Approximate closed form solutions for free vibration of polar orthotropic circular plates

For free vibrations of polar orthotropic plate, simple approximate closed form solutions for mode shapes and its natural frequencies were obtained using the Rayleigh-Ritz method. Coordinate function satisfying the natural boundary conditions and the predetermined coefficients was adapted, which results in compact expressions and enables to readily calculate symmetric and nonsymmetric natural frequencies for arbitrary values of the elastic constants. The derived formulation can be used in designing of circular plates such as wood disk, which are naturally endowed with material orthotropy as well as fiber reinforced composite materials. The model can easily be used for the evaluation of parametric studies on dynamic behaviors and nondestructive methods during the initial design process.     

Letter: On the Global Hyperbolicity of Lorentz 2-Step Nilpotent Lie Groups

This work is concerned with the existence of Lorentz 2-step nilpotent Lie groups having a timelike center and which are not globally hyperbolic. Namely, we prove that any left invariant Lorentz metric with a timelike center on the Heisenberg group H ²ⁿ⁺¹ is not globally hyperbolic.     

三维空间型中闭曲面和测地球面的比较

本文建立关于三维空间型中的可定向闭曲面的某些公式.引入积分绝对平均曲率来描述可定向闭曲面的平均弯曲程度.在此意义上,可定向闭曲面可以与作为包含该闭曲面的凸的测地球的边界测地球面进行比较,这种比较能用于说明空间型自身的某些属性     

The structure of the compound Cd3P2

Structure of Cd₃P₂ (P4₂/nmc, a = b = 8.7390 Å, c = 12.2523 Å) has been solved and refined up to R = 3.78% using precision X-ray diffraction experimental data (λ-MoK _α, graphite monochromator on a primary beam, 11529 reflections). Interatomic distances and valence angles are determined. Phosphorus forms a face-centered cubic lattice in which 3/4 tetrahedral voids are occupied by cadmium atoms in the crystal structure. The structure can be described by two equivalent models in which the positions of cadmium atoms, which occupy tetrahedral voids following the “diamond principle,” are preserved, while the remaining free and occupied voids change their places.     

舰炮对空闭环校射新方法研究

小口径舰炮作为水面舰艇末端防御的主要武器，在近程防空反导中的实时性、快速性、准确性显得极为重要．论文针对目前国内不能很好的解决脱靶量检测问题，从而不能实施大闭环校射的情况，提出了一种闭环校射新方法——虚拟闭环校射方法．该方法通过观测目标未进入有效射击区域之前的实际运动航路，求取一系列逆解射击诸元和顺解射击诸元，从而得出射击诸元误差序列，进而预测出射击诸元误差修正量，实施闭环校射，大大提高了小口径舰炮的作战效能，具有重要现实军事意义．     

浅谈西门子S7-300/400模拟量闭环PID控制应用技巧

本文分析PID控制器的不同变形形式,以及西门子S7-300/400PLC模拟量闭环控制功能和使用系统功能块实现闭环控制方法.     

永磁BLDCM双闭环控制系统技术研究

针对永磁无刷直流电机开环控制误差大,以及单闭环控制动静态特性差的缺点,研究了双闭环控制系统。文中介绍了永磁无刷直流电动机PWM调速的工作原理,阐述了其双闭环控制系统的设计过程,推断了电流调速器和转速调速器的数学模型。仿真结果验证了永磁无刷直流电机双闭环控制具有良好的动静态特性     

相控阵卫星跟踪闭环实验系统设计与实现

相控阵卫星跟踪系统是一个庞大、复杂的闭环系统,主要包含卫星及其发射信号、相控阵接收天线、基带信号处理单元、波束指向控制四个部分。为了解决卫星自跟踪系统在实验中对实时性的要求,降低测试成本和测试难度,设计了一套运动平台卫星自跟踪闭环实验系统。上位机完成了运动目标建模和波束指向控制功能,基于FPGA的四通道硬件板完成接收信号的产生,自跟踪基带数字板完成基带信号的搜索、测角和跟踪滤波,各部分之间通过SMA或串口进行连接。对每一部分作了理论分析和硬件设计描述,并在最后给出了测试结果,表明了演示系统的可行性和有效性。     

中心静脉导管闭式引流胸腔积液在基层医院临床应用研究

目的探讨在基层医院临床中中心静脉导管闭式引流胸腔积液的应用。方法从68例胸腔积液患者中选取55例渗出性积液患者作为研究的对象,采用计算机表法的形式将其分为28例引流组(行引流治疗)和27例常规组(行传统胸腔穿刺抽液术),对两组患者的治疗效果、住院时间以及不良反应发生情况进行指标的记录和对比。结果行引流治疗的引流组其各项指标均优于行常规治疗的27例患者,两组数据对比呈现为P0.05,表明有统计学意义。结论在基层医院中,将中心静脉置管引流胸水应用其中,具有良好的经济以及实用价值,这对于基层医院的临床研究具有重要意义。