无VOC助焊剂的无铅波峰焊工艺探讨

概述了一种新型绿色助焊剂——无挥发性有机化合物(VOC)助焊剂。该助焊剂有无松香、无卤素、免清洗、无污染、方便储存和运输等优点,已经成为助焊剂领域的发展方向。讨论了此助焊剂在无铅波峰焊中应用时需要注意的问题。结果表明,当电路板的预热温度控制在110～120℃,轨道倾角控制在5°～7°,印制板引线脚与焊料的接触时间3～5 s,焊接温度260℃并采用喷雾涂敷方法时,可达到理想的焊接效果。     

Improved model-based magnetic resonance spectroscopic imaging

Model-based techniques have the potential to reduce the artifacts and improve resolution in magnetic resonance spectroscopic imaging, without sacrificing the signal-to-noise ratio. However, the current approaches have a few drawbacks that limit their performance in practical applications. Specifically, the classical schemes use less flexible image models that lead to model misfit, thus resulting in artifacts. Moreover, the performance of the current approaches is negatively affected by the magnetic field inhomogeneity and spatial mismatch between the anatomical references and spectroscopic imaging data. In this paper, we propose efficient solutions to overcome these problems. We introduce a more flexible image model that represents the signal as a linear combination of compartmental and local basis functions. The former set represents the signal variations within the compartments, while the latter captures the local perturbations resulting from lesions or segmentation errors. Since the combined set is redundant, we obtain the reconstructions using sparsity penalized optimization. To compensate for the artifacts resulting from field inhomogeneity, we estimate the field map using alternate scans and use it in the reconstruction. We model the spatial mismatch as an affine transformation, whose parameters are estimated from the spectroscopy data.     

Improved reconstruction for MR spectroscopic imaging

Sensitivity limitations of in vivo magnetic resonance spectroscopic imaging (MRSI) require that the extent of spatial-frequency (k-space) sampling be limited, thereby reducing spatial resolution and increasing the effects of Gibbs ringing that is associated with the use of Fourier transform reconstruction. Additional problems occur in the spectral dimension, where quantitation of individual spectral components is made more difficult by the typically low signal-to-noise ratios, variable lineshapes, and baseline distortions, particularly in areas of significant magnetic field inhomogeneity. Given the potential of in vivo MRSI measurements for a number of clinical and biomedical research applications, there is considerable interest in improving the quality of the metabolite image reconstructions. In this report, a reconstruction method is described that makes use of parametric modeling and MRI-derived tissue distribution functions to enhance the MRSI spatial reconstruction. Additional preprocessing steps are also proposed to avoid difficulties associated with image regions containing spectra of inadequate quality, which are commonly present in the in vivo MRSI data     

基于全天空流星雷达测量动量通量的新方法

全天空流星雷达是新一代的流星雷达, 可以接收流星余迹反射的无线电波, 目前主要应用于探测流星烧蚀区域的大气风场进而研究该区域的大气动力学.文中利用全天空流星雷达, 基于Hocking提出的测量重力波动量通量的新方法来研究昆明地区的大气环境参数—重力波的动量通量.由于采样时间间隔的不同, 该方法仍存在争议.本文主要利用昆明站工作频率为37.5 MHz的全天空流星雷达测得的流星数据来验证采样时间对重力波动量通量取值的影响, 同时利用工作频率分别为37.5 MHz、53.1 MHz的全天空流星雷达在2014年9月份观测的数据对该方法进行了对比分析, 结果表明:采样时间间隔的选取对重力波动量通量的取值有很大影响; 在合适的采样时间间隔内利用全天空流星雷达测量重力波的动量通量的新方法是可行的.     

红外高通量干涉成像光谱仪的设计与验证

介绍了静态高通量干涉成像光谱技术的原理,针对红外波段目标辐射能低、系统易受自身辐射影响等特点,提出了基于分体式Sagnac干涉仪的红外高通量干涉成像光谱仪方案。分析了Sagnac干涉仪的设计方法,特别是分光镜对光线的偏折作用引起的横向剪切量和光线的折射光程不同带来的附加光程差。根据采样定理,计算了成像光谱系统对探测器和后置成像系统的要求。使用长波热像仪(光谱范围7.7~9.3μm)搭建了实验装置,实验结果表明,这种成像光谱系统能够准确测量出样品的光谱,验证了该方案的原理和设计。     

Passive millimetre wave imaging

The authors discuss the current status of passive millimetre wave radiometry as an imaging technique. The major problems are poor spatial resolution and lack of thermal sensitivity. Techniques for overcoming these difficulties are identified, including the use of aperture synthesis, multichannel receivers, correlation and inverse transform techniques     

Estimation of the flux linkage in a direct-torque-controlled drive

An improved integration method is presented for the estimation of the stator flux linkage for speed- and position-sensorless direct-torque-controlled AC machine drives. The method is based on monitoring the scalar product of the estimated stator flux linkage and the measured stator current. The AC part of the scalar product is extracted using filtering and the correction for the estimated stator flux linkage is formed from that part. Adequate performance is obtained by using simple low-pass filtering. By using adaptive filtering in the extraction, the performance of the drive is excellent. Both simulation and laboratory test results are presented, which show that the presented method works well both in steady state and in transients.     

Estimation of dynamically evolving ellipsoids with applications to medical imaging

The estimation of dynamically evolving ellipsoids from noisy lower-dimensional projections is examined. In particular, this work describes a model-based approach using geometric reconstruction and recursive estimation techniques to obtain a dynamic estimate of left-ventricular ejection fraction from a gated set of planar myocardial perfusion images. The proposed approach differs from current ejection fraction estimation techniques both in the imaging modality used and in the subsequent processing which yields a dynamic ejection fraction estimate. For this work, the left ventricle is modeled as a dynamically evolving three-dimensional (3-D) ellipsoid. The left-ventricular outline observed in the myocardial perfusion images is then modeled as a dynamic, two-dimensional (2-D) ellipsoid, obtained as the projection of the former 3-D ellipsoid. This data is processed in two ways: first, as a 3-D dynamic ellipsoid reconstruction problem; second, each view is considered as a 2-D dynamic ellipse estimation problem and then the 3-D ejection fraction is obtained by combining the effective 2-D ejection fractions of each view. The approximating ellipsoids are reconstructed using a Rauch-Tung-Striebel smoothing filter, which produces an ejection fraction estimate that is more robust to noise since it is based on the entire data set; in contrast, traditional ejection fraction estimates are based only on true frames of data. Further, numerical studies of the sensitivity of this approach to unknown dynamics and projection geometry are presented, providing a rational basis for specifying system parameters. This investigation includes estimation of ejection fraction from both simulated and real data.     

Millimeter wave imaging lens antenna

The factors which influence the design of lenses suitable for imaging at millimetre wavelengths are discussed. Examples are given of optical designs for a military tracking application, firstly for use with an open ended waveguide array, and secondly for use with monolithic arrays, each covering a field of view of ±20°.     

Cross-borehole compressional wave imaging of unconsolidatedsediments

Cross-borehole elastic wave tomography offers great potential for nondestructive imaging of near-surface sediments, fluids, and soils. This method was used to create a high-resolution tomographic image of velocity structure from simulated sedimentary layers obtained using compressional wave travel times. The image shows three distinct layers and a water-filled container located in the center of the middle layer. An image obtained from computer-simulated travel times shows good agreement with the image from laboratory data. The high-quality data and the resulting image were due in part to a novel transmitter and receiver design. These were composed of cylindrical piezoelectric transducers individually sealed in oil-filled Teflon containers and stacked to form stringers. The transmitters were extremely efficient at coupling energy into the medium. The transmitter frequency was 135 kHz, velocities ranged from 1.4 to 1.8 km/s, and thus wavelengths were 1.0-1.4 cm. The distance between boreholes was 30 cm, with about 60 cm of vertical coverage     

A generalized series approach to MR spectroscopic imaging

The problem of precise spatial localization of spectral information in magnetic resonance (MR) spectroscopic imaging is addressed. A novel method, called GSLIM (generalized spectral location by imaging), is proposed to make possible the marriage of high-resolution proton imaging with spectroscopic imaging and localization. This method improves on the conventional Fourier series inversion method used in chemical shift imaging (CSI) and the compartmental modeling method used in SLIM by using a generalized series framework for optimal representation of the spectral function. In this way, a priori information extracted from proton imaging can be used, as in SLIM, and the robustness and data consistency of CSI are also retained. Simulation results show that GSLIM can significantly reduce spectral leakage in CSI and inhomogeneity errors in SLIM. It can also reveal compartmental inhomogeneities, and can easily be extended to handle other a priori constraints when necessary. This approach, with some further development, may achieve an optimal combination of sensitivity, quantitative accuracy, speed, and flexibility for in vivo spectroscopy.     

带电粒子在二维中心势和磁通规范场中的运动和分数角动量

以二维中心势和磁通规范势场形成的多连通空间为例,从经典和量子两方面研究了带电粒子的运动轨道和波函数,结果表明：波函数几率云的空间分布与经典轨道完全相同;此外,角动量的分数量子化可以通过量子-经典对应条件唯一确定,而磁通规范势仅使所有波函数产生一共同的拓扑相位,并不改变量子化条件。     

HAADF-STEM imaging with sub-angstrom probes: a full Bloch wave analysis

A full coherent Bloch wave calculation is presented to investigate high-angle annular dark-field image formation for sub-angstrom probes in scanning transmission electron microscopy (STEM). With increasing illumination angle, the contribution of the 1s bound state increases to a maximum at an optimum probe angle, after which we find increasing contributions from high-angle plane wave states around the periphery of the objective aperture. Examination of image contributions from different depths within a crystal shows an oscillatory behavior due to the beating between 1s and non-1s states. The oscillation period reduces with decreasing probe size, while the relative contribution from a specific depth increases. This signifies a changeover from a projection mode of imaging to a depth-slicing mode of imaging. This new mode appears capable of resolving three-dimensional atomic structures in future generation aberration-corrected STEM.     

Estimation of ocean wave spectra using two-antenna SAR systems

Data from synthetic aperture radar, (SAR) systems can be used to estimate ocean wave directional spectra, but the method is limited by nonlinearities associated with the velocity bunching mechanism and the azimuth falloff effect, which reduces the range of azimuth wavelengths that can be observed. A theoretical analysis which suggests that the use of two or more receive antennas, spaced in the along-track direction, may reduce these limitations is presented. Specifically, the band of usable azimuth wavenumbers is shifted by an amount proportional to the antenna spacing, so that a broader range of wavenumbers can be covered by combining the spectrum obtained from the two-antenna signals with the conventional image spectrum     

Quantum flux parametron with magnetic flux regulator

A magnetic flux regulator for a quantum flux parametron (QFP) is proposed. The regulator is formed with a parallel connection of a Josephson junction across activation inductors building an RF-SQUID (superconducting quantum interference device). This junction makes a nonlinear bypass channel for magnetic activation flux and contributes to regulation of applied flux and elimination of undesirable external noises. Using this flux regulator, the activation condition tolerance is not only improved, but also a higher speed transition from a nonactivated state to an activated state is expected. QFP operations with the regulators have been observed with integrated circuits     

Radio wave imaging by an obstacle scanning

A new radio-wave imaging technique was demonstrated with a 9.43 GHz microwave, where an obstacle was scanned in the plane before or behind objects placed in the radiation field and the radio-wave signals disturbed by the obstacle were displayed by a CRT as shadow images.     

Magnitude and phase in thermal wave imaging

In thermal wave imaging on solids both magnitude and phase angle are used. However, a comparison between theoretical and experimental results is lacking even for the simple case of an air-backed metal plate. In the letter we present measurements and point out the difference between sinusoidal and square-wave intensity modulation.     

面向人体安检的毫米波转台成像实验

提出2种基于前视和斜下视圆周扫描的毫米波人体安检方案,该方案采用1对收发分置天线沿圆周扫描,利用矢量网络分析仪、喇叭天线和高精确度转台搭建了毫米波成像实验平台。为了验证2种方案及相应的波数域算法的有效性,通过毫米波转台实验对多个目标反演成像,并得到了良好的实验结果。实验结果表明,本文提出的2种毫米波人体安检方案都具有一定的可行性。