Coil Current Research Articles

Design and Unbiased Control of Nine-Pole Radial Magnetic Bearing

Typical radial active magnetic bearings are structurally symmetric. For example, an eight-pole bearing uses two opposing pairs to control one axis by winding the pair in series. The magnetic force produced by an active magnetic bearing is quadratically proportional to coil currents and inversely proportional to the square of the gap between the bearing and the journal. Bias linearization is widely used to linearize the relationship of coil currents to the magnetic force. However, the bias currents increase ohmic losses and require a larger than necessary capacity of power amplifiers to supply the sum of bias and control currents. Unbiased control of symmetric bearings has the critical issue of slew-rate limiting. Unbiased control of unsymmetrical bearings can eliminate the need for bias currents while avoiding slew-rate singularity except in extreme cases. Although a generalized inversion of the force–current relationship of unbiased unsymmetrical bearings has been proposed previously, no experimental validation is reported. The main objective of this research is to implement the unbiased control strategy and show that exactly the same linear control strategy for eight-pole symmetric bearings can be applied to nine-pole unsymmetrical bearings on industry-scale compressor test rigs. Two test rigs are built: one with eight-pole symmetric bearings and another with nine-pole unsymmetrical bearings. Linear control algorithms are designed and applied. Both control algorithms are linear and consist of lead filters and notch filters. The test results show that the linear control design used for unsymmetrical bearings can achieve the same level of stability that the symmetric bearings provide, satisfying the sensitivity criterion specified by ISO 14839-3.

Exploration of ITER operational space with as-built stiffness of central solenoid modules

The as-built stiffness in the ITER central solenoid (CS) modules (CSM1 thorough to CSM4 are currently manufactured) determines the range of vertical compression forces that can be tolerated by the CS modules during ITER operation. Since the as-built stiffness of the CS modules manufactured (∼32 GPa and ∼34 GPa for CSM1 and CSM2, respectively and similar for the other modules) has been reduced from the design value (53 GPa), the CS axial (vertical) force criteria have been updated assuming a conservative stiffness (25 GPa) with margins for all six CS modules. Initial analysis using the updated CS force criteria has revealed that this reduction affects only the plasma initiation with fully charged CS in the ITER 15 MA Baseline DT scenario, resulting in a slight reduction of poloidal magnetic flux, from 117.5 Wb to 116.2 Wb at initial CS magnetization. Therefore, the 15 MA Baseline scenario has been re-developed with an updated plasma start-up, and then the entire evolution of the CS and poloidal field coil parameters has been validated against all the coil currents, fields and forces criteria. To explore potential risks and opportunities for further optimization of scenarios, the equilibrium operational space (the plasma internal inductance versus the poloidal magnetic flux produced by the coils) at flat-top burn has been analyzed using the CORSICA and DINA codes. The three major ITER reference DT operation scenarios, 15 MA Q = 10 Baseline, 12.5 MA Q > 5 Hybrid and 10 MA Q ∼ 5 Steady-State, satisfy all the coil criteria including the CS force updated reflecting the as-built stiffness. The evolution of the plasma discharge parameters within the equilibrium operational spaces provided a guidance for potential optimization with margins.

Magnetic field simulations of the GOLEM tokamak via the NICE code

In this paper, the direct and inverse equilibrium computation mode of the NICE code is put into operation for the case of the GOLEM tokamak. This enables, for the first time, simulations of GOLEM's magnetic field. The computation mode put into operation simulates the magnetohydrodynamic equilibrium of a tokamak plasma based on a given plasma position or given currents in poloidal magnetic field coils. In order to set up NICE, a virtual model of GOLEM is introduced that includes axially symmetrical approximations of GOLEM's iron core and primary transformer coils. The results of NICE simulations are compared with experimental measurements, and it is shown that NICE simulations produce plasma equilibrium configurations typical for GOLEM. Nevertheless, a more detailed validation of GOLEM's virtual model and of the results of NICE simulations is to be performed in the future.

Machine Learning Based Foreign Object Detection in Wireless Power Transfer Systems

As power electronics continue to drive advancement in electronic devices by managing the system's energy flow, it is advantageous to examine all areas of energy transfer. Wireless power transfer is one section seeing increased adoption in consumer, industrial, and healthcare applications. This consequently increases the concern for safety, reliability, and efficiency. Power transmission to any unintended load is a primary safety issue in high-power wireless power transfer applications, such as unmanned aircraft systems. Foreign object detection is utilized to selectively transmit power to only the intended target to mitigate this issue. This paper seeks to demonstrate a novel alternative method of using machine learning and artificial intelligence techniques for performing foreign object detection solely on the transmitter portion of the system by only monitoring the TX coil current. This method is first evaluated in SPICE simulation software. Then by transitioning to a closed-loop hardware-based testbed designed to be deployed in unmanned aircraft systems. During the test process for the novel FOD method, the aggregate detection accuracy achieved was 83%, including worst-case misalignment conditions with the RX coil. When the outlying worst-case conditions are excluded, the detection accuracy improves to 97%. To measure the FOD effectiveness in preventing temperature rise in a metallic object, the metallic object was placed at the peak power draw location on the transmitter coil. When the metallic object is left at this location for extended periods, the novel FOD method lowers the maximum temperature reached by 86.8°C when compared to no FOD being active. The primary advantage to this novel method is the simplistic hardware additions, which only require minimal alterations to a WPTS without any FOD implemented. Along with this, implementing this foreign object detection method in the transmitter would mitigate the need for a direct communication link between the receiver and transmitter. As a result, the entire system complexity would decrease, thus increasing the UAS power density while maintaining a safe, effective, and reliable wireless power transmission method.

Numerical investigation of toroidal plasma response for ELM control via magnetic perturbations in the DTT Tokamak

Linear plasma response modeling is exploited in this work to assess the effect of different coil configurations on edge localized mode (ELM) stability in full power operational scenarios for the Divertor Tokamak Test (DTT) facility, presently under construction at the ENEA site of Frascati (Italy). The MARS-F code is used to compute, in toroidal geometry and including flow, the resistive plasma response to different vacuum fields with toroidal mode numbers n=1,2,3 . Peeling-like response in particular, correlated with ELM control, is found to be significant for n=2,3 perturbations while n=1 induces a large core response in the investigated scenarios. Two metrics are used to link plasma response to ELM control. Namely the local normal plasma displacement in the x-point region and the Chirikov parameter in PEST-like straight-field-line coordinates. These criteria are used to predict optimal phasing of the active coil arrays and current thresholds based on empirical evidence. Depending on the number of active coils and on the scenario, coil currents between 20 and 40 kAt are predicted to be effective for ELM mitigation in DTT.

Surface corrosion evaluation of steel cable for suspension bridge using electromagnetic method

The cables of suspension bridges are normally made of galvanized steel wires. Corrosion may happen with the galvanized steel wires from the surface after serving for a long time. We developed a compact, low-cost electromagnetic system to evaluate the corrosion of steel cables. AC magnetic field was produced by an excitation coil and eddy current was induced in the steel wires. A detection coil was used to measure the magnetic field produced by the eddy current. In our experiments, the excitation coil was 100 turns with a diameter of 3 cm, the detection coil was 60 turns with a diameter of 1 cm and the excitation frequency was about 80 kHz. A lock-in amplifier was used to get the X signal (same phase signal with the excitation field) and Y signal (90-degree phase different signal with excitation field). From the slope of the X–Y graph, the degree of corrosion can be judged. To make the system portable, the excitation coil, detection coil, amplifier, lock-in amplifier, and AD converter were put in a small box with the size of 85 mm × 170 mm × 60 mm; only a USB cable was used for the power and data transferring. The electric power required for the system was less than 1 W. We prepared some galvanized steel wires with and without corrosion and did some experiments in the laboratory. The degree of corrosion of galvanized steel wires was successfully evaluated using the system.

Enhancing plasma uniformity by employing non-uniform magnetic field modulation in capacitively coupled plasmas

A two-dimensional implicit electrostatic particle-in-cell model is applied for the simulation of non-uniformly magnetized capacitively coupled plasmas. In the absence of a magnetic field, the plasma density is concentrated at the center of the reactor, under the studied parameters. This leads to the formation of a radial density gradient and subsequently generates a radial electric field that causes the deflection of incident ions toward the bottom electrode. Minimizing ion deflection is imperative in order to prevent detrimental effects on plasma processing. In pursuit of this objective, a static magnetic field is applied to the capacitively coupled plasmas by installing direct current coils on the upper section of the reactor. This arrangement has been observed to considerably alter the radial distributions of plasma density and the incident angle of ion bombardment on the bottom electrode.

Casting Machines in Dentistry - A Review Part I.

Dental casting machine is an electrical device used to extrude molten materials to fabricate dental prostheses such as crowns, bridges, intracoronal and extracoronal restorations, and removable partial dentures. The casting process basically include melting and casting. Firstly, the solid material is heated in a crucible in temperature-controlled conditions to melt the material to its smelled form. The dental casting process is a complex one with multifaceted steps and equipment. Different types of casting machines are available to produce heat using different sources and techniques. It includes Arc melting, Open flame casting, and Electrical resistance. Arc melting involves the application of an electric or gas discharge on tungsten electrodes, causing the metal base to melt utilizing the heat produced by arcing. Open flame casting or induction melting employs water-cooled alternate current induction coils to induce heat. However, resistance heating uses electric current to melt precious metals.

A hybrid 2D-FDTD/3D-MoM method used for the analysis of MRI RF coils

The design of radiofrequency (RF) coils is crucial for ultra-high field (UHF) magnetic resonance imaging (MRI) systems. To analyze RF coils, various numerical methods, such as finite-difference time-domain (FDTD) and method of moments (MoM), are usually adopted. In this paper, we present a novel hybrid approach that combines a two-dimensional (2D) FDTD with a three-dimensional (3D) MoM to analyze MRI RF problems. In our algorithm, the MoM is utilized for calculating the coil current, and FDTD is assigned for solving the electromagnetic (EM) fields in the imaging region. The hybrid method achieves superior efficiency and acceptable accuracy than using either method individually. To validate the hybrid method, we analyze an ellipse coil loaded with a uniform phantom and a realistic human head model, with the objective of tailoring the magnetic field intensity by adding a multilayer dielectric pad (DP). The results show an improvement in the magnetic field after optimizing the DP configuration. These simulation studies indicate the potential of the new numerical method for the design and analysis of RF systems for ultra-high field applications.

Predictor-based model predictive control for maglev planar motor with a 2D Halbach array mover

Magnetic levitated planar motors are widely employed in flexible manufacturing and photolithography due to their characteristics of high precision, high acceleration, and low maintenance. In this paper, a novel predictor-based model predictive control is proposed to improve the tracking performance of a maglev planar motor which consists of a 2D Halbach array mover and stationary square coils. A Kalman filter and finite spectrum assignment based on an augmented system model are employed to forecast mover’s location by compensating for the system delay due to data transmission and processing. The predicted location is fed back to model predictive control for control optimization under the limit of coil currents. Hildreth’s quadratic algorithm is then applied to solve for the optimal control effort within the constraints. Comparative experiments demonstrate the effectiveness of the proposed control strategy, the total tracking error of which is much smaller than the result of a proportional–integral–derivative controller and standard model predictive control in all tests. Additionally, experimental results show that the proposed predictor-based model predictive control can tolerate higher speeds of the mover while retaining mover’s steadiness. The experimental results suggest the proposed predictor-based model predictive control is an effective path-tracking strategy for magnetic levitated planar motors subject to delays.

Discrete Modeling of Passive Rectifiers in LCL Tuned Inductive Power Transfer Receivers

This paper presents a new method of analyzing the performance of the secondary side of an LCL tuned secondary in an Inductive Power Transfer (IPT) system using a passive rectifier. Traditional methods of analyzing such systems simplify the dc load into an equivalent ac load. This approximation skews the phase shifts between the primary and secondary currents and introduces errors in the design stage of the system. The proposed method uses a look up table (LUT) to characterize the performance of the LCL tuned secondary network. The results are presented and validated at three different power levels (48 kW, 9 kW, and 4 kW). The LUT closely agrees to the measured output powers, phase angles, and coil currents. A sensitivity analysis also shows that it can reduce errors in the simulation by taking practical component variations into account. As such, the proposed LUT characterization method is a valuable tool to quickly determine how an IPT system's design impacts the secondary side's performance in an LCL tuned network with a passive rectifier for IPT systems.

An Output Power Fluctuation Suppression Method of DWPT Systems Based on Dual-Receiver Coils and Voltage Doubler Rectifier

In Dynamic Wireless Power Transfer (DWPT) systems, the power oscillation phenomena caused by coupling fluctuation are always hard to avoid. Moreover, if the adjacent track coils are excited by different inverters without synchronized drive signals, there will be severe power oscillations due to the frequency difference between the track coil currents. To address these issues, this work proposes a novel DWPT system based on dual-receiver coils and the Voltage Doubler (VD) rectifier. Depending on whether the induced voltages of the two receiver coils are balanced, the rectifier will switch between full-bridge mode and VD mode with no extra controller to maintain the system output under different positions. By designing the dimensions of the coupler, the DWPT system can achieve excellent output stability while avoiding the oscillation caused by the frequency difference between the track coil currents. In addition, the operation characteristic, power divisions, and boundary conditions of each operation mode are clarified. Finally, a 2.2 m 220 V-800 W prototype is built. The experimental results show that even with the track current frequency difference, the output voltage fluctuation is limited to 3.4% during motion, and the system efficiency remains above 90.76% in the case of an 8 cm air gap.

Gas puff imaging measurements during resonant magnetic perturbations on the HL-2A tokamak

The gas puff imaging diagnostic is used for the investigation of the poloidal flow and turbulent fluctuations at the plasma edge and scrape-off layer (SOL) on the HL-2A tokamak. The impact of resonant magnetic perturbations (RMPs) on the edge poloidal velocity ( Vθ ) are investigated and compared during the operation of a stair-like rising RMP coil current ( IRMP ). The application of the RMP is observed to modify the poloidal velocity significantly. When IRMP exceeds 4 kA, the turbulence poloidal velocity at the edge changes direction from electron to ion diamagnetic drift. This phenomenon is explained by the electron loss along the perturbed radial magnetic field (Br) with RMP, with the experimental evidence provided that the edge electric potential increases significantly. A strong impact of the RMP on the properties of plasma density fluctuations in the SOL is also observed. With RMP, both skewness and kurtosis are smaller in the SOL and large-scale turbulence structures (small kr and kθ ) are suppressed in both edge and SOL. These results can improve our understanding of the interaction between RMP and edge turbulence and in particular, the edge poloidal flow.

Research on coil positioning method and magnetic field orientation strategy of wireless power transfer system

In wireless power transfer (WPT) system, the displacement of the receiver coil may cause the problem of efficiency reduction. For this, this paper presents a coil localization method based on the BP neural network, and studies the strategy of using current phase-shift control to realize power orientation in positioning area. In this paper, the distribution of the array coils is designed as overlap, the relationship between the regional coordinate points and the coil current is studied, and the BP neural network is introduced to establish the coil positioning model. In order to effectively improve the localization accuracy, we propose a partition modeling optimization method based on this model. At the same time, a BP neural network magnetic flux model is established to determine the optimal phase of the array coil current when the magnetic flux in the region is the largest. Simulation and experiments show that the average positioning error of the optimization method in the area of 32.9 cm ×32.9 cm is less than 1 cm.

Electron acceleration in a coil target-driven low-β magnetic reconnection simulation

Magnetic reconnection driven by a capacitor coil target is an innovative way to investigate low-β magnetic reconnection in the laboratory, where β is the ratio of particle thermal pressure to magnetic pressure. Low-β magnetic reconnection frequently occurs in the Earth’s magnetosphere, where the plasma is characterized by β ≲ 0.01. In this paper, we analyze electron acceleration during magnetic reconnection and its effects on the electron energy spectrum via particle-in-cell simulations informed by parameters obtained from experiments. We note that magnetic reconnection starts when the current sheet is down to about three electron inertial lengths. From a quantitative comparison of the different mechanisms underlying the electron acceleration in low-β reconnection driven by coil targets, we find that the electron acceleration is dominated by the betatron mechanism, whereas the parallel electric field plays a cooling role and Fermi acceleration is negligible. The accelerated electrons produce a hardened power-law spectrum with a high-energy bump. We find that injecting electrons into the current sheet is likely to be essential for further acceleration. In addition, we perform simulations for both a double-coil co-directional magnetic field and a single-coil one to eliminate the possibility of direct acceleration of electrons beyond thermal energies by the coil current. The squeeze between the two coil currents can only accelerate electrons inefficiently before reconnection. The simulation results provide insights to guide future experimental improvements in low-β magnetic reconnection driven by capacitor coil targets.

Robust Control for Torque Minimization in Wind Hybrid Generators: An H∞ Approach

This study focuses on implementing a wind turbine emulator based on a permanent magnet synchronous machine with excitation auxiliary windings and thoroughly investigates the space harmonics created by this innovative topology in MATLAB/Simulink. A Hybrid Generator (HG) is a robust generator that does not have slip rings or brushes in its structure. Furthermore, the flux of the hybrid generator HG may be easily adjusted as it is created by direct current excitation coils and permanent magnets. Unfortunately, the space harmonic rate in the HG is relatively high. In other words, the mechanical vibrations caused by the electromagnetic torque ripple threaten the drive train’s behaviour and, ultimately, the wind turbine’s lifespan. This study describes two methods for decreasing the ripple in electromagnetic torque. Both circuit architecture and robust H∞ control techniques are considered. After simulating the two approaches, a list of requirements is provided for the maximum allowable amplitude of the inductance and the flux harmonics.

Evaluation of a novel polymer coil for endovascular occlusion of intracranial aneurysms in a rabbit model.

The results of the preclinical study of a novel polymer coil in treatment of elastase induced aneurysms will be presented in this paper. We induced 16 aneurysms in 16 New Zealand white rabbits at the origin of the right common carotid artery at the brachiocephalic trunk. Newly developed polymer coils in both groups for six aneurysms each and platinum coils for two aneurysms each were used. Control angiographies followed in both groups immediately after coiling as well as in the first eight animals 30days after intervention (30days group) and in the other eight animals 90days after (90days group). An explanation and histological evaluation of the treated aneurysms followed. The 12 animals in which the aneurysms were treated with polymer coils showed a complete occlusion (grade IV) in only 6 out of 12 aneurysms (50%), an almost complete occlusion (grade III) in 5 out of 12 (42%) and an incomplete occlusion in the treatment of one aneurysm (8%). Histologically, we observed a significantly more pronounced inflammatory response and neoangiogenesis in aneurysms treated with polymer coils only in the 30days group. Most difficulties and concerns with the polymer coils were related to the flexibility and detachment behaviour. Therefore, and due to the technical challenges of delivery, the novel polymer coil cannot be considered an alternative to the current platinum coils.

Penetration properties of applied resonant magnetic perturbation in HL-2A tokamak

Any arbitrary perturbation on a magnetic field separatrix can cause a structure named homoclinic tangle in tokamaks. Both an edge localized mode (ELM) and a resonant magnetic perturbation (RMP) can lead to a perturbation of the magnetic field on the separatrix. Under the appropriate circumstances, RMP could alleviate or even completely suppress a rapid collapse process of an ELM. The simulation results using the CLTx code, the extended version of the three-dimensional toroidal magnetohydrodynamic (MHD) code (CLT (Ci-Liu-Ti, which means MHDs in Chinese)) with a scrape-off layer, show the structure of the homoclinic tangle with a borderline stochastic region resulting from RMP in HL-2A tokamak. Strongly distorted magnetic field lines with the homoclinic tangle could connect to the tokamak divertors. The footprints of these magnetic field lines on the divertors are consistent with the energy deposit spots in the experiment. From Poincaré plots of escaped magnetic field lines, it is found that the depth of the plasma edge region penetrated by these field lines depends on the RMP coil current, the rotation frequency of the RMP field, and the plasma resistivity.

Locked mode detection during error field identification studies

At the beginning of a machine operation, an assessment of the intrinsic error fields, spurious magnetic field perturbations which can affect plasma dynamics, is often carried out by executing the compass scan method [Scoville J.T. et al. Nucl. Fusion 43 250 (2003)]. This method relies on the application of 3D magnetic fields with various phases, induced by EF correction coils, to trigger a locked mode. The instant of locked mode onset allows the identification of the amplitude and phase of the intrinsic error field, from which the empirical correction currents for its minimization can be deduced. The presence of a locked mode needs to be carefully monitored during this study because of the potential disruptive mode behavior, especially in devices which can tolerate a maximum number of disruptions, as in SPARC and in ITER. A novel method, the so-called non-disruptive compass scan method [Paz-Soldan C. et al., Nuclear Fusion 54 (2014) 073013], avoids the disruption risk, as the name recalls, via magnetic island healing, i.e. stabilizing the locked mode. The magnetic island healing is achieved by switching off the error field correction coil current during the execution of the compass scan and asynchronously by increasing the plasma density. The crucial point of this new method is the detection of the locked mode to initiate the EFCC-density control actions. In this work, the locked mode detector adopted during non-disruptive compass scan test at JET is presented, together with brand-new locked mode metrics, which take into account the actual poloidal deformation due to a locked mode and a class of MHD instabilities, named Beta Alfvén Eigenmodes, that appear in the Mirnov signal in concomitance to the locked mode. The use of multiple metrics for locked mode detection during the execution of the non-disruptive compass scan increases the fidelity of the real-time control system to pinpoint the event, compensating possible magnetic probe failure, and initiate the control sequences to heal the magnetic island.

Effect of resonant magnetic perturbations including toroidal sidebands on magnetic footprints and fast ion losses in HL-2M

Externally applied resonant magnetic perturbations (RMPs), generated by magnetic coils located outside the plasma (referred to as RMP coils), provide an effective way to control the edge localized mode (ELM) in tokamak devices. Due to the discrete nature of the toroidal distribution of these window-frame coils, toroidal sidebands always exist together with the fundamental harmonics designed for ELM control. In this work, the MARS-F code (Liu et al 2000 Phys. Plasmas 7 3681) is applied to investigate the detailed features of the RMP spectra considering both the dominant harmonic (n = 2) and the associated sideband (n = 6), and the impact of the combined fields on magnetic footprints as well as on the fast ion losses for a reference double-null scenario in the HL-2M device. It is found that the sum of the n = 2 and n = 6 RMP fields splits the footprint and widens the footprint area, as compared to the single-n (n = 2) harmonic case. The resistive plasma response breaks the up–down symmetry of the footprint pattern on the outer divertor plates, which is otherwise symmetric assuming vacuum RMP fields. Considering fast ion losses, a threshold value exists for the initially launched radial position of test particles, as well as for the RMP coil current, before the loss occurs. When the threshold criterion is satisfied, the combined n = 2 and n = 6 RMP fields enhance the fast ion loss rate by , as compared to that of the n = 2 component alone. These results illustrate the important role of the sideband of RMP fields on the magnetic footprints and fast ion losses in tokamak plasmas.