Development of a miniature permanent magnetic circuit for nuclear magnetic resonance chip

The existing researches of miniature magnetic circuits focus on the single-sided permanent magnetic circuits and the Halbach permanent magnetic circuits. In the single-sided permanent magnetic circuits, the magnetic flux density is always very low in the work region. In the Halbach permanent magnetic circuits, there are always great difficulties in the manufacturing and assembly process. The static magnetic flux density required for nuclear magnetic resonance(NMR) chip is analyzed based on the signal noise ratio(SNR) calculation model, and then a miniature C-shaped permanent magnetic circuit is designed as the required magnetic flux density. Based on Kirchhoff’s law and magnetic flux refraction principle, the concept of a single shimming ring is proposed to improve the performance of the designed magnetic circuit. Using the finite element method, a comparative calculation is conducted. The calculation results demonstrate that the magnetic circuit improved with a single shimming has higher magnetic flux density and better magnetic field homogeneity than the one improved with no shimming ring or double shimming rings. The proposed magnetic circuit is manufactured and its experimental test platform is also built. The magnetic flux density measured in the work region is 0.7 T, which is well coincided with the theoretical design. The spatial variation of the magnetic field is within the range of the instrument error. At last, the temperature dependence of the magnetic flux density produced by the proposed magnetic circuit is investigated through both theoretical analysis and experimental study, and a linear functional model is obtained. The proposed research is crucial for solving the problem in the application of NMR-chip under different environmental temperatures.     

Thermostabilization of the Frequency of a Proton Relaxometer Based on Nuclear Magnetic Resonance with a Magnetic System on Permanent Magnets

Two variants of automatic correction of temperature variations of a constant magnetic field in the system of a permanent magnet NMR relaxometer are described. In the first version, the magnetic flux correction in the magnetic circuit of the system is used with a magnetic shunt, which is moved by a stepping motor. In the second version, the field is corrected by the current in the coils placed on the magnetic circuit of the magnetic system. Correction of the field is carried out automatically during the tuning of the resonant frequency of the device. The proposed methods make it possible to correct the relative change in the magnetic field within 1%, which covers possible temperature variations of the magnetic field in the entire range of room temperatures when using magnets based on neodymium–iron–boron and samarium–cobalt alloys (NdFeB and SmCo).

     

焊接电流影响GMAW双丝焊电弧等离子体的数值模拟研究

基于电磁学理论和流体力学理论,建立熔化极气体保护焊(Gas metal arc welding,GMAW)双丝焊焊接电弧等离子体三维数学模型,利用流体力学软件Fluent对其进行求解。重点研究焊接电流对GMAW双丝焊电弧等离子体行为的影响规律,获得了电弧温度、电流密度、热通量、磁场分布等结果。研究发现,随着焊接电流的变化,电弧等离子体形状变化显著。随着焊接电流的增大,电弧最高温度和电弧偏转角随之增大,电流密度和工件表面热通量由双峰分布转变为单峰分布,并且热通量峰值随焊接电流的增大而增大。此外,随着焊接电流的增大,磁感应强度和磁场力随之最大,磁场分布由独立两个磁场向耦合磁场转变。为有效、定量地证明模拟结果准确性,开展焊接试验,利用高速摄像监测电弧行为,利用光谱测温测量电弧温度。结果表明模拟结果同试验结果吻合良好,研究结果为合理选择GMAW双丝焊焊接电流参数提供理论依据。     

Temperature dependence dynamical permeability characterization of magnetic thin film using near-field microwave microscopy

A temperature dependence characterization system of microwave permeability of magnetic thin film up to 5 GHz in the temperature range from room temperature up to 423 K is designed and fabricated as a prototype measurement fixture. It is based on the near field microwave microscopy technique (NFMM). The scaling coefficient of the fixture can be determined by (i) calibrating the NFMM with a standard sample whose permeability is known; (ii) by calibrating the NFMM with an established dynamic permeability measurement technique such as shorted microstrip transmission line perturbation method; (iii) adjusting the real part of the complex permeability at low frequency to fit the value of initial permeability. The algorithms for calculating the complex permeability of magnetic thin films are analyzed. A 100 nm thick FeTaN thin film deposited on Si substrate by sputtering method is characterized using the fixture. The room temperature permeability results of the FeTaN film agree well with results obtained from the established short-circuited microstrip perturbation method. Temperature dependence permeability results fit well with the Landau-Lifshitz-Gilbert equation. The temperature dependence of the static magnetic anisotropy H(K)(sta), the dynamic magnetic anisotropy H(K)(dyn), the rotational anisotropy H(rot), together with the effective damping coefficient α(eff), ferromagnetic resonance f(FMR), and frequency linewidth Δf of the thin film are investigated. These temperature dependent magnetic properties of the magnetic thin film are important to the high frequency applications of magnetic devices at high temperatures.     

径向磁力轴承磁极布置对磁场和磁力的影响

使用有限元方法对8极径向磁力轴承的磁场进行了建模,计算了气隙磁通密度和磁力,对比分析了在转子不同的偏心情况下两种磁极布置形式(NSNS交替磁极布置和NNSS成对磁极布置)的磁力轴承的气隙磁通密度和磁力,并通过实验测试验证了有限元磁场建模和计算的准确性.结果表明,在同样大小的电流激励下,NSNS布置比NNSS布置的气隙磁通密度大,偏心时产生的磁力也较大,适合小尺寸的磁力轴承,而NSNS布置磁极之间的磁耦合比NNSS布置形式强,增加了控制系统的复杂性.研究结论对磁力轴承的结构设计和控制系统设计具有一定的指导意义.     

Asymmetric spin echo sequence and requirements on static magnetic field of NMR scanner

The article describes studies with two NMR sequences frequently used for static magnetic field distribution measurement. The gradient echo sequence is suitable for such measurements, it has good sensitivity and fast measurements are possible. The asymmetric spin echo sequence is also suitable for inhomogeneity measurements, but the time taken for measurement is longer. The purpose of the studies was the comparison of the features and the quality of results of both sequences under conditions of an imperfect static magnetic field. The imperfection deliberately was inserted into the static magnetic field of a low-field NMR scanner. Two common measurements were used for comparison, nevertheless due to the low-field NMR scanner the processing of the measured data was not entirely standard and processing using optimisation was required. The experiments revealed that the asymmetric spin echo sequence can provide images of a good quality and also under interference from a static magnetic field where measurement using the gradient echo sequence is not able to provide useable images.     

A proportional counter for low intensity heavy ion flow measurements

A proportional counter designed for heavy ion flux measurements is described. The working gas of the counter is air. The counter is composed of three parallel glass-fiber frames with wire electrodes coiled on them. Based on the counter, there has been developed a diagnostic product measuring the density distribution of accelerated ion flux for radioelectronic device testing. To prevent the dependence of measurement results on weather conditions (atmospheric pressure, temperature, humidity) as well as on the type and energy of ions, there has been applied an automatic correction methodology. In the process of the set-up operation it has been established, that the counter is characterized by a high radiation resource, resistance to extreme downloads, adequate performance of up to 10⁶ ions/s, it is not sensitive either to neutrons or to γ-quanta, and it is convenient to use.     

磁流变液响应时间检测方法及装置研究

磁流变液的响应时间是一个关键指标,关系到磁流变智能执行器的实时可控性能。 然而,MRF 的动态响应面临从电 流、磁场强度及剪切应力等多参数耦合中解耦的难题。 本文提出基于偏置正弦激励电流的相位检测法,通过施加偏置正弦电流 激励正弦磁通密度,由扭矩与磁场相位差获得 MRF 响应时间,无需面临阶跃响应中的复杂解耦问题。 此外,本文通过关键机械 参数设计,突破商用流变仪的剪切率限制,实现了最高 3 000 s -1 下的响应时间测量。 基于此,本文设计和制作了一套 MRF 响应 时间检测装置,分析了装配偏心和磁场探针位置引起的误差;测得多种样品在 1~ 3 A 振幅电流下的响应时间为 23 ~ 140 ms,并 发现响应时间随 MRF 浓度增大而先减小再延长,且与外部磁场强度呈负相关。     

基于磁悬浮轴承高速电主轴的法向磨削力检测方法

利用磁悬浮轴承高速电主轴作为检测器件解决高速磨削法向磨削力实时在线测量困难的问题。通过对磁悬浮轴承高速电主轴转子的动力学分析,得到法向磨削力与电主轴转子位移的两阶导数和转子受到的电磁力的关系。转子的位移数据可由磁悬浮轴承电主轴中的位移传感器的输出得到。为了精确求取电磁力,气隙磁通密度采用图解法求出,消除线性化引入的系统误差;气隙宽度由遍历搜索法标定,消除系统加工装配误差对电磁力测量带来的影响;引入修正系数对由漏磁、磁滞和边缘效应带来的测量误差进行补偿,修正系数由三层神经网络动态实时求出。通过静态和动态试验对电磁力和动态磨削力的检测结果进行验证,试验结果证明基于磁悬浮轴承高速电主轴的法向磨削力检测方法的有效性和准确性。     

脉冲漏磁检测的三维场特征分析及缺陷分类识别

漏磁检测方法广泛应用于石油、运输及化工等行业中金属的缺陷检测.介绍了漏磁检测原理,采用有限元法建立了三维缺陷脉冲漏磁检测模型,分析了缺陷脉冲漏磁场B_x、B_y和B_z分量的特点.结果表明,与传统漏磁检测系统提取缺陷漏磁场水平分量B_x和法向分量B_z进行缺陷识别相比较,三维缺陷脉冲漏磁场分量的提取将提供更多有关缺陷尺寸、位置等信息,尤其是当外加磁场方向与缺陷主平面近似平行时.最后给出了实验验证,实验结果与仿真分析有较好的一致性,这说明有限元仿真分析对实际脉冲漏磁检测系统的设计有重要的指导意义.     

MAGNETIC FIELD AND TEMPERATURE MEASUREMENTS WITH A MAGNETIC FLUID-FILLED PHOTONIC CRYSTAL FIBER BRAGG GRATING

A novel simultaneous measurement method for magnetic field and temperature is proposed based on the two-resonant peaks photonic crystal fiber Bragg grating (PCFBG) filled magnetic fluid. The measuring principles are introduced in detail and verified by experiments. Sensitivity coefficients were achieved theoretically and experimentally and found to be consistent. The errors of temperature and magnetic field sensitivities were indicated theoretically and experimentally. The measurement resolutions were given. The results show that the scheme is feasible. It solves the cross-sensitivity problem between temperature and magnetic field of the fiber grating sensor, realizes dual-parameter measurement using single fiber that reduces greatly the size of the sensing probe, and provides a new method for the two-parameter measurements.     

Characterization of instrument drift using a spherical artifact

Drift is a common and inevitable error source in measurements. Currently there are two main approaches to address instrument drift in image or area-based measurements, drift calibration with target tracking and active feedback correction. We propose an alternative approach to drift calibration for profilometers, particularly high speed instruments such as confocal microscopes or scanning white light interferometers. The method is based on sequential measurements of a spherical artifact whose diameter is larger than the field of view. A best fit sphere algorithm is used to determine the movement of the spherical artifact's center over time. This reduces drift measurement uncertainty because it uses height data over the full field of view, compared to target tracking strategies that involve tracking small features. Simulation results show that under practical conditions, e.g. with typical noise levels and typical drift rates, this method is quite effective and can yield measurements with low uncertainty. The measurement is demonstrated on a commercial confocal microscope to determine drift rate magnitude and direction.     

An automatic setup for measuring low-field temperature dependencies of magnetoresistances of film samples

A simple method for magnetoresistance measurements in weak fields based on using a low-frequency alternating magnetic field is proposed. The special feature of this method is the direct measurement of the difference in resistance ΔR of the sample under study in the field and in the absence of the field, which offers a chance to significantly increase measurement sensitivity. The automatic setup for measuring temperature dependencies of magnetoresistance of film metallic samples with a ΔR sensitivity of up to 10^?7 Ω is described. The temperature dependencies of magnetoresistance of thin-film nanocrystalline Co-Cu alloy samples in weak magnetic fields and in a temperature range of 77–300 K are studied.     

基于FLANN和最小二乘的磁梯度计误差校正

在基于偶极子磁场分量梯度的水下磁异常定位方法中,三轴磁力计自身误差及两磁场坐标系配准误差等是限制水下定位精度的主要因素,因此有必要对其进行校正,补偿磁场分量梯度计测量值。建立了磁场分量梯度计的测量误差模型,提出了基于函数链接型神经网络(functional link artificial neural network,FLANN)和最小二乘法的磁场分量梯度计误差校正方法,给出了误差参数辨识及校正算法,数值仿真和实测数据证明了校正算法具有良好的收敛性,能显著地抑制磁场分量梯度测量误差,该校正方法为提高磁场分量梯度计性能提供了一种可行途径。     

Study of spatial characteristics of near-coastal wind fields using UAV technology

Due to the problems of complex structure, high cost, and limited measurement range of traditional wind field measurement instruments, this paper used a combination of a modified anemometer and a multi-rotor UAV to make realistic measurements of the spatial wind field in near-shore waters. The feasibility of the equipment in the measurement is also demonstrated. To solve the problem that conventional instruments cannot acquire wind field data from multiple measurement points simultaneously, the paper introduces the dimensionless method and demonstrates the feasibility of the method through multiple perspectives. The measured data were noise-reduced and dimensionless processed to obtain the relationships between the dimensionless parameters between measurement points. The comparison analysis was carried out from different heights and directions to investigate the variation of the parameters in each plane and at each measurement point, and it was concluded that the wind field characteristics in a limited range can be inferred from a point within the height in the near-shore area. This conclusion offers the possibility of using a few measurement points to estimate and predict the wind field characteristics in nearshore waters.     

A high-temperature and high-field nuclear magnetic resonance instrument

In the course of constructing an apparatus suitable for studies of nuclear magnetic resonance at high temperatures and high fields we developed a furnace with a new heating element which has been applied for the first time to NMR studies. The furnace can work in a 8-T field of a superconducting magnet system at temperatures up to 2200 K. Because of its geometrical form the heating element does not cause any measurable disturbing magnetic fields resulting from the heating current, and has many advantages compared to conventional heating elements. The temperature at which, up to this moment, we could observe NMR in external fields of 8 T amounts to 1200 K. The specifically adapted NMR probe works at radio frequencies up to 50 MHz. With this apparatus we carried out measurements of the magnitude, sign, and temperature dependence of the Knight shift of Mo in molybdenum metal.     

漏磁法智能管道壁厚检测系统的研究

基于漏磁原理 ,针对目前国内急需的对铁磁性材料管道壁厚快速检测研制成了智能式壁厚快速检测系统 ,通过标定实验进一步验证了该检测方法的可行性。突出特点是采用了电磁励磁的方法 ,避免了永磁励磁法在工作时始终吸紧管道而产生行走难、易磨损、寿命短、成本高的不利因素。通过实验证明该装置具有操作简单 ,易于实现自动检测。     

Modeling magnetic fields measured by surface probes embedded in a cylindrical flux conserver

Calculating magnetic fields at the surface of a flux conserver, perfect conductor, for displaced plasma currents is useful for understanding modes of a Z-pinch. The magnetic fields measured at the flux conserver are a sum of the magnetic fields from the plasma current and the eddy currents which form in the walls to keep the flux constant. While the magnetic field at the wall from the plasma current alone is easily calculated using the Biot-Savart law, finding the eddy currents in the flux conserver which satisfy the boundary conditions can be a tedious process. A simple method of calculating the surface magnetic field for a given Z-pinch displacement off-axis is derived for a cylindrical flux conserver. This relationship does not require the explicit calculation of the eddy currents, saving time when analyzing surface magnetic probe measurements. Analytic expressions can be used to describe the surface magnetic field which increase the understanding of the magnetic probe measurements.     

High precision metrology based microwave effective linewidth measurement technique

A precision microwave effective linewidth measurement technique for magnetic samples has been developed. The measurement utilizes a high-Q cylindrical cavity that contains the sample of interest, a highly stable and programable static magnetic field source, a computer controlled network analyzer for cavity center frequency omega c and quality factor Qc determinations, and the standard metrological substitution ABA method for accurate relative omega c and Qc measurements. Sequential long term ABA measurements show that the time and temperature drifts and random errors are the dominant sources of error, with uncertainties in omega c/2pi and Qc in the range of 50 kHz and 25, respectively. The ABA method is applied to eliminate these drifts and minimize the random errors. For measurements over 25 ABA cycles, accuracy is improved to 0.14 kHz for omega c/2pi and 3 for Qc. The temperature variation over a single ABA cycle is generally on the order of 10(-3)-10(-5) degrees C and there is no need for any further temperature stabilization or correction measures. The overall uncertainty in the 10 GHz effective linewidth determinations for a 3 mm diam, 0.5 mm thick polycrystalline yttrium iron garnet (YIG) disk is 0.15 Oe or less, well below the intrinsic single crystal YIG linewidth. This represents a factor of 10 improvement in measurement accuracy over previous work.     

DEVELOPMENT OF THE CONTROL METHODOLOGY OF THE GIANT MAGNETOSTRICTIVE ACTUATOR BASED ON MAGNETIC FLUX DENSITY

According to the principle of the magnetostriction generating mechanism, the control modelof giant magnetostriction material based on magnetic field and the control method with magnetic fluxdensity are developed. Furthermore, this control method is used to develop a giant magnetostrictivemicro-displacement actuator (GMA) and its driving system. Two control methods whose control vari-ables are current intensity and magnetic flux density are compared with each other by experimentalstudies. Finally, effective methods on improving the linearity and control precision of mi-cro-displacement actuator and reducing the hysteresis based on the controlling magnetic flux densityare obtained.