Analysis of a high Tc superconducting levitation systemwith vibration isolation control

This paper presents a method for controlling vibrations of a levitated high Tc superconducting body subjected to base disturbances. To have the control forces, an actuator consisting of a permanent magnet with an electromagnet was presented. The analytical solution for calculating levitation forces due to the permanent magnet and the control currents in the electromagnet was obtained. The levitation forces obtained coincide with the previously published results. The equation of motion of the levitated body subjected to base disturbances under the control was presented. Nonlinear vibrations of the body were first discussed; then the method of vibration isolation control using the direct disturbance cancellation combining the velocity feedback control was investigated. Numerical calculations were carried out for the levitation forces, with respect to the levitated body subjected to harmonic or pulse base excitations. It was clarified that the present method Is valid for controlling nonlinear systems like the magnetically levitated superconducting body     

Observation of the Field,Current and Force Distributions in an Optimized Superconducting Levitation with Translational Symmetry

The superconducting levitation realized by immersing the high-temperature superconductors (HTSs) into nonuniform magnetic field is deemed promising in a wide range of industrial applications such as maglev transportation and kinetic energy storage. Using a well-established electromagnetic model to mathematically describe the HTS, we have developed an efficient scheme that is capable of intelligently and globally optimizing the permanent magnet guideway (PMG) with single or multiple HTSs levitated above for the maglev transportation applications. With maximizing the levitation force as the principal objective, we optimized the dimensions of a Halbach-derived PMG to observe how the field, current and force distribute inside the HTSs when the optimized situation is achieved. Using a pristine PMG as a reference, we have analyzed the critical issues for enhancing the levitation force through comparing the field, current and force distributions between the optimized and pristine PMGs. It was also found that the optimized dimensions of the PMG are highly dependent upon the levitated HTS. Moreover, the guidance force is not always contradictory to the levitation force and may also be enhanced when the levitation force is prescribed to be the principle objective, depending on the configuration of levitation system and lateral displacement.     

External Flow of an Acoustically Levitated Droplet

One of the major recent advances for experiments in containerless processing is acoustic levitation. Although there are a lot of previous studies for acoustic levitation, characteristic of external flow of an acoustically levitated droplet is not experimentally examined enough. In this study, external flow field has been observed by using high speed camera and Particle Image Velocimetry. In the case of any levitated droplet at a velocity antinode of standing wave, toroidal vortex are generated around levitated droplet. It is found that toroidal vortex around a levitated droplet is strongly affected by viscosity of levitated samples and input voltage. In terms of water droplet, as input voltage is decreased, location of toroidal vortex is moved from bottom to top of levitated samples.     

A self-running standing wave-type bidirectional slider for the ultrasonically levitated thin linear stage

A slider for a self-running standing wave-type, ultrasonically levitated, thin linear stage is discussed. The slider can be levitated and moved using acoustic radiation force and acoustic streaming. The slider has a simple configuration and consists of an aluminum vibrating plate and a piezoelectric zirconate titanate (PZT) element. The large asymmetric vibration distribution for the high thrust and levitation performance was obtained by adjusting the configuration determined by finite elemental analysis (FEA). As a preliminary step, the computed results of the sound pressure distribution in the 1-mm air gap by FEA was com pared with experimental results obtained using a fiber optic probe. The direction of the total driving force for the acoustic streaming in the small air gap was estimated by the sound pressure distribution calculated by FEA, and it was found that the direction of the acoustic streaming could be altered by controlling the vibration mode of the slider. The flexural standing wave could be generated along the vibrating plate near the frequencies predicted based on the FEA results. The slider could be levitated by the acoustic radiation force radiated from its own vibrating plate at several frequencies. The slider could be moved in the negative and positive directions at 68 kHz and 69 kHz, which correspond to the results computed by FEA, with the asymmetric vibration distribution of the slider's vibrating plate. Larger thrust could be obtained with the smaller levitation distance, and the maximum thrust was 19 mN.     

The Influence of Excitation Frequency on Magnetic Levitation Systems with a High-T c Superconductor

In this paper we present a numerical analysis of dynamic features of the levitation system generated by an interaction between a levitated permanent magnet (PM) and a high-T _c superconductor (HTSC) excited by an oscillatory external source. The obtained results show that the value of the frequency (f _free) of the PM displacement in the case of the levitation system generated by an interaction between a levitated PM and a fixed HTSC is equal at the resonance frequency (f _re) of the levitation system generated by an interaction between a levitated PM and HTS excited by an oscillatory external source and the resonance frequency (f _re) is mainly dependent upon the cooling position (Z ₀) and the mass of the PM. The numerical problem in this paper is solved by using the control volume method (CVM).     

Precise Density Measurements for Electromagnetically Levitated Liquid Combined with Surface Oscillation Analysis

A new method is proposed for accurately measuring the densities of high-temperature liquids which involves analyzing the surface oscillations of levitated droplets. This method makes it easy to improve on the accuracy of density measurements obtained by using conventional electromagnetic levitation systems. In addition, the errors in density measurements made on the ground are further reduced by applying a static magnetic field to suppress surface oscillations in levitated liquid droplets. The magnetic field interacts with electrical currents in the levitated droplet, thereby generating a Lorentz force; this force suppresses flow within the liquid droplet. By combining both these methods, the scatter in density measurements for molten Si at temperatures in the range from 1,500 K to 1,900 K is reduced by an order of magnitude compared with previously reported data. Using this new method, the density of molten SiGe has been measured at temperatures from 1,350 K to 1,650 K.     

Measurement of Density and Structural Short-Range Order of Levitated Liquid Metals

The determination of thermophysical properties and structure of undercooled metallic melts must be accomplished by contactless methods due to the high reactivity of the material. It has been shown that electromagnetic levitation provides high-purity conditions to allow deep undercooling. The density and thermal expansion of a levitated drop can be derived from volume measurements using a charge-coupled device (CCD) camera and a digital image processing system. Combining levitation with extended x-ray absorption fine structure (EXAFS) spectroscopy leads to the possibility of studying the local structure of the liquid in a wide temperature range including the deeply undercooled regime.     

Experimental and numerical modeling of particle levitation and movement behavior on traveling-wave electric curtain for particle removal

Traveling-wave electric curtain (EC) has been developed for potential application in particle removal/shield on solar panels and other surfaces. Levitation and transport of a particle in a traveling-wave electric field were simulated. Results show that levitation directions/angles and levitation trajectories differ because of the difference in starting positions and starting times. The particles in the two positive acceleration regions are levitated in opposite directions, and the particles distributed on the dielectric surface are levitated and transported successively rather than simultaneously. Movement trajectories are complex and affected by various factors. In the current paper, movement trajectories are modeled to analyze which motion modes are advantageous or disadvantageous to particle removal. This process is beneficial to elucidate the mechanism of particle removal and provide a guidance for movement control by designing appropriate operating parameters.     

‘Electromaglev’-magnetic levitation of a superconducting disc with a DC field generated by electromagnets: Part 1. Theoretical and experimental results on operating modes, lift-to-weight ratio, and suspension stiffness

We present results of a comprehensive study, both theoretical and experimental, of an ‘electromaglev’ system, in which a high-temperature superconducting bulk sample, e.g. YBa₂Cu₃O_7−δ (YBCO), is levitated stably in a DC magnetic field generated by electromagnets placed underneath the floating object. Results of the zeroth-order theory agree quite well with experimental results on lift-to-weight ratio and suspension stiffness for three bulk samples: (1) a solid YBCO disc; (2) a YBCO annulus; and (3) a YBCO annulus with a neodymium-iron-boron (Nd-Fe-B) permanent magnet disc (PMD) filling the centre. The experiment has also verified the need to satisfy two requirements to achieve stable levitation with a DC magnetic field only: (1) the spatial flow of the supercurrent in the sample must have at least two degrees of freedom; and (2) the electromagnets must generate a magnetic field profile that satisfies spatial requirements for lateral and pitch stability. A permanent magnet disc has only one degree of freedom for its Amperian current, thus it cannot be levitated stably in this system; the experiment has also demonstrated that an HTS solenoid (wound with silver-sheathed BSCCO-2223 tape) cannot be levitated stably, because the solenoid supercurrent flow is also restricted to the azimuthal direction only. The zeroth-order theory together with the Bean model shows that the supercurrent induced in a YBCO sample is independent of the critical current density, J_c, of the material but is directly proportional to the axial component of the field and that the lift of the sample is directly proportional to the product of the axial and radial components of the magnetic field generated by the electromagnets.     

Levitation Properties of Melt-Processed YBCO Bulk with a Linear Notch

In this paper, we report the levitation force properties of a melt-processed YBCO bulk with a linear notch. With the increase of the notch length, levitation force between the YBCO bulk and magnet decreases in exponential law with zero-field cooling (ZFC) condition and while its change obeys the polynomial form when the superconductor is cooled with magnetic field (FC). In addition, it is also found that the change of the area of levitation force hysteretic loop is very small in the case of FC, which means that critical current density is not obviously varied when the YBCO bulk has a linear notch. When the superconductor was cooled in zero magnetic field, the area increases sharply at first and then increases gradually; these results reveal the decrease of the critical current density of the superconducting bulk.     

A stator for a self-running, ultrasonically-levitated sliding stage

Here we propose a self-running, ultrasonically-levitated sliding stage and investigate the levitation and propulsion characteristics of its stator. The stator consists of two aluminum beams and four PZT plates, which have two-paired bimorph configurations. A flexural standing wave was generated along the beam by applying an input voltage to the PZTs, and the stator could be levitated from a flat substrate by the acoustic radiation force generated by its own vibrating beam. The size of the stator was optimized using finite-element analysis (FEA) to maximize the vibration displacement amplitude of the beam. The flexural vibration modes at 24.3 and 102 kHz were the most prominent vibration modes having large displacement amplitudes. The stator was levitated at 23.2 and 96.1 kHz, which are close to the frequencies predicted by the FEA results. A standing wave was observed along the beam. The experimental and the simulated results showed good agreement. The levitation distance h was measured by varying the vibration displacement amplitude of the beam u, and was found to be proportional to u. When a traveling wave was excited along the beam by controlling the temporal phase difference of the two transducers, the stator could be made to hover and to move in the opposite direction to the traveling wave. The stator moved in the positive direction when the phase difference was in the ranges 0 degrees to 200 degrees and 310 degrees to 360 degrees, and in the negative direction when the phase difference was between 220 degrees and 260 degrees.     

Interfacial stability and internal flow of a levitated droplet

Under the microgravity environment, production of new and high quality material is expected. Large droplet is preferable for such a containerless processing in microgravity environment. There are a lot of previous studies for droplet levitation [1]. However, effect of surface instability and internal flow appear remarkable when the droplet becomes large. Elucidation of effect of surface instability and internal flow of the levitated droplet is required for the quality improvement of new material. The objective of present study is to clarify critical conditions of the occurrence of the internal flow and the surface instability. At first, the condition between the stable region and the unstable region of the droplet levitation was evaluated by using the existing critical Weber number theory. The experimental result agreed well with the theory. It was suggested that the stability of droplet can be evaluated by using the theory for the interfacial instability. Finally, two-dimensional visual measurement was conducted to investigate the internal flow structure in a levitated droplet. The effect of physical properties on the internal flow structure of the droplet is investigated by Particle Image Velocimetry (PIV) technique. As the result, it is indicated that the internal flow structure is affected by the physical property such as viscosity.     

Study on Heat Transfer and Flow Characteristic Under Phase-Change Process of an Acoustically Levitated Droplet

Acoustic levitation is one of the levitation technique which is expected to be used for analytical chemistry and manufacturing new materials. Thus, it is important to gather the knowledge about acoustically levitated droplet. The purpose of this study is to investigate the heat transfer and flow behavior under phase change process of an acoustically levitated droplet. The following results were obtained from experiments. Evaporation process and external flow structure of the levitated droplet is visualized by a high speed camera and it is found that they differ by the type of fluid. Toroidal vortices are observed near the surface of the ethanol solution droplet. Heat transfer coefficient is estimated from the volume change and temperature gradient. It is substantially higher than that estimated by the existing experimental correlation.     

Hybrid controller with recurrent neural network for magnetic levitation system

We propose a hybrid controller using a recurrent neural network (RNN) to control a levitated object in a magnetic levitation system. We describe a nonlinear dynamic model of the system and propose a computed force controller, based on feedback linearization, to control the position of the levitated object. To relax the requirement of the lumped uncertainty in the design of the computed force controller, an RNN functions as an uncertainty observer to adapt the lumped uncertainty on line. The computed force controller, the RNN uncertainty observer, and a compensated controller are embodied in a hybrid controller, which is based on Lyapunov stability. The computed force controller, with the RNN uncertainty observer, is the main tracking controller, and the compensated controller compensates the minimum approximation error of the RNN uncertainty observer. To ensure the convergence of the RNN, the adaptation law of the RNN is modified by using a projection algorithm. Experimental results illustrate the validity of the proposed control design for the magnetic levitation system.     

Quantitative analysis of agglomerates levitated from particle layers in a strong electric field

The electrification, agglomeration, and levitation of particles in a strong electric field were analyzed experimentally and theoretically. Particle layers of glass, alumina, and ferrite were formed on a plate electrode and an external voltage was applied. Microscopic observations of the agglomerates levitated from the particle layers revealed that the number of primary particles constituting an agglomerate is affected by particle diameter and electrical resistance, but not by the applied electric field. The electric field distributions in the system were calculated by considering the charges and geometries of the agglomerates formed on the particle layers. The charges of the agglomerates were obtained experimentally. All forces acting on the agglomerates (i.e., gravitational forces, Coulomb forces, interaction forces between polarized particles, image forces, and gradient forces) were analyzed under different conditions, including various electric field distributions and charges of agglomerates. Furthermore, the critical conditions for the levitation of the agglomerates were evaluated using a force balance.     

Aerodynamic Levitation and Inductive Heating – A New Concept for Structural Investigations of Undercooled Melts

The combined application of containerless techniques with X-ray diffraction and absorption at synchrotron sources as well as neutron diffraction enables structural investigations of high-melting-point and/or corrosive liquids above the melting point and in the undercooled state. A variety of containerless techniques are available including electromagnetic and aerodynamic levitation. In the framework of a bilateral project, a new hybrid system combining aerodynamic levitation with inductive heating is being developed. Advantages and concept of the setup are discussed. Different Helmholtz coils and cylindrical coils were used to heat levitated, solid samples. Melting and stable levitation in the liquid state were achieved for aluminum. The general problem of deformation of liquid samples by electromagnetic fields is discussed. Paper presented at the Seventh International Workshop on Subsecond Thermophysics, October 6–8, 2004, Orléans, France.     

Correlations Between Magnetic Flux and Levitation Force of HTS Bulk Above a Permanent Magnet Guideway

In order to clarify the correlations between magnetic flux and levitation force of the high-temperature superconducting (HTS) bulk, we measured the magnetic flux density on bottom and top surfaces of a bulk superconductor while vertically moving above a permanent magnet guideway (PMG). The levitation force of the bulk superconductor was measured simultaneously. In this study, the HTS bulk was moved down and up for three times between field-cooling position and working position above the PMG, followed by a relaxation measurement of 300 s at the minimum height position. During the whole processes, the magnetic flux density and levitation force of the bulk superconductor were recorded and collected by a multipoint magnetic field measurement platform and a self-developed maglev measurement system, respectively. The magnetic flux density on the bottom surface reflected the induced field in the superconductor bulk, while on the top, it reveals the penetrated magnetic flux. The results show that the magnetic flux density and levitation force of the bulk superconductor are in direct correlation from the viewpoint of inner supercurrent. In general, this work is instructive for understanding the connection of the magnetic flux density, the inner current density and the levitation behavior of HTS bulk employed in a maglev system. Meanwhile, this magnetic flux density measurement method has enriched present experimental evaluation methods of maglev system.     

超高层建筑横风向气动刚度研究

为了研究超高层建筑横风向气动刚度,进行了多自由度气弹模型试验,以直接测量模型在不同风速下的振动频率,用该频率相对于自振频率的改变量作为气动刚度的评估指标。分析了结构阻尼比、风场粗糙度、模型密度、折算风速、高宽比、涡振位移等因素对气动刚度的影响。结果表明:在共振临界风速附近,气动刚度造成风振频率改变量可达自振频率的10%;频率改变量随折算风速呈“V”字形变化,在折算风速小于8时,频率改变量通常为正,在共振临界风速附近频率改变量最大,折算风速大于12时,频率改变量保持稳定且略小于结构自振频率。由于涡振位移和气动刚度的相互作用,结构阻尼比越小、风场紊流度越小、模型高宽比越大、密度越小则频率改变量随风速变化的“V”字形越尖锐。最后提出了名义折算风速和实际折算风速的概念,并建立了气动刚度的简化估算模型。     

基于磁悬浮技术建筑物振动测量研究

针对大型基建设备工作时产生的振动对周围建筑和环境的影响,设计了利用磁悬浮技术进行振动测量系统。建立了振子的动力学方程,证明了磁悬浮技术可实现绝对振动测量;对磁悬浮技术振动测量系统的自振和外加振动仿真及功率谱进行了分析;并实测了大型设备工作时产生的振动,得到其振动主频为5 Hz左右的低频分量。分析表明,利用磁悬浮技术测振易于实现绝对式低频振动测量且灵敏度高。     

Affinity two-phase partitioning in acoustically levitated drops

Miniaturized (<1 microL) biospecific affinity two-phase partitioning in an acoustically levitated drop is described. Miniaturization commonly gives unfavorable surface/volume ratios, but in the levitation approach adsorption problems are minimized since the only surrounding wall is the liquid/air interface of the drop. Biotinylated liposomes were partitioned in aqueous poly(ethylene glycol)/dextran two-phase drops with NeutrAvidin-dextran as the affinity ligand. A two-phase drop was trapped and manipulated in a node of a standing ultrasonic wave. Alternatively, a two-phase system was formed by levitation and evaporation of a polymer one-phase drop. Phase mixing was achieved by adjusting the ultrasonic field and phase separation by readjusting the field. NeutrAvidin-dextran brought about the redistribution of biotinylated liposomes from the poly(ethylene glycol)-rich phase into the dextran-rich phase. Thus, an entire affinity two-phase separation procedure, including mixing of the phases and incubation to allow affinity interactions to develop under constant volume, followed by phase separation under controlled evaporation, can be performed in a single levitated drop. This miniaturized technique would allow the separation of biologically active membranes or organelles from individual cells for analysis.