基于自适应逆控制的液压振动台功率谱复现

为了改善液压振动台功率谱密度复现的精度和实时性,引入了自适应逆控制的系统辨识技术。在分析液压振动台功率谱密度均衡原理的基础上,推导时域自适应逆控制(Adaptive inverse control-AIC)和频域逆控制来实时获得振动系统的阻抗函数用以高精度实时修正控制谱密度。文中给出一种多处理器协同工作的硬件平台方案,将实时控制的任务进行分解;最后通过双端口RAM实现数据共享,从而实现功率谱密度的实时复现。仿真实验结果表明：频域自适应逆控制较时域自适应逆控制更利于改善功率谱密度复现的精度和实时性。     

基于协方差本征变换与谐波合成法的随机风场模拟

为解决谐波合成法模拟多变量随机脉动风场计算效率低下,内存易超限的问题,提出了协方差本征变换与谐波合成法相结合的风场模拟新方法。在谐波合成法和协方差本征变换的基础上,采用基于方差比的模态截断准则缩阶互谱密度矩阵,以减少矩阵的Cholesky分解次数,同时运用FFT技术来提高谐波项的合成速度,并给出了该方法的误差控制指标。通过算例表明上述方法可有效减少整个模拟过程的计算时间,采用方差比模态截断准则可保证所有节点位置的样本方差具有较高精度,模拟样本的功率谱密度、相关函数与目标值较为符合,并且具有较好的随机性。证实了所提出方法可以高效、准确地的模拟出随机脉动风场。     

随机振动疲劳寿命评估频域法模型适用性研究

为了研究频域法中的应力幅值概率密度函数(Probability Density Function,PDF)模型在随机振动疲劳寿命评估过程中的适用性，首先介绍常用的5种频域法模型，接着设置由不同谱宽系数、中心频率和功率谱密度(Power Spectral Density,PSD)谱值组合的限带白噪声应力功率谱。在此基础上利用傅里叶逆变换得到对应的时域信号，并将频域法模型计算得到的应力幅值概率密度函数与模拟的时域信号经过雨流循环计数得到的结果进行对比，评估各种频域模型的适用性和精度。结果表明，在整个带宽范围内，ZhaoBaker模型和Dirlik模型都有较高的准确性和适用性，Weibull模型在模拟小谱宽系数(ε<0.15)的单峰谱和谱宽系数大于0.6的宽带PSD时拥有不错的精度；频域模型的精度误差都随中心频率和谱值的增加有不同程度的下降。     

道路模拟试验自适应时域复现控制方法研究

当前国内外关于道路模拟振动试验控制的研究可以分为时域复现、频域复现以及频域幅值域联合复现。其中频域和幅值域联合复现主要应用在道路振动信号中含有较多较大的“尖峰”信号的情况。本文中考虑的道路信号为较为平滑的信号，因此只针对时域复现和频域复现的传统控制方法进行分析。分析得出：传统频域复现方法低频控制精度低，传统的时域复现方法对试验过程中系统特性的变化没有控制能力，从而也影响复现精度，同时当道路信号较长时，试验时间长，试验成本高。因此本文应用了一种自适应的时域复现控制方法，有效地避免了上述出现的问题。试验结果表明，利用该方法实现道路模拟振动试验控制精度可以满足要求，同时缩短了试验时间。     

Blackman-Tukey法的轨道不平顺数值模拟

概述了轨道不平顺的分类和描述方法。分析了估计功率谱密度的Blaekman-Tukey法，并将Blaekman-Tukey法进一步应用于自功率谱情况。根据功率谱密度与轨道随机不平顺的关系，提出了一种基于Blaekman-Tukey法的轨道不平顺数值模拟方法。数值模拟试验表明这种模拟方法简洁、实用、有效。     

一种基于互功率谱密度的频域模态识别法

对各测点响应的互功率谱密度函数进行了详细推导,提出了一种基于互功率谱密度的频域多参点辨识法,这意味着许多利用频响函数的传统频域识别法可以很容易地用于工作状态下的模态识别,大大开阔了未知激励下模态识别的视野。然后采用一飞机模型对该方法进行了试验验证,结果表明,本文所提出的方法确实可行。     

基于流动电磁测量理论的流场重建

提出一种新的流场重建的思想，其理论基础是流动的电磁测量理论。在简要介绍该理论中的重要概念之后，给出了两种实现流场重建的方法：空间域离散和频域展开。进而借助于多相流电容成象中的ＬＢＰ法，处理病态矩阵和提高建象速度。对二维流场的数值模拟例子表明，这种流场重建的方法是可行的。     

平稳随机振动响应的实数域虚拟激励法

针对时不变平稳系统,建议了计算线性随机振动响应功率谱密度的一种实数域虚拟激励算法。基本原理是根据哈特莱变换与傅里叶变换变换关系构造系统的实数型虚拟激励和实数型系统函数,计算结构的实数型虚拟振动响应,由实数型虚拟激励和虚拟响应计算结果确定系统随机振动响应的功率谱密度。研究结果表明,建议的方法可以方便地处理随机激励间的相关性和相干性,与传统的复数型虚拟激励法相比较,只需确定实数型虚拟激励和在频域计算实数型虚拟响应,就能精确算出结构随机振动响应的功率谱密度,计算形式比较简单和可行。     

材料摩擦异响试验台随机振动控制方法研究

为解决车辆在真实道路中的材料摩擦异响测试精度及稳定性问题，开发车用材料摩擦异响试验台系统，基于此系统对其随机振动功率谱密度再现控制方法展开研究。采用变步长频域-X滤波LMS自适应逆控制模型来实时修正系统阻抗函数，实现功率谱密度的高精度再现。采用NI Compact RIO控制器完成数据采集及实时数据处理，对系统振动加速度进行闭环控制，从而实现功率谱密度的实时再现。试验结果表明，采用频域-X滤波LMS自适应逆控制方法可以高精度再现设定功率谱密度，再现误差小于10%，满足实际工程精度要求和实时性要求。     

光纤陀螺随机误差特性仿真与辨识

分析了光纤陀螺的各项随机误差特性,给出了各项随机噪声的仿真方法.运用不同的随机过程模拟各种随机噪声,采用正交小波变换的方法模拟1/f噪声,用白噪声一次离散积分的方法模拟速率随机游走噪声,用一阶马尔可夫过程模拟指数相关噪声,并给出了仿真结果.提出了基于Allan方差、小波分析和功率谱密度的随机误差辨识方法,对光纤陀螺的随机误差特性进行了辨识研究.     

Unique vortex and stripe domain structures in PbTiO3 epitaxial nanodots

The domain structures of PbTiO₃ epitaxial nanodots under the influences of depolarization fields and mismatch strains have been studied using three dimensional phase field simulations. The single-vortex structure and mixed domain configuration, which consisted of zigzag stripe domain and closure dipole flux near the interfaces, were found to be effective in annihilating the depolarization fields in the isotropically tensile and compressive ferroelectric nanodots, respectively. These domain structures were produced by the combined effect of electrostatic and mismatch elastic energies. The width of stripe domain was found to be related to the volume percentage of polarization dipoles along the z-axis, which varied remarkably with the change of compressive mismatch strain. In the case of nanodots under anisotropic mismatch strains, double-vortex domain patterns and stripe domains with nearly straight domain walls were formed. Moreover, the domain structures with electrostatic energy neglected were also studied.     

基于改进马尔可夫链的域名获取方法研究

为解决目前域名获取方法效率低且获取域名数量较少的问题,对前期采集的大量域名进行了统计分析,以发现域名字符的组成规则以及分布特征,并根据这些特征,设计了基于马尔可夫链的域名模型,提出了一种基于改进马尔可夫链的域名生成算法.对生成的域名进行了WHOIS查询验证,以确认域名是否存在.通过大量实验结果证实,该算法具有较高的域名生成准确率,且与其它域名获取方法相比,该方法具有生成域名速率快、域名数量多和顶级域名覆盖广等优点.     

Domain size engineering in tetragonal Pb(In(1∕2)Nb(1∕2))O(3)-Pb(Mg(1∕3)Nb(2∕3))O(3)-PbTiO(3) crystals

The effect of domain size on the dielectric and piezoelectric properties of [111]-oriented tetragonal Pb(In(1∕2)Nb(1∕2))O(3)-Pb(Mg(1∕3)Nb(2∕3))O(3)-PbTiO(3) crystals was investigated. The dielectric permittivity (?(33?) (T)∕?(0)) and piezoelectric coefficient (d(33)) were found to be on the order of 13 800 and 1630 pC∕N, respectively, for samples with domain size of ～500?nm, a 3-fold increase to crystals with domain size of ～50?μm. Rayleigh analysis revealed that the extrinsic contribution to the piezoelectric response increased from ～8% to 30% with decreasing domain size, due to the increased domain wall density and associated irreversible domain wall motion. The enhanced properties were thought to relate to the fine domain structures, however, showing a poor electric field and temperature stabilities with domain size of 500?nm. Of particular significance is that samples with domain size being on the order of 5?μm exhibit field and temperature stabilities, with yet high piezoelectric properties, make it potential for transducer applications.     

Near-field acoustic and piezoresponse microscopy of domain structures in ferroelectric material

Three kinds of near-field microscopy imaging mode including SEAM (Scanning electron acoustic microscopy), PFM (Piezoresponse force microscopy) and SPAM (Scanning probe acoustic microscopy) have been developed to investigate domain structures of ferroelectric ceramics, crystals and thin films in our studies. The domain imaging mechanisms are presented individually in three imaging modes. Sub-surface micro-domain configuration of ferroelectric BaTiO₃ ceramics and single crystal and their dynamic behavior under external fields were clearly visualized by SEAM. Ferroelectric domain structures of ferroelectric PZT thin film and PMN-PT single crystal were characterized by PFM. Nanoscale switching behavior and local field-induced nanoscale displacement behavior of domain structures in ferroelectric thin film were studied by PFM. Antiparallel domain patterns in ferroelectric transparent PLZT ceramics were also characterized by SPAM. The combination of SEAM, PFM and SPAM in application to imaging domain structures undoubtedly enrich our understanding of the nature of piezoelectricity and ferroelectricity at submicro-, even nano-meter scale.     

Nano-sized domain inversion characteristics in LiNbO3 group single crystals using SNDM

This is, we believe, the first report on nano-sized domain inversion characteristics in congruent LiNbO₃ (CLN), MgO doped LiNbO₃ (MgO:LN) and stoichiometric LiNbO₃ (SLN) single crystals brought out using scanning dielectric nonlinear microscopy method. Thin crystal plates for forming nano-sized domain inversions were prepared by chemical mechanical polishing and an ion beam etching process. The small domain dot of 13 nm in radius was formed in CLN. However, the domain dots in CLN tended to disappear. The smallest domain dot of 6.1 nm in radius was obtained in MgO:LN. The domain dot array was also successfully formed at a density of 558 Gbit/in.². In case of higher application voltage, a unique result was obtained in MgO:LN showing that the sizes of domain inversions were almost constant against the voltage pulse width. It is not easy to form domain dot array in SLN since the sidewise growth speed in SLN is very fast.     

结合复小波域和空间总变分的无参考图像清晰度评价

目的针对频域和空域相结合的清晰度评价方法耗时太长、与主观评价一致性不高等问题,提出结合复小波域和空间总变分的感知清晰度评价方法。方法采用低通滤波的方法将原始图像再模糊,得到其参考图像;在复小波域上采用局部相位一致性计算原始图像和参考图像的清晰度图,在空间域上采用总变分计算原始图像和参考图像的清晰度图;分别将原始图像以及参考图像各自的2幅清晰度图进行几何加权平均,得到2幅整体的感知清晰度图;计算2幅感知清晰度图的相似性,得到原始图像的感知清晰度值。结果在LIVE数据库上的SROCC值达到0.9476,CC值达到0.9483,一幅测试图所耗时间为6.003 s。结论实验证明文中算法的评价性能较优,相较同样结合频域和空域的评价方法计算复杂度降低,耗时较少,更加符合人眼视觉主观感知清晰度。     

基于传递路径分析的车身阻尼材料拓扑优化

以某乘用车怠速工况下的车内噪声为研究对象,建立内饰车身的声-固耦合有限元模型,施加实测的加速度激励预测车内噪声响应。通过有限元模型获取系统传递函数,结合实测加速度激励建立传递路径分析模型,分析怠速工况下驾驶员右耳位置121 Hz频率处各路径的声学噪声贡献情况,以贡献量较大的路径为板件贡献量分析的激励输入位置,确定后地板为铺设阻尼的目标板件。以121 Hz处驾驶员右耳声压最小为目标,建立拓扑优化模型,对后地板阻尼进行布局优化。结果表明,怠速工况下121 Hz峰值频率处驾驶员右耳声压级下降5.59 dB(A),传递路径分析对阻尼结构优化设计具有一定指导作用。     

二维平纹机织复合材料弹性性能预测的域分解方法

为了预测二维平纹机织复合材料的弹性性能, 提出了基于有限元重合网格法的域分解方法。域分解方法与传统代表体元法的有限元建模不同, 前者不再建立精细的纤维与基体模型, 而是分别建立二维平纹机织复合材料单胞的整体域与纤维域, 整体域是真实基体体积与纤维体积的叠加, 两区域网格独立剖分, 互不影响。采用MSC. Nastran中的多节点约束在纤维节点与基体节点之间建立位移协调来模拟纤维和基体单元的位移函数关系, 实现了纤维域和基体域的耦合计算。研究表明, 域分解方法大大简化了机织复合材料细观力学建模的复杂性, 降低了建模时间, 采用域分解方法预测的二维平纹机织复合材料弹性常数与试验值吻合较好, 充分说明了该预测模型与方法的正确性。研究了不同纤维体积分数下, 域分解方法预测二维平纹机织复合材料的弹性常数的变化趋势, 结果表明, 随纤维体积分数增加, 模量呈上升趋势, 泊松比呈降低趋势。     

Translational positioning of a magnetic domain wall in ferromagnetic nanowires using a stray field filter

Understanding the interaction between the magnetic domain wall and the various artificial defects in ferromagnetic nanowires has been of utmost importance for the future realization of the spintronic devices based on the magnetic domain wall motion in nanowires. In this work, the chirality filter effect of the magnetic domain wall in T-shaped ferromagnetic nanowires with a stray field filter was investigated via micromagnetic simulation. A tapered wire was attached to the flat nanowires to form a potential barrier or well for the domain wall propagating along them. For the domain wall passing through the potential barrier or the potential well, the spin structure of the domain wall and the interaction between the domain wall and the potential barrier/well were investigated in detail. The chirality-dependent translational positioning of the domain wall was intensively examined for the potential barrier and potential well cases. The domain wall chirality transmission on relatively long length scales using a series of potential wells was explored.     

近化学计量比LiNbO3晶体电畴结构观察和机理探讨

采用双坩埚提拉法(DCCZ)生长了各种不同成分的近化学计量比LiNbO₃晶体, 并用腐蚀法观察了其电畴结构. 结果表明, 化学成分对未经极化处理晶体的电畴结构起决定性作用, 当Li₂O 含量处于49.4mol%附近时, 晶体z面电畴呈现特殊的三次对称反畴; 当晶体中Li₂O含量为49.7mol%时, 晶体为完全单畴. 本文对其形成机理进行了探讨, 认为在由顺电相向铁电相转变 时, 局部铁电畴的极性方向与该处沿z轴方向的温度梯度正负密切相关, z轴生长晶体时, 由于相变发生所处位置离生长界面的距离受LiNbO₃晶体计量比影响, 所处温场固有温梯也 随之不同, 在此基础上解释了不同成分晶体的电畴结构形成原因. 最后讨论了控制铁电畴结构的工艺措施.