Stepwise magnetization control strategy for DC-magnetized memory machine

Memory machine (MM) equipped with hybrid permanent magnets (PMs), i.e., NdFeB and low coercive force (LCF) PMs, exhibits acceptable torque capability at low speeds and high efficiency at high speeds. Previous literatures have addressed that the constant power speed range (CPSR) of MMs can be further extended by online PM flux control, and the requirement of flux-weakening (FW) inverter current can be reduced as well. Nevertheless, how to coordinate the d-axis vector FW and PM magnetization control over a whole operating range in a reasonable manner remains unreported. Therefore, this paper proposes and implements a stepwise magnetization control strategy on a DC-magnetized memory machine based on the operating characteristics under various PM magnetization states and speed ranges. The configuration, principle and mathematical model of the investigated machine are introduced first. Then, the proposed control strategy is established by dividing the operating envelop into several FW regions, and an appropriate FW control scheme is utilized at each stage. It demonstrates that the CPSR can be effectively extended by simply applying the demagnetizing current pulses in several steps. This simplifies the control efforts considerably without resorting to continuous PM flux control and frequent actions of the switching devices. The effectiveness of the proposed control strategy is verified by experimental results

Modeling, Analysis and Control of a Variable Flux Machine

Electric motors are the key elements in electric propulsion systems. The performance of Electric vehicles (EVs) significantly depends on the electric motors. Permanent magnet synchronous machines (PMSMs) with rare-earth magnets are widely used in EV applications because they fulfill most requirements of EV motors. However, low efficiency at high speed, limited resources and fluctuating prices of rare-earth permanent magnets (PMs) have forced industries to develop alternatives to rare-earth machine technologies. Recently, Variable-Flux PMSMs (VF-PMSMs) also known as memory motors have been introduced to overcome the drawbacks of PMSMs. This thesis focuses on the modeling, analysis and control of the Aluminum-Nickel-Cobalt (AlNiCo) magnet-based VF-PMSMs. This thesis presents the effect of different magnetization pulse widths and methods on the magnetization level, back-EMF and no-load losses of the VF-PMSM. The injection of the magnetization or de-magnetization current pulse will change the magnet flux linkage and back-EMF harmonics. An adaptive nonlinear filter is used to estimate the back-EMF during the motoring mode. The harmonics present in the machine back-EMF due to different magnetization and de-magnetization current pulse widths and magnetization methods are analyzed. Besides, the quality of the back-EMF for different speeds and machine no-load losses are presented for different magnetization states (MSs)

Novel Lorentz Force Velocimetry system based on bulk high-temperature superconductors

Die Lorentzkraftanometrie (LKA) ist eine Technik zur Messung der Geschwindigkeit von elektrisch leitfähigen Flüssigkeiten. Sie ist eine nicht-invasive Messtechnik, die besonders vorteilhaft für heiße, opake und aggressive Elektrolyte ist. Die LKA wurde für Salzwasser als Modellelektrolyt erfolgreich mit Dauermagnetanordnungen (DM) ohne magnetischen Rückschluss, aber mit gezielter Flussführung (Halbach-Array) und hochpräzisen Kraftmesssytemen (KMS) auf Basis der interferometrischen Messung der Auslenkung des Magnetsystems und der elektromagnetischen Kompensation der Auslenkung demonstriert. Um die LKA für schwach leitfähige Elektrolyte zu erweitern, ist eine Magnetfelderzeugung von > 1 T erforderlich. Ein Hochtemperatursupraleiter-Bulk (Bulk-HTS) kann ein Magnetfeld von mehreren Tesla erzeugen und somit die LKA-Leistung deutlich verbessern und die bisher genutzten DM ersetzen. Diese Arbeit zielt deshalb darauf ab, Bulk-HTS's in der LKA unter Berücksichtigung der kritischen Verbindungen zwischen der Funktionalität von Bulk-HTS's und dem KMS einzusetzen und ein LKA-System mit Bulk-HTS's zu entwerfen, herzustellen und zu testen. Die Ergebnisse wurden für die Entwicklung eines neuartigen LKA-System auf Basis eines Bulk-HTS als Magnetfeldquelle und einer Torsionswaage als Kraftmesssystem genutzt. Dieses System - Superconducting High-precision Lorentz Force Measurement System (Super-LOFOS) - wurde dann erfolgreich aufgebaut und getestet. Bei Kühlung mit flüssigem Stickstoff bzw. Helium werden auf der Stirnfläche des Super-LOFOS magnetische Flußdichten von B_T = 100 mT bzw. B_T = 1,2 T erzeugt. Damit erweitert die vorliegende Arbeit die Einsetzbarkeit der LKA für gering elektrisch leitende und langsam strömende Fluide auf (σ · u) = 1-10) S s⁻¹, sowie macht hochpräzise Kraftmessungen bis 1 nN unter kryogenen Bedingungen möglich. Darüber hinaus stellt das entwickelte Messsystem Super-LOFOS einen tragbaren Magnetfeldgenerator dar, der für NMR- und MRT-Technologien, Drug Targeting, und magnetische Trennungsverfahren einsetzbar ist.Lorentz Force Velocimentry (LFV) is a technique to measure the velocity of electrically conducting fluids. The advantage of LFV is that this non-invasive measurement method is particularly well suited for use on hot and opaque liquids as well as aggressive electrolytes. LFV for saline water - the model electrolyte - was successfully demonstrated using permanent magnet (PM) configurations without an iron yoke magnetic flux guide, but with targeted magnetic flux guidance (Halbach-array) and a high-precision force measurement system (FMS) based on interferometric measurements of magnet system (MS) deflection and electromagnetic force compensation balance. To extend the LFV for weakly conductive electrolytes, it is required to generate a magnetic field over 1 T. A bulk high-temperature superconsuctors (HTSs) can generate the magnetic field of several teslas and therefore can significantly enhance LFV performance, replacing previously used PM. This thesis aims to integrate the bulk HTS into the LFV considering the critical links between the functionality of bulk HTS and the FMS with the end goal to design, manufacture, and test a novel LFV system using bulk HTS. The obtained results were used to develop a novel LFV system using bulk HTS as magnetic field generator and a torsion balance as FMS. This LFV system - Superconducting High-precision Lorentz Force Measurement System (Super-LOFOS) - was then successfully designed, manufactured, and tested. Using nitrogen and helium cryogenic liquids, the magnetic flux density of B_T = 100 mT and B_T = 1.2 T were generated at the Super-LOFOS front surface, respectively. This thesis also extends the LFV applicability for weakly-conducting and slow-flowing electrolytes (σ · u) = (1-10) S s⁻¹ as well as enabling high-precision force measurements up to 1 nN under cryogenic temperatures. Furthermore, the developed Super-LOFOS provides a portable magnetic field generator, which can be used for NMR and MRI technologies, magnetic separation, and drug targeting applications

Towards nanoscale magnetic memory elements : fabrication and properties of sub - 100 nm magnetic tunnel junctions

The rapidly growing field of spintronics has recently attracted much attention. Spintronics is electronics in which the spin degree of freedom has been added to conventional chargebased electronic devices. A magnetic tunnel junction (MTJ) is an example of a spintronic device. MTJs consist of two ferromagnetic layers separated by a thin insulating barrier. The tunnel current that flows through the barrier depends on the relative alignment of the magnetization in the ferromagnetic layers. As a consequence of this dependence, the MTJ exhibits two different resistance values that distinguish a logical ‘0’ and a logical ‘1’, corresponding to anti-parallel and parallel magnetization. Due to these two distinct states, the MTJs can be used as magnetic memory elements, and serve as bits for information storage in magnetic random access memories (MRAMs). MRAMs can lead to instant-on computers and longer battery lifetimes for mobile devices, which gives MRAMs the potential to replace the current RAM technologies. However, for the current RAM technologies to be replaced by MRAMs, the dimensions of the MTJs have to decrease to sub - 100 nm in order to achieve a high enough areal density and to match the semiconductor technology. Therefore, the research in this Thesis aims at fabricating sub - 100 nm MTJs and investigating the influence of the reduced dimensions on the modification of magnetic and electronic properties. The MTJ is incorporated in an engineered multilayer stack to promote stability and reproducibility of the magnetic and electric response of the MTJ. For the structuring of these layers, we have used top-down nanofabrication techniques to produce the sub - 100 nm MTJs. For MTJs with a surface area of less than 0.01 µm2, the Al2O3 barrier has an approximate thickness of a nanometer to ensure an appropriate tunnel current. Therefore, we have concentrated on the plasm oxidation of sub -nm thin Al layers to produce Al2O3 barriers. We have shown that over-oxidation of sub -nm thin Al2O3 barriers of MTJs can be observed in real-time using in situ differential ellipsometry measurements. The change in ellipsometry signal of Al layers grown on CoFe films, is proportional to the amount of oxidized metallic material. As a result, the derivative of this signal is a direct measure of the oxidation rate. Further analysis of this oxidation rate allowed us to determine the onset of the CoFe oxidation. We have found the onset to be proportional to the deposited Al layer thickness. The amount of CoO determined from in situ X-ray Photoelectron Spectroscopy data on identical samples is found to be proportional to that obtained from ellipsometry. In short, this means that the point in time on which over-oxidation starts can be precisely determined. This is a critical necessity in producing exact, well-functioning MTJs. We have used electron beam (EB) lithography to pattern the sub - 100 nm features. With EB lithography features of only a few nanometer in size can be defined, hence EB lithography enables the exploration of the fundamental boundaries of magnetic and electric scaling properties. For the structuring of large-area samples with ultra-dense arrays of sub - 100 nm MTJs the throughput is limited because of the sequential writing process. However, with the developed special high-speed EB writing strategy we could pattern a sample area of 16 mm2 with ultra-dense arrays of sub - 100 nm elliptical features in 10 minutes. The strategy is employed to define the pattern of hard Ta masks with sub - 100 nm features. The Ta masks are etched at -50 ¿C in a SF6/O2 plasma to an etch depth that can be controlled with nanometer precision. Ar+ ion beam milling is used to transfer the pattern and to produce dense arrays of sub - µm MTJs. Insight in the magnetic switching behavior of nanoscale MTJs as a function of the size, shape and thickness is vital for MRAM application. Especially, the collective properties of high areal density arrays of MTJs are of interest, because magnetostatic coupling mechanisms between elements can be a limiting factor for applications. In order to understand the effects of geometry and coupling mechanisms, the switching of 5 nm thick polycrystalline nanoscale Co dots is examined using SQUID measurements and the switching of sub - µm MTJs is studied with MOKE measurements. An array consists of approximately 108 elements with a width ranging from 50 to 300 nm, and a length to width aspect ratios of 1.5 to 2.5, arranged on a rectangular lattice. The measured switching fields of the Co dots and MTJs were low compared to predictions using the Stoner-Wohlfarth model. The deviations of the Stoner-Wohlfarth behavior could be explained in term of interdot coupling and edge roughness. Comparison with the outcome of OOMMF simulation of the switching of a single dot revealed that the interdot coupling has a major influence on the magnetic switching behavior of arrays of nanoscale magnetic elements. This implies that for the feasibility of ultra-high areal density arrays of nanoscale MTJs for information storage new strategies are needed. For example, a more complex toggle MTJ multilayer stacks can be used for the nanostructuring of sub - µm MTJs. Faceting of the etch mask due to physical sputtering of the mask material is a problem during deep etching of ultra-high bit density arrays of sub - µm MTJs for MRAMs. Besides that, chlorinated etch residues can reduce the magnetization of patterning magnetic materials substantially, and therefore constitutes a considerable concern. To get more insight into the magnetization losses, CoFeB dots were etched in a high ion density Cl2- based plasma with a width ranging from 0.3 to 6.4 µm. The magnetic properties of the CoFeB dots were measured by SQUID magnetometry. The sub-µm CoFeB dots showed significant magnetization reductions, despite H2O rinsing. Scanning electron microscopy (SEM) studies revealed that etching in a Cl2-based plasma caused faceting of the masks, leading to sloped sidewalls. SEM pictures were used to determine the geometric volume which was compared to the effective magnetic volume resulting from the magnetometry measurements. The SEM data are in good agreement with the magnetometry data, and a chloride penetration depth of only a few nanometer could be derived, indicating that the postetch rinsing is sufficient to prevent considerable corrosion of the CoFeB dots. This means that the chlorinated etch residues could be removed from the samples, without severely effecting the magnetic properties. The I-V characteristics of nanometer thin AlOx barriers are measured by applying a voltage over a 1×1 µm2 square MTJ pillar located at the cross-point of the bottom and top electrode. The I-V response of the MTJs showed in principle three different characteristics, that is an ohmic like response, a response resembling breakdown, and a tunneling response. Approximately 30 % of the measured 32 MTJs showed a tunneling response and had a resistance-area product between approximately 20 and 50 k µm2. Furthermore, the conductance showed roughly a parabolic behavior implying an asymmetrical barrier. As a consequence, we used the Brinkman formula to fit the experimental data. The fits yielded average barrier heights between 1.2 and 2.3 eV, and an asymmetry parameter ranging from 0.3 to 0.8 eV, which are close to reported observations in literature. Resistance measurements yielded no significant magnetoresistance for the 1 µm2 MTJs. Probably, as a consequence of interface roughnesses, a strong N´eel coupling originates, through which an independent switching of the two magnetic layers of the MTJ is hindered. As an alternative electrical characterization technique, the conductive atomic force microscopy (c-AFM) technique can used to measure the local electrical transport properties of nanoscale MTJs. We have explored this c-AFM technique and performed I-V measurements by applying a constant bias voltage to the bottom electrode and measuring the tunnel current through the barrier. However, due to resist remains no significant bias voltage dependency was observed

A novel linear motor for a linear refrigeration compressor: modelling, measurement and sensor-less stroke detection

With the increasing global awareness of the environmental conservation, linear compressors have attracted growing attention with their applications in domestic and cryogenic refrigeration systems. A linear compressor is driven directly by a linear motor and the free-piston design allows piston stroke to be variable. An active control of stroke prevents piston-cylinder collision and enables efficient cooling capacity modulation. This thesis introduces the performance of a novel moving magnet type linear compressor/motor and investigates the approaches to sensor-less stroke detection. An experimental test facility incorporating the linear compressors into a vapour compression refrigeration system was introduced, in which piston displacement was measured with a displacement sensor. The piston stroke and offset were controlled with PID controllers implemented in LabVIEW. To investigate the characteristics of the moving magnet linear motor, a finite element analysis (FEA) model was built in ANSYS Maxwell 19.2. Simulations were validated through static force measurements. Force constant was given by the static shaft force against current. Saturation can be observed with the increase of current. A smaller saturation current was shown for a larger armature displacement. For the purpose of increasing cooling capacity of the linear compressor, operations with small axial clearance volumes were considered. Refrigeration performance using R1234yf as refrigerant with various clearance volumes and with an offset of 0 mm were experimentally compared. The cooling capacity for a pressure ratio of 2.5 and a stroke of 13 mm increases by 12% as the clearance decreases from 1.07 mm to 0.4 mm. Piston stroke detection without a displacement sensor reduces the cost and facilitates the stroke control especially in miniature linear compressors. An artificial neural network (ANN) based stroke detection was presented. Fast Fourier transform (FFT) analysis was performed on current and voltage signals to extract harmonic terms as inputs of the neural network model to predict the stroke. The ANN technique can achieve a good accuracy for most of the cases, but reliability remains a problem. A more reliable sensor-less stroke detection technique based on flux linkage variation using inductive coils was proposed. The technique requires resonant operation. A 1D (One-Dimensional) electromagnetic model and a 3D (Three-Dimensional) FEA model were built to compute the flux linkage variations. The open-circuit flux linkage in each core produced by NdFebB magnets varies linearly with the piston displacement. Flux linkage difference at two zero-crossing points of current was used to infer stroke. The proposed low-cost sensor-less stroke detection technique can achieve error of only 4%. The adoption of this novel technique is crucial to the commercialization of free-piston machines for high efficiency

非線形弾性要素による内部力補償に基づく無段階変位–力変換機構の創生 ― 微小操作力で強大な磁気力・把持力・制動力・張力を制御可能とするロボット要素 ―

Tohoku University博士（情報科学）thesi

Neural Network Based Torque Control of Switched Reluctance Motor for Hybrid Vehicle Propulsion

Considering the extensive non-linearities in the switched reluctance motor (SRM) drive, variation in the DC bus voltage and specific requirements of the hybrid electric vehicles (HEVs) traction application, a feed-forward back propagation neural network (BPNN) based torque controller is proposed. By using proposed controller, the torque ripple has been effectively reduced at low speeds while the power efficiency has been optimized at high speeds range. The problem of multi-valuedness related with the neural network based direct inverse control has been targeted by designing a bank of two-hidden-layer neural network controllers. And the problem of torque oscillation due to the change of control mode and step change of firing angle has been solved by using dead-band filtering and nearly continuous changing of firing angle and phase currents. Computed results are presented to demonstrate the effectiveness of the proposed control scheme

The 2016 oxide electronic materials and oxide interfaces roadmap

Lorenz, M. et al.Oxide electronic materials provide a plethora of possible applications and offer ample opportunity for scientists to probe into some of the exciting and intriguing phenomena exhibited by oxide systems and oxide interfaces. In addition to the already diverse spectrum of properties, the nanoscale form of oxides provides a new dimension of hitherto unknown phenomena due to the increased surface-to-volume ratio. Oxide electronic materials are becoming increasingly important in a wide range of applications including transparent electronics, optoelectronics, magnetoelectronics, photonics, spintronics, thermoelectrics, piezoelectrics, power harvesting, hydrogen storage and environmental waste management. Synthesis and fabrication of these materials, as well as processing into particular device structures to suit a specific application is still a challenge. Further, characterization of these materials to understand the tunability of their properties and the novel properties that evolve due to their nanostructured nature is another facet of the challenge. The research related to the oxide electronic field is at an impressionable stage, and this has motivated us to contribute with a roadmap on ‘oxide electronic materials and oxide interfaces’. This roadmap envisages the potential applications of oxide materials in cutting edge technologies and focuses on the necessary advances required to implement these materials, including both conventional and novel techniques for the synthesis, characterization, processing and fabrication of nanostructured oxides and oxide-based devices. The contents of this roadmap will highlight the functional and correlated properties of oxides in bulk, nano, thin film, multilayer and heterostructure forms, as well as the theoretical considerations behind both present and future applications in many technologically important areas as pointed out by Venkatesan. The contributions in this roadmap span several thematic groups which are represented by the following authors: novel field effect transistors and bipolar devices by Fortunato, Grundmann, Boschker, Rao, and Rogers; energy conversion and saving by Zaban, Weidenkaff, and Murakami; new opportunities of photonics by Fompeyrine, and Zuniga-Perez; multiferroic materials including novel phenomena by Ramesh, Spaldin, Mertig, Lorenz, Srinivasan, and Prellier; and concepts for topological oxide electronics by Kawasaki, Pentcheva, and Gegenwart. Finally, Miletto Granozio presents the European action ‘towards oxide-based electronics’ which develops an oxide electronics roadmap with emphasis on future nonvolatile memories and the required technologies. In summary, we do hope that this oxide roadmap appears as an interesting up-to-date snapshot on one of the most exciting and active areas of solid state physics, materials science, and chemistry, which even after many years of very successful development shows in short intervals novel insights and achievements.This work has been partially supported by the TO-BE COST action MP1308. J F acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (SEV-2015-0496) and MAT2014-56063-C2-1R, and from the Catalan Government (2014 SGR 734). F.M.G. acknowledges support from MIUR through the PRIN 2010 Project ‘OXIDE’.Peer reviewe