A state-of-the-art review on low-frequency nonlinear vibration isolation with electromagnetic mechanisms

Vibration isolation is one of the most efficient approaches to protecting host structures from harmful vibrations, especially in aerospace, mechanical, and architectural engineering, etc. Traditional linear vibration isolation is hard to meet the requirements of the loading capacity and isolation band simultaneously, which limits further engineering application, especially in the low-frequency range. In recent twenty years, the nonlinear vibration isolation technology has been widely investigated to broaden the vibration isolation band by exploiting beneficial nonlinearities. One of the most widely studied objects is the “three-spring” configured quasi-zero-stiffness (QZS) vibration isolator, which can realize the negative stiffness and high-static-low-dynamic stiffness (HSLDS) characteristics. The nonlinear vibration isolation with QZS can overcome the drawbacks of the linear one to achieve a better broadband vibration isolation performance. Due to the characteristics of fast response, strong stroke, nonlinearities, easy control, and low-cost, the nonlinear vibration with electromagnetic mechanisms has attracted attention. In this review, we focus on the basic theory, design methodology, nonlinear damping mechanism, and active control of electromagnetic QZS vibration isolators. Furthermore, we provide perspectives for further studies with electromagnetic devices to realize high-efficiency vibration isolation.

     

Comparison of Proton Acceptor and Proton Donor Properties of H2O and H2O2 in Organic Crystals of Drug-like Compounds: Peroxosolvates vs. Crystallohydrates

Two new peroxosolvates of drug-like compounds were synthesized and studied by a combination of X-ray crystallographic, Raman spectroscopic methods, and periodic DFT computations. The enthalpies of H-bonds formed by hydrogen peroxide (H₂O₂) as a donor and an acceptor of protons were compared with the enthalpies of analogous H-bonds formed by water (H₂O) in isomorphic (isostructural) hydrates. The enthalpies of H-bonds formed by H₂O₂ as a proton donor turned out to be higher than the values of the corresponding H-bonds formed by H₂O. In the case of H₂O₂ as a proton acceptor in H-bonds, the ratio appeared reversed. The neutral O∙∙∙H-O/O∙∙∙H-N bonds formed by the lone electron pair of the oxygen atom of water were the strongest H-bonds in the considered crystals. In the paper, it was found out that the low-frequency Raman spectra of isomorphous crystalline hydrate and peroxosolvate of N-(5-Nitro-2-furfurylidene)-1-aminohydantoin are similar. As for the isostructural hydrate and peroxosolvate of the salt of protonated 2-amino-nicotinic acid and maleic acid monoanion, the Raman spectra are different.     

浮筏的主动吸振控制实验研究

浮筏在低频减振方面存在较大不足,而这恰是主动减振的优势.将两者相结合,研究了浮筏的主动吸振控制问题.研制了一种主动吸振器,建立了以DSP F2812为核心的控制系统;提出了符号LMS算法,与广泛使用的滤波x-LMS等算法相比,该算法不需要精确的模型信息;在此基础上设计了自适应前馈主动吸振控制律,并在浮筏实验平台上进行主动吸振控制实验.实验结果表明,通过主动吸振控制,浮筏的低频减振能力得到了显著的改善.     

InGaN/GaN多量子阱结构发光二极管发光机理转变的低频电流噪声表征

本文对InGaN/GaN多量子阱结构发光二极管开启后的电流噪声进行了测试, 结合低频电流噪声的特点和载流子之间的复合机理, 研究了低频电流噪声功率谱密度与发光二极管发光转变机理之间的关系. 结论表明, 当电流从0.1 mA到10 mA逐渐增大的过程中, InGaN/GaN发光二极管的电流噪声行为从产生-复合噪声逐渐接近于低频1/f噪声, 载流子的复合机理从非辐射复合过渡为电子与空穴之间载流子数的辐射复合, 并具有标准1/f噪声的趋势, 此时多量子阱中的电子和空穴之间的复合趋向于稳定. 本文的结论提供了一种表征InGaN/GaN多量子阱发光二极管发光机理转变的有效方法, 为进一步研究发光二极管中载流子的复合机理、优化和设计发光二极管、提高其发光量子效率提供理论依据.     

Far infrared sub-nanosecond pulse generation in GaP with a time-synchronized mode-locked double-frequency Nd: Glass laser system

Light pulses of 149 m wavelength and 700 ps duration are generated by non-collinear phase-matched difference frequency mixing of laser pulses at 1053.5 and 1061 nm in a (110) cut GaP crystal. The pump laser pulses are generated in a time-synchronized mode-locked double-frequency Nd:glass laser system consisting of a silicate glass branch and a phosphate glass branch. A photon conversion efficiency of 4 × 10^–6 is achieved. The non-linear susceptibility constant of GaP is determined to be d ₁₄ = (10 ± 1) pm V^–1.     

Parametric amplification at low-frequency pumping and generation of four-mode entangled states

We show that parametric amplification at low-frequency pumping can be implemented in aperiodic nonlinear photonic crystals. In this process, the intensities of the waves with frequencies higher than the pumping frequency increase with increase in the interaction length as in the case of the standard process of parametric amplification at high-frequency pumping. The process under consideration includes a nondegenerate parametric down-conversion followed by two parametric up-conversions in the same pumping wave. As a result, generation at four new frequencies arises. Quantum analysis of the process demonstrates the presence of entanglement in the states of four modes. Talk presented at the oral issue of J. Russ. Laser Res. dedicated to the memory of Professor Vladimir A. Isakov, Professor Alexander S. Shumovsky, and Professor Andrei V. Vinogradov held in Moscow February 21–22, 2008.     

Noise in porous silicon structures in air and in conditions of gas adsorption

Low-frequency noise, in the range 1–500 Hz, is measured at room temperature in samples of Au/PS/SCS/Al structure with a layer of porous silicon. Numerical values of the Hooge parameter α_H are estimated and the dependence of α_H on the composition of the gaseous environment, where the sample is placed, is determined. Possible reasons of observed high values of α_H for the samples in air and of the increase in α_H in conditions of gas adsorption are discussed. Introducing of carbon oxide in air changes the shape of spectrum.     

点阵式上转换板在激光光斑采集中的应用研究

 通过在镜头前加装上转换板,利用其快速的红外上转换功能可以提高CCD对于低频窄脉冲光斑的采集效率。然而在利用传统面阵式工艺制成的上转换板采集光斑的过程中,普遍存在因上转换材料内部漫反射及局部饱和效应造成的光斑形状和光强分布的失真。针对上述问题,提出了一种新的点阵式上转换板制作工艺,并设计了点阵式上转换板光斑采集成像质量对比实验。实验结果表明,点阵式上转换板对低频窄脉冲光斑有更好的采集效果。为评价点阵式上转换板所采集光斑的成像质量,建立了相应的评价模型。     

New Pool Boiling Heat Transfer in the Presence of Low-Frequency Vibrations Into a Vertical Cylindrical Heat Source

A vertical cylinder was applied as a heat source into a water pool; the vibrations were imposed into the heater with different heat fluxes, and the frequencies were adjusted at 10, 15, 20, and 25 Hz. An imaging system was employed to observe the produced bubbles around the cylindrical heat source. The results showed that the boiling heat transfer was enhanced under the vibrations with a shorter transient process, and the wall temperature also decreased. The best enhancement ratio was achieved at the frequency of 25 Hz and a heat flux value of 30 kW/m² as a consequence of imposed vibrations.     

数字控制DC-DC Buck变换器中低频振荡现象分析

在考虑采样保持器的作用下,建立了数字控制Buck变换器系统的z域模型,由此揭示了系统发生低频分岔现象的原因.根据系统特征值的变化趋势,预测了该数字控制变换器失稳时分岔点的位置和类型,同时,推导出系统发生低频振荡的频率以及振幅,即系统波动方程.最后,通过PSpice电路仿真实验验证了理论分析结果的合理性与有效性. 关键词： 低频振荡 数字控制 变换器 z域模型')" href="#">z域模型