实现小波变换图像识别的光电混合实验系统

本文依据联合变换相关器,设计一套实现小波变换图像识别的光电混合实验系统,联合变换功率谱及相关输出强度分布分别用CCD采集并输入计算机进行处理,证明小波变换可以明显改善光学图像相关识别能力;同时,该系统提高了光学相关识别实验的灵活性和可操作性。     

一种基于干扰重构的自适应干扰对消算法

从干扰信号的波束域和时域特征入手,建立了基于四阶累积量联合对角化的干扰估计模型,提出了联合干扰重构和自适应对消的主瓣干扰抑制算法,给出了处理流程和工程应用条件,分析了干扰重构的相对误差,对信噪比损失性能进行了仿真分析,同时评估了对测角性能的影响。仿真结果表明,在干噪比大于40 dB、干扰与目标夹角大于1/8 波束宽度的条件下,该算法可有效抑制主瓣干扰,其重构的干扰信号与真实干扰信号相对误差小于2% ,干扰抑制后目标回波信噪比损失小于3 dB,测角精度恶化小于波束宽度的1/30。     

基于比累对切透镜的联合菲涅耳变换相关器特性研究

相关器的特性变化规律对其实际应用中参数的设定具有重要的指导意义。针对基于比累对切透镜的联合菲涅耳变换相关器,通过数值仿真详细分析了比累对切透镜缝宽变化对相关器输出特性和空变特性的影响。结果表明,比累对切透镜缝宽能一定程度地控制联合菲涅耳变换功率分布的有效面积、相关峰的尖锐度及其空变特性的敏感度。实验采用纯相位空间光调制器编码比累对切透镜相位掩模替代实体透镜的方式搭建了相关器,实验结果与仿真结果一致性好,进一步验证了上述结论的正确性。     

Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu₆Sn₅ particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders. The bulk solder strength decreased rapidly as the fine Cu₆Sn₅ precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solderjoints, especially during system use at high temperatures.     

Application of an asymmetrical four point bend shear test to solder joints

The asymmetrical four-point bend shear (AFPB) test method was used to measure the shear strength and creep properties through the stress relaxation experiments using three different Pb-free solder joint compositions in an assolidified condition. Since it was difficult to shear the uniform specimens and the local bending usually occurs at the inner loading points, the notches were introduced at the joint line to preferentially weaken this region. The stress analysis by finite element modeling showed that the straight notches transform the parabolic shear stress distribution in the uniform specimen into a relatively uniform shear distribution along the bond line in the notched specimens. Therefore, the shear strength results from the notched specimens are expected to be much more accurate. Experiments showed that both the Sn-3.6Ag-1Cu (wt.%) and Sn-3.6Ag-1Cu-0.45Co joints have superior strength and creep properties as compared to the Sn-3.5Ag joint. However, there was no statistical difference between the shear strength of the Sn-3.6Ag-1Cu and Sn-3.6Ag-1Cu-0.45 Co joints. Moreover, the difference between the creep resistance of these two types of joints was small.     

SMT焊点质量自动检测与智能鉴别技术

在分析SMT焊点组装质量常用检测技术的基础上,提出并介绍了基于SMT焊点虚拟成形技术的SMT焊点质量自动检测与智能鉴别技术的基本原理和方法,并对该技术的研究意义、主要研究内容和研究思路等进行了探讨。     

基于块稀疏贝叶斯模型的ISAR成像方法

传统ISAR稀疏成像主要针对独立散射点散射系数的重构问题,然而实际情况下目标散射点之间并不是独立存在的,而是以区域或块的形式存在,在该情形下利用常用的稀疏重构算法并不能完全地刻画块状目标的真实结构,因此该文考虑采用块稀疏重构算法进行目标散射系数重建。基于块稀疏贝叶斯模型和变分推理的重构方法(VBGS),包含了稀疏贝叶斯学习(SBL)方法中参数学习的优点,其利用分层的先验分布来表征未知信号的稀疏块状信息,因而相对于现有的恢复算法能够更好地重建块稀疏信号。该方法基于变分贝叶斯推理原理,根据观测量能自动地估计信号未知参数,而无需人工参数设置。针对稀疏块状目标,该文结合压缩感知(CS)理论将VBGS方法用于ISAR成像,仿真实验成像结果表明该方法优于传统的成像结果,适合于具有块状结构的ISAR目标成像。     

基于小波包变换与自适应阈值的SMT焊点图像去噪

为提高表面组装技术焊点图像去噪效果,提出一种基于小波包变换和自适应阈值的焊点图像噪声去除新方法.利用小波包变换对图像进行多层分解,通过对图像小波包树系数的分析,对小波包树系数高频部分和低频部分进行Wiener滤波;为提高去噪性能,通过计算小波包系数对应的中值绝对方差估计,提出一种改进的自适应小波包阀值算法,并对图像进行...     

发电机定子绕组接头焊接质量检测方法及判断标准

通过对鲁布革电厂发电机定子绕组接头焊接质量检测分析,探讨了接头焊接质量检测方法及判断标准。     

模糊控制在直流电机调速中的应用

本文主要介绍了以模糊控制理论为基础的直流电机调速模糊控制系统。通过把人们用自然语言描述的控制直流电机调速经验转换成模糊控制规则,通过这些规则经过模糊推理和模糊决策得到控制量。实验证明,模糊控制直流电机调速性能高于一般的直流电机调速。