Simulation of Arbitrarily-shaped Magnetic Objects

We propose a novel method for simulating rigid magnets in a stable way. It is based on analytic solutions of the magnetic vector potential and flux density, which make the magnetic forces and torques calculated using them seldom diverge. Therefore, our magnet simulations remain stable even though magnets are in close proximity or penetrate each other. Thanks to the stability, our method can simulate magnets of any shapes. Another strength of our method is that the time complexities for computing the magnetic forces and torques are significantly reduced, compared to the previous methods. Our method is easily integrated with classic rigid-body simulators. The experiment results presented in this paper prove the stability and efficiency of our method.     

Oscillatory motion-based miniature magnetic walking robot actuated by a rotating magnetic field

Magnetic micro-robots have been proposed for use in biomedical applications. These studies focus on locomotion control using a gradient, alternating, and rotating magnetic fields at the sub-micro scale. However, this study focuses on a basic mechanism of active locomotion for diagnostic robots. Furthermore, the digestive intestine in the human body has a complex path in which locomotion methods can become either swimming or walking according to the inner condition. Therefore, we propose a new simple mechanism for amphibious locomotion within a rotating magnetic field using the three-axis Helmholtz coil system. The proposed magnetic robot consists of NdFeB permanent spherical magnets, flexible silicone tubes, and legs. Successive changes of actuation of yaw and roll motions cause alternating and walking motions. Direction of movement is decided by rotating the direction of the magnetic field (clockwise or counter-clockwise). In addition, turning directions are decided by the plane of the rotating magnetic field. A magnetic torque between the rotating magnetic field and the magnetic moments produce a constant walking pattern similar to a trotting gait. In addition, an oscillatory motion of the flexible robot body can generate a thrust force in the liquid. Finally, through the various experiments, we evaluate the capability of the locomotion.     

Modeling of ferrofluid magnetic actuation with dynamic magnetic fields in small channels

One of the most important and promising research areas in biomedical and micropumping applications is magnetic actuation of ferrofluids with dynamic magnetic fields. For ensuring the use of ferrofluids in various applications in engineering fields, their flows generated by magnetic fields should be extensively investigated and simulated. In this study, simulations of ferrofluid actuation with dynamic magnetic fields were performed by modeling it using the COMSOL Multiphysics software, and iron oxide nanoparticle-based ferrofluids at different angles of rotating magnets were considered to provide insight into ferrofluid flow in small channels. Ferrofluid flows were modeled at different magnetic flux densities provided by rotating magnets, and velocity profiles inside the channel were analyzed. It was shown that ferrofluid actuation can be considered as a futuristic micropumping alternative, simulation results matched well with the experimental results of previous work, and the established model could serve as a tool to analyze ferrofluid flows generated by dynamic magnetic fields. The results of the model show that flow rates up to 100 µl/s can be reached at a rotation angle of 30° by using dynamic magnetic fields. Various applications including biomedical applications might be envisaged.     

Theoretical analysis of a new, efficient microfluidic magnetic bead separator based on magnetic structures on multiple length scales

We present a theoretical analysis of a new design for microfluidic magnetic bead separation. It combines an external array of mm-sized permanent magnets with magnetization directions alternating between up and down with μm-sized soft magnetic structures integrated in the bottom of the separation channel. The concept is studied analytically for simple representative geometries and by numerical simulation of an experimentally realistic system geometry. The array of permanent magnets provides long-range magnetic forces that attract the beads to the channel bottom, while the soft magnetic elements provide strong local retaining forces that prevent captured beads from being torn loose by the fluid drag. The addition of the soft magnetic elements increases the maximum retaining force by two orders of magnitude. The design is scalable and provides an efficient and simple solution to the capture of large amounts of magnetic beads on a microsystem platform.     

A bidirectional magnetic microactuator using electroplatedpermanent magnet arrays

A novel bidirectional magnetic microactuator using electroplated permanent magnet arrays has been designed, fabricated and characterized. To realize a bidirectional microactuator, CoNiMnP-based permanent magnet arrays have been fabricated first on a silicon cantilever beam using a new electroplating technique. In the fabricated permanent magnets, the vertical coercivity and retentivity have been achieved up to 87.6 kA/m (1100 Oe) and 190 mT (1900 G), respectively by applying magnetic field during electroplating. A prototype bidirectional magnetic microactuator has been realized by integrating an electromagnet with a silicon cantilever beam, which has permanent magnet arrays on its tip. By applying a do current of 100 mA and altering its polarity, bidirectional motion on the tip of the cantilever beam has been successfully achieved in the deflection range of ±80 μm     

Electromagnetically force balanced polymer accelerometer

An acceleration sensor from polymer has been developed which balances a proof mass by magnetic forces. The sensor is fabricated from a polyimide membrane with conductor paths from gold patterned by photolithography and etching, a frame manufactured by ultrasonic hot embossing, and permanent magnets fixed to the frame. Except the conductor path and permanent magnets, all components are made of polymers on a planar substrate, and then the frame is kinked forming the desired three-dimensional structure. In a first try, a sensitivity of 0.46 V/(m/s²) was achieved, and cross axis sensitivity error was less than 3 %.     

Fabrication and integration of microscale permanent magnets for particle separation in microfluidics

Microfluidic magnetophoresis is an effective technique to separate magnetically labeled bioconjugates in lab-on-a-chip applications. However, it is challenging and expensive to fabricate and integrate microscale permanent magnets into microfluidic devices with conventional methods that use thin-film deposition and lithography. Here, we propose and demonstrate a simple and low-cost technique to fabricate microscale permanent magnetic microstructures and integrate them into microfluidic devices. In this method, microstructure channels were fabricated next to a microfluidic channel and were injected with a liquid mixture of neodymium (NdFeB) powders and polydimethylsiloxane (PDMS). After the mixture was cured, the resulted solid NdFeB–PDMS microstructure was permanently magnetized to form microscale magnets. The microscale magnets generate strong magnetic forces capable of separating magnetic particles in microfluidic channels. Systematic experiments and numerical simulations were conducted to study the geometric effects of the microscale magnets. It was found that rectangular microscale magnets generate larger \(({\mathbf {H}}\cdot \nabla ) {\mathbf {H}}\) which is proportional to magnetic force and have a wider range of influence than the semicircle or triangle magnets. For multiple connected rectangular microscale magnet, additional geometric parameters, including separation distance, height and width of the individual elements, further influence the particle separation and were characterized experimentally. With an optimal size combination, complete separation of yeast cells and magnetic microparticles of similar sizes (\(4\;\upmu \hbox {m}\)) was demonstrated with the multi-rectangular magnet microfluidic device.     

Permanent magnet configuration in design of an eddy current damper

This paper discusses the design of an eddy current passive damper using different configurations of permanent magnets. Motional eddy current damping effect is used for the development of a passive damper. Eddy currents are generated in a conductor in a time-varying magnetic field. They are induced either by movement of the conductor in a static field or by changing the strength of the magnetic field, initiating motional and transformer electromotive forces, respectively. The conceived eddy current damper consists of a conductor as an outer tube, and an array of axially magnetized, ring-shaped permanent magnets (PMs), separated by iron pole pieces as a mover. The relative movement of the magnets and the conductor causes the conductor to undergo motional eddy currents. Using this concept, damping characteristics of the new damper is obtained through analytical modeling, and verified by experimental analysis. The optimum PMs’ size and configuration are also derived using analytical and finite element analysis, respectively. A damping coefficient as high as 53 kg/s is achievable with the proposed design specifications.     

A fully integrated micromachined magnetic particle separator

A prototype micromachined magnetic particle separator that can separate magnetic particles from suspended liquid solutions has been realized on a silicon wafer. The requisite magnetic field gradients are generated by integrated inductive components in place of permanent magnets, which yields several advantages in design flexibility, compactness, electrical and optical monitoring, and integration feasibility (thus enabling mass production). Preliminary experiments have been performed on aqueous suspensions of magnetic beads. At 500 mA of dc current, approximately 0.03 Tesla of magnetic flux density is achieved at the gap between the quadrupoles, and the magnetic particles rapidly move toward the quadrupoles, separate from the buffer solution, and clump on the poles. The magnetic particles clumped on the poles are also easily released when the dc current is removed, achieving the primary purpose of a separator. The device shows that micromachined magnetic components have a high potential in biological or biomedical applications, especially in separating small amounts of cells or DNA that are marked with magnetic beads, especially when close monitoring and control of the process is important     

Superparamagnetic particle dynamics and mixing in a rotating capillary tube with a stationary magnetic field

The dynamics of superparamagnetic particles subject to competing magnetic and viscous drag forces have been examined with a uniform, stationary, external magnetic field. In this approach, competing drag and magnetic forces were created in a fluid suspension of superparamagnetic particles that was confined in a capillary tube; competing viscous drag and magnetic forces were established by rotating the tube. A critical Mason number was determined for conditions under which the rotation of the capillary prevents the formation of chains from individual particles. The statistics of chain length was investigated by image analysis while varying parameters such as the rotation speed and the viscosity of the liquid. The measurements showed that the rate of particle chain formation was decreased with increased viscosity and rotation speed; the particle dynamics could be quantified by the same dimensionless Mason number that has been demonstrated for rotating magnetic fields. The potential for enhancement of mixing in a bioassay was assessed using a fast chemical reaction that was diffusion-limited. Reducing the Mason number below the critical value, so that chains were formed in the fluid, gave rise to a modest improvement in the time to completion of the reaction.     

Dynamics of rotating paramagnetic particle chains simulated by particle dynamics,Stokesian dynamics and lattice Boltzmann methods

Paramagnetic particles, when subjected to external unidirectional rotating magnetic fields, form chains which rotate along with the magnetic field. In this paper three simulation methods, particle dynamics (PD), Stokesian dynamics (SD) and lattice Boltzmann (LB) methods, are used to study the dynamics of these rotating chains. SD simulations with two different levels of approximations—additivity of forces (AF) and additivity of velocities (AV)—for hydrodynamic interactions have been carried out. The effect of hydrodynamic interactions between paramagnetic particles under the effect of a rotating magnetic field is analyzed by comparing the LB and SD simulations, both of which include hydrodynamic interactions, with PD simulations in which hydrodynamic interactions are neglected. It was determined that for macroscopically observable properties like average chain length as a function of Mason number, reasonable agreement is found between all the three methods. For microscopic properties like the force distribution on each particle along the chain, inclusion of hydrodynamic interaction becomes important to understand the underlying physics of chain formation. Any opinion, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.     

Construction and characterisation of a modular microfluidic system: coupling magnetic capture and electrochemical detection

This work presents the fabrication and characterisation of a versatile lab-on-a-chip system that combines magnetic capture and electrochemical detection. The system comprises a silicon chip featuring a series of microband electrodes, a PDMS gasket that incorporates the microfluidic channels, and a polycarbonate base where permanent magnets are hosted; these parts are designed to fit so that wire bonding and encapsulation are avoided. This system can perform bioassays over the surface of magnetic beads and uses only 50 μL of bead suspension per assay. Following detection, captured beads are released simply by sliding a thin iron plate between the magnets and the chip. Particles are captured upstream from the detector and we demonstrate how to take further advantage of the system fluidics to determine enzyme activities or concentrations, as flow velocity can be adjusted to the rate of the reactions under study. We used magnetic particles containing β-galactosidase and monitored the enzyme activity amperometrically by the oxidation of 4-aminophenol, enzymatically produced from 4-aminophenyl-β-d-galactopyranoside. The system is able to detect the presence of enzyme down to approximately 50 ng mL⁻¹.     

A stationary apparatus of magnetic abrasive finishing using a rotating magnetic field

The precision and efficient surface finishing processes have recently become highly demanded by industries. Magnetic Abrasive Finishing (MAF) is studied as an effective method for a surface finishing. The MAF process can produce a smoothly finished surface by means of relative motion between a magnetic abrasive and the workpiece surface. To successfully finish the surface, it is critical to control the magnetic abrasive motion on the workpiece surface. The magnetic abrasive is suspended in a magnetic field generated by the electromagnets located underneath the workpiece. In this paper, a finishing system using a rotating magnetic field, with stationary electromagnets and workpiece, has been developed for the surface finishing. This system utilizes a rotating magnetic field to control the force and dynamic motion of the Magnetic Abrasive Particles (MAPs). This approach eliminates any mechanical motion of the electromagnets and the workpiece, to make the system more controllable, reliable, and cost-effective. The mechanism and characteristics of this system are summarized in this paper. The finite element method with COMSOL Multiphysics^® Simulation software was used to analyze the magnetic flux distribution, magnetic flux density, and magnetic forces in the working area.

     

Design of a yield stress characterizing platform for magnetorheological fluid magnetized by permanent magnets

In this paper we proposed a platform for measuring shear force of magnetorheological (MR) fluid by which the relationship of yield stress and magnetic flux density of specific materials can be determined. The device consisted of a rotatable center tube placed in a frame body and the magnetic field was provided by two blocks of permanent magnets placed oppositely outside the frame body. The magnitude and direction of the magnetic flux were manipulated by changing the distance of the two permanent magnets from the frame body and rotating the center tube, respectively. For determining the magnetic field of the device, we adopted an effective method by fitting the finite element method result to the measured one and then rebuilt the absent components to approximate the magnetic field, which was hardly to be measured as different device setup were required. With the proposed platform and analytical methods, the drawing shear force and the corresponding yield stress contributed by MR fluid in respect to the magnitude and direction of given magnetic flux density could be evaluated effectively for specific designing purposes without the requirement of a large, complex, and expensive instrument.

     

Resin-bonded NdFeB micromagnets for integration into electromagnetic vibration energy harvesters

A micromachining technique is presented for the fabrication of resin-bonded permanent magnets in the microscale. Magnetic paste is prepared from NdFeB powder and an epoxy resin, filled into lithographically defined photoresist molds or metal molds, and formed into resin-bonded magnets after curing at room temperature. A coercivity of 772.4 kA/m, a remanence of 0.27 T, and a maximum energy product of 22.6 kJ/m³ have been achieved in an NdFeB disk micromagnet with dimensions of Φ200 μm×70 μm. Based on the developed micro-patterning of resin-bonded magnets, a fully integrated electromagnetic vibration energy harvester has been designed and fabricated. The dimensions of the energy harvester are only 4.5 mm×4.5 mm×1.0 mm, and those of the micromagnet are 1.5 mm×1.5 mm×0.2 mm. This microfabrication technique can be used for producing permanent magnets tens or hundreds of micrometers in size for use in various magnetic devices.     

用于微型阀的磁性双稳态系统作用力研究

为了降低微型阀功耗,提出了一种新型磁性双稳态系统。该系统由两块钕铁硼材料环形永磁体和一片软磁片组成,其中软磁片选用钢和坡莫合金作为考查材料。采用有限元仿真的方法对软磁片受到的磁场力进行分析,结果显示:当软磁片半径较永磁体半径偏大10%左右时能够获得磁场力极值,同时通过调整两块永磁体之间的距离,软磁片所受到的磁场力能够比单永磁体作用下的受力大30%以上。对软磁片受力进行了测试并与有限元分析结果进行了比较,吻合较好,实验表明:该磁性双稳态系统能够很好地应用在微型阀当中。     

体内微型机器人的全方位旋进驱动特性

提出了一种由外旋转磁场驱动的体内微机器人．它以相邻径向异向磁化瓦状多磁极圆筒形NdFeB永磁体为外驱动器，以机器人内嵌同结构的NdFeB永磁体为内驱动器，外驱动器旋转时产生旋转磁场，通过磁机耦合作用于内嵌驱动器形成机器人驱动力矩，在本体外表面螺纹与流体动压力的作用下，实现机器人在管道内的在线旋进．在建立微机器人游动模型的基础上，以垂直管道为试验环境，研究了机器人的全方位驱动特性，试验结果表明机器人可以实现管道内全方位驱动．     

Control aspects of a tracked magnetic levitation high speed test vehicle

The paper deals with the control system of the high speed test vehicle KOMET. The control hardware configuration consisting of digital computer, interface, sensors, magnet drivers and magnets is described. Control system synthesis is performed based on the state space approach and the classical approach of the z-transform. It leads to various control concepts, which are evaluated with regard to their responses to guideway irregularities, external forces and their sensitivity to plant parameter variations. The measurements used to reconstruct the state vector are magnet gaps, vehicle accelerations and/or magnet currents. To be efficient, the magnets have to be operated on small magnet gaps. This demands a fast system response to external disturbances and guideway inputs. This in turn may lead to the excitation of the elastic modes of the system. The higher order modes of vehicle and guideway are therefore included in the control synthesis. The speed range of the KOMET extends to 400 km/hr. Results from high speed testing are evaluated with regard to system responses, power requirements, and loss in magnetic force due to eddycurrents.     

Numerical simulation and optimization of planar electromagnetic actuators

The design rules and characterization of planar electromagnetic actuators are presented focusing on numerical force simulation and maximization of efficiency. Theoretical simulations are carried out for circular concentric, circular eccentric and rectangular concentric placements of the magnet with respect to the coil yielding the highest forces for the circular concentric arrangement. As a result larger magnets or larger coils are not linked to higher forces in any case, thus an optimum magnet diameter for a given coil radius can be calculated and vice versa. With respect to the vertical distance of the coil to the magnet the optimum parameters are given. Dimensions and forces are derived in generalized units to facilitate scaling to other geometries. The simulations are excellently confirmed by experimental data of different coils and magnets.     

On-chip manipulation and trapping of microorganisms using a patterned magnetic pathway

We demonstrate on-chip manipulation and trapping of individual microorganisms at designated positions on a silicon surface within a microfluidic channel. Superparamagnetic beads acted as microorganism carriers. Cyanobacterium Synechocystis sp. PCC 6803 microorganisms were immobilized on amine-functionalized magnetic beads (Dynabead^® M-270 Amine) by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)–N-hydroxysulfosuccinimide coupling chemistry. The magnetic pathway was patterned lithographically such that half-disk Ni₈₀Fe₂₀ (permalloy) 5 μm elements were arranged sequentially for a length of 400 micrometers. An external rotating magnetic field of 10 mT was used to drive a translational force (maximum 70 pN) on the magnetic bead carriers proportional to the product of the field strength and its gradient along the patterned edge. Individual microorganisms immobilized on the magnetic beads (transporting objects) were directionally manipulated using a magnetic rail track, which was able to manipulate particles as a result of asymmetric forces from the curved and flat edges of the pattern on the disk. Transporting objects were then successfully trapped in a magnetic trapping station pathway. The transporting object moves two half-disk lengths in one field rotation, resulting in movement at ~24 μm s^?1 for 1 Hz rotational frequency with 5 μm pattern elements spaced with a 1 μm gap between elements.