关键词： density functional tight-binding method   divide-and-conquer method   molecular dynamics   metadynamics  

摘要： Dcdftbmd is a Fortran 90/95 program that enables efficient quantum mechanical molecular dynamics (MD) simulations using divide-and-conquer density functional tight-binding (DC-DFTB) method. Based on the remarkable performance of previous massively parallel DC-DFTB energy and gradient calculations for huge systems, the code has been specialized to MD simulations. Recent implementations and modifications including DFTB extensions, improved computational speed in the DC-DFTB computational steps, algorithms for efficient initial guess charge prediction, and free energy calculations via metadynamics technique have enhanced the capability to obtain atomistic insights in novel applications to nanomaterials and biomolecules. The energy, structure, and other molecular properties are also accessible through the single-point calculation, geometry optimization, and vibrational frequency analysis. The available functionalities are outlined together with efficiency tests and simulation examples. (c) 2019 Wiley Periodicals, Inc.

关键词： internal stability   maximal feedback linearization   mechanical systems   nonlinear control systems   zero dynamics  

摘要： For systems that are not feedback linearizable, a natural question is: how to find the largest feedback linearizable subsystem and, if the partial linearization is not unique, what are the control-theoretic properties of various partial linearizations. In this paper, we will consider the problem of how to choose a partially linearizing output that renders the zero dynamics asymptotically stable and when such an output exists. We will state general results solving completely the problem for systems whose linearizability defect is one by identifying and describing two classes of systems. For the first class, all maximal partial linearizations lead to the same zero dynamics. For the second class, any asymptotic behavior of the zero dynamics can be achieved by a suitable choice of a partially linearizing output. In the second part, we apply our results to mechanical systems with two-degrees-of-freedom and provide a detailed study of their partial linearizations. We illustrate the obtained results by examples of Acrobot (which belongs to the second class) and Pendubot (which belongs to the first class).

关键词： 制盐   循环泵   机械密封   优化设计  

摘要： 针对80万吨制盐装置循环泵机械密封频繁损坏的问题,分析了导致机械密封频繁损坏的原因,确定了主要原因为机械密封存在设计缺陷,提出了对机械密封进行优化设计的改造方案,并将优化后的机械密封运用于80万吨制盐装置上,大大降低了循环泵机械密封的损坏频率。

关键词： 农业机械化   农业现代化   作用  

摘要： 科技的快速发展使农业机械化技术水平得以提高,农业机械化是农业现代化的标志,是发展农业现代化必经之路。我国目前正处于由传统农业向现代化农业过渡的关键时期,需要大力发展农业生产机械化,以为实现农业现代化奠定基础。本文在分析农业机械化在农业现代化作用的基础上探讨大力发展农业机械化的对策,以实现农业经济的健康发展。

关键词： 工程机械   装载机   概念机   纽荷兰   TETRA  

摘要： 在今年的德国慕尼黑bauma展上,凯斯纽荷兰工业集团(纽交所代码:CNHI/米兰证交所代码:CNHI)旗下品牌凯斯工程机械发布了甲烷动力概念轮式装载机——TETRA,让公众洞见了工程机械在可持续发展、人机互联和技术进步方面的未来。

关键词： 劳动力   规模经营   诱致性变迁理论   机械技术选择   水稻播种方式  

摘要： 利用2016年江西省、江苏省的农村入户调查数据,从劳动力要素禀赋和规模经营演进的视角,以水稻机插秧技术为例,分析了农户机械技术选择的作用机制和影响程度。研究结果表明:在经营规模偏小的情况下,机插秧是非理想的农户技术选择,资本节约型播种技术强化了非机械化技术的选择路径。农户水稻经营规模的增加显著促进农户机插秧技术的选择,水稻种植规模每增加1亩,农户机插秧技术的选择概率会平均增加0.13%。农业生产劳动力数和劳动雇工均价对农户机插秧技术选择具有显著的负向影响,在其他条件不变的情况下,家庭农业生产劳动力数和劳动雇工均价每增加1个单位,机插秧技术的选择概率分别平均降低32.87%和4.53%。因此,未来地方政府推进水稻全程机械化的进程中,相关政策重心应在加大技术服务供给和规范流转契约这两方面。

关键词： 锚杆钻机   机械臂   动力学方程   区间分析   不确定性估计  

摘要： 矿用锚护钻机是现代化矿井巷道支护过程中高效、安全的自动化设备,其中锚护机械臂是完成支护作业的关键部件。因加工装配误差、材料缺陷、使用磨损、井下工作环境恶劣和操作人员工作强度大等因素,将导致矿用锚杆钻机-机械臂的动力学系统的不确定性问题更为突出。以锚护机械臂为研究对象,基于拉格朗日理论推导出了锚护机械臂的动力学模型。将区间数学理论引入到锚护机械臂的动力学系统的不确定性分析中,提出了基于区间数学的机械臂的动力学系统不确定性分析方法,主要构造了区间扫描法和张量积法2种代理模型来分析讨论机械臂各个关节上的力矩与关节角度、关节角速度之间的关系。由于机械臂的杆件2和杆件4在实际操作中离钻架重心距离远、受力大、结构易变等原因,重点研究了杆件2和杆件4的力矩存在不确定参数时,相应输出的角度、角速度的响应区间和效率变化。结果表明,扫描法的时间消耗为张量积法的0.1734~0.2888倍,同时计算结果与理论值更接近,精度较高,误差较小,更加适用于工程实际中。试验研究了力矩存在不确定性参数时的机械臂的动力学系统动态响应,并与理论结果进行对比分析,两者得到的系统动态响应规律基本一致,验证了该区间算法的可实施性和准确性。含区间不确定性参数的动力学特性研究为后续锚护机械臂的控制做进一步的铺垫,也成为预测和判断机械臂定位精度的新方法,进一步保障机械臂对顶板和侧帮的支护效果。

关键词： 甘薯产业   劳动力   成本   机械化   湖南  

摘要： 近年来湖南省甘薯种植面积呈下降趋势,但其总产值下降并不显著。为分析该现象产生的原因,本研究通过调研和统计分析,结合劳动力成本、流向、年龄分布等因素分析了甘薯产业产量、种植面积、产值等发生变化的原因,进而分析了劳动力变化过程中甘薯机械化生产的优势和主要问题。结果发现湖南省甘薯产值主要受劳动力价格影响,种植面积减少主要受劳动力成本限制,劳动力流出比例大、成本逐年增涨、劳动力严重老龄化是影响甘薯产业发展的重要限制因素。机械化生产将能显著降低生产成本,但是同样受劳动力价格和劳动者技能掌握程度等因素影响。劳动力成本的增涨,迫使产业寻求更优化的生产工具和生产方式,分析发现在达到一定种植规模时,机械化生产的节本增效效果显著,然而机械化生产也存在自身的限制因素,生产过程对劳动者的技术要求高、农民购买力不足是当前产业发展不可忽视的限制因素。

关键词： CRISPR-Cas9   RNA splicing   Genome editing   Group II intron   Spliceosome  

摘要： Noncoding RNA (ncRNA) has a key role in regulating gene expression, mediating fundamental processes and diseases via a variety of yet unknown mechanisms. Here, we review recent applications of conventional and enhanced Molecular Dynamics (MD) simulations methods to address the mechanistic function of large biomolecular systems that are tightly involved in the ncRNA function and that are of key importance in life sciences. This compendium focuses of three biomolecular systems, namely the CRISPR-Cas9 genome editing machinery, group II intron ribozyme and the ribonucleoprotein complex of the spliceosome, which edit and process ncRNA. We show how the application of a novel accelerated MD simulations method has been key in disclosing the conformational transitions underlying RNA binding in the CRISPR-Cas9 complex, suggesting a mechanism for RNA recruitment and clarifying the conformational changes required for attaining genome editing. As well, we discuss the use of mixed quantum-classical MD simulations in deciphering the catalytic mechanism of RNA splicing as operated by group II intron ribozyme, one of the largest ncRNA structures crystallized so far. Finally, we debate the future challenges and opportunities in the field, discussing the recent application of MD simulations for unraveling the functional biophysics of the spliceosome, a multi-mega Dalton complex of proteins and small nuclear RNAs that performs RNA splicing in humans. This showcase of applications highlights the current talent of MD simulations to dissect atomic-level details of complex biomolecular systems instrumental for the design of finely engineered genome editing machines. As well, this review aims at inspiring future investigations of several other ncRNA regulatory systems, such as micro and small interfering RNAs, which achieve their function and specificity using RNA-based recognition and targeting strategies.

关键词： 间隔棒   拆装工具   设计  

摘要： 以100公里的330kV^750kV电压等级线路来算,三相分裂导线需安装约5000个间隔棒,工作量巨大。每当自然灾害发生时,线路会发生舞动、脱冰跳跃等剧烈活动现象,致使大量间隔棒在强外力作用下发生损坏。对发生损坏的间隔棒需及时更换。更换间隔棒有带电作业和停电作业两种方式,设计一种能轻松快速安装、更换间隔棒的工具提高功效,为输电线路基建、检修工人减负是本文所讨论的设计的目标。