Linearized Proximal Alternating Direction Method of Multipliers for Parallel Magnetic Resonance Imaging

In this study, we propose a linearized proximal alternating direction method with variable stepsize for solving total variation image reconstruction problems. Our method uses a linearized technique and the proximal function such that the closed form solutions of the subproblem can be easily derived.In the subproblem, we apply a variable stepsize, that is like Barzilai-Borwein stepsize, to accelerate the algorithm. Numerical results with parallel magnetic resonance imaging demonstrate the efficiency of the proposed algorithm.

Magnetic Method Surveying and Its Application for the Concealed Ore-Bodies Prospecting of Laba Porphyry Molybdenum Ore Field in Shangri-La, Northwestern Yunnan Province, China

Recently, a number of large molybdenum (-copper) deposits have been discovered successively in the Laba area, Shangri-La county, northwestern Yunnan province. The investigation confirmed that there is a superlarge porphyry-skarn hydrothermal vein type molybdenum-polymetallic- metallogenic system with the total prediction reservoir of more than 150 mt molybdenum. The porphyry intrusions contributed to the mineralization closely, the superficial little vein molybdenum (-copper, lead, silver) ore-bodies are usually located in faults and fractures, and the deep porphyry type ore-bodies occurred in the granodiorite porphyries, the skarn type ore-bodies occurred in the contact zone intrused into Triassic limestone or Permian basalts. Laba ore block is a new exploration area with great prospecting potential. In order to reduce the target area and guide the further exploration work, the magnetic method measurement about 3.3 square kilometres was carried out in the ore field. This paper presents an application of analyzing the horizontal and vertical derivative, using Fast Fourier Transform (FFT) filter (FFT high-pass, low-pass, cosine roll-off, suscepbility), calculated spectra frequency energy to predict the depth and intensity of the apparent remanence magnetization of source (Hilbert). The calculated results and magnetic anomalous show that the remanence anomaly is caused by the intrusions into the Triassic limestone and Permian basalts with small anomalies, and the depth of located source is not great. We have identified a number of positions to the three drilled well, the drilled result specify interpretation with very high accuracy. The magnetic method is helpful to identify porphyry mineralization, and judge the shape and depth of the concealed ore-bearing intrusive bodies under the similar geological condition.

Generalized Nonconvex Low-Rank Algorithm for Magnetic Resonance Imaging (MRI) Reconstruction

In recent years,utilizing the low-rank prior information to construct a signal from a small amount of measures has attracted much attention.In this paper,a generalized nonconvex low-rank(GNLR) algorithm for magnetic resonance imaging(MRI)reconstruction is proposed,which reconstructs the image from highly under-sampled k-space data.In the algorithm,the nonconvex surrogate function replacing the conventional nuclear norm is utilized to enhance the low-rank property inherent in the reconstructed image.An alternative direction multiplier method(ADMM) is applied to solving the resulting non-convex model.Extensive experimental results have demonstrated that the proposed method can consistently recover MRIs efficiently,and outperforms the current state-of-the-art approaches in terms of higher peak signal-to-noise ratio(PSNR) and lower high-frequency error norm(HFEN) values.

双并列转子永磁直驱电机转子不平衡力分析与优化

双并列转子永磁直驱电机受轴间距限制,设计时会切掉部分定子铁心和绕组,造成转子电磁力不平衡。为最大限度地抑制转子所受的不平衡电磁力,该文从双并列转子结构出发,分析了电机绕组电感参数不对称的原因,提出通过合理设计线圈跨距和分布平衡三相电感。在此基础上,构建转子所受总电磁力的数学模型,理论分析各个分力的来源及影响因素,研究各个分力对转子所受总电磁力的影响,提出了优化占空角、优化耦合间距、采用定子端部不对称结构、采用转子偏心结构和切除部分单元电机五种优化方法,实现对转子所受电磁力的优化,探讨了不同方法的优化效果,以及其对转矩脉动的影响,总结了不同优化方法的优缺点。

面向光栅刻划机的负刚度结构隔振性能优化

大面积光栅刻划机实验室存在的复杂振动因素,会对它的运行及刻画精度造成影响,针对光栅刻划机设计一种正负刚度并联的被动隔振结构,改善仅有空气弹簧时对低频隔振无效的现象,拓宽隔振频带。为模拟隔振器隔振性能,建立系统的动力学模型和动力学方程,使用仿真软件MATLAB进行仿真,为磁致负刚度弹簧进行尺寸结构设计,对比并联磁致负刚度弹簧前后的隔振效果。仿真结果表明采用空气弹簧和磁致负刚度结构并联后能够有效的隔离低频振动,并且磁吸致负刚度和磁斥致负刚度进行组合能够有效的改善单一磁吸致负刚度或磁斥致负刚度结构的非线性。

考虑初始风速与机端故障稳态电压跌落程度的直驱风机故障响应特性分类及其判别方法研究

对电力系统进行安全分析时,往往需要对各种预想场景进行仿真。随着风电的大量接入,对风电场提出适用于预想故障分析的等值建模方法变得十分重要,而根据风机故障后的动态特性对其进行分群则是等值建模的基础。各风机在故障后的动态响应特性不仅受其控制方式的影响,还受到运行风速和故障严重程度的影响,而一般情况下风机控制的响应速度较快,可以认为在故障持续期间风机能够运行至故障稳态。基于此,提出考虑初始风速及机端故障稳态电压跌落程度的风电机组故障响应特性分类及判别方法,以此作为风机分群的依据。首先针对永磁直驱风机(direct-driven permanent magnet synchronous generator,PMSG)故障过程中无功优先控制、故障恢复期间有功斜坡恢复的控制策略,分析其在不同风速条件下故障后所有可能的响应特性,将初始风速与故障稳态电压作为分类指标,从机理上找到直驱风机进入各类响应特性的边界条件;然后,提出利用预想故障信息的风机机端故障稳态电压计算方法,在此基础上提出适用于预想故障的直驱风机分群方法;最后,通过对风电场进行详细建模仿真,验证所提分群方法的正确性。

基于调制系数最优法的TLDMC-PMSM新型直接转矩控制

针对矩阵变换器永磁同步电机系统(MC-PMSM)直接转矩控制方法出现的转矩磁链脉动大、幅值失调、系统鲁棒性弱等问题,提出了一种基于调制系数最优法的三电平直接矩阵变换器永磁同步电机(TLDMCPMSM)新型直接转矩控制方法。首先描述了MC-PMSM直接转矩控制系统的原理;其次用三电平直接矩阵变换器替换两电平矩阵变换器,转速环通过复叠式螺旋滑模控制、转矩磁链环通过超螺旋滑模直接转矩控制替换传统的比例积分控制,并且引入调制系数最优法来抑制电机转矩波动逐渐增大,进而实现对永磁同步电机的精确控制;最后通过MATLAB/Simulink仿真软件和硬件实验验证所提方法的可行性与优越性。

Graph Regularized Sparse Coding Method for Highly Undersampled MRI Reconstruction

The imaging speed is a bottleneck for magnetic resonance imaging( MRI) since it appears. To alleviate this difficulty,a novel graph regularized sparse coding method for highly undersampled MRI reconstruction( GSCMRI) was proposed. The graph regularized sparse coding showed the potential in maintaining the geometrical information of the data. In this study, it was incorporated with two-level Bregman iterative procedure that updated the data term in outer-level and learned dictionary in innerlevel. Moreover,the graph regularized sparse coding and simple dictionary updating stages derived by the inner minimization made the proposed algorithm converge in few iterations, meanwhile achieving superior reconstruction performance. Extensive experimental results have demonstrated GSCMRI can consistently recover both real-valued MR images and complex-valued MR data efficiently,and outperform the current state-of-the-art approaches in terms of higher PSNR and lower HFEN values.

Two-Level Bregman Method for MRI Reconstruction with Graph Regularized Sparse Coding

In this paper, a two-level Bregman method is presented with graph regularized sparse coding for highly undersampled magnetic resonance image reconstruction. The graph regularized sparse coding is incorporated with the two-level Bregman iterative procedure which enforces the sampled data constraints in the outer level and updates dictionary and sparse representation in the inner level. Graph regularized sparse coding and simple dictionary updating applied in the inner minimization make the proposed algorithm converge with a relatively small number of iterations. Experimental results demonstrate that the proposed algorithm can consistently reconstruct both simulated MR images and real MR data efficiently, and outperforms the current state-of-the-art approaches in terms of visual comparisons and quantitative measures.

大型永磁直驱发电机磁固耦合特性研究

该文开展大型永磁直驱发电机磁固耦合特性研究。采用共形映射方法,计算考虑齿槽效应的气隙磁场标量磁势;基于离散电流元和磁等效电路模型,定量模拟发电机定转子局部的磁饱和效应,通过数值迭代得到气隙磁场强度分布;引入一系列均布的磁等效弹簧,模拟定转子之间的耦合作用,建立永磁直驱发电机磁固耦合有限元分析模型。以实际在役兆瓦级永磁直驱发电机为对象,通过电磁有限元计算结果验证所提出的气隙磁场强度分析方法;基于金风兆瓦级永磁发电机测试平台,开展转/定子耦合系统模态测试,与理论模型得到的固有频率和模态振型进行对比。在此基础上,分析气隙长度和转子转速对耦合系统固有频率的影响。研究结果对于准确评估发电机振动能量、规避危险共振点指导机组设计等均具有重要的参考价值。

利用海底磁场观测数据估算海浪运动方向

本文提出一种利用海底磁场观测数据获得海浪运动方向信息的方法。首先,基于电磁感应原理,推导出海底地形水平条件下三维海浪感应磁场表达式,据此可以计算海浪感应磁场,并可以得到海浪感应磁场方向。理论分析表明,海浪感应磁场的方向与海浪运动方向一致,这为利用海浪感应磁场估计海浪运动方向提供了方法理论基础。本文从南黄海海底观测磁场数据中提取了海浪感应磁场,然后采用时频方向谱方法(Spectral-Directionalogram,简称SD)估算出海浪运动方向谱,即海浪运动方向,并与第三代海浪模式WAVEWATCHⅢ的海浪运动方向进行对比。在平潮期内两者吻合较好,误差在10°以内,但在涨落潮时,基于海底磁场数据估计的海浪运动方向谱精度较低。

矿用永磁直驱电机异响声信号降噪方法研究

针对矿用永磁直驱电机异响声信号噪声干扰大,有用信号被噪声淹没难以提取的问题,提出一种融合改进VMD与小波软阈值的降噪方法。首先,利用粒子群算法优化变分模态分解算法得到分解层数k和惩罚因子α的最优参数组合,基于最优参数组合分解获得矿用永磁直驱电机异响声信号k个本征模态分量(IMF)。其次,利用加权裕度指标筛选出有效信号分量和需进一步分解的含噪分量,基于小波软阈值对含噪分量进一步降噪。最后,将有效信号分量与小波软阈值降噪后的分量重构得到最终降噪信号。应用此方法分别对仿真信号和矿用永磁直驱电机异响声信号降噪,并与其他方法对比。试验结果表明,该方法能将仿真信号信噪比提升至27.5247 dB,均方根误差降低至0.0855,实测信号信噪比提升至34.7153 dB,均方根误差降低至0.0067,降噪效果较好,为后续的故障特征提取与故障诊断工作提供数据基础。

永磁直驱风电全功率变流器并联控制方法研究

为提高永磁直驱风电全功率变流器并联控制精度,研究了永磁直驱风电全功率变流器并联控制方法。首先分析永磁直驱风电全功率变流器并联拓扑结构,建立永磁直驱风电全功率变流器模型,再计算无功功率受有功功率波动造成电压幅值产生波动,结合虚拟阻抗控制方法,实现对永磁直驱风电全功率变流器的并联控制。实验结果表明,采用该方法控制永磁直驱风电全功率变流器并联运行的机侧环流与网侧环流波动较为稳定,且环流控制精度较高,最高可达到99%,验证了该方法的控制性能。

针对预想故障的风电集群电磁暂态等值建模方法

基于现有针对单个风电场的等值建模方法,提出一种适用于风电集群的等值建模方法,利用CloudPSS-XStudio套件完成了风电集群等值建模系统开发,该系统将预想故障选择、等值参数计算和结果分析进行了集成,为含大规模风电集群的电力系统的动态安全分析提供了支撑。以各风电场平均风速以及预想故障作为输入,基于等值模型的迭代仿真得到各个风电机组的分群指标,并完成对风电集群的分群等值建模。以某实际风电集群拓扑接入IEEE 39节点系统作为测试算例,验证了所提方法的正确性。

A Generalized Two-Level Bregman Method with Dictionary Updating for Non-Convex Magnetic Resonance Imaging Reconstruction

In recent years, it has shown that a generalized thresholding algorithm is useful for inverse problems with sparsity constraints. The generalized thresholding minimizes the non-convex p-norm based function with p < 1, and it penalizes small coefficients over a wider range meanwhile applies less bias to the larger coefficients.In this work, on the basis of two-level Bregman method with dictionary updating(TBMDU), we use the modified thresholding to minimize the non-convex function and propose the generalized TBMDU(GTBMDU) algorithm.The experimental results on magnetic resonance(MR) image simulations and real MR data, under a variety of sampling trajectories and acceleration factors, consistently demonstrate that the proposed algorithm can efficiently reconstruct the MR images and present advantages over the previous soft thresholding approaches.

融合局部低秩先验与Bloch流形约束的磁共振指纹重建算法

为了实现快速成像,磁共振指纹(Magnetic Resonance Fingerprinting,MRF)技术通常使用非笛卡尔稀疏采样模板对K空间进行高度欠采样,从而获得稀疏K空间信号.然而,从稀疏的K空间信号重建像空间数据是一个病态不适定问题,重建出的MRF像空间数据存在大量的混叠伪影,直接影响到组织生理参数的重建准确度.为此需要将各种先验知识引入重建模型之中,以缓解MRF重建问题的不适定性.针对上述问题,本文提出一种融合局部低秩先验与Bloch流形约束的MRF重建模型,并使用交替方向乘子法(Alternating Direction Method of Multipliers,ADMM)求解模型中的非凸MRF重建问题.本文算法在引入MRF像空间数据的局部低秩先验的同时,使用预先构建的字典为重建指纹提供流形约束.一方面通过空域局部低秩约束有效抑制混叠伪影的产生,另一方面利用字典先验避免指纹的时域流形特征在迭代重建过程中丢失.仿真实验结果表明,相较于引入了全局低秩先验与Bloch流形约束的其他同类算法,本文算法可以提供更高的组织参数重建准确度.

基于非局部低秩约束的改进灵敏度编码重建算法

灵敏度编码(Sensitivity encoding,SENSE)是一种应用广泛的并行磁共振成像(Magnetic resonance imaging,MRI)重建模型。目前已有的针对SENSE模型的改进方法的重建图像中依然有较多伪影,尤其在较高加速因子时很难重建出比较清晰的图像。因此,本文基于非局部低秩约束(Nonlocal low-rank,NLR),提出了一种改进的SENSE模型,称为NLR-SENSE。该模型使用加权核范数作为秩代理函数,并使用交替方向乘子法(Alternating direction multiplier method,ADMM)进行求解。仿真实验结果表明,与其他几种并行磁共振成像方法相比,NLR-SENSE方法在视觉比较和3个不同的客观指标上均表现优异,能有效提升重建图像的质量。

Compute extremely low-frequency electromagnetic field exposure by 3-D impendance method

A 3-D impedance method has been introduced to compute the electric currents induced in a human body exposed to extremely low-frequency electromagnetic field. The 3-D impedance method has been deduced from Maxwell equations and is put into the computation and simulation effectively to the visible human body model, which has 196×114×626 cells and more than 40 types of tissues. As the result, two representative cases are investigated. One is exposure of the human body to 100 μT （1 000 mG）, the limit recommended by the International Commission on Non-Ionizing Radiation Protection for the public and the other one is the exposure of human body to 0.4 laT （4 mG）, the level at which a statistical link appears with a doubled risk of development of childhood leukaemia. The distribution of induced current density can be obtained and the maximum of induced current are found to be 16 mA/m^2 and 0.07 mA/m^2.

磁共振方法在堤坝渗漏探测中的实验

为精确定位堤坝渗漏点位置,介绍了一种能直接定量探测水的地球物理方法——磁共振法。通过阐述磁共振方法的基本原理和反演算法,确定适用于堤坝渗漏探测的线圈类型和测量方式。采用吉林大学自主研制的JLMRS型磁共振探水仪,在辽宁桓仁崔家街堤坝坝顶铺设4匝边长为12.5m的正方形线圈,连续测量15个点,测量深度达35m。经过数据处理和反演解释,得到坝体二维含水量剖面图,为消除上游水体对实验结果的影响提出了修正方法,进而得到坝体二维相对含水量剖面。实验结果表明:崔家街大坝存在3个渗漏点(距溢洪道140、180和240m处)和2个渗透区(距溢洪道100～150m和170～250m处);深度为-20～-10m,为砾卵石层渗漏,与工程地质资料和下游渗水情况相符。

低磁纬度地区ΔT异常解释的途径与方法

我国海南省诸岛礁及广阔的南海海域,位于磁赤道带两侧的低磁纬度区内,在以水平磁化为主的环境中,磁性物体所产生的ΔT异常特征与处于中纬度地区的中国大陆广大地区的ΔT异常特征差别较大,长期以来,对其认识与解释都感到困惑。通过对实测资料的应用与研究,分析了不同磁化条件下,ΔT、Za、Ha异常的关系,提出了低磁纬度地区ΔT异常的解释思路、“倒相180°解释方法”,改进了低磁纬度地区化赤、化极等磁化方向转换的稳定性和变倾角转换问题,论述了ΔT总导数模量定性与定量解释方法等。认为不仅可解决低磁纬度地区的磁异常解释问题,而且在对全球磁异常的认识与解释时也有重要作用。