IDENTIFICATION OF MODAL PARAMETERS FROM MEASURED OUTPUT DATA USING VECTOR BACKWARD AUTOREGRESSIVE MODEL

In this paper, a modal identification system that is based on the vector backward autoregressive (VBAR) model has been developed for the identification of natural frequencies, damping ratios and mode shapes of structures from measured output data. The modal identification using forward autoregressive approach has some problems in discriminating the structure modes from spurious modes. On the contrary, the VBAR approach provides a determinate boundary for the separation of system modes from spurious modes, and an eigenvalue filter for the selection of physical modes is existent in the proposed method. For convenience of application, the backward state equation established from VBAR model is transformed into a forward state equation, which is termed as transformed VFAR model in this paper. In addition, the extraction of equivalent system matrix of state equation of motion for structures from the transformed VFAR model has been developed, and then the normal modes can be calculated from the identified equivalent system matrix. Two examples of modal identification are carried out to demonstrate the availability and effectiveness of the proposed backward approach: (1) Numerical modal identification for a three-degree-of-freedom dynamic system with noise level in 20% of r.m.s of measured output data; (2) experimental modal identification of a cantilever beam. Finally, to show the advantage of the proposed VBAR approach on the selection of physical modes, the modal identification by stochastic subspace method was performed. The results from both methods are compared.     

Identification of the structure parameters using short-time non-stationary stochastic excitation

In this paper, we propose an approach to the flexural stiffness or eigenvalue frequency identification of a linear structure using a non-stationary stochastic excitation process. The idea of the proposed approach lies within time domain input-output methods. The proposed method is based on transforming the dynamical problem into a static one by integrating the input and the output signals. The output signal is the structure reaction, i.e. structure displacements due to the short-time, irregular load of random type. The systems with single and multiple degrees of freedom, as well as continuous systems are considered.     

On the algebraic reconstruction of the Duffing's mechanical system

The identification of the unknown parameters of the Duffing's mechanical system, based on an algebraic approach, is presented. This approach is fast, accurate, and simple to numerically implement. Also, the method, combined with a suitable invariant filter, can became robust against high frequency output measurement noises. Our method uses the availability of one measurable output and produces an exact formula for the unknown parameters, which may be realized in terms of iterated convolutions. First, we show that the Duffing's system parameters are linearly identifiable with respect to the position variable, then we obtain a linear system where the unknowns are the unavailable parameters. Suitable algebraic operations on the output differential equations makes the identification schema independent of the unavailable initial conditions of the underlying nonlinear dynamical system.     

一种基于三角形几何结构的星图识别算法

提出了一种新的以星三角形为基本识别单元的几何结构星图识别算法。通过对观测误差的预处理,将该算法星三角形的匹配过程转换为一个没有误差的三角形识别问题。引入一个能部分描述三角形结构的特征量,将导航三角形的识别所需的特征匹配数量降低,而多三角形的组合有效地提高了正确识别率。仿真实验表明,该算法所需导航星库容量小,识别效率高,误识率低,实时性和鲁棒性优于传统的三角形星图识别算法。     

一种多分辨率高维图像特征匹配算法

通过对图像进行特征提取和变换,图像的相似性匹配可以转换为高维向量空间内的点匹配. 为了解决高维数据的维数灾难问题,提出一种基于多分辨率数据结构的向量近似方法. 从低分辨率开始计算距离下限,如果距离下限大于目前结果集中的最大距离,则不需要在高分辨率上计算其距离而将其排除掉,从而降低了向量近似方法的运算复杂度. 提出应用此方法的近邻搜索算法并运用到图像数据库的特征匹配中,实验证明：新方法搜索效率优于传统的向量近似方法.     

全腔输出相对论磁控管输出模式转换结构的理论设计和数值模拟

采用全腔输出结构后,相对论磁控管径向尺寸显著减小,轴向长度也有较大幅度的缩短.但是,由于输出结构为三个相对独立的扇形波导,实际应用时,一般需要对微波输出模式进行转换.针对全腔输出相对论磁控管,本文研究了两种输出模式转换结构并利用三维全电磁粒子模拟程序对其进行了研究.首先研究了将三个扇形波导角向增宽从而渐变或者突变为一个同轴波导的情况,研究结果表明,两种情况下输出微波功率均大于采用传统三个独立扇形波导输出时的90%,输出模式主要是TEM模.其次研究了输出区由三个扇形输出波导分别变换为三个截面大小与之接近的矩形输出波导的可行性,研究结果表明,注入扇形波导中的TE₁₁模式几乎全部转换为矩形波导中的TE₁₀模式.实际应用时,可根据需要选择上述输出模式转换结构.     

A new approach to design a control system for a FGR furnace using the combination of the CFD and linear system identification techniques

A new approach to design control systems for an industrial furnace with flue gas recirculation (FGR) is presented. To facilitate the control system design, a linear dynamic model is needed for the furnace. Full-scale computational fluid dynamics (CFD) simulations are used to generate the required small signal input and output data sets. Subsequently, a least squares based system identification technique is used to obtained the linear dynamic models. After model validation, feedback controller is designed based on these linear dynamic models. Finally, the performance of the designed closed-loop control system is also evaluated using both linear dynamic model and full-scale nonlinear CFD model. The comparison shows that the control system designed using the proposed approach can minimize the deviation of nitric oxides (NO) emission from the design point by minimize the dynamic NO formation, hence to prevent any excessive NO formation in the combustion process when the system subjects to disturbances.     

Equivalent source method for identification of panel acoustic contribution in irregular enclosed sound field

For the interior sound field formed by the complex vibrating structure,an identification approach of panel acoustic contribution based on equivalent source method(ESM)was presented.The normal velocity on the surface of vibrating structure was first reconstructed by using interior nearfield acoustic holography based on ESM and the prediction of whole interior enclosed sound field was realized.Then the sound pressure produced by each panel at the interested field point was respectively replaced by the radiated pressure of the enclosed interior sound field which is formed by the equivalent virtual sources located near the surface of the cavity.Combining with the reconstructed normal surface velocity,the acoustic contribution of each panel to any position in the cavity was obtained by transforming the complex enclosed non-free field into the simple interior free field.Numerical simulations and experiments are conducted,and the influences of the number of the equivalent sources and the distance between them and the reconstructed surface have been investigated.The results show that the proposed method is easier to be implemented with the same accuracy than the traditional analysis method.     

复杂封闭声场面板声学贡献度识别的等效源法

针对由复杂结构振动形成的封闭空间声场,提出了一种基于等效源法的面板声学贡献度分析方法。该方法首先利用基于等效源法的内部声全息技术,重构出振动结构表面的法向振速并实现对整个内部封闭声场的预测。再将振动结构的每个面板在腔体内部场点产生的声压分别用位于空腔表面附近的等效源在该点产生的辐射声压代替,将复杂的封闭非自由声场问题转化为简单的内部自由场问题,结合重建出的结构表面法向振速进而识别出封闭振动结构各面板对腔体内任意位置的声学贡献度。通过对复杂结构内声场的数值仿真和验证实验,分析了等效源的数量及与重建面距离等参数对重建精度的影响,结果表明所提方法不仅能够达到传统数值分析方法的计算精度,而且具有更简单的求解过程。      

高效强流径向分离腔振荡器研究

根据同轴虚阴极的结构特点,提出了一种紧凑型径向分离腔振荡器,该径向分离腔的特点是输入电子束的电流可以很大,从而可以得到较高的微波输出功率. 综合考虑影响微波输出的各种因素,在电子束为410kV,电流为35kA的条件下,模拟得到的平均功率大于50GW,频率为146GHz,电子束功率效率达到348%. 关键词： 径向分离腔 高功率微波 同轴虚阴极 电子束分布     

THE ENERGY MOBILITY

The energy mobility method is presented and applied both numerically and experimentally to thin plates of high modal density. The energy mobility is defined as the ratio of the frequency averaged quadratic velocity at one point of a structure to the frequency averaged active power injected at one point of the structure. It can be calculated using only classic input and transfer mobilities. The property of energy additivity of the contributions of several external loads is shown to be a good approximation in the case of a single structure. To quantify the approximations on typical structures one is interested in, numerical simulations are presented on plates. A good agreement is found between the predictions using the energy mobility and the exact calculations. An experiment conducted on a plate with a real excitation confirms that the prediction of the energy mobility approach can be quite satisfactory. Finally, a remarkable property for the energy mobility (contrary to standard mobility) is noted: the insensitivity to the presence of a mass heterogeneity at driving points. This property allows one to simplify considerably the characterization of industrial structures that have in general a lot of heterogeneities. The energy mobility concept is further applied to the vibrational behaviour of an assembly. For each subsystem the connection is described by adding injected power and coupling power into the energy additivity. The equality of the frequency averaged quadratic velocity and the power flow balance at each coupling point allows one to calculate the coupling power at connection points with energy mobilities of uncoupled subsystems. A system of two plates has been used to compare the energy mobility calculation and the exact one given by the classic mobility method. The comparison shows that a connectivity factor must be introduced to keep the same formalism generally used with classic mobilities. A generalized definition for the energy mobility is then established.     

Frequency domain analysis and identification of block-oriented nonlinear systems

Block-oriented nonlinear models including Wiener models, Hammerstein models and Wiener-Hammerstein models, etc. have been extensively applied in practice for system identification, signal processing and control. In this study, analytical frequency response functions including generalized frequency response functions (GFRFs) and nonlinear output spectrum of block-oriented nonlinear systems are developed, which can demonstrate clearly the relationship between frequency response functions and model parameters, and also the dependence of frequency response functions on the linear part of the model. The nonlinear part of these models can be a more general multivariate polynomial function. These fundamental results provide a significant insight into the analysis and design of block-oriented nonlinear systems. Effective algorithms are therefore proposed for the estimation of nonlinear output spectrum and for parametric or nonparametric identification of nonlinear systems. Compared with some existing frequency domain identification methods, the new estimation algorithms do not necessarily require model structure information, not need the invertibility of the nonlinearity and not restrict to harmonic inputs. Simulation examples are given to illustrate these new results.     

Damage identification in plates using finite element model updating in time domain

A response sensitivity-based approach is presented for identifying the local damages in isotropic plate structures from the measured structural dynamic responses. The local damage is simulated by a reduction in the elemental Young's modulus of the plate. In the forward analysis, the forced vibration responses of the plate under external force are obtained from Newmark direct integration. In the inverse analysis, a response sensitivity-based finite element model updating approach is used to identify local damages of the plate in time domain. The damage identification results are obtained iteratively with the penalty function method with Tikhonov regularization using the measured structural dynamic responses. Two numerical examples are investigated to illustrate the correctness and efficiency of the proposed method. Both single damage and multiple damages cases are studied. The effects of measurement noise and measurement point on the identification results are investigated. Studies in this paper indicate that the proposed method is efficient and robust for both single and multiple damages for plate structures. Good identified results can be obtained from the short time histories of a few number of measurement points.     

Identification of hybrid ARX-neural network models for three-dimensional simulation of a vibroacoustic system

Acoustic noise in industrial areas, typically generated by compressors and vacuum pumps, may be mitigated by the combined use of passive and active noise control strategies. Despite its widespread use, the traditional Active Noise Control (ANC) technique requires error feedback and has been proven to be effective only within a small spatial region. When the movement of human ears is required within a large region and error feedback is difficult to be accomplished, new cancelling strategies have to be devised to achieve acceptable levels of spatial coverage. In the pursuit of this goal, this paper proposes a vibroacustic model to predict noise radiated from machinery. The model output is the sound signal of the noise at a given point inside a closed room. The two model inputs are the vibration signal at the noise source and the spatial coordinates of the intended point. Experimental output data were measured at several points inside a region defined by a solid rectangle. A fixed-order ARX model was chosen (AutoRegressive with eXogenous input), and for each spatial point and its corresponding pair of input-output signals, a set of parameter values was estimated. To integrate all these models into a single one, a neural network was employed to associate or approximate each set of parameters to its spatial coordinates. With this approach, the total number of parameters is expected to be greatly reduced, when considering the original separated models. Experimental results are presented and comparisons with other models are established on the basis of least-square error metrics and parsimony of parameters. A qualitative perspective for employing the proposed model in the design of large-region ANC strategies is also offered.     

基于六角形晶格的光子晶体零折射率特性及其光学应用研究

运用色散特性分析的方法,在六角形晶格光子晶体能带结构的布里渊区中心找到了狄拉克点,用等效媒质理论验证了在这一点上光子晶体的等效介电常量和等效磁导率同时为零,即存在狄拉克频点,用此六角形晶格光子晶体可以实现零折射率材料.随后运用有限元数值仿真方法,研究了此六角形晶格结构在狄拉克频点及其附近所呈现的零折射率特性.将此种六角形晶格光子晶体经过合理设计用以制作凹透镜,发现可以实现远场的亚波长聚焦.此外在凹透镜的入射面处引入防反射结构可以在一定程度上增强聚焦效果,提高了系统的分辨率.     

A simplified Time Reversal method used to localize vibrations sources in a complex structure

The main issue while dealing with problems due to structural vibrations is the identification of the sources that create annoyance. One of the experimental processes that permit to tackle this reverse problem uses Time Reversal method to localize the origin of the vibration detected on the surface of a structure.The Time Reversal experiment is based on a principle of time symmetry of waves propagation in a media. Using transceivers located on the structure, one can record its state of vibration. If all the signals recorded by the transceivers are reversed in time and reemitted from the position where they have been recorded, the resulting vibration will converge back to the point where it was originally emitted. In the standard approach, localization is observed both in time and space. We propose here a simplified localization process based on space localization only. We will apply this method on a complex industrial structure, stimulated with bursts. We shall show in the article the influence of certain parameters such as the number of transceivers or the structure complexity. Finally, all the tests presented hereby will allow us showing that the Time Reversal method is a very efficient and very easy to use method.     

基于热膨胀效应的可调光功率分束器设计

可调光功率分束器是集成光子系统中的重要功能器件,其光功率输出具有动态可调控特性,诸多光子系统对其有着广泛需求.本文基于空隙槽型Y分支波导结构,利用热膨胀效应,提出了一种新型可调光功率分束器实现方法,通过调控温度来改变分支处空隙槽的宽度,以实现其分支波导的光功率输出的动态变化.采用有限元方法,对其热膨胀形变和光学性能进行了模拟分析.其模拟结果表明,该器件能实现大范围光分束调控,且对波长和偏振态依赖性低;同时,该器件具有结构简单、易于设计和制作、易于调控等优点,这种新型器件在各种光子系统中具有重要应用前景.     

Structural damping identification based on an iterative regularization method

A time domain structural damping identification method is presented in this paper. The damping of a structure is assumed to be time-invariant Rayleigh damping, time-variant Rayleigh damping and modal damping, respectively, and an iterative regularization method is proposed to identify these three types of damping in turn. A sensitivity approach is adopted in the formulation of the ill-posed inverse problem. A new constraint is imposed on the identified iterative increment as well as on the unknown structural parameters to ensure their physical meaning in the identification process is not lost. The proposed method is verified in numerical studies with a space frame structure and laboratory measurements of accelerations with accurate results.     

Experimental modal substructuring to couple and uncouple substructures with flexible fixtures and multi-point connections

Modal substructuring or component mode synthesis (CMS) has been standard practice for many decades in the analytical realm, yet a number of significant difficulties have been encountered when attempting to combine experimentally derived modal models with analytical ones or when predicting the effect of structural modifications using experimental measurements. This work presents a new method that removes the effects of a flexible fixture from an experimentally obtained modal model. It can be viewed as an extension to the approach where rigid masses are removed from a structure. The approach presented here improves the modal basis of the substructure, so that it can be used to more accurately estimate the modal parameters of the built-up system. New types of constraints are also presented, which constrain the modal degrees of freedom of the substructures, avoiding the need to estimate the connection point displacements and rotations. These constraints together with the use of a flexible fixture enable a new approach for joining structures, especially those with statically indeterminate multi-point connections, such as two circular flanges that are joined by many more bolts than required to enforce compatibility if the substructures were rigid. Fixture design is discussed, one objective of which is to achieve a mass-loaded boundary condition that exercises the substructure at the connection point as it is in the built up system. The proposed approach is demonstrated with two examples using experimental measurements from laboratory systems. The first is a simple problem of joining two beams of differing lengths, while the second consists of a three-dimensional structure comprising a circular plate that is bolted at eight locations to a flange on a cylindrical structure. In both cases frequency response functions predicted by the substructuring methods agree well with those of the actual coupled structures over a significant range of frequencies.     

星图识别算法的优化设计与实现

在INS／CNS组合导航系统中,为了有效实时地用CNS的输出姿态信息校正INS的陀螺漂移,必须提高CNS中星图识别算法的运行速度。提出了一种改进的三角形星图识别方法,基三角形识别成功后,增加一颗附加检测星,计算出附加检测星和基三角形的三个星点的角距值。分析结果表明传统的星图识别算法在主频为600MHz的DSP6414上运行需要300ms左右的时间,算法改进后,只需40-80ms,数据输出率提高到10Hz。改变的算法在不存在伪星的情况下成功识别率近100％,存在伪星时,可以有效去除伪星。同时,解决了传统星图识别算法在嵌入式系统中实现难、耗时长的问题。该算法可以实时地校准惯导系统的误差。