Published papers

Conventional acoustic metamaterials for elastic waves are mainly based on mechanical systems with local resonant structures, but this study aims to reduce vibration by replacing the unit cell with a resonant structure of a coupled electrical-structural system. As a basic study, we confirmed the vibration-damping property of acoustic metamaterials including electrical systems by numerical simulation and further clarified the resonance characteristics of unit cells by experiment.

This study deals with the effect of an electromagnetic shunt damper on the vibration suppression and capturing performance of the superconducting magnetic docking system. It utilizes the property of a superconductor to hold a magnet in an equilibrium position due to the magnetic flux pinning effect. This system enables non-contact and non-controlled docking, reducing energy consumption and impact compared to conventional systems. However, there are problems such as the possibility of docking failure and the large free vibration during docking due to the low damping to the magnet. The important factors affecting the vibration suppression and capturing performance of superconducting magnetic docking systems are the damping characteristics and potential energy distribution, and it is important to improve these factors. We have focused on electromagnetic shunt dampers as a way to increase the damping of the system. The electromagnetic shunt damper is a type of dynamic absorber that has an electric circuit consisting of a coil, resistor, and capacitor in the subsystem. By converting the kinetic energy of the main system into electrical energy of the subsystem, vibration damping can be increased. The potential energy distribution of a superconductor shows asymmetry with respect to the equilibrium position of the magnet due to hysteresis, but this is not taken into account in the advanced mirror image method. Therefore, the capturing performance could not be calculated by numerical analysis, but we have devised a new mirror image model that takes the asymmetry into account. In this study, we investigate the effect of parameters in an electromagnetic shunt damper on the damping characteristics and capturing performance of the superconducting magnetic docking system by numerical analysis.

This study aims to verify the effectiveness of a gyroscopic damper on the nonlinear resonances of a magnetic levitation system with superconductors at several resonance points. Magnetic levitation systems using high-temperature superconductors provide controlled and stable levitation, but because they are non-contact, they have low damping and are easily affected by external disturbances. In addition, the magnetic force of superconductivity tends to cause nonlinear vibrations, and reducing these vibrations is an important development issue from the perspective of structural safety in system adaptation. We focus on the use of gyroscopic damper as a passive vibration suppression method that takes advantage of the property of the superconducting magnetic levitation system to be stable without control. Our previous studies have shown that gyroscopic damper is effective for simultaneous vibration suppression in the translational and tilting directions by optimizing the setting parameters of the gyroscopic damper. The vibration suppression effect in primary resonance has also been theoretically and experimentally confirmed. In this study, in addition to a reduction of approximately 10% in both the translational and tilting directions with respect to the excitation frequency component of secondary resonance, it was confirmed that the amplitude of what appears to be a subharmonic component oscillating at 1/2 and 3/2 of the excitation frequency was reduced by almost 100%. Thus, it was demonstrated that the gyroscopic damper can suppress secondary resonance in addition to primary resonance.

High-temperature superconducting pinning maglev (HTSPM) systems offer efficient, low-energy solutions for transportation due to their stable levitation characteristics. However, the nonlinear hysteresis properties of the levitation forces, coupled with the lower inherent stiffness of the levitation system compared to wheel-rail contacts, tend to induce nonlinear low-frequency vibrations. These issues pose challenges to operational safety and ride smoothness, especially under external perturbations, which result in a broad spectrum of sensitive frequencies that enhance these vibrations. This paper integrates experimental measurement, theoretical derivation and numerical simulation to elucidate the system's resonance characteristics and the frequencies that may induce resonance. Initially, a mathematical model of the levitation force of HTSPM is developed based on experiments and simulations. The study then uses the multiscale method to explore the nonlinear behaviors of HTSPM under different perturbation conditions. Forced vibration experiments are conducted at various externally excited frequencies to identify the system's sensitive frequencies. The results show that the system responds at its natural frequency, double the natural frequency, and the sum of the natural and the excitation frequencies. Specific damping values that can suppress nonlinear vibrations are also disscussed. This study provides a theoretical foundation for dynamics investigations of HTSPM trains, considering the effects of nonlinear levitation forces.

This study deals with the vibration suppression effect of an LCR parallel electromagnetic shunt damper on a superconducting magnetic levitation system. Superconducting magnetic levitation systems have the advantages of low energy loss due to friction because of the non-contact nature of the system and, furthermore, stable levitation without control. Therefore, it is expected to be applied to magnetically levitated trains and magnetic formation flights. On the other hand, the occurrence of resonance with large amplitude due to low attenuation can be a problem, so suppression of the amplitude is an important issue. We focused on the LCR electromagnetic shunt damper, which has the advantage of being non-contact, as a method of suppressing vibration. However, the coupling of the mechanical and electrical systems via a velocity term is essential for the parallel type, making conventional modal analysis inapplicable. Therefore, we used the impedance expression for the circuit equation of the subsystem. As a result, the velocity term coupling was eliminated, thereby allowing the application of conventional modal analysis. The analysis showed that the two natural frequencies depend on the excitation frequency, and based on this fact, the dependence of the frequency response of the system on the capacitance and the resistance was able to be determined by numerical analysis. Furthermore, the amplitude reduction effect predicted by this numerical analysis was confirmed experimentally. In summary, in this study, the vibration suppression of the superconducting magnetic levitation system by the LCR electromagnetic shunt damper was analyzed using vibration modal analysis and verified experimentally.

This study investigated the effect of electromagnetic shunt dampers on the resonance amplitude reduction in a superconducting magnetic levitation system. There are two types of electromagnetic shunt dampers, series type and parallel type, depending on the configuration of the electric circuit, and their damping characteristics may differ depending on the external resistance value in the circuit. In this study, after discussing the vibration-suppression effects of both types according to the governing equations, vibration experiments were conducted using both dampers with different resistance values. As a result, it was confirmed that, for the larger resistance value, the amplitude reduction effect is smaller in the series-type damper, while it remained high in the parallel type. We also performed numerical integrations, including the nonlinearity of magnetic force in the superconducting magnetic levitation system. As a result, it was numerically confirmed that the parallel-type damper can also be expected to reduce amplitude at a resonance caused by nonlinearity.

This paper presents experimental verification of the vibration suppression effect of a gyroscopic damper on a system supported elastically by a superconducting magnetic levitation system when subjected to vibration. While superconducting magnetic levitation systems are capable of uncontrolled and stable levitation, they are prone to multi-degree-of-freedom vibration and nonlinear vibration of the levitating body due to their low damping. In our previous study, we have numerically predicted that a gyroscopic damper, a passive vibration control method, may be effective in simultaneously suppressing translational and tilting vibrations by optimizing the design parameters. In this paper, this vibration suppression effect predicted was investigated through experiments. In the experiment, nonlinear vibration of the levitating body occurred, which may have been caused by the magnetic force of the superconductivity, but the motion coupling of the gyroscopic damper and the levitating body was found to reduce that vibration by 30% in the translational direction and by 40% in the tilting direction. The gyroscopic damper was thus demonstrated to be effective in simultaneously suppressing the translational and tilting vibration of the levitating body.

In this paper, suppression effect of a gyroscopic damper on vibrations of a body levitated by magnetic force acting from superconductors is investigated by numerical calculations. Because of the low damping property of the superconducting magnetic levitation system, simultaneous suppression of multi-degree-of-freedom vibration due to the coupled translational and tilting motions of the levitating body and suppression of nonlinear vibration are important development issues. A gyroscopic damper has been considered as a passive vibration suppression method that takes advantage of the control-free features of superconducting magnetic levitation systems, and if the design parameters are optimized, it may be possible to simultaneously suppress not only tilt vibration but also translational vibration. In this paper, the governing equations are first derived from the analytical model, and then numerical calculations are performed. As a result, the simultaneous suppression effect of the gyroscopic damper on the translational and tilt vibrations of the levitating body was numerically confirmed. Furthermore, the suppression effect of the gyroscopic damper on the nonlinear vibration of the levitating body was also confirmed numerically.

境界上に非線形性を有する長大構造物内を超音波ガイド波が伝播する際に，内部共振に起因して発生しうる累積的高調波を，多重尺度法を用いて理論的に解析し，従来用いられていた摂動法よりも厳密に解析し，伝播に伴う振幅の変動について考察した．

© 2002-2011 IEEE. This paper reports a unique resonance phenomenon that can occur when a superconductive magnetic bearing with nonuniform magnetization distribution is forced to vibrate in a single direction. The dynamics are studied by deriving the equations of motion, and a unique resonance is predicted both theoretically and numerically. A simple experiment is also conducted. The results show that a combination resonance can occur when the difference between the rotation frequency and the vibration frequency is equal to the natural frequency of the rotor.

© 2018 Elsevier B.V. We analytically investigate the influence of damping on Lamb waves, which are a specific type of guided wave in two-dimensional plates. Considering material attenuation, we suppose that Lamé constants are complex numbers. This leads to the associated wavenumbers being complex, with the imaginary part of the wavenumber being associated with effect of attenuation of the guided wave. In this paper, we show how dispersion curves and attenuation coefficients can be obtained using the self-adjointness and the method of multiple scales (MMS), which is a type of perturbation method. Using the self-adjointness and the MMS, we can calculate the frequency- and wavenumber-dependent attenuation coefficients from the integral values and boundary values of a corresponding eigenfunction with respect to each propagation mode. This analytical method can yield not only dispersion curves but also mode-by-mode attenuation coefficients regardless of the numerical initial values, unlike numerical approaches using the Newton method. Thus, the proposed method can more easily calculate the attenuation coefficients with respect to a particular mode than conventional methods. Furthermore, the results obtained by proposed method were in good agreement with those obtained by the semi-analytical finite element (SAFE) method, which validates the proposed method.

© 2002-2011 IEEE. In this paper, we propose a new type of electromagnetic shunt damper where an air core coil is set above a floating magnet in a superconducting levitation system. The coil is connected to a shunt circuit consisting of a capacitance and a resistance. We have derived the optimal parameter values of capacitance C and resistance R from the fixed point theory after linearizing the governing equations and experimentally confirmed the effectiveness of the damper. We have also confirmed the electromagnetic coupling coefficient, which couples the mechanical system and the electric system and can be treated as a constant; as it is enhanced by the presence of the superconductor. Further, we have oscillated the superconductor and observed the amplitude of the floating magnet with and without the shunt damper. By the result, we have confirmed effective amplitude reduction by the shunt damper.

Taking into account damping effects, the propagation-distance dependence of the amplitude of nonlinear guided waves arising from an internal resonance was investigated. In the analysis of Lamb-wave propagation, we introduced nonlinear quadratic effects derived from both geometric and material nonlinearities, referred to as classical nonlinearities, and damping effects by expressing the Lamé constants as complex numbers. Assuming a small detuning from the phase matching condition with respect to wavenumber and frequency, and using the method of multiple scales and self-adjointness of the governing equations with boundary conditions, the amplitude equations describing the nonlinear coupling of the two wave modes are derived as solvability conditions. This derivation also provides both physical and mathematical insights into both the phase and group velocity matching conditions. Numerical results obtained from these amplitude equations show that because of the nonlinear coupling an internal resonance between the two modes can occur under the above matching conditions. Without detuning, one mode increases monotonically in amplitude while the other decreases monotonically. Nevertheless, their changes gradually decrease with propagation distance, and each amplitude converges to a finite value. However, with wavenumber and frequency detuning, both amplitudes increase and decrease cyclically with propagation distance, resulting in a weakening of this resonance. Furthermore, results with damping effects show an attenuation of both amplitudes with propagation distance, which limits the distance resonance waves can propagate.

超電導磁気軸受で支持された回転体のふれ回り振幅の低減のために有効な電磁シャントダンパの構成要素であるコイルおよび永久磁石の設計指針を検討した．

回転体である永久磁石とそれを非接触に支持する可動な高温超電導体との間での運動の磁気的非線形連成による内部共振により生じうる，危険速度付近での回転体の振幅低減について，数値計算および実験によって調べた．

音響場における気泡クラスターの体積振動および並進運動を観察する実験方法を提案し，気泡クラスターにはたらく第２Bjerknes力および撮影結果の画像解析手法を示し，画像解析から得られた気泡クラスター中心の軌跡と理論値との比較を示した．

音響放射圧を介して連成する二つの球形気泡の体積振動に関して，多重尺度法に基づく非線形モード解析を行った．振動振幅および位相の定常解において，エネルギーの局在化に至る分岐が生じうることを示し，その発生領域を音波周波数および二気泡間の距離の関数として求めた．

円柱ガイド波の減衰について，入力周波数と伝播モードに依存する減衰定数を解析的に求め，さらに電磁超音波探触子EMATによる測定から実験的に求めた．

ガイド波の高次モードを発生させるための電磁超音波探触子 (EMAT) について，理論的に検討したうえで実験による検証を行った．

超電導磁気軸受で支持された回転体について，磁化分布の非一様性に起因する分数調波共振の発生を調べた．