also estimates the magnitude, phase angle, and frequency of the signal and adaptively follows the variations in all these three variables. The characteristics of the filter, including its mathematical equations, stability analysis, steady state, and dynamic responses, are discussed in this paper. The filter highly attenuates the harmonics, unknown interharmonics, and distortions. However, an extension of the filter for full removal of harmonics and unknown interharmonics is also presented, and its operating principles are discussed. The structural simplicity and robustness of the filter make it desirable for power system applications. In addition to the wide applications in power systems, it can specifically be used as an adaptive antialiasing filter for three-phase applications.