Dynamic Modeling and Analysis of 5‑PSS/UPU Parallel Mechanism with Elastically Active Branched Chains

To study the characteristics of the 5-prismatic–spherical–spherical(PSS)/universal–prismatic–universal(UPU)parallel mechanism with elastically active branched chains,the dynamics modeling and solutions of the parallel mechanism were investigated.First,the active branched chains and screw sliders were considered as spatial beam elements and plane beam element models,respectively,and the dynamic equations of each element model were derived using the Lagrange method.Second,the equations of the 5-PSS/UPU parallel mechanism were obtained according to the kinematic coupling relationship between the active branched chains and moving platform.Finally,based on the parallel mechanism dynamic equations,the natural frequency distribution of the 5-PSS/UPU parallel mechanism in the working space and elastic displacement of the moving platform were obtained.The results show that the natural frequency of the 5-PSS/UPU parallel mechanism under a given motion situation is greater than its operating frequency.The maximum position error is -0.096 mm in direction Y,and the maximum orientation error is -0.29°around the X-axis.The study provides important information for analyzing the dynamic performance,dynamic optimization design,and dynamic control of the 5-PSS/UPU parallel mechanism with elastically active branched chains.

Dynamic Accuracy Analysis of a 5PSS/UPU Parallel Mechanism Based on Rigid-Flexible Coupled Modeling

In order to improve the low output accuracy caused by the elastic deformations of the branch chains,a finite element-based dynamic accuracy analysis method for parallel mechanisms is proposed in this paper.First,taking a 5-prismatic-spherical-spherical(PSS)/universal-prismatic-universal(UPU)parallel mechanism as an example,the error model is established by a closed vector chain method,while its influence on the dynamic accuracy of the parallel mechanism is analyzed through numerical simulation.According to the structural and error characteristics of the parallel mechanism,a vector calibration algorithm is proposed to reduce the position and pose errors along the whole motion trajectory.Then,considering the elastic deformation of the rod,the rigid-flexible coupling dynamic equations of each component are established by combining the finite element method with the Lagrange method.The elastodynamic model of the whole machine is obtained based on the constraint condition of each moving part,and the correctness of the model is verified by simulation.Moreover,the effect of component flexibility on the dimensionless root mean square error of the displacement,velocity and acceleration of the moving platform is investigated by using a Newmark method,and the mapping relationship of these dimensionless root mean square errors to dynamic accuracy is further studied.The research work provides a theoretical basis for the design of the parameter size of the prototype.

Multi-objective Trajectory Planning Method based on the Improved Elitist Non-dominated Sorting Genetic Algorithm

Robot manipulators perform a point-point task under kinematic and dynamic constraints.Due to multi-degreeof-freedom coupling characteristics,it is difficult to find a better desired trajectory.In this paper,a multi-objective trajectory planning approach based on an improved elitist non-dominated sorting genetic algorithm(INSGA-II)is proposed.Trajectory function is planned with a new composite polynomial that by combining of quintic polynomials with cubic Bezier curves.Then,an INSGA-II,by introducing three genetic operators:ranking group selection(RGS),direction-based crossover(DBX)and adaptive precision-controllable mutation(APCM),is developed to optimize travelling time and torque fluctuation.Inverted generational distance,hypervolume and optimizer overhead are selected to evaluate the convergence,diversity and computational effort of algorithms.The optimal solution is determined via fuzzy comprehensive evaluation to obtain the optimal trajectory.Taking a serial-parallel hybrid manipulator as instance,the velocity and acceleration profiles obtained using this composite polynomial are compared with those obtained using a quintic B-spline method.The effectiveness and practicability of the proposed method are verified by simulation results.This research proposes a trajectory optimization method which can offer a better solution with efficiency and stability for a point-to-point task of robot manipulators.

准噶尔盆地东北部石炭系碎屑岩天然气成藏条件及勘探方向

准噶尔盆地东北部石炭系是该盆地天然气重要勘探领域,但长期以来火山岩被作为主要勘探对象,而忽视了源储共生的碎屑岩勘探和研究,造成对石炭系碎屑岩成藏条件及勘探潜力认识不足。通过对东北部石炭系烃源岩评价、有效源灶刻画、碎屑岩储层评价、油气源对比及成藏模式构建等,取得了3点认识:(1)东北部石炭系发育滴水泉组、松喀尔苏组B段、石钱滩组3套优质气源岩,主要存在三南凹陷—滴水泉凹陷、五彩湾凹陷—大井地区、东道海子凹陷—白家海凸起3个烃源中心;(2)东北部石炭系发育砂砾岩、砂岩、浊积岩、云质岩、页岩等多类型规模储集体,总体属于低孔低渗—超低孔超低渗储层,2 900~4 500 m深度段存在溶蚀孔隙发育带;(3)油气源对比揭示3套气源岩对东北部天然气均有贡献,近源成藏特征明显,石炭系碎屑岩存在源内非常规和近源常规2类天然气藏,预测存在页岩气、致密砂岩气、常规天然气藏有序聚集的成藏模式。研究认识揭示盆地东北部石炭系碎屑岩源储配置条件优越,源内、近源碎屑岩天然气藏是该区重要勘探方向。

Transition-metal catalyzed asymmetric reactions under continuous flow from 2015 to early 2020

During last two decades,transition-metal catalyzed asymmetric reactions under continuous flow system have attracted a widespread attention.With its prominent advantages including higher safety and efficiency,precise control,better heat/mass transfer,easier scale-up and better sustainability,it has impressed both the academia and related industries.However,comparing with the prosperous development of transition-metal catalyzed asymmetric reactions in batch,utilization of this type of chemistry under continuous flow is still at an early stage.To inspire more potential industrial application and further studies of this chemistry in flow,this review sum-marized the recent advances of transition-metal catalyzed asymmetric reactions under continuous flow.Moreover,we further discussed the encountered challenges including reactor engineering,catalyst design,catalyst deacti-vation,pressure drop,clogging,side reactions etc.,as well as their corresponding solutions and achievements in this mini-review.