Permanent Magnet Assembly Research Articles

Design and Manufacture of a Near 3 T High Field Permanent Magnet Assembly

This paper introduces a design methodology for a multi-tesla permanent magnet. Normally the permanent magnet is designed to provide the magnetic fields below 2 T. By employing high performance materials and the 3D magnetic circuit design method, a prototype magnet has been built and generates maximum 2.987 T field strength in 0.008 m air gap with less than 20 kg total weight. It is expected a practical, compact and light-weight high field source for NMR, MRI or other instruments.

Experiments on a Modular Magnetic Refrigeration Device

An experimental magnetic refrigeration test device has been built at Riso DTU. The device is designed to be modular, and thus all parts of the device can easily be replaced depending on the experiment. This makes the device highly versatile, with the possibility of performing a wide variety of different experiments. The test device is of the reciprocating type, and the magnetic field source is provided by a permanent Halbach magnet assembly with an average flux density of 1.03 Tesla. This work presents experimental results for flat plate regenerators made of gadolinium and sintered compounds of La(Fe,Co,Si)13 and experimentally investigates the effect of thermal conduction through the regenerator housing walls. Each regenerator was tested over a range of hot reservoir temperatures under no load conditions for a regenerator comprised of gadolinium. The test machine was also tested with two different compositions of La(Fe,Co,Si)13 compounds. Test results are presented for a regenerator made of a single La(Fe,Co,Si)13 material and a two-material regenerator, and the results are compared to the same system using gadolinium.

Optimization of Permanent Magnet Assemblies Using Genetic Algorithms

This paper describes the utilization of genetic algorithms to optimize the topology of assemblies of permanent magnets. Conventional genetic algorithms are modified to employ pixel refinement to improve the computational efficiency of the optimization process. An example case is demonstrated for optimizing the figure of merit of a permanent magnet dipole. By using only four magnetization directions, a figure of merit value of 0.11 is achieved using the proposed optimization method.

Numerical Simulation and Analysis on Magnetizing Exciter for Magnetic Flux Leakage

Magnetic flux leakage (MFL) is a non-destructive testing method used to inspect the pipe and magnetization of the pipe wall to saturation is essential for anomalies to be reliably and accurately detected and characterized. Axial components of magnetic flux density obtained during the MFL inspection have been simulated using three-dimensional finite element analysis and the effects of magnetizing exciter parameters on magnetic flux density are investigated. The pipe modeled in this paper has an outer diameter of 127mm (5 in.) with a wall thickness of 9 mm (0.354 in.). According to numerical simulations, an increase in the magnetic flux density of pipe wall is observed with an increase in the permanent magnet length and height. It clearly illustrates that Nd-Fe-B permanent magnet assembly with 70 mm length and 40 mm height may magnetize pipe wall to near saturation.

Three-Dimensional Nonaxisymmetric Pole Piece Shape Optimization for Biplanar Permanent-Magnet MRI Systems

The pole pieces of a permanent-magnet system for magnetic resonance imaging (MRI) are optimized for maximum magnetic field homogeneity in the imaging volume. Axisymmetric and nonaxisymmetric optimized pole piece designs based on the same geometrical parameterization are presented. The optimization algorithm demonstrated is based on field calculations by the three-dimensional finite-element method and takes into account the nonlinearity of the magnetic materials involved. The necessity of the nonaxisymmetric design in obtaining suitable field homogeneity over a large imaging volume is demonstrated through comparison of various pole piece designs for a novel biplanar permanent-magnet assembly with reduced pole dimensions as required in the development of an integrated linear accelerator and MRI system for real-time image-guided adaptive radiotherapy. The sensitivity of the field inhomogeneities to geometrical variations in the nonaxisymmetric design surface is explored and statistical parameters quantifying this sensitivity are approximated.

Structure and Performance Analysis of Regenerative Electromagnetic Shock Absorber

This paper analyze d the structure and principle of a regenerative electromagnetic shock absorber in detail. The innovative shock absorber resembles linear generator in principle and can generate electric power through the relative reciprocating motion between coil assembly and permanent magnet assembly . A t the same time, the damping can remove discomfort caused by road roughness. The regenerated electric power can be recovered through battery. Analysis of magnetic flux density of the permanent magnet array of the innovative shock absorber was performed using ANSYS software based on the structure parameters given in the paper,then the performance parameters of the shock absorber was determined . Analysis and calculation results prove the viability of this shock absorber .

Design and Optimization of a Novel Bored Biplanar Permanent-Magnet Assembly for Hybrid Magnetic Resonance Imaging Systems

We present a novel design for a biplanar permanent-magnet assembly suitable for use in hybrid magnetic resonance imaging (MRI) systems. The key feature of our design is a large cylindrical hole that is longitudinally bored through the entire magnet assembly. The presence of the bore permits the potential inclusion of additional peripheral devices within or near the magnet structure that may benefit from being placed along, or oriented parallel to, the main magnetic field. In particular, the magnet assembly can be considered for use in an integrated system consisting of a 6 MV medical linear accelerator (linac) coupled to an MRI system for state-of-the-art real-time image-guided adaptive radiotherapy. We use magnetic field calculations based on the finite-element method (FEM) to quantify the detrimental effects of the bore on the field homogeneity in the imaging volume for pole-piece designs found commonly in industry. We then demonstrate that shape optimization of the pole pieces yields novel pole designs that lead to suitable levels of field homogeneity. We examined the resultant magnetic field within the bore for the optimized design and found that it has maximum field homogeneity.

Permanent magnet assembly producing a strong tilted homogeneous magnetic field: towards magic angle field spinning NMR and MRI

We introduce a cylindrical permanent magnet design that generates a homogeneous and strong magnetic field having an arbitrary inclination with respect to the axis of the cylinder. The analytical theory of 3 D magnetostatics has been applied to this problem, and a hybrid magnet structure has been designed. This structure contains two magnets producing a longitudinal and transverse component for the magnetic field, whose amplitudes and homogeneities can be fully controlled by design. A simple prototype has been constructed using inexpensive small cube magnets, and its magnetic field has been mapped using Hall and NMR probe sensors. This magnet can, in principle, be used for magic angle field spinning NMR and MRI experiments allowing for metabolic chemical shift profiling in small living animals.

Comparison of adjustable permanent magnetic field sources

A permanent magnet assembly in which the flux density can be altered by a mechanical operation is often significantly smaller than comparable electromagnets and also requires no electrical power to operate. In this paper five permanent magnet designs in which the magnetic flux density can be altered are analyzed using numerical simulations, and compared based on the generated magnetic flux density in a sample volume and the amount of magnet material used. The designs are the concentric Halbach cylinder, the two half Halbach cylinders, the two linear Halbach arrays and the four and six rod mangle. The concentric Halbach cylinder design is found to be the best performing design, i.e. the design that provides the most magnetic flux density using the least amount of magnet material. A concentric Halbach cylinder has been constructed and the magnetic flux density, the homogeneity and the direction of the magnetic field are measured and compared with numerical simulation and a good agrement is found.

Drive unit for electro-acoustic converter with radially magnetized permanent magnetic bars

An electro-acoustic drive unit includes at least one permanent magnet assembly and at least an inner pole piece and an outer pole piece forming a pair of pole pieces, the permanent magnet assembly together with the pair of pole pieces forming a magnetic circuit with a pole gap for exciting a voice coil assembly. The pole gap is arranged to provide a magnetic field directed radially with respect to a center axis of the loudspeaker, and the permanent magnet assembly includes at least two permanent magnetic bars, with radially extending magnetization direction with respect to the center axis of the loudspeaker.

DISCUSSION ABOUT THE MAGNETIC FIELD PRODUCED BY CYLINDRICAL HALBACH STRUCTURES

This paper uses a three-dimensional analytical approach based on the Coulombian model for studying the magnetic field produced by cylindrical Halbach structures. Such structures, commonly used in magnetic couplings or in electrical machines, are composed of tile permanent magnets with rotating magnetizations. Such assemblies of tile permanent magnets allow one to easily optimize the radial field shape in the air gap of electrical machines. In addition, Halbach structures can be used in magnetic couplings for improving the torque transmitted between the two rotors. Analytical studies dealing with the optimization of such structures generally use a two- dimensional analytical approach for calculating either the magnetic field produced by tile permanent magnets or the forces exerted between them. These two-dimensional expressions are useful because they have a very low computational cost. However, their accuracy depends greatly on the structure dimensions. We propose in this paper to use a three-dimensional analytical model based on the Coulombian model for determining the exact shape of the magnetic field produced by a Halbach structure. Such an approach also allows one to determine the demagnetizing magnetic field inside the tile permanent magnets. This element of information is important for the design of tile permanent magnets. In addition, we show that some effects cannot be predicted with the linearized analytical model. This implies that a linearized dimensional optimization is not accurate. This study has been carried out without any simplifying assumptions. Therefore, the calculations of the three magnetic field components are exact for all points in space, whatever the magnet dimensions. We can say that such a three-dimensional analytical approach is a good alternative to a finite element one because it has a lower computational cost and is more accurate.

MODELLING AN IRONLESS LOUDSPEAKER BY USING THREE-DIMENSIONAL ANALYTICAL APPROACHES

MODELLING AN IRONLESS LOUDSPEAKER BYUSING THREE-DIMENSIONAL ANALYTICALAPPROACHESR. Ravaud and G. LemarquandLaboratoire d'Acoustique de l'Universite du Maine, UMR CNRS 6613Avenue Olivier Messiaen, 72085 Le Mans, Franceguy.lemarquand@univ-lemans.frAbstract |This paper presents some improved three-dimensionalexpressions of the magnetic ¯eld created by tile permanent magnetsuniformly and radially magnetized for the design of ironlessloudspeaker structures. All the expressions determined have beenreduced to compact forms. We use these expressions for theoptimization of ironless loudspeaker structures in which the radial¯eld must be radially uniform. Indeed, as ring permanent magnetsradially magnetized are rather di±cult to manufacture, these magnetsare replaced by assemblies of tile permanent magnets radiallyand uniformly magnetized. We present an example of ironlessloudspeaker structure that has been optimized with our three-dimensional approaches.1. INTRODUCTIONTile permanent magnets can be used for manufacturing ring permanentmagnets radially magnetized by stacking them together. Such a wayof manufacturing a ring is commonly realized because ring permanentmagnets radially magnetized are di±cult to fabricate. Consequently,this alternative method consists in using tile permanent magnetsuniformly magnetized. However, they do not exhibit the sameproperties as tiles radially magnetized and can lower the quality ofthe radial ¯eld created by the permanent magnet structure.Manyauthorshavestudiedthe¯eldcreatedbyring[1]-[6], cylinder[7]-[9], tile permanent magnets [10]-[12] or by disk conductors [13]-[15]. All these analytical methods have enabled authors to study thesti®ness or the force between permanent magnets for magnetic bearings[16][17] or couplings [18][19]. More generally, optimizations can becarried out with analytical or semi-analytical approaches [20]-[22]. An

MAGNETIC FIELD IN MRI YOKELESS DEVICES: ANALYTICAL APPROACH

This paper presents a three-dimensional analytical expressions for studying the static magnetic fleld produced by Magnetic Resonance Imaging structures. This medical imaging technique uses a very high and uniform magnetic fleld produced by ring permanent magnets with rotating polarizations. However, the manufacturing of such ring permanent magnets is di-cult to realize. Consequently, such ring permanent magnets are replaced by assemblies of tile permanent magnets uniformly magnetized. Unfortunately, the magnetic fleld produced by these tile permanent magnets uniformly magnetized is both less important and less uniform than the one produced by an idealized ring permanent magnet. We propose in this paper to study the in∞uence of the number of tile permanent magnets used on the magnetic fleld properties.

Multiobjective Optimization Based on Response Surface Model and Its Application to Engineering Shape Design

A combined method is presented to deal with the practical engineering problems of multiobjective optimization. The nondominated sorting genetic algorithm II (NSGA-II) is adopted as a searching tool for the Pareto-optimal solutions, which is improved by using a new crossover operator. The response surface model (RSM) based on the radial basis function is used to reduce the computational effort. The application of the method to the shape optimization process of a permanent magnet assembly for magnetic resonance imaging devices is described, and the numerical results show that the method is feasible and efficient for the engineering shape optimization.

Measurement of NdFeB permanent magnets demagnetization induced by high energy electron radiation

Demagnetization of NdFeB permanent magnets has been measured as function of radiation dose induced by high energy electrons. The magnet samples were of different intrinsic coercive forces, ≃ 12 and ≃ 20 KOe , dimensions and direction of magnetization. 5 GeV electron beam from 12 GeV Cornell Synchrotron was used as a radiation source. A calorimetric technique was employed for radiation dose measurement. Results indicated that depending on the sample intrinsic coercive force, shape and direction of magnetization the radiation dose causing 1% of demagnetization of the sample varies from 0.0765 ± 0.005 Mrad to 11.3 ± 3.0 Mrad , i.e., by more than a factor of 100. Experimental data analysis revealed that demagnetization of the given sample induced by radiation is strongly correlated with the sample demagnetizing temperature. This correlation was approximated by an exponential function with two parameters obtained from the data fitting. The function can be used to predict the critical radiation dose for permanent magnet assemblies like undulator magnets based on its demagnetizing temperature. The latter (demagnetization temperature) can be determined at the design stage from 3-D magnetic modeling and permanent magnet material properties.

Electromagnetic Two-Dimensional Scanner Using Radial Magnetic Field

In this paper, we present the design, fabrication, and measurement results of a two-dimensional electromagnetic scanning micromirror actuated by radial magnetic field. The scanner is realized by combining a gimbaled single-crystal-silicon micromirror with a single turn electroplated metal coil, with a concentric permanent magnet assembly composed of two concentric permanent magnets and an iron yoke. The proposed scanner utilizes the radial magnetic field rather than using a lateral magnetic field oriented 45° to the horizontal and vertical scan axes to achieve a biaxial magnetic actuation. The single turn coil fabricated with electroplated copper achieves a nominal resistance of 1.2Ω. A two-dimensional scanner with mirror size of 1.5mm in diameter was fabricated. Maximum optical scan angle of 8.8° in horizontal direction and 8.3° in vertical direction were achieved. Forced actuation of the gimbal at 60 Hz and resonant actuation of the micromirror at 19.1-19.7kHz provide slow vertical scan and fast horizontal scan, respectively. The proposed scanner can be used in raster scanning laser display systems and other scanner applications.

The optimal design method of permanent magnet assemblies with completely open architecture for MRI

This paper presents a flexible design method to facilitate the construction of permanent magnet assemblies with completely open architectures for the Magnetic Resonance Imaging (MRI). The initial design is based on the ideally analytical assumptions. Then the dimensions and directions of the magnetization of each permanent element are optimized by using a modified genetic algorithm. Numerical implementations show that the proposed method provides a main magnetic field with modified homogeneity and strength at the imaging region, which is defined outside of the magnet structure, and minimizes the cost of the MRI equipment.

Finite Element Analysis of Permanent Magnet Assembly With High Field Strength Using Preisach Theory

This paper presents a method to simulate the characteristics of permanent magnet materials with a magnetic field varying arbitrarily. Combining Preisach model and finite element analysis, a dynamic magnetic field computation is implemented for a permanent magnet assembly in a process of assembling. The hysteresis characteristic including local loop and rotational magnetization are considered in the formulation of the method proposed. The numerical results are compared with the ones of a conventional method, the electric current sheet equivalent, and the measured values to verify the validity of the presented method

Innovative design of a magnetocaloric system

In the present paper we consider the problem of optimizing the cooling of a magnetocaloric refrigerator. In this work we first theoretically and then experimentally study the performance of a single material regenerator under different operating conditions. The basic principles of the design and implementation of our magnetic refrigerator prototype are presented as well as a new magnetic assembly of NdFeB permanent magnets. The design of the equipment uses a movement of relative displacement optimized for the phases of activation and inactivation of the magnetic field. Each part of the equipment is implemented in order to be controlled separately and to allow a large variety of the tests: gear pumps with individual control, sequence of programmable magnetocaloric cycle, unit control by programmable controller and application programming interface by color LCD touch screen, real-time processing data acquisition using a National Instruments System implemented on Independent PC, expelled heat using different standard heat exchangers.

Efficiency of permanent magnet assemblies for MRI devices

Highly uniform fields in a cavity are critical for a successful magnetic resonance imaging device. There are various permanent magnet assemblies with specially configured arrangements to generate uniform fields. This paper compares the efficiency of these designs and their applications for different field ranges.