基于现代谱估计理论的工况模态分析

大型结构如涡轮发动机，桥梁和淑海建筑等通常都处于变化非常大的一种工作环境之下，承受某种随机的环境激励，这时就需要直接利用工况条件下的响应数据提取模态参数。根据环境激励技术（NExT)的相关理论，线性结构中白噪声响应之间的相关函数与系统的脉冲响应函数具有类似的数学表达形式。本文首先将上述结论推广到复模态中，然后采用现代谱理论，运用谐波恢复的扩充的Prony方法，从相关函数中提取环境激励下的结构的模态参数，该方法由于对实模态和复模态进行了统一的处理，因而对于实模态和复模态都适用，通过仿真数值算例表明，该辨识方法对于环境激励下的系统辨识是合理有效的。     

基于维纳估计的光谱反射率重建优化算法研究

目的研究光谱反射率重建算法,解决各种物体颜色的光谱反射率重建精度问题。方法通过高精度多光谱成像系统获取实验样本的系统响应值,分光光度计获取样本的光谱反射率,采用Wiener估计法、自适应维纳估计法和提出的优化维纳估计法,对待测样本实验数据进行光谱重建,并评价重建结果。结果在3种光谱重建算法仿真实验中,提出算法的均方根误差平均值为0.0355,平均CIE1976色差为1.4349,优于其他2种算法。结论在光谱重建算法的研究中,基于优化的维纳估计算法可以有效提高光谱的重建精度,可应用于实际的多光谱成像复制中。     

Power Measurement in DVB-T Systems: On the Suitability of Parametric Spectral Estimation in DSP-Based Meters

Performance assessment and large-scale monitoring of terrestrial digital video broadcasting (DVB-T) transmission apparatuses are currently a pressing need due to the rapid growth of related broadcasting networks in many countries. Power measurements, in particular, play a very important role since RF and IF signal power, RF channel power, RF and IF power spectrum, noise (or unwanted) power, and power efficiency are relevant parameters to be measured as accurately as possible. Although modern spectrum analyzers and high-performance vector signal analyzers exhibit satisfying accuracy and repeatability, their cost, weight, and size make them unsuitable for the purpose. This paper is focused on the design and realization of a digital signal processing-based meter for power measurement in DVB-T systems that is capable of granting good accuracy, satisfying repeatability, reduced measurement time, and cost effectiveness. This paper deals with the digital signal processing algorithm to be implemented in the meter, paying attention to parametric power spectral density (PSD) estimators for their reduced memory requirement and potentially limited computational burden. In particular, a parametric estimation algorithm that is capable of assuring a fast measurement rate is implemented and made operative. The simulation and emulation stages are properly designed to regulate the most relevant parameters of the adopted PSD estimators according to the specific features of the signals involved; a number of experiments on actual DVB-T signals are conducted, the results of which are compared to those provided by competitive measurement solutions.      

A Damage Estimation Method for Packaging Systems Based on Power Spectrum Densities Using Spectral Moments

Packaging systems always endure some type of damage during transportation, and Wöhler curves are typically used to determine the mechanical wear evolution of a packaging system. Damage estimations can be performed using a Rainflow method, but continuous stress recording is required. However, this recording is not always possible, so global transportation vibrations are represented by the power spectral density (PSD). In this paper, the damage of a system according to the Wöhler curve was studied. The Basquin model was used to determine the mechanical wear evolution of the system, and the mechanical behaviour was established by extracting the Basquin coefficient b and the constant system C from the Wöhler curves. A Wöhler curve was further realized for a packaging system, and its Basquin coefficient b and system constant C were also extracted. Damage is estimated by the accumulation of the stress cycles using the Palmgren–Miner rules. In this paper, the maximum peaks of the cycle were detected in a signal. The density probability of the maximum stress apparition was also constructed from a multivariate Gaussian model. PSDs have the same statistical properties as the corresponding temporal events, so these statistical properties were extracted by the spectral moment method. Furthermore, a damage equation was proposed using the PSD, and the damage induced by different PSDs was determined from the damage equation and compared with the actual duration on a vibration table.     

Tolerance chart optimization

A tolerance chart is a graphical representation of a process plan and a manual procedure for controlling tolerance stackup when the machining of a component involves interdependent tolerance chains. This heuristic, experience-based method of allocating tolerances to individual cuts of a process plan can be embodied in a computer-based module. This paper introduces a graph theoretic representation for the tolerance chart. A special path tracing algorithm is used to identify tolerance chains from this graph. Optimal tolerance allocation among individual cuts is achieved using a linear goal programming model instead of existing heuristic methods. A more comprehensive mixed integer programming model is developed to incorporate linear tolerance cost functions and alternative process selection.     

一种基于质量目标的统计公差和统计公差带的表达

在对德、中两国统计尺寸公差标准和现有统计公差及公差带表示方式简要介绍的基础上, 重点讨论了作者提出的一种基于质量目标的统计公差模式和统计公差带的表达。指出这种新的面向质量目标的统计公差技术由于其在技术要求、质量要求和工序质量控制之间建立起了标准化的表达界面, 它所建立的质量指标体系、统计公差数据库和统计公差带的标准化图表具有普遍的应用价值     

Tolerance design of datum systems

A majority of mechanical products and devices can be viewed as a collection of parts engineered to assemble so that mating parts and features satisfy some predetermined spatial relationships. Designers typically satisfy the constraints that arise from these relationships by using a set of specific physical features to create on each mating part an accurate reference frame from which all other relevant geometric elements are located. The accuracy and precision with which two mating parts assemble and influence subsequent alignment of other features are controlled by the nominal dimensioning and tolerancing scheme imposed on the geometric elements used to create the two coincident reference frames. The paper presents the development and implementation of a computer aid to assist designers in the allocation of design sizes and tolerances that satisfy functional translation, rotation and assembly constraints imposed on four datum systems commonly used in design.     

Spectral ambiguity of Allan variance

The phase-noise spectrum determines the Allan variance by a well-known integral formula. It is shown here that unique inversion of this formula is not possible in general because the mapping from spectrum to Allan variance is not one-to-one. A necessary and sufficient condition for two distinct phase spectra to have the same Allan variance is given     

Spectral efficiency of spectrum-pooling systems

The authors investigate the idea of using cognitive radio to reuse locally unused spectrum to increase the total system capacity. The authors consider a multiband/wideband system in which the primary and cognitive users wish to communicate to different receivers, subject to mutual interference and assume that each user knows only his/her channel and the unused spectrum through adequate sensing. The basic idea under the proposed scheme is based on the notion of spectrum pooling. The idea is quite simple; a cognitive radio will listen to the channel and, if sensed idle, will transmit during the voids. It turns out that, although its simplicity, the proposed scheme showed very interesting features with respect to the spectral efficiency and the maximum number of possible pairwise cognitive communications. We impose the constraint that users successively transmit over available bands through selfish water filling. For the first time, our study has quantified the asymptotic (with respect to the band) achievable gain of using spectrum pooling in terms of spectral efficiency compared with classical radio systems. The authors then derive the total spectral efficiency as well as the maximum number of possible pairwise communications of such a spectrum-pooling system.     

Spectral reflectance of silicon photodiodes

A precision spectrometer was used to measure the spectral reflectance of a silicon photodiode over the wavelength range from 250 to 850 nm. The results were compared with the corresponding values predicted by a model based on thin-film Fresnel formulas and the known refractive indices of silicon and silicon dioxide. The good agreement at the level of 2 x 10(-3) in the visible wavelength range verifies that the reflection model can be used for accurate extrapolation of the spectral reflectance and responsivity of silicon photodiode devices. In addition, characterization of the photodiode reflectance in the ultraviolet region improves the accuracy of the spectral irradiance measurements when filter radiometers based on trap detectors are used.     

Spectral decomposition of anisotropic elasticity

Summary A newly developed approach, based on the spectral decomposition principle, which is especially useful in crystallography, is applied in this paper. The compliance fourth-rank tensor of crystalline media belonging to the monoclinic system is spectrally decomposed, its eigenvalues are evaluated, together which its elementary idempotent tensors, which expand uniquely the fourth-rank tensor space into orthogonal subspaces. Next, the compliance tensor is spectrally analysed for anisotropic media of the orthorhombic, tetragonal, hexagonal and cubic crystal systems, by regarding these decompositions as particular cases of the spectral decomposition for monolinic media. Consequently, the characteristic values and the idempotent fourth-rank tensors are derived, as well as the stress and strain second-rank eigentensors for all the above mentioned symmetries.     

Spectral albedos of mid-latitude snowpacks

Spectral albedos of impure-nonhomogeneous snowpacks, typical for mid-latitudes, at wavelengths from 400 to 2200 nm are modeled through a numerical solution of the radiative transfer equation in the two-stream approximation. Discrete depth-dependent values of density, grain size and impurity concentration are used to characterize the snowpacks. The model is for diffuse incident radiation, and the numerical method is based on doubling and invariant imbedding. The effect of soot impurities on snowpack albedos is illustrated: when a snowpack is semi-infinite (thickness of ten centimeters or more), soot reduces the albedos at visible wavelengths; however, for smaller snowpack thicknesses soot may increase the albedos at visible wavelengths. By adjusting soot content and snow grain size, good quantitative agreement with some observations at the Cascade Mountains (Washington) and at Point Barrow (Alaska) are obtained; however, the model grain sizes are found to be fifty to four hundred percent larger than the measured values. For satellite snowcover observations, a model for effective albedo of partially snowcovered areas is developed and compared with some NOAA-2 observations of the southeastern United States.     

Spectral analysis of polyharmonic vibrations

This article examines methods of evaluating the harmonic components of deterministic vibrations. The methods are based on different techniques for additional treatment of the results of spectral analysis. The added treatment makes it possible to more accurately determine the parameters of the harmonic components. Translated from Izmeritel'naya Tekhnika, No. 1, pp. 32–34, January, 1996.     

Spectral characterization of eucalyptus wood

The main difficulties in wood and pulp analyses arise principally from their numerous components with different chemical structures. Therefore, the basic problem in a specific analytical procedure may be the selective separation of the main carbohydrate-derived components from lignin due to their chemical association and structural coexistence. The processing of the wood determines some structural modification in its components depending on the type of wood and the applied procedure. Fourier transform infrared (FT-IR) spectrometry and X-ray diffraction have been applied to analyze Eucalyptus g. wood chips and unbleached and chlorite-bleached pulp. The differences between samples have been established by examination of the spectra of the fractions obtained by successive extraction (acetone extractives, acetone free extractive samples, hemicelluloses, and lignins) by evaluating the derivative spectra, band deconvolution, etc. The energy and the hydrogen bonding distance have been evaluated. The relationship between spectral characteristics and sample composition has been established, as well as the variation of the degree of crystallinity after pulping and bleaching. The integral absorption and lignin/carbohydrate ratios calculated from FT-IR spectra of the IR bands assigned to different bending or stretching in lignin groups are stronger in the spectrum of eucalyptus chips than those from brown stock (BS) pulp spectra because of the smaller total amount of lignin in the latter. FT-IR spectra clearly show that after chlorite bleaching the structure of the wood components is partially modified or removed. Along with FT-IR data, the X-ray results confirmed the low content of lignin in the pulp samples by increasing the calculated values of the crystalline parameters. It was concluded that FT-IR spectroscopy can be used as a quick method to differentiate Eucalyptus globulus samples.     

The Verification of Composite Position Tolerance

An algorithm for the verification of composite position tolerances is proposed and its computer implementation is described. It is shown that a prevalent graphical verification method does not adequately reflect the standard. Composite position tolerance requirements are mathematically formulated as a non-linear optimization problem with a linear objective function and quadratic constraints. The method of feasible directions is used as the basis for an efficient computer implementation. Its performance is evaluated under various test conditions as well as against a simplex pattern search.     

Fatigue and Damage Tolerance of Glare

Methods have been developed to describe the fatigue initiation and propagation mechanisms in flat panels as well as mechanically fastened joints and to determine the residual strength of large flat panels. Glare shows excellent crack growth characteristics due to the mechanism of delamination and fibre bridging. The fatigue insensitive fibres restrain the crack opening and transfer load over the crack in the metal layers. During the initiation phase fibre bridging does not occur and the behaviour is dominated by the metal initiation properties. Mechanically fastened joints introduce additional effects such as secondary bending, load transfer and aspects related to the fastener installation. The residual strength of Glare is dependent on the amount of broken fibres and the delamination size and can be described with the R-curve approach.The impact resistance of Glare is related to the aluminium and glass/epoxy properties and is significantly higher than the impact resistance of monolithic aluminium. The same has been proven for fire resistance. Depending on the Glare grade and thickness, the outer aluminium layer will melt away, whereas the other layers will remain intact due to carbonisation of the glass/epoxy layers and delamination of the laminate. The air in the delaminations will act as insulation, keeping the temperatures at the non-exposed side relatively low.