Coil Driver Research Articles

High-Frequency Magnetic Pulse Generator for Low-Intensity Transcranial Magnetic Stimulation

This paper presents a high-frequency (HF) magnetic pulse generator designed for low-intensity transcranial magnetic stimulation (LI-TMS) applications. HF pulse stimulation can induce a strong electric field with minimal current and enhance the penetration depth of the electric field in human tissue. The HF magnetic pulse generator was designed and fabricated using a microcontroller unit, gate driver, full-bridge coil driver, and stimulation coil. Measurements with a full-bridge circuit supply voltage of 10 V demonstrated an electric field intensity of 6.8 Vpp/m at a frequency of 1 MHz with a power dissipation of 2.45 W. Achieving a similar electric field intensity at a frequency of 100 kHz required approximately ten times the coil current. Additionally, a quasi-resonant LC load was introduced by connecting a capacitor in series with the stimulation coil, which set the resonant frequency to approximately 10% higher than the frequency of 1 MHz. This approach reduced the coil impedance, achieving higher current with the same bias supply voltage. Experimental results showed an enhanced electric field intensity of 19.1 Vpp/m with a supply voltage of only 1.8 V and reduced power dissipation of 1.11 W. The proposed HF pulse train with quasi-resonant coil system is expected to enable a low-power LI-TMS system.

Low-cost bidirectional coil driver with microamp precision

We describe a compact constant current power supply with µA precision designed to drive coils. The unit generates currents from −125 mA to 125 mA with a load up to 10 Ω using a precision 16-bit digital to analogue converter, driven from a microcontroller (e.g. Raspberry Pi Pico). All power for the unit is derived from the 5 V of the microcontroller. As a demonstration of the capability of the power supply, it was applied to spin manipulation in a helium spin echo system.

Directional eddy current probe configuration for in-line detection of out-of-plane wrinkles

Real-time monitoring of carbon fibre composites during Automated Fibre Placement (AFP) manufacturing remains a challenge for non-destructive evaluation (NDE) techniques. A directional eddy-current (EC) probe with asymmetric transmit and differential receive (Tx-dRx) coils is designed, constructed and characterized to evaluate the detectability of out-of-plane wrinkles. Initial studies were conducted to determine suitable excitation frequencies and to analyse the impact of relative orientations of driver and pickup coils on wrinkle detectability. The probe configurations are evaluated experimentally and employ a new finite element modelling approach to better understand the relationship between eddy-current density and defect detection. The findings indicate that a probe configuration with an asymmetric driver coil normal to the material surface and aligned with the fibre directions, and with differential pickup coils 90° to the scanning direction, shows the best capability for out-of-plane wrinkle detection, with SNR >20 for wrinkles over 1.3 mm in amplitude.

Wireless Power Transfer System for Mobile Robots with Impedance Matching by Adjusting Position of Driver Coil

Wireless power transfer based on magnetic resonant coupling can be used to supply power to a mobile robot within a few meters of a transmitter coil. However, when the robot moves or its power consumption fluctuates, its input impedance varies significantly and, in case of impedance mismatch, power reflection may occur. To address this issue, we employed a driver coil on the transmitter side to match input impedance. By regulating the coupling coefficient between the driver and the transmitter coils, the input impedance is matched and power reflection is eliminated. Input impedance regulation is achieved by moving the driver coil mechanically. As confirmed by our experiments, the transmitting efficiency matched with the analytically calculated value, and the input impedance was matched for a vast range of distances from 0 to 1 m and load resistances from 4 to 210 Ω by moving the driver coil properly. Therefore, the proposed system successfully addresses the power reflection problem in wireless power transfer for mobile robots.

Investigation of rolling contact fatigue cracks using the transmitter-receiver eddy current testing under moving conditions

ABSTRACT Rolling contact fatigue (RCF) cracks online detection using eddy current testing (ECT) is in urgent demand. However, RCF cracks detection and evaluation in this way under moving conditions remains challenging due to the velocity effect in ECT. This paper aims to study the response of a transmitter-receiver eddy current probe to cracks under moving conditions and evaluate the depth and inclination angle of RCF cracks. In this paper, a high-speed eddy current testing system is developed to experimentally investigate the influence of coil gap, detection speed, and the lift-off on the eddy current probe’s response under moving conditions. In addition, the temporal and amplitude features of the eddy current signal are extracted to characterise the depth and inclination angle of the RCF cracks. The experimental results indicate the eddy current probe’s response can be improved by increasing the coil gap (coil centre distance) suitably, which can be done to compensate for the attenuation of the eddy current signal caused by detection speed and lift-off. The probe’s response hardly changes with an increase in detection speed when the driver and pick-up coils of the eddy current probe completely overlap. The crack depth and inclination angle can be evaluated under moving conditions.

Ultra-low noise, bi-polar, programmable current sources.

We present the design process and implementation of fully open-source, ultra-low noise programmable current source systems in two configurations. Although originally designed as coil drivers for Optically Pumped Magnetometers (OPMs), the device specifications make them potentially useful in a range of applications. The devices feature a bi-directional current range of ±10 and ±250 mA on three independent channels with 16-bit resolution. Both devices feature a narrow 1/f noise bandwidth of 1Hz, enabling magnetic field manipulation for high-performance OPMs. They exhibit a low noise of 146 pA/Hz and 4.1 nA/Hz, which translates to 15 and 16 ppb/Hz noise relative to full scale.

A method to measure the electric parameters of the driver in a fusion-relevant RF ion source

RF ion sources for the neutral beam injection application contain a part called the driver where the plasma is generated. Measurement of the driver's external electrical parameters, such as coil current Icoil and driver equivalent impedance ZD, is useful for assessing the RF power coupling process and estimating the internal plasma parameters. The direct measuring method using RF I-V sensors at the driver's input terminal is inconvenient since it requires the installation of I-V sensors with very high phase (difference) accuracy near the driver. Thus, an indirect measuring method is developed for the RF ion sources equipped with the transformer-isolated Γ-type matching network in this paper. The method uses the electric parameters measured before the matching network and an impedance matching model of the real-world matching network. In the matching model, the matching transformer is treated as a mutual inductance circuit, and the parasitic impedance of the matching network is considered. Then the method is applied to an RF ion source, HINT. With the real-time impedance and power measured before the matching network as inputs, the driver's electric parameters and the RF power transfer efficiency η of HINT are monitored. Without plasma discharge, Icoil and ZD measured indirectly are consistent with those measured directly. With discharges (at transmitted power of 5∼12 kW and filling pressure of 0.45∼0.7 Pa), the deviations of indirect measurements from direct measurements are below 10% in Icoil, and then below 0.06 (absolute) in η. The comparisons show that the indirect measuring method can be used as an alternative to evaluate the driver's electric parameters and η in HINT.

Model of Ferrite-cored Driver-pickup Coil Probe Application of TREE Method for Eddy Current Nondestructive Evaluation

An analytical model of a driver-pickup coil probe, consists of a cylindrical ferrite core, located above a layered conductor is presented. The truncated region eigenfunction expansion (TREE) method is used and the solution region is truncated with a certain radius around markedasmathitalicz axis. First, the magnetic vector potential of each region of filamentary coil problem is derived and solved with variables separation method using boundary and interface conditions, and then the rectangular cross-section coil problem is solved with superposition method. The expression of induced voltage in pickup coil is obtained and can be calculated with software such as Matlab or Mathematica. Using the proposed analytical model, the influence of the excitation frequency and excitation current in the driver coil on the responses of the pickup coil is examined. Experiments are performed, and the changes of voltage induced in the pickup coil due to the conductor are measured at different excitation frequencies and excitation currents. The analytical calculation results agree with the experimental results very well, verifying the correctness of the proposed analyticalmodel.

Integration and evaluation of magnetic stimulation in physiology setups.

A large number of behavioral experiments have demonstrated the existence of a magnetic sense in many animal species. Further, studies with immediate gene expression markers have identified putative brain regions involved in magnetic information processing. In contrast, very little is known about the physiology of the magnetic sense and how the magnetic field is neuronally encoded. In vivo electrophysiological studies reporting neuronal correlates of the magnetic sense either have turned out to be irreproducible for lack of appropriate artifact controls or still await independent replication. Thus far, the research field of magnetoreception has little exploited the power of ex vivo physiological studies, which hold great promise for enabling stringent controls. However, tight space constraints in a recording setup and the presence of magnetizable materials in setup components and microscope objectives make it demanding to generate well-defined magnetic stimuli at the location of the biological specimen. Here, we present a solution based on a miniature vector magnetometer, a coil driver, and a calibration routine for the coil system to compensate for magnetic distortions in the setup. The magnetometer fits in common physiology recording chambers and has a sufficiently small spatial integration area to allow for probing spatial inhomogeneities. The coil-driver allows for the generation of defined non-stationary fast changing magnetic stimuli. Our ex vivo multielectrode array recordings from avian retinal ganglion cells show that artifacts induced by rapid magnetic stimulus changes can mimic the waveform of biological spikes on single electrodes. However, induction artifacts can be separated clearly from biological responses if the spatio-temporal characteristics of the artifact on multiple electrodes is taken into account. We provide the complete hardware design data and software resources for the integrated magnetic stimulation system.

Examination of the arrangement and shape of coils for permittivity measurement of dielectric sandwiched between carbon fiber reinforced plastics using electromagnetic induction testing

The arrangement and shape of coils for permittivity measurement of a dielectric sandwiched between carbon fiber reinforced plastics (CFRPs) using electromagnetic induction testing (EIT) is investigated here. First, the influence of the electromagnetic shielding effectiveness of the CFRPs on the permittivity measurement using EIT was examined by finite element analysis (FEA). The effectiveness was lower than that of general metals, indicating that EIT can be used for permittivity measurement. Next, the coil arrangement and shape suitable for permittivity measurement were investigated by FEA. The use of two rectangular spiral coils, one as a driver coil and the other as a pickup coil, was adopted. Finally, the permittivity was measured experimentally using the coil arrangement, geometry, and test conditions determined based on the FEA results. The experimental results showed that the output voltage increased as the permittivity increased, demonstrating the effectiveness of the proposed method.

Differential ECT Probe Design and Investigation for Detection of Rolling Contact Fatigue Cracks With Different Orientations

Eddy current testing(ECT) is widely applied to detect surface defects on metallic materials. In ECT for cracks on complex curved surfaces, immunity to lift-off and the detection ability of the eddy current probe are critical challenges. In this study, we designed and developed a novel differential transformer ECT probe using AD698 signal conditioning to detect cracks with different orientations in rail treads. It adopts flexible printed circuit board (FPCB) technology for complex curved surfaces. The new ECT probe consists of four-square driver coils and two 8-shaped pick-up coils with antiserial connections that can sense opposite magnetic field changes in different directions. The proposed probe can solve the detection ability and lift-off problem of crack with different orientation on complex curved surfaces. The theoretical analysis which is based on transformer principles, is described in this paper. The detection ability of the new probe at different frequencies was evaluated by numerical simulation. Upon completion, the numerical simulations analyzed the effects of crack with different orientation and lift-off on the detection ability of the proposed probe. The experimental system was built to verify the detection ability of different crack with different orientations and immunity to lift-off by the proposed probe. The results of the simulation and experiment indicated that the proposed probe has a good ability to detect crack with different orientation whereas its performance decreases as crack orientation increasing. The proposed probe can also substantially suppress lift-off noise.

Magnetic Field Simulator Control System for Nanosatellites

Nano- and microsatellites are becoming more and more popular in the last ten years. The main reason for this is the low cost of their development and launch. Two such satellites of the CubeSat format have already been developed at the Igor Sikorsky Kyiv Polytechnic Institute. They have successfully completed their missions in orbit. Work on a new satellite is underway. Before flying into orbit, all satellites must pass a large number of ground tests. An important part of almost every satellite is its subsystem of orientation and stabilization. Since it contains magnetic sensors and electromagnets, special magnetic field simulators are used to test it. Such a simulator must create a uniform magnetic field with the required parameters. The most common design used to simulate a magnetic field is a Helmholtz cage. It was built in the nanosatellite laboratory of the Igor Sikorsky Kyiv Polytechnic Institute. This paper deals with the development of a control system for it. In the beginning, the design, parameters, and operation principle of the Helmholtz cage were described. It consists of six electromagnetic square shape coils with a side of 1.5 meters. Each coil has 52 turns of 1.29 mm wire. Such dimensions make it possible to obtain a spherical zone of uniformity of the magnetic field with a radius of 293 mm. The control system should regulate the current in the coils, change its direction and also control the parameters of the generated magnetic field. The existing solutions for such a system were analyzed. As a result, it was decided to use an H bridge circuit and pulse width regulation. The structural and schematic diagrams of the control system were developed. Were selected all necessary components, such as power module, central processing unit, microcontroller for PWM signal generation, and magnetometer for magnetic field control. To reduce the ripple of the output current, an RLC filter for the coil driver was developed. Its frequency response, current and voltage curves of its load were calculated analytically and coincided with practical results. An experimental layout of the control system was assembled. As a result of the tests, it was found that the system regulates the current in the coil from 0 to 6.8 A and can change its direction. The current ripple in the coils does not exceed 1 mA (peak-to-peak) at an average current of 0,574 A. The duration of the transient in the system is about 23 ms. These values meet the requirements and allow simulating any mode of satellite flight in orbit. The developed control system for the magnetic simulator allows testing the magnetic orientation systems of nanosatellites developed at Igor Sikorsky Kyiv Polytechnic Institute and will help in their preparation for the flight.

A Fully Integrated Low-Power Hall-Based Isolation Amplifier With IMR Greater Than 120 dB

A CMOS Hall-based fully integrated isolation amplifier for differential voltage sensing is presented in this work. The design is fabricated in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.35 \mu \text{m}$ </tex-math></inline-formula> CMOS process in which the high voltage (HV) side of the amplifier contains a coil driver while the low voltage (LV) side includes a Hall-effect sensor, low-noise amplifier, programmable-gain amplifier, filter, and chopper switches. Another Hall sensor performs the digital isolation using the on-off keying (OOK) technique for clock recovery. The introduced prototype achieves above 120 dB of isolation mode rejection (IMR) at 60 Hz and operates at a continuous isolation working voltage of 0.6 kV. It has also a maximum nonlinearity of 0.64 %, an input-referred offset of 1 mV, a 40 dB full-scale signal-to-noise ratio over a 40 kHz bandwidth, and a spurious-free dynamic range of 64 dB. The silicon area for each of the two separate dices employed for the HV and LV side of the isolation amplifier is 1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with a power consumption of 7.6 mW and 9.9 mW respectively. The achieved miniaturized size of the isolation components, as well as their significantly low-power consumption, ensure the suitability of the proposed isolation amplifier for multi-channel readout circuit applications.

Local Defect Detection of Ferromagnetic Metal Casing Based on Pulsed Eddy Current Testing

Remote-field eddy current testing (RFECT)-based methods have been widely used in conductive pipe evaluation. However, the response signals of RFECT are affected both by the casing thicknesses around the driver coil and in the vicinity of the pick-up coil, making it difficult to invert and separate the thicknesses around the coils. Moreover, the relative permeability difference between the near-field and the remote-field zones causes a further deviation in the thickness inversion. To avoid these problems, a near-field pulsed eddy current testing method is proposed for defect evaluation. Time-domain transient features, which can be applied for defect detection, are analyzed based on the theoretical data of pulsed eddy current testing. A prototype of the testing apparatus and a defective ferromagnetic metal casing are designed to verify the feature performance for defect evaluation. The experimental results indicate that the time-related features are sensitive to circumferential defects and insensitive to the axial defects, and the magnitude-related feature is sensitive to both the circumferential and axial defects. Therefore, the magnitude-based method can be used to detect the local defects of ferromagnetic metal casings.

Eddy-current array-probe technique for imaging near-surface and deep-lying delaminations in multidirectional carbon fiber composites

An eddy-current array-probe method was proposed to visualize delamination occurring within multidirectional carbon fiber-reinforced plastics. First, samples with a near-surface delamination were scanned by a coil array probe comprising a single driver coil and multiple pickup coils, and the detectability of the delamination was evaluated by the signal-to-noise ratio. An appropriate selection of the image phase and the distance between the driver and pickup coils enabled detection of the delamination suppressing the signal variation in the non-defective region. Furthermore, the experimental results suggested that the distance between the driver coil and the most sensitive pickup coil correlated with the delamination size. Second, the array-probe technique was used to detect a delamination located at various depths. The detectability of the delamination was found to be strongly affected by the rotation angle of the array probe. The optimum probe rotation angle depended on the depth of the delamination, indicating that a delamination occurring in deeper regions can be visualized by setting an appropriate probe rotation angle.

Design and application of wireless power transfer using Class-E inverter based on Adaptive Impedance-Matching Network

Class-E inverter, which is assumed as an ideal exciter for wireless power transfer system due to their low power losses and suitability for high-frequency operation, can operate under the proper switching conditions when the distance between the transmitter and the receiver and the load is constant. The change in the distance between the coils or the load value removes the optimum switching conditions and causes high switching losses. This paper describes all design procedures and the inductive coupling wireless power transfer system based on a Class-E inverter used as a coil driver. In case the distance between the coils changes, the optimum switching condition of the Class-E inverter and the impedance matching between the transmitter and receiver have been dynamically sustained with an adaptive capacity array on the transmitter side. The best combination of the array involving a serial/parallel capacitor network is configured using a novel searching algorithm for different coil distances. In addition, this study investigates both coupling efficiency and the efficiency of Class-E inverter that is remarked by a few studies in the existing literature. The developed simulation studies and the laboratory prototype with power of 1 kW demonstrate the providing of the zero voltage-switching requirement for Class-E inverter and the impedance matching of the wireless power transfer system in case of variable coupling coefficient. Therefore, the efficiency of power transfer achieved as 91.13% for the implementation circuit.

A Galvanic Isolated Amplifier Based on CMOS Integrated Hall-Effect Sensors

A novel galvanic isolated amplifier based on CMOS integrated Hall sensors is presented in this paper. Two serially connected Hall-effect sensors are integrated along with their instrumentation amplifiers using the TSMC 65nm process. A printed-circuit board is employed to validate the proposed isolation amplifier by assembling the chip with chopper modulator, coil driver, miniature coil, variable gain amplifier, and anti-aliasing filter. Because of the miniaturized size of isolation components, this approach can be packaged in chip for industrial applications. This solution replaces the need of bulky/frequency dependent current transformers, complex isolation amplifiers with embedded analog to digital converters, and allows proposed sensors to be used in voltage and current sensing applications. The introduced prototype achieves an input referred offset of 1 mV, 50 dB full-scale signal-to-noise ratio in a 10 kHz bandwidth, and spurious-free dynamic range of 53 dB, while satisfying continuous isolation working voltage of 550 V.

Design and Fabrication of Compact Arrayed Magnet for Biological EPR Imaging

A portable and compact electron paramagnetic resonance (EPR) imaging magnet was developed for building a 750 MHz EPR imaging system. To realize low-cost fabrication, it was shown that the magnetic homogeneity required for redox EPR imaging using nitroxyl probes can be approximately reproduced by regularly arranging small neodymium magnets that are easy to handle. A five-channel coil driver consisting of operational amplifiers and switching power supplies was developed for field sweep, field modulation, and three field gradients to downsize the imaging system. The EPR imaging of 4-oxo-2,2,6,6-tetramethylpiperidine-d16-1-oxyl (tempone-d16) using a phantom was successfully performed by employing a 3D radial sequence controlled by a digital EPR console.

Time-domain modeling analysis of pulsed eddy current testing on ferromagnetic casing.

Pulsed eddy current has attracted increasing attention to ferromagnetic metal material evaluation since it possesses low power consumption and abundant frequency spectrum advantages. However, when a current source is applied to generate the pulse excitation signal in eddy current testing, the inductance of driver coil induces a large reverse electromotive force in the excitation circuit, which distorts the pulse excitation signal as the excitation circuit hardly satisfies the severe power and stability requirements for counteracting the reverse induced electromotive force. Therefore, a pulsed eddy current field excited by a voltage-driven coil placed concentrically to a ferromagnetic casing is studied, and the analytic solutions of it are formulated in this paper. In contrast to current source based pulsed eddy current testing methods, the electromotive force induced by the coil's inductance and eddy current is analyzed. Furthermore, a new pulse excitation function is adopted and induced electromotive force equations of pick-up coils are formulated based on the superposition principle of magnetic field. Finally, the theoretical results are verified by experiments.

Wideband Magnetic Excitation System for Atomic Force Microscopy Cantilevers with Megahertz-Order Resonance Frequency

Small cantilevers with a megahertz-order resonance frequency provide excellent sensitivity and speed in liquid-environment atomic force microscopy (AFM). However, stable and accurate oscillation control of a small cantilever requires the photothermal excitation, which has hindered their applications to the studies on photo-sensitive materials. Here, we develop a magnetic excitation system with a bandwidth wider than 4 MHz, enabling a light-free excitation of small cantilevers. In the system, a cantilever with a magnetic bead is driven by a magnetic field generated by a coil. In the coil driver, a differentiation circuit is used for compensating the frequency dependence of the coil impedance and keeping the current constant. By implementing several differentiation circuits with different frequency ranges, we enable to drive various cantilevers having different resonance frequencies with sufficient excitation efficiency. In contrast to the conventional coil driver with a closed-loop circuit, the developed one consists of an open-loop circuit and hence can be stably operated regardless of the coil design. With the developed system, atomic-resolution imaging of mica in liquid using a small cantilever with a megahertz-order resonance frequency is demonstrated. This development should lead to the future applications of AFM with small cantilevers to the studies on various photo-sensitive materials and phenomena.