关键词： 机械手   提升工况   动力学仿真   动态特性  

摘要： 以垃圾车机械手提升工况作为主要研究对象,对其进行动态特性研究,首先,采用理论力学分析法建立提升工况数学模型,并计算提升工况扭矩变化;其次,通过虚拟样机方法对机械手进行动力学仿真,分析机械手在提升工况下的运动规律。结果表明,理论计算与仿真分析结果误差在可接受范围内,验证了结构设计合理性及仿真分析的有效性,为机械手部件设计选型提供依据。

关键词： 恒定转速比   冗余机械臂   运动规划   神经动力学   龙格-库塔法  

摘要： 提出一种基于神经动力学的恒定转速比(CRR)算法,用于解决冗余机械臂运动规划末端精度问题.在运动规划中,通过神经动力学求解出雅可比矩阵伪逆,由4阶龙格-库塔法得出实时精准的关节角速度,从而获得各关节平均速度比,并将其比值关系带入CRR算法中求解出最优关节角度.与传统的加权伪逆运动规划不同,CRR算法是在关节角位置层面求解,这也正是提高冗余机械臂末端精度的关键环节.在文末,通过平面3自由度和平面6自由度冗余机械臂模型进行运动规划仿真,仿真结果验证了算法的有效性.

关键词： 振动   收获   模态分析   激振频率   振动载荷  

摘要： 为了提高林果振动收获的作业效率,根据果实在振动过程中的动态响应,研究机械振动采收林果时的最佳激振参数,降低收获时的功耗。该文以开心果振动采收为研究对象,建立果树的三维模型,进行模态分析得出前15阶固有频率和模态振型,并进行了振动响应特性仿真实验。结果表明:果树在10.5Hz、15.3Hz、17.3Hz时振型响应一致性较好,且双体多向性加载方式更适合开心果的振动采收。通过线性分析增大激振力以达到果实脱落条件比不断提高力作用点高度更为高效。同时激振频率为10~12Hz,振动载荷为1600N时可以确保大多数果实脱落。该研究可为林果振动收获机械设计提供理论参考。

关键词： 原子力显微镜   传动装置   电动汽车   《基于原子力显微镜的纳米机械加工与检测技术》   纳米机械加工   动力匹配   整体优化  

摘要： 随着社会经济的快速发展,人们对于能源储存量与环境污染问题的关注度越发热门,而汽车作为人们生活、工作中必不可少的交通工具,其能源消耗与市场发展走向也成为社会研究分析的重点;在此情景下,电动汽车的产生与发展得到人们的高度重视。电动汽车动力匹配与整体优化过程中,内置动力系统的创新研究是非常关键的突破环节之一.

关键词： 机械通气   每搏连续无创血压监测系统   脉搏指示连续心排血量监测   血流动力学  

摘要： 目的对比每搏连续无创血压监测系统(CNAP)与脉搏指示连续心排血量监测(PiCCO)技术测定机械通气重症患者血流动力学参数的差异及一致性,评估CNAP无创监测血流动力学的可行性.方法采用前瞻性观察性自身对照研究方法,选择福建省立医院重症医学四科2018年6月至2019年3月收治的机械通气且需行血流动力学监测的重症患者,在入科后立刻置入PiCCO导管,先采用热稀释法监测血流动力学参数,分别在0 h和24 h获得平均动脉压(MAPPiCCO)、心排血指数(CIPiCCO)、脉压变异度(PPVPiCCO)及外周血管阻力指数(SVRIPiCCO),然后再用CNAP监测获取相同参数(MAPCNAP、CICNAP、PPVCNAP及SVRICNAP),两种方法均连续测量3次取平均值;比较两种方法监测各血流动力学参数的差异.采用Pearson检验分析两种方法获取血流动力学参数的相关性;采用Bland-Altman法分析指标的一致性.结果共有38例患者入选,排除24 h内死亡1例和自动出院2例、心房颤动2例、数据采集丢失1例,最终纳入32例患者.32例患者中女性12例,男性20例;年龄26~84岁,平均(66.8±19.1)岁;体重指数(23.7±3.9)kg/m2;急性生理学与慢性健康状况评分Ⅱ(APACHEⅡ)为(19.5±5.3)分;序贯器官衰竭评分(SOFA)为(9.7±4.1)分.两种方法监测结果显示,CICNAP与CIPiCCO、PPVCNAP与PPVPiCCO差异无统计学意义〔CI(mL·s-1·m-2):59.8±12.6比58.5±14.2,PPV:(14.7±6.8)%比(14.0±6.8)%,均P>0.05〕;MAPCNAP、SVRICNAP均明显高于MAPPiCCO、SVRIPiCCO〔MAP(mmHg,1 mmHg=0.133 kPa):65.6±9.4比60.1±9.2,SVRI(kPa·s·L-1·m-2):206.2±53.9比179.5±57.8,均P<0.01〕.相关性分析显示,两种方法测得的MAP、CI、PPV及SVRI均呈显著正相关(r值分别为0.624、0.864、0.835及0.655,均P<0.05).一致性分析显示,CICNAP与CIPiCCO、PPVCNAP与PPVPiCCO的一致性较好,平均差值分别为1.2 mL·s-1·m-2、0.5%,95%可信区间(95%CI)分别为-12.8~15.3 mL·s-1·m-2、-7.1%~8.2%;MAPCNAP与MAPPiCCO、SVRICNAP与SVRIPiCCO的一致性较差,平均差值分别为5.5 mmHg、26.8 kPa·s·L-1·m-2,95%CI分别为-10.4~21.3 mmHg、-64.5~118.0 kPa·s·L-1·m-2.结论监测机械通气重症患者的CI和PPV时,CNAP与PiCCO可以相互替代;而两种方法监测的MAP、SVRI可能存在差异,需正确谨慎解读.

关键词： Water effect   Silicates   Cracking   Mechanical properties   Molecular dynamics  

摘要： Understanding the effects of water on the mechanical properties of silica glass is important for many applications of silicate glasses. In this study, the effects of water on both elastic and plastic properties of pure silica glass are investigated. The introduction of molecular water leads to an increase of Young's modulus of silica glass at low water content, while hydroxyl exhibits an opposite effect. While hydroxyl groups decrease both strength and fracture toughness of the glass via destructing the silica network connectivity, molecular water undermines these properties by effectively driving the silica network to a "strained" configuration in absence of external stress. The water effect can be characterized in terms of the change of Si-O bond length and Si-O-Si bond angle. The plateau in the stress-strain curves of silica with the existence of molecular water in compact tension is associated with the Si-O bond breaking followed by formation of silanol groups. Moreover, the introduction of molecular water lowers the critical tensile stress where the plateau occurs. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

关键词： Graphene-like BC3   Molecular dynamics simulations   Uniaxial tension   Notches   Cracks   Temperature effects  

摘要： Experimentally fabricated two-dimensional materials have lately evoked significant attention in the nanodevice fabrication industry. In this manuscript, we study the mechanical response of crystalline boron-carbide with BC3 stoichiometry, which is a novel two-dimensional (2D) graphene-like material. Its excellent electrical, thermal and mechanical properties make it an exceptional candidate for a wide range of applications. However, different type of defects created during the production process or device assembly may deteriorate its remarkable mechanical properties. Hence, the purpose of this study is to investigate the mechanical properties of pristine and defective BC3 nanosheets through classical molecular dynamics (MD) simulations. Therefore, we study for the first time the influence of several crack lengths and notch diameters on the mechanical response at different temperatures under uniaxial tensile loading. Our results indicate that larger cracks and notches decrease the strength of 2D graphene-like BC3 nanosheets. Additionally, it was revealed that a temperature increase induces a weakening effect on the tensile strength of BC3 monolayer. Our MD results not only highlight the outstanding mechanical properties of graphene-like BC3, but also reveal its advantages regarding its thermo-mechanical properties, which are critical for the design of nanodevices.

关键词： Polymer   Friction and wear   Molecular dynamics simulation  

摘要： Mechanical and tribological properties of polytetrafluoroethylene (PTFE) reinforced with carbon nanotubes (CNTs) are studied by a molecular dynamics (MD) simulation to explore inherent interactions and wear mechanisms of polymer nanocomposites from an atomic scale. The friction models of polymer sliding against metal are developed to examine the friction coefficient and wear rate. The mechanical properties of pure PTFE are also analyzed to explain the inherent mechanisms of friction reduction and wear resistance improvement. The MD simulated results show that the Young's modulus and shear stress of PTFE increase by 136.58% and 236.3% after carbon nanotubes reinforcement, respectively. The average friction coefficient of PTFE in the steady stage sliding against Cu layer under normal conditions decreases from 0.169 to 0.127 after CNTs reinforcement. The interaction potential energy between CNT and PTFE was calculated to interpret the improvement effect of CNT. The inherent mechanisms of the enhanced mechanical and tribological properties of PTFE are emphatically discussed and interpreted from an atomic view by analyzing the variations of the radius distribution function, relative concentrations and temperature in the thickness direction.

关键词： mechanochemistry   ball milling   Knoevenagel condensation   kinetics  

摘要： We focus on understanding the kinetics of a mechanically activated Knoevenagel condensation conducted in a ball mill, that is characterized by sigmoidal kinetics and the formation of a rubber-like cohesive intermediate state coating the milling ball. The previously described experimental findings are explained using a phenomenological kinetic model. It is assumed that reactants transform into products already at the very first collision of the ball with the wall of the jar. The portion of reactants that are transformed into products during each oscillation is taken to be a fraction of the amount of material that is trapped between the ball and the wall of the jar. This quantity is greater when the reaction mixture transforms from its initial powder form to the rubber-like cohesive coating on the ball. Further, the amount of reactants processed in each collision varies proportionally with the total area of the layer coating the ball. The total area of this coating layer is predicted to vary with the third power of time, thus accounting for the observed dramatic increase of the reaction rate. Supporting experiments, performed using a polyvinyl acetate adhesive as a nonreactive but cohesive material, confirm that the coating around the ball grows with the third power of time.

关键词： Epoxy   Gamma irradiation   Molecular dynamic   Dielectric spectroscopy   Mechanical properties  

摘要： The chemical structures, molecular dynamics and mechanical properties of diglycidyl ether of bisphenol-A (DGEBA)/triethylenetetramine (TETA) resins under gamma irradiation and physical ageing have been investigated. Fourier transform infrared (FTIR) spectroscopy results indicated that an amount of amines and oxidized carbonyl-containing products were generated during the irradiation process. The oxidized skin-like layer was confirmed via micro-FTIR, and the thickness of the oxidized layer increased linearly with irradiation dose. Dynamical mechanical analysis and broadband dielectric spectroscopy were used to analyze the molecular dynamics of DGEBA/TETA resins under different irradiation doses. Three distinct relaxations were observed for all polymers. The irradiation process resulted in an increase of local mobility corresponding to beta and gamma relaxations. The cooperative mobility associated with alpha relaxation was significantly shifted to lower temperature with irradiation. Meanwhile, alpha relaxation broadened considerably with increasing irradiation dose, demonstrating an increasingly heterogeneous environment. The gamma irradiation process resulted in an increase of threshold stress, but a decrease of strain at break, which was accounted for a destruction of the cross-linking networks associated with the increase of the heterogeneity environments. Meanwhile, the increase of the flexural and tensile modulus with the irradiation dose was due to the generation of the abundant hydrogen bonds acted as physical cross-linking points. (C) 2019 Published by Elsevier Ltd.