北川体育场高空悬挑看台梁板模板设计与施工

通过四川北川县体育场高空悬挑看台梁板支模工程的实践,验证了高大空间复杂外形工程条件下扣件式钢管脚手架的适用性.明确了脚手架结构验算的部位,表明了轴心受压对立杆稳定性的影响,指出了施工重点部位.扣件式脚手架用于复杂外形结构有易于操作、经济性好和适用性强的优点.     

基于SMO的PMSM磁极位置检测技术

为构造低成本、高可靠性的永磁同步电机(PMSM)无位置传感器控制系统,对基于滑模观测器(SMO)的PMSM磁极位置检测技术进行研究.基于滑模变结构和矢量控制理论设计了滑模观测器,用于估算电机的反电势,并构造了采用锁相环(PLL)结构的磁极位置检测单元.仿真和实验结果表明,所设计的观测器能较准确地估算电机反电势,但存在一定的高频抖振,而锁相环单元能在很大程度上减小高频分量对角度估算的影响,提出的方法能实现较高精度的磁极位置检测.     

定参数Butterworth滤波器的改进设计

针对Butterworth低通滤波器归一化后的传递函数动态性能指标的单一化,提出了可以根据需要改变其动态性能指标的低通滤波器。基于牛顿迭代法对Butterworth低通滤波器的极点的位置进行迭代计算,使重新分布极点的滤波器的传递函数动态性能指标满足设备的需求,并对改进后的低通滤波器进行阶跃响应分析。     

超空泡航行体纵向运动的鲁棒极点配置控制

由于空泡及滑行力的存在,使得超空泡航行体的建模及控制问题变得非常复杂,在其模型简化过程中会产生一定的建模误差,采用鲁棒极点配置算法进行超空泡航行体控制可以有效克服这一缺点.对超空泡航行体所受的流体动力、尤其是滑行力的动态特性进行了分析整理;建立了超空泡航行体纵平面内的动力学模型;最后利用鲁棒极点配置算法设计了控制器.仿真结果表明,该控制系统具有较强的鲁棒性及稳定性.对各控制变量的控制效果进行了对比分析,分析结果表明空化器比尾舵具有更好的控制效果.     

Influence of lithium-based products proposed for counteracting ASR on the chemistry of pore solution and cement hydrates

Low- and high-alkali cement pastes were made with or without LiNO₃ or a Li-bearing glass. The [Li]/[Na+K] molar ratio was kept constant to 0.74. The specimens were stored at 23, 38, and 60 °C in sealed containers. After 3, 7, 28, and 91 days, their pore solutions were extracted and analysed, and their residual water contents were obtained by drying. The Li glass was found to react quite slowly, and the corresponding [Li⁺] in solution progressively increased with time, temperature, fineness (as-received glass vs. ground glass), and the [Na⁺+K⁺] concentration in solution. This glass increased the pH by about 0.1, and by about 0.2 after it was finely ground. In contrast, LiNO₃ decreased the pH by about 0.1, despite significantly increasing the [Na⁺+K⁺] in the pore solution. The higher the total %Na₂O_e content (including Li) in the original mixtures, the higher the total alkali content incorporated in the cement hydrates. The [Li⁺]-[Na⁺+K⁺] ratio in solution was about half of the initial ratio (0.74), while this ratio in the cement hydrates was always over 1.1. Li is the alkali most preferentially incorporated into the cement hydrates, while K is the least.     

Low-temperature synthesis of cements from rice hull ash

Rice hull is an agricultural by-product containing about 20% of silica. Usually, this material is burned at the rice fields generating small silica particles, which may cause respiratory and environmental damage. This work describes the use of rice hull ash as a raw material to prepare Ca₂SiO₄-related cements, which is a component of commercial Portland cement. Rice hull was heated at 600 °C rendering silica with a surface area of 21 m² g⁻¹. This material was mixed with CaO and BaCl₂·2H₂O in several proportions, added stoichiometricaly in order to keep a ratio (Ca+Ba)/Si=2. The solids were mixed with water 1:20 (w/w) and sonicated for 60 min. The suspensions were dried and heated at several temperatures (from 500 to 1100 °C). The resulting solids were analyzed by FT-IR spectroscopy and X-ray diffraction. Cements with structure similar to that of β-Ca₂SiO₄ were obtained at temperatures as low as 700 °C, according to the composition.     

Impedance spectroscopy study of hardened Portland cement paste

In this paper, the differential impedance analysis (DIA) has been applied to the study of the dielectric properties of hardened Portland cement paste. Two time constants are found in the impedance spectra obtained in the frequency region form 100 kHz to 15 MHz. One time constant has been attributed to the solid matrix and the other one to the liquid phase filling the pores. The effect of the cement paste-electrodes interface has been quantified using two different experimental set-ups. Measurements using direct contact between electrodes and cement paste have been compared with measurements using an air gap technique in which the specimen “floats” between the electrodes. The two referred time constants have been found in both types of measurements. The influence of drying on the dielectric parameters is also studied.     

Experimental study and numerical simulation on the formation of microstructure in cementitious materials at early age

The formation of microstructure in early age cement paste and concrete was examined with an ultrasonic experimental set-up. Research parameters included the influence of curing temperature (isothermal curing at 20, 30 and 40 °C), water/cement ratio (0.40, 0.45 and 0.55) and amount of aggregate. In parallel with the experiments, the cement hydration model HYMOSTRUC was utilized to simulate the formation of the microstructure. In this study, the cement paste was considered as a four-phase system consisting of water, unhydrated cement, hydration products and that part of the hydration product that causes the contact between the hydrating cement grains (so called “bridge volume”). A correlation has been found between the growth of bridge volume calculated with the model and the changes in the pulse velocity. It is believed that ultrasonic pulse velocity (UPV) measurements can represent a valuable tool to investigate the development of the microstructure at early age.     

线性规划的可行解集合的几何特征

本文提出平凡解是基可行解,也是极点,对基可行解与极点是等价概念给出一种证明方法,指出E~n中的可行解集合有类似R~2中的凸多边形的一些特征。     

自动化电解去内交叉孔毛刺过程的仿真与实验

针对零件内部交叉孔相贯线处的毛刺去除困难的现状,利用电解加工的原理,采用插孔固定阴极式的电极布局形式,提出基于电流检测的自动电解去毛刺的方法.建立电解去毛刺的数学模型,用仿真和实验研究电解去毛刺过程中加工间隙、加工电压及加工时间对毛刺去除的影响,分析去毛刺过程中电流密度随时间的变化规律.在该电极布局形式下,毛刺与工件间的平均电流随着毛刺的去除逐渐下降,且电流曲线的斜率在毛刺完全去除后趋近于0.以此作为去除过程是否完成的判断依据,研制了自动电解去毛刺的控制系统并完成了自动化去毛刺实验.实验结果表明:该方法能够有效地对毛刺大小进行判断,可以控制加工时间进行精确的毛刺去除,成本低、安全可靠.