Ring vibrations in an acoustic medium as a source of ultrasonic radiation

In this paper, the vibrational characteristics of ultrasonic transducers consisting of a ceramic ring filled with a plastic disk are considered. For axially symmetric resonances, the vibrational amplitude on the surface of the plastic core is approximately described by a truncated Bessel function of zero order and first kind J(0), leading to an ultrasonic beam which is nondiffracting near the transducer. The near- and far-field pressure distributions of such transducers are measured and compared with model predictions. In the near zone, the width of the measured main lobe is narrow, which is in accordance with the theoretical results for circular transducers with a truncated Bessel function amplitude distribution. By changing the inner diameter of the ring, it is possible to control the resonance frequency of the plastic core. On the other hand, the resonance frequency of the ring may be regulated by varying its width. The existing coupling of resonant vibrations of the ceramic ring and plastic core enables variations of the bandwidths of the considered circular transducers.     

Finite element analysis of relative shaft vibrations of two-pole induction motors with static rotor eccentricity

The paper shows a computational methodology for calculating the relative shaft vibrations in the sleeve bearings of two-pole induction machines regarding excitation due to an electromagnetic force, which is caused by static rotor eccentricity. For a worst case calculation concerning the height of exciting magnetic force electromagnetic field damping effects and magnetic resistance concerning the homopolar flux are neglected. The calculated magnetic force, acting on the rotor core with double supply frequency in direction of the smallest air gap, is implemented into a finite element rotor dynamic model. With this model the influence of the rotor speed as well as influence of the direction of the magnetic force on the relative shaft displacements can be analyzed. Therefore the paper shows a computational methodology to check, whether the rotor-bearing design is sensitive for electromagnetic excitations due to static rotor eccentricity and prepares therefore the possibility to introduce improvements during the design phase of the induction motor.     

Proposal of DC shield reactor type superconducting fault current limiter

Saturated DC reactor type superconducting fault current limiter (SFCL) had been proposed two years ago. It was classified to rectifier type SFCL. The changing inductance value with the operating mode has superior characteristics to reduce voltage sag during step increase of the load current. But it has the disadvantage of its weight. In this paper, rectifier type SFCL with shielded reactor has been proposed. The reactor which has superconducting ring or tube inside its winding is substituted to the DC link of the rectifier. The configuration looks like an air core transformer with secondary short winding. When the current through the bulk shield-ring reaches to a certain level, the flux penetrates to the shield body and finite impedance appears in the primary winding. In other words, when the surface flux density exceeds its critical flux density, the flux penetrates into the bulk superconductor, and increases equivalent inductance. The equivalent transient resistance of the shield was represented as a function of exponential of the time. Using this equivalent transient resistance, the transient impedance was expressed. The transient wave analysis using EMTDC (electro-magnetic transients in DC systems) has been described. Simulated waveforms are shown considering the source inductance, the leakage inductance, the coupling coefficient and the forward voltage drop of the semiconductor. And voltage sag was also investigated with 50% step load increase.Preliminary design was also performed. The coil size and number of turns are designed to obtain adequate inductance for the current limitation, and the central magnetic field of the coils are calculated. There is optimal aspect ratio to minimize the magnetic field with restriction in outer diameter of the coil.     

Differential inductive ratio transducer with short-circuiting ringfor displacement measurement

This paper describes a differential inductive ratio transducer (DIRT) with a short-circuiting ring (SCR) which is smaller in size, lighter in weight, has 500 turns in each coil, has a laminated core of mumetal, is rugged in construction, and is lower in cost. It has linear and hysteresis-free response with immunity to undesirable variations in excitation conditions and influencing parameters. Due to the differential ratio mode of operation, there is a cancellation of common factors in the numerator and denominator. These factors have the terms containing the influence of constant errors due to leakage flux and finite resistance of the SCR, and variable errors due to changes in the excitation conditions and temperature. This process makes the transducer smart in its behavior. It has a full range of displacement up to ±37.5 mm, sensitivity of 0.0933 V·mm^-1, resolution of 0.1 μm, temperature drift of 4.6 ppm/°C for a change in temperature from 30 to 70°C, stability within ±0.09% for an excitation variation from 1 to 5 V, and within ±0.015% for a frequency variation from 1 to 10 kHz. It is suitable for use in all types of environments, including hostile conditions     

Flux Quantization and Aharonov-Bohm Effect in Superconducting Rings

Superconductivity is a macroscopic coherent state exhibiting various quantum phenomena such as magnetic flux quantization. When a superconducting ring is placed in a magnetic field, a current flows to expel the field from the ring and to ensure that the enclosed flux is an integer multiple of h/(2|e|). Although the quantization of magnetic flux in ring structures is extensively studied in literature, the applied magnetic field is typically assumed to be homogeneous, implicitly implying an interplay between field expulsion and flux quantization. Here, we propose to decouple these two effects by employing an Aharonov-Bohm-like structure where the superconducting ring is threaded by a magnetic core (to which the applied field is confined). Although the magnetic field vanishes inside the ring, the formation of vortices takes place, corresponding to a change in the flux state of the ring. The time evolution of the density of superconducting electrons is studied using the time-dependent Ginzburg-Landau equations.     

Local Magnetic Measurements of Trapped Flux Through a Permanent Current Path in Graphite

Temperature- and field-dependent measurements of the electrical resistance of different natural graphite samples suggest the existence of superconductivity at room temperature in some regions of the samples. To verify whether dissipationless electrical currents are responsible for the trapped magnetic flux inferred from electrical resistance measurements, we localized them using magnetic force microscopy on a natural graphite sample in remanent state after applying a magnetic field. The obtained evidence indicates that at room temperature a permanent current flows at the border of the trapped flux region. The current path vanishes at the same transition temperature \(T_c\approx 370\) K as the one obtained from electrical resistance measurements on the same sample. This sudden decrease in the phase is different from what is expected for a ferromagnetic material. Time-dependent measurements of the signal show the typical behavior of flux creep of a permanent current flowing in a superconductor. The overall results support the existence of room-temperature superconductivity at certain regions in the graphite structure and indicate that magnetic force microscopy is suitable to localize them. Magnetic coupling is excluded as origin of the observed phase signal.     

环状永磁体表面径向磁通密度分布的模拟计算

利用AutoCAD软件构建四极磁环的三维模型,采用Ansoft Maxwell软件的有限元分析法对各向异性粘结钕铁硼(NdFeB)四极环状磁体表面径向磁通密度分布进行数值模拟。研究了各向异性粘结NdFeB永磁材料的剩磁B_r和矫顽力H_(cb)及环状磁体的高度、厚度和内外径,对环状磁体表面径向磁通密度与磁特性的影响。结果表明,环状磁体表面径向磁通密度与NdFeB材料的矫顽力H_(cb)及磁体的厚度密切相关,而NdFeB材料的剩磁B_r及磁体的高度对其影响较小。     

Schematics of magnetically-elastic pressure transducers

Conclusions Experiments have shown that all three types of transducers with an appropriate selection of parameters have virtually linear output characteristics. Transducers with a variable inductance have a larger output power compared with the type of transducers made by the Swedish firm ASEA. The load resistances carry the difference of two voltages, the original biasing voltage and the voltage drop across a ballast lamp or the collector circuit of a transistor.The output power of a transistor with a variable inductance operating in a condition of free magnetization can be raised by measuring the magnetizing current increment and then amplifying it by means of the variable resistance which shunts the load and consists of a transistor or a magnetic amplifier with a negative current feedback through the transducer circuit.A suitable transducer circuit can be selected on the basis of specific conditions and requirements and by means of the comparison tests described in this work.N. P. Kunitskii and S. V. Krayushkin participated in the investigation of transducers with a variable inductance operating in a free or forced magnetization condition.     

An analog core memory for pulse-width modulated signal

This paper describes the results of studies on an analog core memory with the aim of applying them to the process controllers. The analog information is stored as the flux level of the square-loop magnetic core. The "write-in" and "read-out" of flux level are achieved by the pulse-width modulation techniques. The storage errors are introduced by the deviations from the ideal square-loop characteristics. However, improvements of stability and linearity are obtained by the following methods: 1) negative dc bias ampere-turn is applied to the square-loop magnetic core, 2) two magnetic cores are connected in the push-pull circuit configuration, 3) the pulse-width modulated (PWM) signal is employed as the "write-in" and "read-out" signal. The experimental results of this device show that the accuracy of analog information storage is better than 0.5 percent at the cycle-time of 1 ms. The PWM signal has both analog and digital characteristics and is suitable for the sequential analog arithmetic computation. An analog-digital hybrid computing control system is outlined to show how these devices can be utilized.     

Development and Characterization of a Diaphragm-Shaped Force Transducer for Static Force Measurement

There are different types of force transducers, used over the years, comprising of different principles and have different grades of uncertainty. Force transducers may be analog or digital and may be strain gauged, tuning fork type, piezoresistive type, Hall Effect based etc. Though different types of force transducers have been developed in last few decades, still strain gauged force transducers are used over the larger scale due to practical viability and reliability for force measurement. The paper describes low-cost indigenous development of a force transducer, which has been developed for a nominal capacity of 5 kN with ease of design and manufacturing. It has strain gauges applied at suitable locations to form a Wheatstone bridge. The force transducer has been calibrated according to the standard calibration procedures based on ISO 376 and IS 4169 standards. Measurement results and an uncertainty analysis have been presented. The uncertainty of measurement of the force transducer is found up to 0.06%. The paper also discusses the implications of applying multiple strain gauges at a single location in series to enhance the sensitivity of the force transducer. Two strain gauges are arranged in series for the given location and the sensitivity of the force transducer is enhanced. It also indicates that the uncertainty of measurement of the force transducer in such case is enhanced up to 0.05% or better to make force transducer suitable for most of metrological and industrial applications.     

The application of multistable magnetic cores for dialing purposes in telephony

Multistable magnetic cores have the well-known property that a certain amount of flux, stored in the core, remains unchanged for a long time. This amount of flux can be broken down by feeding a number of voltage pulses, with a constant volt-second integral and the right polarity, to a core winding. Each of these readout pulses will lower the flux level in the core until there is no flux left, and from that moment a readout pulse only causes a small reversible change in flux, setting up a very short induction EMF in the core windings. A simple integrating network can be used to judge the output voltage of a winding, and it gives no output if the former results from a reversible flux change. If the pulse generator has a repetition frequence of ten cycles per second, the output pulses of the integrating network, after being shaped by a monostable trigger, will be suitable for dialing purposes in conventional telephone systems. A predetermined amount of flux, corresponding to one of the numbers one to ten, can easily be obtained by discharging a loaded condenser in series with a resistance through a core winding.     

Magnetic Flux Penetration and Motion in Antiferromagnetic Superconductors

The paper reviews a concept of induced spin-flop domain inside vortices in an antiferromagnetic superconductor. Such phenomenon may occur when an external magnetic field is strong enough to flip over magnetic moments in the core of the vortex from their ground state configuration. The formation of the domain structure inside vortices modifies the surface energy barrier of the superconductor. During this process the entrance of the flux is stopped and a newly created state exhibits perfect shielding. Such behavior should be visible as a plateau on the dependence of flux density as a function of the external magnetic field. The end of the plateau determines the critical field, which has been called the second critical field for flux penetration. Moreover, it is predicted and described how this phenomenon modifies flux creep in layered superconductors. The various scenarios of changing the creep regime from thermal to quantum and vice versa at constant temperature are discussed.     

强磁场作用下材料扩散型固态相变的微观机理

为探讨强磁场对物质原子尺度行为(电子运动、离子扩散)的影响,采用光学显微镜研究强磁场作用下Fe-0.12%C合金的扩散型固态相变;采用数字多用表测量强磁场作用下的纯铝板电阻研究其电子分布.结果表明:随磁感应强度增强,Fe-0.12%C合金室温显微组织中,铁素体晶粒平行于磁场方向伸长并呈链状排列的趋势增强,珠光体团的长轴方向平行于磁场方向伸长的程度也增强;纯铝板的电阻在平行于磁场方向放置时减小,垂直于磁场方向放置时电阻有增加趋势.这是由于组成金属晶体的自由电子和排列成晶格状的金属离子在磁场作用下受到洛伦兹力的作用,随磁感应强度增强,沿磁场方向的电子浓度、金属离子扩散有增强趋势,导致磁场作用下材料扩散型相变的室温组织出现形状各向异性.     

An experimental study of flux presetting-resetting characteristics of magnetic cores with rectangular hysteresis loop

When magnetic cores with a square hysteresis loop are used for a time delay Crcuit or a pulse counter circuit, and if these circuits are designed by the use of the static hysteresis loop, the ac magnetization curve, or the control magnetization curve (CMC) which is the basic characteristic of a magnetic amplifier, there is apt to be a marked discrepancy between their design values and measured values. Since, in the flux control of magnetic cores used for a time delay circuit or a pulse counter circuit, the flux, contrary to the case with a magnetic amplifier, arrives at the negative saturation from the positive saturation by many intermittent signal voltages, a new characteristic quite different from CMC and other characteristics must be adopted as the standard of the design in this case. It was for this reason that instantaneous relation between the rate of flux change and the exciting current was examined in the case in which real flux resetting behaviors of a time delay circuit or a pulse counter circuit were taken into consideration, by using grain-oriented 50 per cent permalloy as the sample, with a view to obtaining core characteristics serviceable enough for exact design methods of a time delay circuit or a pulse counter circuit. Various useful results have been obtained as to: 1) how the correlation between rate of flux changedPhi/dtand instantaneous values of exciting current icchanges in proportion to the flux values, 2) how the amount of reset fluxDeltaPhi_{r}will change according to the flux valuePhiand the rate of flux changedPhi/dt, and 3) how the above-mentioned relations are connected to the tape thickness of the magnetic core. This paper reports the results of this investigation.     

Finite Element Method Applied in Electromagnetic NDTE: A Review

The paper contains an original comprehensive review of finite element analysis (FEA) applied by researchers to calibrate and improve existing and developing electromagnetic non-destructive testing and evaluation techniques, including but not limited to magnetic flux leakage (MFL), eddy current testing, electromagnetic-acoustic transducers (EMATs). Premium is put on the detection and modelling of magnetic field, as the vast majority of ENDT involves magnetic induction, either as a primary variable MFL or a complementary phenomenon (EC, EMATs). FEA is shown as a fit-for-purpose tool to design, understand and optimise ENDT systems, or a Reference for other modelling algorithms. The review intentionally omits the fundamentals of FEA and detailed principles of NDT. Strain-stress FEA applications in NDT, especially in ultrasonography and hole-drilling methodology, deserve as well a separate study.     

Viscous flux motion in anisotropic magnetic superconductors

Magnetic field penetrates in the form of flux lines or vortex thread into type-II magnetic superconductors (MSC) and induces magnetization of magnetic subsystem over a distance of an order of the London penetration depth surrounding the normal cores. When a flux line moves by, surrounding magnetization moves as a whole through the sample and a free motion of vortices is subjected to magnetic viscous drag, giving rise to dissipation. The flux flow resistance in the mixed state of anisotropic MSC has been studied on the basis of the London theory. Expressions for the dissipation and viscosity coefficient associated with the change of the magnetic subsystem as a vortex moves about are derived.     

Force and Stiffness of Passive Magnetic Bearings Using Permanent Magnets. Part 2: Radial Magnetization

This paper deals with the calculation of the force and the stiffness between two ring permanent magnets whose polarization is radial. Such a configuration corresponds to a passive magnetic bearing. The magnetic force exerted between ring permanent magnets is determined by using the Coulombian model. The expressions obtained are semianalytical and we show that it is not possible to find an exact analytical expression of the force between two ring permanent magnets. Then, thanks to these semianalytical calculations, the ring dimensions are optimized in order to have a great force or a great stiffness. Moreover, we show that the relative position of the rings for which the force is the strongest depends on the air gap dimension. This result is new because the curvature effect is taken into account in this paper. We can say that such semianalytical expressions are more precise than the numerical evaluation of the magnetic forces obtained with the finite-element method. Moreover, semianalytical expressions have a low computational cost whereas the finite-element method has a high one. Thereby, as shown in this paper, such calculations allow an easy optimization of quadripolar lenses or devices using permanent magnets.      

Magnetic hysteresis model

A dynamical model is described which permits calculation of the excitation currentIas a function of time in a laminated grain-oriented (G-O) steel transformer core. The independent variable is the magnetic flux density or, equivalently, the coil voltage less theIRdrop associated with the resistanceRof the windings. Recent observations on flux reversal mechanisms in GO steel indicate that, in the range of magnetic field intensities typically present in transformer cores, the important reversal processes are the motion of long 180° domain walls continuous across grain boundaries and the motion of 90° walls within individual grains. These processes are represented in the model by two subcircuits connected in series. Each subcircuit consists of an inductive element in parallel with a linear resistor which accounts for the eddy current losses accompanying the flux change. The properties of each inductive element (flux vs. current) reflect the two wall motion mechanisms, respectively, in the limit of zero frequency. This model is capable of faithfully simulating minor loop behavior as well as the response to complex waveforms; e.g., the superposition of two or more frequencies. The circuit equations are solved, and some results of computer calculations using a program that implements this model are presented.     

A Comparative Study on the Selected Area Electron Diffraction Pattern of Fe Oxide/Au Core-shell Structured Nanoparticles

The selected area electron diffraction （SAED） pattern of magnetic iron oxide core/gold shell nanoparticles has been studied. For the composite particles with mean size less than 10 nm, their SAED pattern is found to be different from either the pattern of pure Fe oxide nanoparticles or that of pure Au particles. Based on the fact that the ring diameters of these composite particles fit the characteristic relation for the fcc structure, the Au atoms on surfaces of the concerned particles are supposed to pack in a way more tightly than they usually do in pure Au nanoparticles. The driving force for this is the coherency strain which enables the shell material at the heterostructured interface to adapt the lattice parameters of the core.