The optimum bandwidth of chirp signals in ultrasonic applications

A new formula for the pulse-compression ratio achievable with linear FM pulses (chirp signals) in ultrasonic applications is derived. The calculations take into account the frequently omitted effects of the transfer function of an ultrasonic transducer. For a squared-cosine type transducer spectrum, the optimum chirp signal bandwidth is 1.14 times the 6 dB bandwidth of the transducer. The pulse-compression ratio is 2.24 dB lower than in the case of an ideal broadband transducer. Measurements show a close agreement with the presented theory.     

25ps可见喇曼光探针

给出了受激喇曼效应中,有关脉冲压缩的主要关系式。选择适应的实验条件可以将聚焦光束受激喇曼散射脉冲压缩大于10倍。     

一种矢量量化编码的快速搜索算法

提出了一种矢量量化编码的快速匹配算法。利用码书的拓扑结构和柯西 施瓦兹不等式的性质推导出了消除不必要匹配操作的条件,并根据这个条件提出了快速搜索算法。实验结果表明,该算法不仅明显降低了矢量编码过程的复杂程度,同时也保证了与全搜索编码相同的图像编码质量。与其它搜索方法的操作次数和存储空间进行了比较。     

Changes in electrical resistance of carbon‐black‐filled silicone rubber composite during compression

We studied the changes in the electrical resistance of carbon black filled silicone rubber composite, which is the sensitive element of the flexible force sensor, as a function of time during compression. The experimental results show that there is a sudden increase of the electrical resistance along with the sudden increase of the stress immediately after the compression. When the sample strain is kept constant, the electrical resistance and the stress both decay with time. The data of the stress relaxation and the resistance relaxation both can be fitted by the linear combination of two exponential functions. Based on the shell structure theory, the experimental phenomena are explained from the view that the uniaxial pressure induces the changes in the effective conductive paths. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2700–2706, 2007     

二维核磁共振数据的噪声剔除与数据压缩

讨论了对二维核磁共振数据进行噪声剔除与数据压缩的一般原理与方法,介绍在Sun图形工作站上的实现情况,并给出实验结果与分析。     

Numerical validation of the shear compression specimen. Part I: Quasi-static large strain testing

The shear compression specimen (SCS), which is used for large strain testing, is thoroughly investigated numerically using three-dimensional elastoplastic finite element simulations. In this first part of the study we address quasi-static loading. A bi-linear material model is assumed. We investigate the effect of geometrical parameters, such as gage height and root radius, on the stress and strain distribution and concentration. The analyses show that the stresses and strains are reasonably uniform on a typical gage mid-section, and their average values reflect accurately the prescribed material model. We derive accurate correlations between the averaged von Mises stress and strain and the applied experimental load and displacement. These relations depend on the specimen geometry and the material properties. Numerical results are compared to experimental data, and an excellent agreement is observed. This study confirms the potential of the SCS for large strain testing of material.     

Warpage prediction of optical media

The warpage of injection–compression‐molded optical media, such as compact discs and digital video discs, due to asymmetric cooling during production is predicted. Thermally induced stress is calculated with a nonisothermal compressible flow simulation with a viscoelastic constitutive model. A finite element analysis is formulated with axisymmetric plate elements based on Kirchhoff thin‐plate theory to simulate the warpage of the disc due to the asymmetric thermal stress and gravity after demolding. Simulation results of warpage for compact‐disc‐recordable moldings are compared with experimental observations under different processing conditions, such as the melt temperature, mold temperature, and packing pressure, with an optical grade of polycarbonate. The comparison shows that the simulation well predicts the effects of various processing conditions. Both the simulation and experiment indicate that of the processing conditions, the mold temperature has the greatest effect on warpage. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 859–872, 2003     

基于幅度统计多分辨率分析的SAR图像灰度压缩

基于幅度统计多分辨率分析的原理，针对SAR图像特殊的显示要求，提出了一种新的灰度压缩方法，解决了常规灰度级压缩方法不能很好保持原始图像数据直方图分布的问题，实验结果表明该方法能有效保证灰度压缩前后图像数据直方图分布形状的一致性，较好地保持原始图像的视觉特征。     

Compression of powerful microwave frequency-modulated e-polarized pulses by autocollimative gratings

Electromagnetic pulse compressors using gratings in autocollimation regime for the central frequency of a pulse for p- polarization having greater electrical reliability are theoretically investigated. The theory is built on the basis of the numerical decision of electromagnetic field equations by a method of the integral equation. A case of aberrationless approximation of bell-like pulse compression with quadratically modulated phase is analytically considered. The numerical experiment shows the presence of strong distortions of a pulse at compression, caused by cubic and senior members in decomposition of phase frequent dependence, got by a wave in the compressor. Conditions of optimum pulse compression with the account of aberrations are investigated.     

辐射驱动内爆温度源脉冲整形研究

辐射驱动温度源脉冲整形是获得高压缩比的重要途径。本文从脉冲整形的基本原理出发，采用数值模拟方法，给出适合于神光Ⅲ装置内爆实验的辐射驱动温度源脉冲整形曲线。