混有协同自适应巡航控制车辆的异质交通流稳定性解析与基本图模型

针对传统车辆和协同自适应巡航控制(cooperative adaptive cruise control,CACC)车辆构成的异质交通流,研究其稳定性与基本图模型.应用实车测试验证的CACC模型和智能驾驶员模型(intelligent driver model)分别作为CACC车辆和传统车辆的跟驰模型,建立异质流稳定性解析框架,研究不同平衡态速度、不同CACC车辆比例时的异质流稳定性.推导异质流基本图模型,并进行数值仿真实验.研究结果表明,在传统车辆稳定的速度范围,异质流处于稳定状态.在传统车辆不稳定的速度范围,CACC车辆比例增加以及平衡态速度远离9.6—18.6 m/s速度范围,均能够改善异质流的不稳定性.通行能力随着CACC车辆比例的增加而提高.此外,CACC模型的期望车间时距越大,异质流稳定域越大,但通行能力降低.因此,恒定车间时距CACC控制策略下的期望车间时距取值应权衡异质流稳定域和通行能力两个方面的影响.     

NO dissociation pathways on Rh(1 0 0), (1 1 0), and (1 1 1) surfaces: A comparative density functional theory study

The chemisorption and dissociation pathways of NO on the Rh(1 0 0), (1 1 0), and (1 1 1) surfaces are studied by the plane-wave density functional theory (DFT) with CASTEP program. In addition, the electronic and geometrical effects that affect the NO dissociation reactions have been investigated in detail. The calculation results are presented as following: The effective activation energies of the best NO dissociation pathways on the Rh(1 0 0), the Rh(1 1 0), and the Rh(1 1 1) are 0.63, 0.66 and 1.77 eV, respectively. The activity of the Rh planes for NO dissociation is in the order of Rh(1 0 0) ≈ Rh(1 1 0) > Rh(1 1 1). The low dissociation barrier for Rh(1 0 0) and Rh(1 1 0) is associated with the existence of a lying-down NO structure which acts as a precursor for dissociation. By Mulliken population analysis and structure analysis, both electronic and geometrical effects are found to affect the NO dissociation reactions, but the geometrical effect exceed the electronic. The energy decomposition scheme has been used to provide further insight into the NO dissociation reactions. Based on the calculations, the interaction energy between N and O in the transition state on the Rh(1 1 1) is found much larger than that on the Rh(1 0 0) and the Rh(1 1 0). The major differences of should originate from the variation of the bonding competition effect.     

Optical properties of Dy ions in sodium gadolinium tungstates crystal

Dy³⁺-doped NaGd(WO₄)₂ crystal with sizes of about Φ20×40 mm² was grown by the Czochralski technique along the (0 0 1) orientation. Polarized absorption spectra, fluorescence spectra, and fluorescence decay curve of Dy³⁺-doped NaGd(WO₄)₂ have been recorded at room temperature. Based on the Judd-Ofelt (J-O) theory, the intensity parameters from the measured line strengths were evaluated. The J-O parameters were used to predict radiative transition probabilities, radiative lifetimes and branching ratios for various excited levels of Dy³⁺-doped NaGd(WO₄)₂ crystal. The luminescent quantum efficiency of the ⁴F_9/2 level was determined to be approximately 63% for this material. The emission cross-section of the ⁴F_9/2→⁶H_13/2 transition was estimated by using the Füchtbauer-Ladengurg method.     

Stable multiwavelength and passively mode-locked Yb-doped fiber laser based on nonlinear polarization rotation

Based on the nonlinear polarization rotation (NPR) technique and a fiber Mach–Zehnder interferometer (MZI), a fiber laser which can work in two different modes has been demonstrated. In one mode, it works as a continuous wave (CW) multiwavelength laser, and stable dual-, triple- and tetra-wavelength lasing with the wavelength spacing of 1.1 nm are obtained. In the other mode, it works as a passively mode-locked fiber laser, and the generated pulses have a repetition rate of 10.6 MHz, an FWHM width of 800 ps and the largest energy of 1 nJ. The multi-functional property makes this fiber laser a very convenient, cost effective and multi-purpose laser source.     

牵引变流器网侧电流谐波抑制研究

作为电力机车牵引传动系统核心部件的牵引变流器是牵引供电网的主要谐波来源之一。牵引变流器采用的单相四象限整流器,输出含有二次脉动,且工作开关频率较低,导致变流器网侧电流谐波问题严重。目前单相四象限整流器的控制脉冲产生主要采用SPWM技术。与SPWM相比,SHEPWM（特定谐波消除PWM）可以针对性的消除特定的谐波,谐波含量更小,效率更高,但其无法消除调制波中本身就存在的谐波。针对SHEPWM调制策略进行了分析设计,同时对陷波滤波器技术、PR控制策略和LC回路在降低谐波含量中的作用进行深入分析。经过合理的方案选择,有效的降低了网侧电流谐波含量,并通过MATLAB仿真实验进行了分析验证。     

基于共振型高灵敏度光声光谱技术探测痕量乙炔气体浓度

乙炔气体作为判断变压器运行状态的一种故障气体,其浓度的高低反映了变压器的运行状况,因此对其浓度的探测在变压器的维护中具有重要意义。为了准确探测变压器运行过程中产生的乙炔气体浓度,为变压器的维护提供技术参数,针对基于DFB激光器的共振型光声光谱技术痕量乙炔气体检测技术开展研究,对传统的光声光谱探测系统进行改进。根据光声光谱技术的理论可知,光声信号的强度与入射激光的功率成正比,所以在光声池的出射窗口采用一个平面反射镜将红外光再次反射到光声池中以增加入射光功率,增强光声信号强度,进一步提高了光声系统的探测灵敏度。通过一定浓度的乙炔气体在不同调制频率和不同调制深度下光声信号强度的变化,确定光声探测系统的最佳调制频率和最佳调制深度为767 Hz和0.3 mV。利用不同浓度乙炔气体对系统进行标定,然后采用最小二乘法对光声信号与气体浓度进行拟合,二者具有很好的线性度。通过Allan方差计算可知,系统在平均时间达到200 s时,能够达到最低探测极限浓度。实验表明,在一个大气压下,积分时间为10 ms时,改进后的共振型光声光谱探测系统对乙炔气体的最低探测极限浓度达到了0.3 μL·L-1。还将小波去噪技术引入到低浓度下乙炔气体的光声信号处理中,有效消除了低浓度气体光声信号中的噪声,提高了信噪比。设计的共振型光声光谱探测系统操作简单,最低探测浓度符合国标中对变压器维护过程中对乙炔气体的探测需求,在变压器维护领域具有广阔的应用前景。     

Creep deformation measurement using quartz optical fiber

This paper describes an optical system for high temperature creep strain measurement using quartz optical fiber, super long working distance microscope and digital image processing techniques. In this system one end of the quartz optical fibers is arrayed in a small area on the specimen surface and the other end is illuminated by a laser beam. The fiber ends on the specimen surface form the spot array. The small optical spots on the specimen are tracked by a CCD camera and the images are processed by digital image processing software. The diameter of each quartz fiber is 100 μm and the fibers can be arrayed in a small area. The local strains are determined by measuring the variety of relative distance between two spots. Experimental results of local creep strain on the welding joints of 15CrMo and HK40 at 850°C are obtained.     

除环上矩阵的子矩阵的秩的恒等式与不等式

本文提供了除环上矩阵的一些子矩阵的秩的若干恒等式与不等式,出现在本文中的这些结果,在处理涉及某些应用方面的复矩阵(作为除环上矩阵的特例)问题以及某些纯数学问题将起重要作用。     

ICP－AES法测定钨酸钇钾激光晶体中的Er和Nd

应用ＩＣＰ－ＡＥＳ法测定钨酸钇钾［ｋＹ（ＷＯ４）２］激光晶体中的Ｅｒ和Ｎｄ。试样用磷酸分解，Ｔｍ被选作内标元素。Ｅｒ和Ｎｄ的回收率分别为９８．６％－１０５．３％和９６．５％－１０５．８％，相对标准偏差分别为１．９０％和１．６０％。     

Synthesis and Raman scattering of GaN nanorings, nanoribbons and nanowires

Low-dimensional GaN materials, including nanorings, nanoribbons and smooth nanowires have been synthesized by reacting gallium and ammonia using Ag particles as a catalyst on the substrate of MgO single crystals. They were characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). EDX, XRD indicated that the low-dimensional nanomaterials were wurtzite GaN. New features are found in Raman scatterings for these low-dimensional GaN materials, which are different from the previous observations of GaN materials. Received: 16 November 2000 / Accepted: 17 November 2000 / Published online: 21 March 2001