Characteristics of bowl-shaped coils for transcranial magnetic stimulation
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Abstract:
Transcranial magnetic stimulation (TMS) has recently been used as a method for the treatment of neurological and psychiatric diseases. Daily TMS sessions can provide continuous therapeutic effectiveness, and the installation of TMS systems at patients' homes has been proposed. A figure-eight coil, which is normally used for TMS therapy, induces a highly localized electric field; however, it is challenging to achieve accurate coil positioning above the targeted brain area using this coil. In this paper, a bowl-shaped coil for stimulating a localized but wider area of the brain is proposed. The coil's electromagnetic characteristics were analyzed using finite element methods, and the analysis showed that the bowl-shaped coil induced electric fields in a wider area of the brain model than a figure-eight coil. The expanded distribution of the electric field led to greater robustness of the coil to the coil-positioning error. To improve the efficiency of the coil, the relationship between individual coil design parameters and the resulting coil characteristics was numerically analyzed. It was concluded that lengthening the outer spherical radius and narrowing the width of the coil were effective methods for obtaining a more effective and more uniform distribution of the electric field.Keywords:
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A new measurement method to determine ac losses in superconducting coils has been proposed, and its validity confirmed experimentally. In this method, the ac losses in the coil can be obtained from measured Poynting's vectors around the coil windings. The Poynting's vectors can be obtained by local magnetic fields and electric fields around the coil. Local magnetic fields and electric fields are measured by pick-up coils and potential leads, which are mounted around the sample coil. These pick-up coil and potential lead pairs are arranged on the inner side, outer side, top, and bottom of the coil windings to cover the entire coil windings. This measurement method has the benefit that ac losses in a coil in a superconducting coil system consisting of multiple coils can be measured. Not only ac losses in the coils but also information on local electromagnetic phenomena in the coils can be obtained from profiles of the Poynting's vectors. In this paper, the details of the measuring method are described, and experiments using a coil wound with Bi-2223 multifilamentary tapes were carried out in liquid nitrogen to confirm the validity of the measurement method.
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We have investigated the performance of a normal conducting transfer coil (n-coil) for nondestructive evaluation with a SQUID. The transfer efficiency, which depends on frequency and wire impedance, has been calculated for several coil designs to determine the performance of the n-coil. We have fabricated a 30 mm diameter n-coil and examined its performance with a high-TC SQUID. If both the pick-up coil and the input coil have 50 turns, then at high frequencies the magnetic field produced by the input coil becomes 50% of the magnetic field at the pick-up coil, with a low cut-off frequency of 2 kHz. The experimental result agrees well with a numerical calculation, which suggests a magnitude of magnetic field detected by our SQUID equal to the magnitude of the quasi-uniform magnetic field produced by the input coil. We also confirmed that the thermal noise of the n-coil, produced by its normal resistance, does not become higher than the intrinsic magnetic field noise of the SQUID.
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In this paper, a planar displacement sensor with inductive spiral coils is presented. The sensor is composed of two inductive coils of spiral shape. One coil is a stationary coil with input terminals, whereas the other coil is a moving (short-circuited) coil. The moving coil physically moves with respect to the stationary coil in x-z and y-z planes. The input inductance changes when the moving coil moves with respect to the stationary coil and it is measured between the terminals of the stationary coil. The inductive operating principle of the sensor was tested using the prototypes fabricated in printed circuit board technology. Two prototypes were designed and fabricated: the prototype without a gap in the stationary coil and the prototype with the gap inserted in the segments of the stationary coil.
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The improved magnetic induction tomography (MIT) coil system which consists of the two-arm Archimedean spiral coil (TAASC) as excitation coil and the solenoid as receiver coil has much better performance in coil system sensitivity than the conventional MIT coil system which uses the solenoids as excitation coil and receiver coil. In this paper the theoretical sensitivity property for improved MIT coil system are studied fully. The magnetic fields produced by TAASC and solenoid are derived approximately based on the Biot-Savart law. The relations between the coil system sensitivity and the parameters (amplitude of excitation current, maximum outer radius and number of turns of TAASC, and number of turns and length of solenoid) of coil system are calculated. The results show that the sensitivity for improved MIT coil system is proportional to the number of turns of solenoid and electric current of TAASC, and can be improved with the increase of maximum outer radius of TAASC and radius of solenoid. The sensitivity is decreased as the length of solenoid increasing, and not significantly associated with the number of turns of TAASC.
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The problem that misalignment between the transmitting coil and the receiving coil significantly impairs the transmission power and efficiency of the system has been attached more and more attention. In order to improve the uniformity of the magnetic field between the two coils to solve this problem, a new type of coil called pan-shaped coil is proposed. Three-dimension simulation models of the planar-core coil and the pan-shaped coil are established using Ansoft Maxwell software. The coupling coefficient between the transmitting coil and the receiving coil is obtained by simulating the magnetic field with the receiving coil misalignment or not. And the maximum percentage difference strength along the radial direction which is defined as the magnetic field uniformity factor is calculated. According to the simulation results of the two kinds of coil structures, it is found that the new type of coil structure can obviously improve the uniformity of the magnetic field, coupling coefficient and power transmission properties between the transmitting coil and the receiving coil.
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A kind of miniature Rogowski coil which measures high frequency and heavy current is introduced,and the principle of work of Rogowski coil is analyzed.The influence of non-uniform laying of the miniature Rogowski coil,including the biased laying and the inclined laying on the electric current survey performance is discussed.In order to reduce the influence of the coil laying on the capriciousness,the coil may be wound thread or increased evenly from the integral coefficient.The Rogowski coil of the hollow multi-layer circles reduces the coil stray capacity in the manufacture process.The design project of coil is proposed,the current waveform is tested by using many kinds of structures of the coil,and the massive test data are obtained.Through comparing the overseas standard electric current test and head test data of the probe,the current coil design is improved unceasingly.The test using magnetic material can enhance the coil actually from the integral criterion function,but the used magnetic material should have saturation induction density and consider its working band.
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The measuring coil geometry has a direct influence on the magnetic field distribution, and the magnetic field distribution affects the detection performance of the eddy current sensor. The influence of coil inner diameter and thickness on magnetic induction intensity of the coil was theoretically analyzed. The three-dimensional finite element model of the main spool of the overflow valve was measured using ANSYS Maxwell. The other factors such as coil turns and excitation current intensity were analyzed. Under the condition of the coil inner and outer diameter and thickness influence on the magnetic induction of the coil, the influence of coil geometry on the linearity and sensitivity of the coil is obtained. The results show that the coil geometry influences the detection performance of the sensor, and the linearity and sensitivity of the sensor can be improved by changing the inner and outer diameter and thickness of the coil.
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There is no uniform external magnetic field in the actual working environment of a current transformer.In this paper,a new method for analyzing the interference of non-uniform magnetic field on the Rogowski coil of current transformer is proposed.Magnetic field is created by adjacent power lines.According to the magnetic direction at both near and far sides of Rogowski coil,the coil structure is divided into two parts for respective analysis,and the total induced electromotive force is calculated to be zero,which indicates that the non-uniform magnetic field will not interfere with the Rogowski coil on the premise of uniform turns density and cross-sectional area of the coil.Two experiments are conducted,and the results show that the interference of non-uniform magnetic field on the Rogoski coil can be clearly suppressed by increasing the uniformity of the coil's turns density and cross-sectional area,and adding a shield.
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This review describes induction coil sensors, which are also known as search coils, pickup coils or magnetic loop sensors. The design methods for coils with air and ferromagnetic cores are compared and summarized. The frequency properties of coil sensors are analysed and various methods for output signal processing are presented. Special kinds of induction sensors, such as Rogowski coil, gradiometer sensors, vibrating coil sensors, tangential field sensors and needle probes are described. The applications of coil sensors as magnetic antennae are also presented.
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