A chiral elastic metamaterial beam for broadband vibration suppression

One of the significant engineering applications of the elastic metamaterial (EMM) is for low-frequency vibration attenuation because of its unusual low-frequency bandgap behavior. However, the forbidden gap from many existing EMMs is usually of narrow bandwidth which limits their practical engineering applications. In this paper, a chiral-lattice-based EMM beam with multiple embedded local resonators is suggested to achieve broadband vibration suppression without sacrificing its load-bearing capacity. First, a theoretical beam modeling is suggested to investigate bandgap behavior of an EMM beam with multiple resonators. New passbands due to dynamic interaction between resonators are unpleasantly formed, which become a design barrier for completely broadband vibration suppression. Through vibration attenuation factor analysis of the resonator, an EMM beam with section-distributed resonators is proposed to enable broadband vibration attenuation function. Required unit number of the resonator in each section is quantitatively determined for complete vibration attenuation in a specific frequency range. Finally, the chiral-lattice-based EMM beam is fabricated, and experimental testing of the proposed structure is conducted to validate the design.     

Wave propagation characterization and design of two-dimensional elastic chiral metacomposite

In this work, a chiral metacomposite is proposed by integrating two-dimensional periodic chiral lattice with elastic metamaterial inclusions for low-frequency wave applications. The plane harmonic wave propagation in the proposed metacomposite is investigated through the finite element technique and Bloch's theorem. Band diagrams are obtained to illustrate wave properties of the chiral metacomposite. Effective dynamic properties of the chiral metacomposite are numerically calculated to explain low-frequency bandgap behavior in the chiral metacomposite. Interestingly doubly negative effective density and modulus of the chiral metacomposite are found in a specific frequency range, where a pass band with negative group velocity is observed. Tuning of the resulting low-frequency bandgaps is then discussed by adjusting microstructure parameters of the metamaterial inclusion and lattice geometry. Specifically design of a metacomposite beam structure for the broadband low-frequency vibration suppression is demonstrated.     

Forming Low-Frequency Complete Vibration Bandgaps in a thin Nonmetallic Elastic Metamaterial Plate

Acoustical Physics - Low-frequency vibration-bandgaps in elastic metamaterials open new possibilities to minimize low-frequency vibration and noise. Unfortunately, fabricating a complete vibration...     

磁悬浮－气囊主被动混合隔振装置理论和实验

为了更有效地控制舰船动力机械宽频和低频线谱振动的传递,提出了一种将磁悬浮作动器与气囊隔振器集成应用的磁悬浮-气囊主被动混合隔振装置。通过对磁悬浮作动器机电耦合特性和混合隔振系统动力学特性的分析研究,确定了满足线谱振动控制要求和满足混合隔振装置性能要求的参数设计方法。针对主动控制时,FxLMS (filtered-x least mean square)算法在小阻尼系统上需用高阶FIR滤波器建模,运算量大的问题,提出了分频段控制的改进FxLMS算法,并有效地解决了作动器的非线性效应问题。样机实验结果表明:理论分析是正确的,该项技术控制力需求小,装置稳定性好,具有优良的宽频隔振和低频线谱振动控制效果。      

船舶机械磁悬浮气囊混合隔振技术

有源无源混合隔振是控制船舶低频线谱噪声的重要技术,但工程应用的实例还非常少见。在磁悬浮-气囊混合隔振理论和原理样机研究的基础上,针对船用机械低频线谱的隔振需求,进一步突破了体积小、输出力大、功耗低、频响平直、波形失真度低等磁悬浮作动器工程化设计技术;解决了混合隔振器的稳定性和冲击、摇摆适应性等技术难题;研究了工程实用的控制算法,采用非线性逆模型补偿使控制系统线性化,并提出了窄带Fx-Newton时域算法,可在机械设备运行时的多线谱、多通道耦合、线谱振幅非稳态等情况下实现快速稳定控制;研制了船用200 kW柴发机组混合隔振装置,实验结果表明该技术具有优良的宽频隔振效果和低频线谱控制能力,性能可满足工程实用要求。      

X形超阻尼局域共振声子晶体梁弯曲振动带隙特性

以声子晶体理论为基础,设计了一种具有超阻尼特性的X形局域共振结构,分析了周期性附加X形局域共振的梁弯曲振动传播特性.利用拉格朗日方程分析了X形局域共振结构动力学等效特性,揭示了该结构的阻尼放大的机理,分析了几何结构参数对于带隙特性的影响,并利用有限元法验证了X形局域共振结构的超阻尼特性.研究结果表明,周期性附加X形局域结构能够有效地抑制低频弯曲振动在梁中的传播,产生超阻尼特性,实现低频、宽带的减振效果,为结构的低频减振提供了一个新的设计方案.     

Unidirectional amplification and shaping of optical pulses by three-wave mixing with negative phonons

A possibility to greatly enhance frequency-conversion efficiency of stimulated Raman scattering is shown by making use of extraordinary properties of three-wave mixing of ordinary and backward waves. Such processes are commonly attributed to negative-index plasmonic metamaterials. This work demonstrates the possibility to replace such metamaterials that are very challenging to engineer by readily available crystals which support elastic waves with contra-directed phase and group velocities. The main goal of this work was to investigate specific properties of indicated nonlinear optical process in short-pulse regime and to show that it enables elimination of fundamental detrimental effect of fast damping of optical phonons on the process concerned. Among the applications is the possibility of creation of a family of unique photonic devices such as unidirectional Raman amplifiers and femtosecond pulse shapers with greatly improved operational properties.     

Lowering plasma frequency by enhancing the effective mass of electrons： A route to deep sub-wavelength metamaterials

Deep sub-wavelength metamaterials are the key to the further development of practical metamaterials with small volumes and broadband properties. We propose to reduce the electrical sizes of metamaterials down to more sub-wavelength scales by lowering the plasma frequencies of metallic wires. The theoretical model is firstly established by analyzing the plasma frequency of continuous thin wires. By introducing more inductance elements, the effective electron mass can be enhanced drastically, leading to significantly lowered plasma frequencies. Based on this theory, we demonstrate that both the electric and the magnetic plasma frequencies of metamaterials can be lowered significantly and thus the electrical sizes of metamaterials can be reduced to more sub-wavelength scales. This provides an efficient route to deep sub-wavelength metamaterials and will give rigorous impetus for the further development of practical metamaterials.     

Vibration suppression of a cantilever beam using magnetically tuned-mass-damper

Eddy currents are induced by the movement of a conductor through a stationary magnetic field or a time varying magnetic field through a stationary conductor. These currents circulate in the conductive material and are dissipated, causing a repulsive force between the magnet and the conductor. These electromagnetic forces can be used to suppress the vibrations of a flexible structure. A tuned mass damper is a device mounted in structures to reduce the amplitude of mechanical vibrations and is one of the effective vibration suppression methods. In the present study, an improved concept of this tuned mass damper for the vibration suppression of structures is introduced. This concept consists of the classical tuned mass damper and an eddy current damping. The important advantages of this magnetically tuned mass damper are that it is relatively simple to apply, it does not require any electronic devices and external power, and it is effective on the vibration suppression. The proposed concept is designed for a cantilever beam and the analytical studies on the eddy current damping and its effects on the vibration suppression. To show the effectiveness of the proposed concept and verify the eddy current damping model, experiments on a cantilever beam are performed. It is found that the proposed concept could significantly increase the damping effect of the tuned mass damper even if not adequately tuned.     

基于声学超材料的低频可调吸收器

在当今社会,噪声污染已经成为人类健康的一大威胁,如何有效地控制和消除噪声污染一直是科研领域的一个重要话题.本文以开口环嵌套结构为模型,设计并制备了一种声学超材料.通过理论分析、数值模拟和实验测试,发现由于模型内部空腔的强烈耦合共振效应,该超材料可以在低频区域实现接近完美的吸声效应.此外,通过简单地绕轴旋转其内腔开口方向,即可改变该超材料的相对阻抗值,进而在较宽的频带范围内实现对吸收峰位置的可调控制.由于该超材料具有深亚波长的尺寸,因此非常有利于低频吸声器件的小型化和集成化,同时该模型也为宽带吸收器的设计奠定了基础.     

MODELLING DYNAMICS OF A CONTINUOUS STRUCTURE WITH A PIEZOELECTRIC SENSORACTUATOR FOR PASSIVE STRUCTURAL CONTROL

This paper presents the concept of a vibration control system in which motions of a continuous structure with piezoelectric sensors/actuators can be suppressed (or activated) through transforming mechanical energy to electrical one and vice versa. The study is focused on distributed parameter structures, in which electromechanical variables are spatially dependent, and therefore traditional methods of design of piezoelectric transformers do not apply. In this case, a different approach is necessary to account for the spatial dependency of the variables. To examine the feasibility of the proposed vibration control system, we have performed the vibration suppression analysis of the cantilevered beam with piezoelectric sensors/actuators subjected to an exciting force/moment(s). The experimental results indicate that the damping of the composite system increases by 8-10 times in comparison with the mechanical system.As a result, the paper significantly expands the concept of passive damping mechanism for structural systems to take into account the dynamics of a continuous elastic structure piezoelectrically coupled to electrical network.     

Double-negative dynamic properties in one-dimensional multi-phase metamaterial based on the symmetrical equivalent layer

Acoustic metamaterials have important potential applications in engineering by their unusual properties. Based on the concept of symmetrical equivalent layer, it is found that layered acoustic metamaterial which exhibits simultaneously negative effective modulus and density can be existed. By using the transfer matrix method, the continuous double-negative dynamic properties can be achieved through a multi-phase microstructure considering viscous damping, and the left-handed wave propagation property appears during the double-negative frequency regions. This will be a reference for the design of 1D acoustic negative refraction metamaterials.     

Passive damping enhancement of a two-degree-of-freedom system through a strongly nonlinear two-degree-of-freedom attachment

This work reports on the first experimental study of the broadband targeted energy transfer properties of a two-degree-of-freedom (two-DOF) essentially nonlinear energy absorber. In particular, proper design of the absorber allows for an extended range of energy over which it serves to significantly enhance the damping observed in the structural system to which it is attached. Comparisons of computational and experimental results validate the proposed design as a means of drastically enhancing the damping properties of a structure by passive broadband targeted energy transfers to a strongly nonlinear, multidegree-of-freedom attachment.     

Optimized electric networks for vibration damping of piezoactuated beams

This paper studies the multimodal vibration damping of an elastic beam equipped with multiple piezoelectric actuators connected to an electric network. Two analytical models of the electromechanical coupled structure are considered: a homogenized one, accurate when a large number of actuators is employed, is used to derive simple design criteria for the electric network; and a discrete one, able to face real situations when few actuators are employed, is adopted to test the network performance, defined as the exponential time-decay rate of the free vibrations of the controlled structure. Some electric networks are presented and compared in simulation to networks previously proposed in the literature, in order to evaluate their performances in broadband vibration control.     

Vibration isolation via a scissor-like structured platform

More and more attentions are attracted to the analysis and design of nonlinear vibration control/isolation systems for better isolation performance. In this study, an isolation platform with n-layer scissor-like truss structure is investigated to explore novel design of passive/semi-active/active vibration control/isolation systems and to exploit potential nonlinear benefits in vibration suppression. Due to the special scissor-like structure, the dynamic response of the platform has inherent nonlinearities both in equivalent damping and stiffness characteristics (although only linear components are applied), and demonstrates good loading capacity and excellent equilibrium stability. With the mathematical modeling and analysis of the equivalent stiffness and damping of the system, it is shown that: (a) the structural nonlinearity in the system is very helpful in vibration isolation, (b) both equivalent stiffness and damping characteristics are nonlinear and could be designed/adjusted to a desired nonlinearity by tuning structural parameters, and (c) superior vibration isolation performances (e.g., quasi-zero stiffness characteristics etc.) can be achieved with different structural parameters. This scissor-like truss structure can potentially be employed in different engineering practices for much better vibration isolation or control.     

Robust stability characterizations of active metamaterials with non-Foster loads

Active metamaterials incorporating with non-Foster elements have been considered as one of the means of overcoming inherent limitations of the passive counterparts, thus achieving broadband or gain metamaterials. However, realistic active metamaterials, especially non-Foster loaded medium, would face the challenge of the possibility of instability. Moreover,they normally appear to be time-variant and in unsteady states, which leads to the necessity of a stability method to cope with the stability issue considering the system model uncertainty. In this paper, we propose an immittance-based stability method to design a non-Foster loaded metamaterial ensuring robust stability. First, the principle of this stability method is introduced after comparing different stability criteria. Based on the equivalent system model, the stability characterization is used to give the design specifications to achieve an active metamaterial with robust stability. Finally, it is applied to the practical design of active metamaterial with non-Foster loaded loop arrays. By introducing the disturbance into the nonFoster circuit(NFC), the worst-case model uncertainty is considered during the design, and the reliability of our proposed method is verified. This method can also be applied to other realistic design of active metamaterials.     

Optimal synthesis of 2D phononic crystals for broadband frequency isolation

The spatial distribution of material phases within a periodic composite can be engineered to produce band gaps in its frequency spectrum. Applications for such composite materials include vibration and sound isolation. Previous research focused on utilizing topology optimization techniques to design two-dimensional (2D) periodic materials with a maximized band gap around a particular frequency or between two particular dispersion branches. While sizable band gaps can be realized, the possibility remains that the frequency bandwidth of the load that is to be isolated might exceed the size of the band gap. In this paper, genetic algorithms are used to design squared bi-material unit cells with a maximized sum of band-gap widths, with or without normalization relative to the central frequency of each band gap, over a prescribed total frequency range of interest. The optimized unit cells therefore exhibit broadband frequency isolation characteristics. The effects of the ratios of contrasting material properties are also studied. The designed cells are subsequently used, with varying levels of material damping, to form a finite vibration isolation structure, which is subjected to broadband loading conditions. Excellent isolation properties of the synthesized material are demonstrated for this structure.     

A comparison of two nonlinear damping mechanisms in a vibration isolator

The vibration transmissibility characteristics of a single-degree-of-freedom (SDOF) passive vibration isolation system with different nonlinear dampers are investigated in this paper. In one configuration, the damper is assumed to be linear and viscous, and is connected to the mass so that it is perpendicular to the spring (horizontal damper). The vibration is in the direction of the spring. The second configuration is one in which the damper is in parallel with the spring but the damping force is proportional to the cube of the relative velocity across the damper (cubic damping). Both configurations are studied for small amplitudes of excitation, when some analysis can be conducted based on analytical expressions, and for large amplitudes of excitation, where the analysis is based on numerical simulations. It is found that the two nonlinear systems can outperform the linear system when force transmissibility is considered. However, for displacement transmissibility, the system with the horizontal damper exhibits some desirable properties, but the system with cubic damping does not.     

Optimal synthesis of 2D phononic crystals for broadband frequency isolation

The spatial distribution of material phases within a periodic composite can be engineered to produce band gaps in its frequency spectrum. Applications for such composite materials include vibration and sound isolation. Previous research focused on utilizing topology optimization techniques to design two-dimensional (2D) periodic materials with a maximized band gap around a particular frequency or between two particular dispersion branches. While sizable band gaps can be realized, the possibility remains that the frequency bandwidth of the load that is to be isolated might exceed the size of the band gap. In this paper, genetic algorithms are used to design squared bi-material unit cells with a maximized sum of band-gap widths, with or without normalization relative to the central frequency of each band gap, over a prescribed total frequency range of interest. The optimized unit cells therefore exhibit broadband frequency isolation characteristics. The effects of the ratios of contrasting material properties are also studied. The designed cells are subsequently used, with varying levels of material damping, to form a finite vibration isolation structure, which is subjected to broadband loading conditions. Excellent isolation properties of the synthesized material are demonstrated for this structure.     

Suppression of random vibration in flexible structures using a hybrid vibration absorber

A design method is proposed to suppress stationary random vibration in flexible structures using a hybrid vibration absorber (HVA). While the traditional vibration absorber can damp down the vibration mainly at the pre-tuned mode of the primary structure, active damping is generated by the proposed HVA to damp down all resonant modes of interest of the vibrating structure and the spatial average mean square motion of the vibrating structure can be minimized. Only one absorber and one feedback signal are required to achieve global vibration suppression of a flexible structure under stationary random excitation. A special pole-placement controller is designed such that all vibration modes of the flexible structures become critically damped. It is proved analytically that the proposed HVA damps the vibration of the entire structure instead of just the attachment point of the absorber. The proposed optimized HVA is tested on a beam structure and it shows a superior performance on global suppression of broadband vibration in comparison to other published designs of passive and hybrid vibration absorbers.