Mobility Analysis of Altmann Overconstrained Linkages by Modified Grübler-Kutzbach Criterion

Developing a general mobility method/formula is a hot topic lasting for more than 150 years in kinematics. It is necessary to apply any mobility method to puzzling overconstrained mechanisms for verification of its generality. Altmann linkages are such recognized puzzling mechanisms that their mobility analysis is of important significance. A necessary condition for judging a general mobility method is that the method can be fit for Altmann linkages. Firstly, this study classes Altmann linkages into 17 types in terms of the numbers and types of kinematic pairs, and then Altmann overconstrained linkages are further classified into 4 types. Secondly, the mobility of Altmann overconstrained linkages is systematically analyzed by the Modified Grübler-Kutzbach criterion based on screw theory, where passive freedoms are defined as limb passive freedoms and mechanism passive freedoms. In addition, the full-cycle mobility is judged, which overcomes the shortcoming of instantaneous property of screw theory. It is shown that Modified Grübler-Kutzbach criterion not only obtains the correct numerical mobility, but also gives the mobility character by resolving reciprocal screws for the constraint system. This study lays the foundation of verification for the generality of Modified Grübler-Kutzbach criterion. Besides, Altmann overconstrained linkages almost comprise all kinds of modern parallel mechanisms and some classical mechanisms, which provides an important reference for mechanism mobility calculation.     

Mechanical Fault Diagnosis Based on Band-phase-randomized Surrogate Data and Multifractal

The vibration signals of machinery with various faults often show clear nonlinear characteristics.Currently,fractal dimension analysis as the common useful method for nonlinear signal analysis,is a kind of single fractal form,which only reflects the overall irregularity of signals,but cannot describe its local scaling properties.For comprehensive revealing of internal properties,a combinatorial method based on band-phase-randomized(BPR) surrogate data and multifractal is introduced.BPR surrogate data method is effective to eliminate nonlinearity in specified frequency band for a fault signal,which can be utilized to detect nonlinear degree in whole fault signal by nonlinear titration method,and the overall nonlinear distribution of fault signal is displayed in nonlinear characteristic curve that can be used to analyze the fault signal qualitatively.Then multifractal theory as a quantitative analysis method is used to describe geometrical characteristics and local scaling properties,and asymmetry coefficient of multifractal spectrum and multifractal entropy for fault signals are extracted as new criterions to diagnose machinery faults.Several typical faults include rotor misalignment,transversal crack,and static-dynamic rubbing fault are analyzed,and the results indicate that those faults can be distinguished by the proposed method effectively,which provides a qualitative and quantitative analysis way in the field of machinery fault diagnosis.     

新型液压多泵在液压调速系统中的节能分析

针对液压系统的节能问题,提出了几种基于新型液压元件（多泵）的调速回路节能方法。这几种方法采用的核心元件是多泵,该泵可在一个泵体内形成多个不同流量的泵,有多个进油口和多个出油口,可根据执行元件的不同工况要求切换油路的连接方式,使泵的输出功率与负载功率合理匹配,从而达到提高回路效率、减少能耗的目的。阐述了单作用多泵双压节流调速回路、双作用多泵数字节流调速回路、多泵数字容积调速回路等几种新型回路的工作原理,并对多泵数字节流调速回路在恒载下的功率特性进行了分析比较,结果表明,这几种回路不同程度上减小了功率损失,提高了回路的效率。     

VC轧辊板形控制的力学原理

根据实验和有限元分析结果,分析了控制压力与油腔长度对VC轧辊的静载凸度及其分布特性的影响规律,在此基础上,阐明了VC轧辊板形控制的力学原理。在以VC轧辊为支撑辊的轧机系统中,板形控制的机理是,VC轧辊的胀辊效应可以有效地减小工作辊与支撑辊之间的接触区域,在工作辊两端形成与支撑辊弯曲方向相反的弯矩。板形控制的技术关键在于选择合适的油腔长度,消除VC支撑辊静载凸度特性中的等凸度区。     

2-RRR+RRS球面并联机构超静定力学全解

针对2-RRR+RRS球面并联机构,利用拆杆法建立其4次超静定的静力学平衡方程;考虑构件弹性,利用小变形叠加原理,建立机构的4个变形协调方程。基于所建方程,利用MAPLE软件分析求得在不同输入力和力矩情况下,机构构件所受力与机构位姿关系曲线图,明晰了不同外力或外力矩输入情况下对机构影响的差异。研究结果为该机构的工程设计及实际应用提供了静力学理论基础和依据。     

微电网中三相逆变器孤岛运行控制技术

分布式电源通常采用无互联线的传统“功率一电压一电流”三环下垂控制器来实现并联系统问的功率均分,但采用三环下垂控制不仅控制器复杂,而且其控制效果受连线阻抗阻感比影响严重。尤其是在线路短、阻抗小的微电网系统中,过小的连线阻抗会严重影响功率均分效果甚至会导致系统不稳定。为此设计一种基于虚拟阻抗的“电压一电流”双环下垂控制方法,使并联系统在连线阻抗很小且不对称,传统功率下垂控制方式已不能稳定工作的情况下,仍然能够维持良好的电流均分效果。对比传统三环下垂方法,其具有稳定裕度大,动态响应快,实现简单等特点。最后在理论分析的基础上进行实验研究,通过与三环下垂控制方式的对比,验证了双环下垂控制的有效性。     

三相并网逆变器脱网运行电压控制技术

IEEE Std.1547规定并网逆变器需要具备孤岛运行的能力以维持负载电压稳定。分析了比例积分控制、比例谐振控制和比例复数积分控制3种线性电压控制策略从系统跟随特性、抗扰特性以及系统主导极点分布对3种电压控制方案的稳态特性和动态特性进行了对比,并在Matlab/Simulink中进行了仿真研究。最后在TMS320F2812数字信号处理器数字化控制平台上验证了理论分析的正确性。     

基于证据融合理论的配电网经济运行指标权重的闭环计算方法

为有效评价配电网经济运行指标,需要确定合理的运行指标权重,为此提出了基于证据融合理论的配电网经济运行指标权重闭环计算方法。首先邀请专家给出指标主观权重,剔除离异程度大的专家意见,并对不可靠意见进行折扣处理,利用证据融合理论确定专家主观权重。若该主观权重符合证据充分条件,则该权重即为最终权重;否则利用熵值法和主成分分析法...     

ZQMnl2-8-3-2高锰铝青铜在2．4％NaCl溶液中的空蚀行为

利用磁致伸缩空蚀试验机研究了ZQMnl2—8—3—2高锰铝青铜在2．4％NaCl溶液中的空蚀行为．结果表明：空蚀作用致使ZQMnl2—8—3—2高锰铝青铜的自腐蚀电位正移．在腐蚀与空蚀的交互作用中,力学因素起了至关重要的作用,腐蚀因素作用相对较小．微裂纹首先在α／κ相界的α相部分形成;随着空蚀的进行,这些微裂纹在α相内...     

快速凝固Ti-48Al合金的组织及纳米硬度(英文)

研究冷却速度对熔体旋转法制备的快速凝固Ti-48%Al合金组织的影响。结果表明:同传统凝固相比,快速凝固明显细化了Ti-48%Al合金的组织,并使合金的成分变得均匀。随着冷却速度的增加,合金的晶粒明显细化。研究薄带厚度、辊速与冷却速度的关系。随着辊速的增加,冷却速度增加,薄带厚度呈减小的趋势。由于细晶强化的作用,快速凝固薄带的纳米硬度随着冷却速度的增加而增加。