移动质量与梁耦合系统固有频率的计算与分析

建立了在移动质量减速运动情况下,通过获取梁的挠度响应曲线,数值仿真实验获得耦合系统的基频;在移动质量匀速运动情况下,通过获取梁的跨中挠度响应曲线,数值仿真实验获得梁的基频;以及用来计算耦合系统各阶固有频率的特征值方法的理论。数值算例结果表明,这类耦合系统的各阶固有频率不仅与质量比有关,而且与位置比也有关,若用梁的固有频率取代耦合系统的固有频率或用实验值代替理论值有时会产生较大的误差．     

The effect of system parameter variations on natural frequencies

In the paper the relationships between variations of mass and stiffness matrices of system and variation of natural frequencies vector are analysed. Results are applied to finite element method and to stiff finite element method. The example of influence of masses and mass inertia moments variations of support beam on its natural frequencies illustrates the analysis.     

A review of mass matrices for eigenproblems

Various nonconsistent mass matrices have been presented to achieve more accurate natural frequencies in eigenproblems of the finite element analysis. The matrices are obtained as a linear combination of lumped and consistent mass matrices. For an improved accuracy, the consistent mass should be more weighted than the lumped mass. Instead of the mass combination, the interpolation functions can be combined to give nonconsistent mass matrices, which show the same tendency. To find a nonsingular lumped mass matrix for the bending vibration of beams, a translational inertia has been proposed for rotational degrees of freedom. The inertia effect is highly overestimated and hence lower natural frequencies are obtained. When combined with the consistent mass matrix, however, the modified lumped mass matrix gives a significant improvement for the natural frequencies of intermediate and higher modes. A simple corrective method was applied to get a better estimation of the natural frequencies through the use of the frequency dependent stiffness and mass matrices. The method shows high accuracy without complicated calculations.     

The transformation of the modal equations into the force equations

For discrete, linear elastic systems in free vibration, the existing methods of problem solution employ as input values, the system mass coefficients and either its stiffness or influence coefficients. The output values for these methods are, the system natural frequencies, and corresponding member displacements, which are its mode shapes. By transforming the modal equations into the problem force equations, using the same input values, the output values become the system natural frequencies and the corresponding member forces. Since stresses are most easily calculated from forces, the force equations appear to be ideally suited for engineering design.     

A pocket calculator program for some simple vibration problems

An earlier buckling program [1] has been adapted to give the natural frequencies and mode shapes for free axial vibrations of bars, transverse vibrations of taut wires and torsional vibrations of shafts. The cross section of the bar, wire or shaft can vary at intervals along its length, and so can its distributed mass (or inertia for the shaft), which is allowed for in a classically exact way. Lumped masses (or rotors for shafts) and spring supports can be attached anywhere along the length, and the ends can be free or clamped. The program just fits on a Hewlett-Packard HP-25 pocket calculator, which can store only 49 program steps and 13 numbers.     

Vibration analysis of axially moving magnetic tape with comparisons to static and dynamic experimental results

The demand for greater storage capacity is prompting the development of data storage tape with greater volumetric and areal density. As the data tracks on these tapes become narrower, minimization of the lateral tape motion (LTM) becomes more important to prevent loss of data due to read/write errors. In order to identify and minimize sources of LTM, a vibration model is developed which simplifies the tape drive to a fixed-fixed Euler-Bernoulli beam model with axial velocity. The effects of varying axial velocity, tension, free span length, and tape thickness were investigated. The calculated natural frequencies are compared to those obtained by both static and dynamic experiments. Effects of varying the length, tape thickness, and tension were studied in the static experiment while the effects of changing speed and tension were studied in the dynamic experiment.     

Free flexural vibration behavior of laminated angle-ply elliptical cylindrical shells

This paper deals with the free flexural vibration characteristics of anisotropic laminated angle-ply elliptical cylindrical shells using finite element approach. The formulation is based on first-order shear deformation theory. The present model accounts for in-plane and rotary inertia effects. A detailed study is carried out to highlight the effects of shell geometry, cross-sectional properties, lap-up and ply-angle on the natural frequencies pertaining to different types of modes of vibrations of non-circular shell structures.     

基于神经网络的多传感器融合PDR定位方法

针对当前行人航位推算系统因行人随意性行走、传感器漂移等造成行人步长估计不精确、方向计算误差累积问题,提出了一种基于神经网络和智能手机内置多传感器融合的PDR室内定位方法.首先利用加速计采集的传感器数据和移动距离数据训练BP神经网络,将训练好的BP神经网络模型进行行人移动距离预测,然后根据行人行走步伐的连续性特点和传感器输出之间的相关性,设计了一种微航向角融合的方向估计算法.该算法通过对行走过程中的情况进行分类以获得可靠的传感器源,利用3种微航向角进行分类加权融合,最终获得行人行走方向的精确估计.实验结果表明,通过行人移动距离预测和微航向角融合算法能够实现得较好的定位效果.     

一类有静差系统的结构特征

通过分析惯性导航系统初始对准方案的静差特性发现,实际工程中存在着这样一类系统,无论为它设计什么样的动态补偿器,其被调节量总是有静差的。且其静差只依赖开环系统的结构参数,而与动态补偿器的结构及其参数的选择无关。我们称这类系统为有差系统。本文分析了有差系统的结构特征,并且给出了通过开环结构参数计算静差的公式。     

Dynamics-based analysis and synthesis of human locomotion

One of the best ways to synthesize realistic human motions is to animate characters from captured motion data that inherently respect motion laws. Retargeting and interpolation methods are often used to adapt these motions to different representations of the character and to various environmental constraints but they may introduce physical inaccuracies, although the synthesized motions are natural looking. This paper presents a method for evaluating the physical correctness of retargeted and interpolated locomotions using an inverse dynamics analysis. Furthermore, we propose to improve an initial database with analysed motions that are synthesized again by using a forward dynamics approach. The analysis algorithm consists in determining the resulting forces and torques at joints. With this intention, we develop an automatic creation process of the mass/inertia model of the character. Then using support phase recognition, we compute resulting forces and torques by an inverse dynamics method. The retargeting and the interpolation methods change the physics of the motions. This change is evaluated by using the results of our analysis on artificial and real motions and by using literature results and experimental data from force plates. The evaluation relies on the study of several retargeting and interpolation parameters such as the global size of the character or the structure of the model. The output of this evaluation, the resulting forces and torques at joints, are used to produce physically valid motions by using forward dynamics simulation. With this purpose, we introduce forces and torques normalizations, and finally the synthesized motions may improve the initial database.     

Substructure synthesis methods for dynamic analysis of multi-body systems

The formulation for the dynamic analysis of flexible multi-body systems that undergo large rigid body motion, leads to geometrically non-linear inertia properties due to large rotations. These inertia non-linearities that represent the coupling between gross rigid body motion and small elastic deformation, are dependent on the assumed displacement field. As alternatives to the finite element methods, deformable body shape functions and shape vectors are commonly employed to describe elastic deformation of linear structures. In this paper, substructure shape functions and shape vectors are used to describe elastic deformation of non-linear inertia-variant multi-body systems. This leads to two different representations of inertia nonlinearities; one is based on a consistent mass formulation, while the other is a lumped mass technique. The multi-body systems considered are collections of interconnected rigid and flexible bodies. Open and closed loop systems are permitted.     

Silicon retina with correlation-based, velocity-tuned pixels

A functional two-dimensional silicon retina that computes a complete set of local direction-selective outputs is reported. The chip motion computation uses unidirectional delay lines as tuned filters for moving edges. Photoreceptors detect local changes in image intensity, and the outputs from these photoreceptors are coupled into the delay line, where they propagate with a particular speed in one direction. If the velocity of the moving edges matches that of the delay line, then the signal on the delay line is reinforced. The output of each pixel is the power in the delay line signal, computed within each pixel. This power computation provides the essential nonlinearity for velocity selectivity. The delay line architecture differs from the usual pairwise correlation models in that motion information is aggregated over an extended spatiotemporal range. As a result, the detectors are sensitive to motion over a wide range of spatial frequencies. The design of functional one- and two-dimensional silicon retinas with direction-selective, velocity-tuned pixels is described. It is shown that pixels with three hexagonal directions of motion selectivity are approximately (225 mum)(2) in area in a 2-mum CMOS technology and consume less than 5 muW of power.     

六自由度并联式机器人拉格朗日动力方程

本文对六自由度并联机器人的动力学问题进行了研究.文中根据一二阶影响系数矩阵,导出了仅依赖于系统的质量分布和几何特性的广义惯性张量和广义惯性功率模型矩阵,建立了多回路系统的拉格朗日动力方程和运动控制方程.最后给出了实例计算.     

磁悬浮刚性转子系统振动机理分析与动力学建模

磁悬浮惯性执行机构采用磁轴承支承,可通过主动控制实现极微振动,但磁悬浮惯性执行机构仍存在频谱分量丰富的振动.首先在转子动静不平衡和Sensor Runout振动机理分析的基础上,重点分析了Magnet Runout产生振动机理;然后,建立包含多振动源的系统动力学建模,并将整个动力学模型分解为平动和转动子系统,分析表明转子动静不平衡、Sensor Runout和Magnet Runout是通过不同的途径产生振动,不仅产生同频振动还包含倍频振动;最后,提出磁悬浮刚性转子系统主动振动控制的要求,为以后的主动振动控制研究奠定基础.     

Dynamic stiffness matrix of non-symmetric thin-walled curved beam on Winkler and Pasternak type foundations

Using the technical computing program Mathematica, the dynamic stiffness matrix for the spatially coupled free vibration analysis of thin-walled curved beam with non-symmetric cross-section on two-types of elastic foundation is newly presented based on the power series method. For this, the elastic strain energy considering the axial/flexural/torsional coupled terms, the kinetic energy including the rotary inertia effect, and the energy due to the elastic foundation are introduced. Then, equations of motion are derived from the energy principle and explicit expressions for displacement parameters are derived based on power series expansions of displacement components. Finally, the exact dynamic stiffness matrix is determined using force–displacement relations. In order to demonstrate the validity and the accuracy of this study, the natural frequencies of thin-walled curved beams with mono-symmetric and non-symmetric cross-sections are evaluated and compared with the analytical solutions and finite element solutions using Hermitian curved beam elements and ABAQUS’s shell elements. In addition, some results by a parametric study are reported.     

Finite element modal analysis of a rotating disk–spindle system in a HDD with hydrodynamic bearings considering the flexibility of a complicated supporting structure

This paper presents a method to analyze the free vibration of a rotating disk–spindle system in a HDD with hydrodynamic bearings (HDBs) considering the flexibility of a complicated base structure by using finite element method. Finite element equations of each component of a HDD spindle system from the spinning flexible disk to the flexible base plate are consistently derived by satisfying the geometric compatibility in the internal boundary between each component. The rigid link constraints are also imposed at the interface area between the sleeve and hydrodynamic bearings to describe the physical motion at this interface. A global matrix equation obtained by assembling the finite element equations of each substructure is transformed to a state-space matrix-vector equation, and both damped natural frequencies and modal damping ratios are calculated by solving the associated eigenvalue problem by using the restarted Arnoldi iteration method. The validity of the proposed method is verified by comparing the calculated damped natural frequencies and modes with the experimental results. This research also shows that the supporting structure which includes the stator, housing and base plate plays an important role in determining the natural frequencies and mode shapes of a HDD spindle system     

Nonlinear free vibration of fixed off-shore framed structures

The dynamical behavior of fixed off-shore framed structures is studied using the Wittrick-Williams algorithm to solve the nonlinear eigenvalue problem. The effects of shear deformation and rotary inertia as well as axial static loading are considered in this study of nonlinear free vibration.

The members are assumed to be rigidly connected and the added water mass is assumed equal to the mass of the water displaced. The structural modeling is based on a two-dimensional representation of the three-dimensional tower assuming a constant dimension equal to the base length perpendicular to the plane. The distributed masses of the members in the plane of the frame are computed by summing up the structural mass, the mass of the water contained in the tube, and the mass of the water displaced. The member masses in the plane perpendicular to the frame are assumed to be lumped at the horizontal cross-brace levels.

The results of the study indicate that while the first two frequencies obtained from the nonlinear and linear eigenvalue solutions agree closely, the effect of nonlinear eigenvalue solution is significant for the higher frequencies. The results also highlight the significant effects of the axial static force in the dynamic tangent stiffness matrix in the free vibration study of the off-shore structure. Fields for further research include (i) soil-structure interaction studies for gravity off-shore structures, buried pipelines, and (ii) nuclear power plant structures.     

Construction and accuracy analysis of images of the daily-mean mixed-layer depth

Abstract

Using satellite data only, a unique model of the upper-ocean mixed-layer has been developed recently by Yan for calculating the thermal inertia (or the heat flux change per unit temperature change per day) of the mixed-layer from which daily mean mixed-layer depth is estimated. Thus digital maps of the oceanic daily mixed-layer depth in the vicinity of the Sargasso Sea for 14 May-30 August, 1982 have been generated from visible, near-infrared, thermal-infrared and microwave observations made by radiometers on board the NOAA-7, -8, GOES and Nimbus-7 satellites. The errors incurred in producing such maps are of two general type: measurement from satellites and model simplification. The sensitivity of the modelled mixed-layer depth to simplifications in the model (modelling errors) and to errors in data used for forcing the model (data errors) is considered here. To emph isize the oceanographic relevance of these errors, we express the errors in mixed-layer depth as a function of model errors and data errors, and compare these errors with the daily mixed-layer depth changes of in situ data. We also discuss some of the error sources in this paper. Thus the applications and practical limitations of the model are evaluated. The results of this study show that errors in the surface temperature field cause the largest errors in the predicted mixed-layer depth, because the surface temperature errors affect both the heat flux estimation and the thermal inertia estimation. The errors in wind speed field are less severe although these errors may become more significant at wind speed higher than 8.5 ms^?1, below which most of our field comparisons were made. The smallest uncertainty is found to occur with remotely estimated changes in sea-surface albedo as this error in a very small way affects the heat flux. These results imply that the errors due to direct atmospheric radiative effects or those which determine radiative fluxes and the determination of sea surface temperature should be reduced to improve prediction of mixed-layer thermal inertia and mixed-layer depth in summer using satellite data.     

Chain Vibration and Dynamic Stress in Three-Dimensional Multibody Tracked Vehicles

A three-dimensional computational finite element procedure for the vibration and dynamic stress analysis of the track link chains of off-road vehicles is presented in this paper. The numerical procedure developed in this investigation integrates classical constrained multibody dynamics methods with finite element capabilities. The nonlinear equations of motion of the three-dimensional tracked vehicle model in which the track link s are considered flexible bodies, are obtained using the floating frame of reference formulation. Three-dimensional contact force models are used to describe the interaction of the track chain links with the vehicle components and the ground. The dynamic equations of motion are first presented in terms of a coupled set of reference and elastic coordinates of the track links. Assuming that the structural flexibility of the track links does not have a significant effect on their overall rigid body motion as well as the vehicle dynamics, a partially linearized set of differential equations of motion of the track links is obtained. The equations associated with the rigid body motion are used to predict the generalized contact, inertia, and constraint forces associated with the deformation degrees of freedom of the track links. These forces are introduced to the track link flexibility equations which are used to calculate the deformations of the links resulting from the vehicle motion. A detailed three-dimensional finite element model of the track link is developed and utilized to predict the natural frequencies and mode shapes. The terms that represent the rigid body inertia, centrifugal and Coriolis forces in the equations of motion associated with the elastic coordinates of the track link are described in detail. A computational procedure for determining the generalized constraint forces associated with the elastic coordinates of the deformable chain links is presented. The finite element model is then used to determine the deformations of the track links resulting from the contact, inertia, and constraint forces. The results of the dynamic stress analysis of the track links are presented and the differences between these results and the results obtained by using the static stress analysis are demonstrated.     

分段式自适应确定BP网络隐层结点数 ——以遥感图像分类为例

隐层数和隐层结点数直接关乎BP网络的学习能力，但目前对隐层结点数的选择尚无适用的理论，一般凭经验或试凑确定。本文提出一种分段式自适应确定隐层结点数的算法，它通过评估网络输出相对误差相应调整隐层结点数，通过迭代运算在使网络输出相对误差逐步减小的同时，逼近可能的最优隐层结点数。通常这个最优结点数即网络输出相对误差出现震荡的起点对应的结点数，以这个结点数决定的网络结构能够在网络输出精度与运算开销之间取得较佳平衡。