Periodic Image Method for Open Boundary Axisymmetrical Magnetic Field Problems

A magnetic levitation isolation system applied for the active control of micro-vibration in space requires actuators with high accuracy, linear thrust and low power consumption. The magneto-force-thermal characteristics of traditional electromagnetic actuators are not optimal, while actuators with a Halbach array can converge magnetic induction lines and enhance the unilateral magnetic field. To improve the control effect, an accurate magnetic field analytical model is required. In this paper, a magnetic field analytical model of a non-equal-size Halbach array was established based on the equivalent magnetic charge method and the field strength superposition principle. Comparisons were conducted between numerical simulations and analytical results of the proposed model. The relationship between the magnetic flux density at the air gap and the size parameters of the Halbach array was analyzed by means of a finite element calculation. The mirror image method was adopted to consider the influence of the ferromagnetic boundary on the magnetic flux density. Finally, a parametric model of the non-equal-size Halbach actuator was established, and the multi-objective optimization design was carried out using a genetic algorithm. The actuator with optimized parameters was manufactured and experiments were conducted to verify the proposed analytical model. The difference between the experimental results and the analytical results is only 5%, which verifies the correctness of the magnetic field analytical model of the non-equal-size Halbach actuator.

Download Full-text

Formation of Majorana fermions in finite-size graphene strips

SciPost Physics ◽

10.21468/scipostphys.3.1.002 ◽

2017 ◽

Vol 3 (1) ◽

Cited By ~ 4

Author(s):

Vardan Kaladzhyan ◽

Cristina Bena

Keyword(s):

Magnetic Field ◽

Boundary Conditions ◽

Finite Size ◽

Inhomogeneous Magnetic Field ◽

Orbit Coupling ◽

Open Boundary ◽

Majorana Fermions ◽

Spin Orbit Coupling ◽

Spin Orbit ◽

Open Boundary Conditions

We investigate the formation of Majorana fermions in finite-size graphene strips with open boundary conditions in both directions, in the presence of an in-plane magnetic field and in the proximity of a superconducting substrate. We show that for long enough strips the Majorana states can form in the presence of a Rashba-like spin-orbit coupling, as well as in the presence of an inhomogeneous magnetic field. We find that, unlike infinite graphene ribbons in which Majorana states arise solely close to the bottom of the band and the Van Hove singularities, for finite-size systems this can happen also at much smaller doping values, close to the Dirac points, and depends strongly on the type of the short edges of the systems (e.g. armchair vs. zigzag), as well as on the width of the ribbons.

Download Full-text

Improved Concept and Model of Eddy Current Damper

Journal of Vibration and Acoustics ◽

10.1115/1.2172256 ◽

2005 ◽

Vol 128 (3) ◽

pp. 294-302 ◽

Cited By ~ 82

Author(s):

Henry A. Sodano ◽

Jae-Sung Bae ◽

Daniel J. Inman ◽

W. Keith Belvin

Keyword(s):

Magnetic Field ◽

Theoretical Model ◽

Eddy Current ◽

Eddy Currents ◽

Opposite Polarity ◽

Image Method ◽

Time Varying ◽

Transverse Beam ◽

Eddy Current Damper ◽

Improved Model

When a conductive material experiences a time-varying magnetic field, eddy currents are generated in the conductor. These eddy currents circulate such that they generate a magnetic field of their own, however the field generated is of opposite polarity, causing a repulsive force. The time-varying magnetic field needed to produce such currents can be induced either by movement of the conductor in the field or by changing the strength or position of the source of the magnetic field. In the case of a dynamic system the conductor is moving relative to the magnetic source, thus generating eddy currents that will dissipate into heat due to the resistivity of the conductor. This process of the generation and dissipation of eddy current causes the system to function as a viscous damper. In a previous study, the concept and theoretical model was developed for one eddy current damping system that was shown to be effective in the suppression of transverse beam vibrations. The mathematical model developed to predict the amount of damping induced on the structure was shown to be accurate when the magnet was far from the beam but was less accurate for the case that the gap between the magnet and beam was small. In the present study, an improved theoretical model of the previously developed system will be formulated using the image method, thus allowing the eddy current density to be more accurately computed. In addition to the development of an improved model, an improved concept of the eddy current damper configuration is developed, modeled, and tested. The new damper configuration adds significantly more damping to the structure than the previously implemented design and has the capability to critically damp the beam’s first bending mode. The eddy current damper is a noncontacting system, thus allowing it to be easily applied and able to add significant damping to the structure without changing dynamic response. Furthermore, the previous model and the improved model will be applied to the new damper design and the enhanced accuracy of this new theoretical model will be proven.

Download Full-text