Optimum design of linear tuned mass dampers for structures with nonlinear behaviour

This paper investigates the optimum design of tuned mass dampers (TMDs) for the seismic protection of inelastic structures. A single linear TMD is treated and is assumed to be applied to a single nonlinear degree of freedom system, described by the Bouc–Wen hysteretic model. The seismic load is modelled by a stationary filtered stochastic process to consider its intrinsic stochastic nature. The optimization problem is set by taking into consideration three different possible objective functions (OFs): the maximum of the peak structural displacement standard deviation, the average hysteretic dissipated energy of a protected building with reference to an unprotected one, and a functional damage that considers the two indexes previously described. Different numerical examples and parametric analysis are shown to confront the three optimization criteria and to determine the best tuning frequency and damping ratio of the TMDs to be used in the structure. Results confirm that the application of a TMD system reduces the amount of the hysteretic dissipated energy, which is a direct measure of damage in the structure, and so it is beneficial for the protection of buildings that develop a nonlinear behaviour under severe dynamic loadings.     

Velocity profile effects in Coriolis mass flowmeters: Recent findings and open questions

The aim of this paper is to discuss velocity profile effects in Coriolis flowmeters and to review related research work. The measurements made by Coriolis flowmeters are dependent upon the steady flow velocity distribution within them whenever certain features of the fluid vibrational fields are not uniform inside the measuring tube. This dependence is confirmed by simulation results on two straight tube configurations, one operating in a beam-type mode and the other in a shell-type mode. Findings to date and open questions regarding velocity profile effects in Coriolis flowmeters are discussed for both fully developed and disturbed inlet flow conditions.     

Effect of streamwise and preferential diffusion on cylindrical Burke-Schumann flames

Effect of streamwise and preferential diffusion on cylindrical Burke-Schumann flame has been analyzed using perturbation method and Green’s function technique. Results show that for large Peclet number, streamwise diffusion has little effect, while for small Pe, it is balanced with radial diffusion such that a finite minimum flame height exists. Preferential diffusion induces flame temperature variation along the flame surface and the results agree qualitatively with existing experimental data. It also shows that the symmetry in flame temperature variation by the sign of (Le-1) in two dimensional case does not hold in cylindrical flames due to focusing and defocusing effects of mass and thermal diffusion by the radial curvature of flame.     

采用线性求角的旋变轴角解码与激磁系统

设计了一种基于单片DSP的旋变轴角解码与激磁系统并进行了实验验证.由于轴角解码中反正切运算需要很大运算量或很多数据空间存储查表值,甚至需要外扩存储芯片,因此,提出一种反正切运算补偿成分区间的线性运算求角方法.同时,在保证测角精度的基础上降低旋变激磁的电压幅值,从而实现了旋变轴角解码与激磁系统的高集成度与低功耗.实验结果显示:测角精度可达0.013°,框架电机速率精度达到0.000 4°/s.体积、质量、功耗与原激磁电源模块JD20-D15C36MK和轴角解码模块19XSZ2413相比均减少80%以上,很好地满足了航天应用小体积、轻质量和低功耗的要求.     

Scheduling problems with the nonlinear effects of learning and deterioration

In this paper, we present both nonlinear job deterioration and nonlinear learning which exist simultaneously. Job deterioration and learning co-exist in many realistic scheduling situations. By the effects of learning and deterioration, we mean that the processing time of a job is defined by the increasing function of its execution start time and position in the sequence. The following objectives are considered: single-machine makespan and sum of completion times (square) and the maximum lateness. For the single-machine case, we derive polynomial time optimal solutions. For the case of an m-machine permutation flowshop, we present polynomial time optimal solutions for some special cases of the problems to minimize makespan and total completion time.     

An investigation on effects of traveling mass with variable velocity on nonlinear dynamic response of an inclined Timoshenko beam with different boundary conditions

In this paper, the nonlinear dynamic response of an inclined Timoshenko beam with different boundary conditions subjected to a traveling mass with variable velocity is investigated. The nonlinear coupled partial differential equations of motion for the bending rotation of cross-section, longitudinal and transverse displacements are derived using Hamilton’s principle. These nonlinear coupled PDEs are solved by applying Galerkin’s method to obtain dynamic response of the beam under the act of a moving mass. The appropriate parametric studies by taking into account the effects of the magnitude of the traveling mass, the velocity of the traveling mass with a constant acceleration/deceleration and effect of different beam’s boundary conditions are carried out. The beams’ large deflection has been captured by including the stretching effect of its mid-surface. It was seen that the existence of quadratic-cubic nonlinear terms in the governing coupled PDEs of motion renders hardening/stiffening behavior on the dynamic responses of the beam when traversed by a moving mass. In addition, the obtained nonlinear results are compared with those from the linear analysis.     

Computer-controlled susceptometer for investigating the linear and nonlinear dielectric response

A fully automatized alternating current (ac) susceptometer is constructed for simultaneous measurements of the phase resolved complex linear and complex nonlinear ac susceptibilities of lossy and dispersive dielectric materials. This relatively simple setup allows measurements over a wide range of experimental variables, such as ac amplitudes up to 40 V, frequencies from 10(-2) to 10(3) Hz, and temperatures from 100 to 600 K utilizing only current/voltage and analog/digital converters and a personal computer. In contrast with the commonly used analysis of the charge accumulated on a standard capacitor in series with the sample our method is based on the analysis of the current flowing directly through the sample. Absence of any capacitive voltage dividers in the measurement circuit eliminates uncontrolled phase shifts. This is why the instrument provides high quality, nonlinear susceptibility data and in particular appears as a very convenient tool for discrimination between continuous and discontinuous phase transitions when determining the sign of the real part of the third order dielectric susceptibility.     

Simultaneous observation of acoustic resonance phenomena at both surfaces of a plate coated with thin layers

Acoustic resonance phenomena at the front and back surfaces of a plate coated with thin layers were successfully observed in the amplitude spectrum of the back surface echo. The amplitude ratio of spectra with and without layers takes its maximum and minimum values at the resonant frequencies of the front and back coatings and both frequencies can clearly be distinguished from each other. As an application, the thicknesses of the front and back coatings on a steel plate were measured simultaneously using their resonant frequencies, thus verifying the validity of the principle.     

An experimental and mathematical study on the effects of ignition energy and system on the flame kernel development

A constant volume combustion chamber is used to investigate the flame kernel development of gasoline air mixtures under various ignition systems, ignition energies and spark plugs. Three kinds of ignition systems are designed and assembled, and the ignition energy is controlled by the variation of the dwell time. Several kinds of spark plugs are also tested. The velocity of flame propagation is measured by a laser deflection method, and the combustion pressure is analyzed by the heat release rate and the mass fraction burnt. The results represent that as the ignition energy is increased by enlarging either dwell time or spark plug gap, the heat release rate and the mass fraction burnt are increased. The electrodes materials and shapes influence the flame kernel development by changing he transfer efficiency of electrical energy to chemical energy. The diameter of electrodes also influences the heat release rate and the burnt mass fraction.     

伺服系统线性特性和非线性摩擦的解耦辨识方法研究

针对伺服进给机构包含线性结构和非线性摩擦特性的特点,提出一种将线性模型和非线性摩擦特性进行分步解耦辨识的方法.首先分别确定伺服系统的线性模型和非线性模型,将伺服系统结构模型转化为线性模型加非线性摩擦反馈的结构.为了消除非线性摩擦力对线性模型的影响,采用2路同向非过零速信号对系统激励,利用2组系统输入和输出信号的差值作为线性参数的辨识数据对线性参数进行估计.获得线性模型后进一步利用系统稳态输出实现对非线性Coulomb摩擦幅值特性的估计.系统仿真和实验都证明了该辨识方法对提高伺服系统的辨识精度及控制器设计的有效性和可靠性.     

The effects of spherical aberration on multiphoton fluorescence excitation microscopy

Multiphoton fluorescence excitation microscopy is almost invariably conducted with samples whose refractive index differ from that of the objective immersion medium, conditions that cause spherical aberration. Due to the quadratic nature of multiphoton fluorescence excitation, spherical aberration is expected to profoundly affect the depth dependence of fluorescence excitation. In order to determine the effect of refractive index mismatch in multiphoton fluorescence excitation microscopy, we measured signal attenuation, photobleaching rates and resolution degradation with depth in homogeneous samples with minimal light scattering and absorption over a range of refractive indices. These studies demonstrate that signal levels and resolution both rapidly decline with depth into refractive index mismatched samples. Analyses of photobleaching rates indicate that the preponderance of signal attenuation with depth results from decreased rates of fluorescence excitation, even in a system with a descanned emission collection pathway. Similar results were obtained in analyses of fluorescence microspheres embedded in rat kidney tissue, demonstrating that spherical aberration is an important limiting factor in multiphoton fluorescence excitation microscopy of biological samples.     

Effects of a one-way clutch on the nonlinear dynamic behavior of spur gear pairs under periodic excitation

Nonlinear behavior analysis was used to verify whether a one-way clutch is effective for reducing the torsional vibration of a paired spur gear system under periodic excitation. The dynamic responses were studied over a wide frequency range by speed sweeping to check the nonlinear behavior using numerical integration. The gear system with a one-way clutch showed typical nonlinear behavior. The oscillating component of the dynamic transmission error was reduced over the entire frequency range compared to a system without a one-way clutch. The one-way clutch also eliminated unsteady continuous jump phenomena over multiple solution bands, and prevented double-side contact, even with very small backlash. Installing a one-way clutch on both sides of the gear system was more effective at mitigating the negative effects of external periodic excitation and various parameter changes than a conventional gear system without a one-way clutch.     

Effect of the hereditary kernel on the solution of linear and nonlinear dynamic problems of hereditary deformable systems

A differential dependence between stresses and deformations is shown to be equivalent, even in the most general case, to integral dependences with regular hereditary kernels that have a serious disadvantage. Numerical results are presented that were obtained by solving dynamic problems of hereditary deformable systems using exponential and weakly singular hereditary kernels.     

Identification of the nonlinear excitation force acting on a bowed string using the dynamical responses at remote locations

For achieving realistic numerical simulations of bowed string instruments, based on physical modeling, a good understanding of the actual friction interaction phenomena is of great importance. Most work published in the field including our own has assumed that bow/string frictional forces behave according to the classical Coulomb stick-slip model, with an empirical velocity-dependent sliding friction coefficient. Indeed, the basic self-excited string motions (such as the Helmholtz regime) are well captured using such friction model. However, recent work has shown that the tribological behavior of the bow/string rosin interface is rather complex, therefore the basic velocity-dependent Coulomb model may be an over-simplistic representation of the friction force. More specifically, it was suggested that a more accurate model of the interaction force can be achieved by coupling the system dynamical equations with a thermal model which encapsulates the complex interface phenomena. In spite of the interesting work performed by Askenfelt [32], a direct measurement of the actual dynamical friction forces without disturbing the string motion is quite difficult. Therefore, in this work we develop a modal-based identification technique making use of inverse methods and optimization techniques, which enables the identification of the interface force, as well as the string self-excited motion, from the dynamical reactions measured at the string end supports. The method gives convincing results using simulated data originated from nonlinear computations of a bowed string. Furthermore, in cases where the force identifications are very sensitive to errors in the transfer function modal parameters, we suggest a method to improve the modal frequencies used for the identifications. Preliminary experimental results obtained using a basic bowing device, by which the string is excited with the stick of the bow, are then presented. Our identifications, from the two dynamical string reactions, are consistent as attested by the comparison of the two available versions of the string dynamical motion and of the friction force. Furthermore, the method seems adequate to investigate the interface force for the bowed string.     

Influence of foundation mass on the nonlinear damped response of orthotropic shallow spherical shells

The effect of the mass of the foundations on the large-amplitude response of clamped and simply-supported orthotropic shallow spherical shells continuously supported by Winkler and Pasternak elastic foundations, subjected to uniform step pressure and sinusoidal pulse loadings has been investigated, with and without damping. Nonlinear governing partial differential equations of motion are derived and solved in space and time-domains using Chebyshev polynomials and an implicit Houbolt time-marching scheme, respectively. Results show a significant influence of the foundation inertia on the amplitude and time-period of the response of orthotropic shallow spherical shells.     

Experimental studies of laser excitation of normal acoustic waves in ferromagnetic metals at the curie temperature

Laser excitation of normal (Lamb) waves in ferromagnetic plates and the effect of magnetic phase transition on their main characteristics—amplitude, vibration frequency, and propagation velocity—are studied experimentally.     

Time interval measurement device based on surface acoustic wave filter excitation, providing 1 ps precision and stability

This article deals with the time interval measurement device, which is based on a surface acoustic wave (SAW) filter as a time interpolator. The operating principle is based on the fact that a transversal SAW filter excited by a short pulse can generate a finite signal with highly suppressed spectra outside a narrow frequency band. If the responses to two excitations are sampled at clock ticks, they can be precisely reconstructed from a finite number of samples and then compared so as to determine the time interval between the two excitations. We have designed and constructed a two-channel time interval measurement device which allows independent timing of two events and evaluation of the time interval between them. The device has been constructed using commercially available components. The experimental results proved the concept. We have assessed the single-shot time interval measurement precision of 1.3 ps rms that corresponds to the time of arrival precision of 0.9 ps rms in each channel. The temperature drift of the measured time interval on temperature is lower than 0.5 ps/K, and the long term stability is better than +/-0.2 ps/h. These are to our knowledge the best values reported for the time interval measurement device. The results are in good agreement with the error budget based on the theoretical analysis.     

On the distribution of nonlinear effects in locally nonlinear one-dimensional chain type structures

Nonlinear effect is a typical phenomenon for nonlinear systems, which is characterized as the energy of the system input transferred from the frequency modes in the input to the output frequency locations, which are either below or above the input frequency band. The present work is devoted to investigating the distribution of nonlinear effects in one-dimensional chain type structures with local nonlinearity in which only one or few components are of nonlinear property. The results reveal that the positions of both the nonlinear components and the external input play crucial roles in determining the distribution of nonlinear effects in the locally nonlinear structures, and provide a new insight into the complex dynamic of nonlinear systems in frequency domain. The results are of great significance to the analysis, design and control of the mechanical systems whose few components are of nonlinear property.     

Stiffness effects on repeatability of positioning of linear axes

This paper studies the effect of stiffness (including slider stiffness and bearing stiffness) on the repeatability of the positioning of linear axes. First, a mathematical model between stiffness and the repeatability of the positioning of linear axes is established. The effects of the stiffness of the slider and bearing on the repeatability of positioning are analysed. The theoretical results show that the repeatability of the positioning decreases as the slider component stiffness and bearing stiffness increases. Second, a finite element analysis is implemented to calculate the effect of slider stiffness on the repeatability of positioning, the simulation results are found to be consistent with the theoretical results. Finally, the effect of bearing stiffness on the repeatability of positioning is experimentally investigated. The experimental and theoretical analysis results have been observed to coincide. Thus, the accuracy of the theoretical analysis is verified by both simulation and experimental results. The derived results afford designers and engineers a scientific basis for the stiffness design theory.