连续体单孔手术机器人的建模与优化分析

提出了一种新构型的6自由度连续体单孔手术机器人．其形变骨架采用超弹性材料一体化成型加工,具有一系列十字交叉镂空结构的弹性铰链．建立了机器人的运动学模型,分析了机器人的可达工作空间,提出一种手术作业空间的优化搜索方法,获得了作业空间中关键位置的操作灵活性．进而以操作灵活性为目标,以关节形变能力为约束,优化分析连续体机构形变骨架的几何参数,形成提升连续体单孔手术机器人操作灵活性的参数优化方法．最后进行连续体机器人的运动控制实验,经过测试,机器人的末端位置误差为2.23 mm,角度误差为2.06°,验证了模型的准确性．     

Studying the viscosity of methane fluid for different resolution levels models using Poiseuille flow in a nano-channel

In this work, the viscosity–temperature–density relationship of methane is investigated for three fully atomistic models (RISM, OPLS and MOPLS) and the corresponding coarse-grained models (CGRISM, CGOPLS and CGMOPLS) by studying the Poiseuille flow inside a silicon nano-channel using molecular dynamic simulation. In order to solve the interaction problem of nano-channel atom and methane molecule, the coarse-grained methane model and the classical Berthelot–Lorentz mixing rules are employed. The optimized coarse-grained methane models are determined using the relative entropy minimization method. The density distribution, stress force profile and velocity profile of coarse-grained models are compared with the fully atomistic models for different nano-channel widths. It is concluded that the results of coarse-grained model are in reasonable agreement with those of the corresponding fully atomistic model. Furthermore, the value of viscosity is calculated by fitting the velocity profile to the continuum solution from the Navier–Stokes equations and then compared to experimental value. The results show that the MOPLS and CGMOPLS models could well predict the viscosity of methane fluid, while the OPLS model performed worst. All simulation results indicate that the coarse-grained models can be used to predict the viscosity of methane fluid accurately.     

Construction of models of dispersive elastodynamic behavior of periodic composites: A computational approach

A finite element method is described for the construction of continuum models of elastodynamic behavior of periodic composites. The method yields a hierarchy of models including ones which can be used to predict the phenomenon of dispersion of elastic waves in bulk composites; prediction of this effect of small-scale heterogeneity on the macroscopic behavior of composite materials is beyond the scope of the classical effective modulus theories.     

An energy model for the optimal design of linear continuum structures

An analytical model is presented for the optimal design of linearly elastic continuum structures. To facilitate the expression of the combined analysis and design problem in general form, a basis is introduced covering a general set of energy invariants. Both internal (strain) energy and the expression of generalized cost are represented conveniently in terms of this basis, and as a result the optimality conditions for the design problem have a particularly simple form. Present developments comprise a reinterpretation and an extension of existing models where the design variable is the material modulus tensor, and where cost is represented in a general form. The conventional potential energy statement for linear continuum elastostatics is restated in the form of an isoperimetric problem, as a preliminary step. This interpretation of the mechanics is then incorporated in a max-min formulation applicable for the general design of linear continuum structures. To exemplify its application, the model is interpreted as it would apply for certain materials with particular geometric structure, e.g. crystalline forms. Also problems treated earlier where optimal material properties are predicted for the case where unit cost is proportional to the trace of the modulus tensor are identified as examples within the generalized formulation. The application of a recently developed technique to predict optimal black-white structures, i.e. designs having sharp topological features, is considered in the setting of the present generalized model.     

Investigation of multiscale fluid flow characteristics based on a hybrid atomistic–continuum method

A computer program based on a molecular dynamics–continuum hybrid method has been developed in which the Navier–Stokes equations are solved in the continuum region and the molecular dynamics in the atomistic region. The coupling between the atomistic and continuum is constructed through constrained dynamics within an overlap region where both molecular and continuum equations are solved simultaneously. The simulation geometries are solved in three dimensions and an overlap region is introduced in two directions to improve the choice of using the molecular region in smaller areas. The proposed method is used to simulate steady and start-up Couette flow showing quantitative agreement with results from analytical solutions and full molecular dynamics simulations. The prepared algorithm and the computer code are capable of modeling fluid flows in micro and nano-scale geometries.     

Finite element model of ionic nanowires with size-dependent mechanical properties determined by ab initio calculations

A finite element procedure for modeling crystalline nanostructures such as nanowires is proposed. The size effects exhibited by nanoobjects are captured by taking into account a surface energy, following the classical Gurtin–Murdoch surface elasticity theory. An appropriate variational form and a finite element approach are provided to model and solve relevant problems numerically. We describe a simplified technique based on projection operators for constructing the surface elements. The methodology is completed with a computational procedure based on ab initio calculations to extract elastic coefficients of general anisotropic surfaces. The FEM continuum model is validated by comparisons with complete ab initio models of nanowires with different diameters where size-dependent mechanical properties are observed. The FEM continuum model can then be used to model similar nanostructures in ranges of sizes or geometries where analytical or atomistic model is limited. The validated model is applied to the analysis of size effects in the bending of an AlN nanowire.     

Optimization of axisymmetric elastic modulus distributions around a hole for increased strength

Holes in engineering structures cause stress concentrations that often lead to failure. In nature, however, blood vessel holes (foramina) in load-bearing bones are not normally involved in structural failures. It has been found that this behavior is linked to the material distribution near the hole. In the present paper, we have investigated the effectiveness of optimizing the radial distribution of the isotropic elastic modulus around a circular hole to increase load-carrying capacity. Bezier curves were used to describe the radial distribution of the elastic modulus. Since changing the elastic modulus usually affects the strength, the ratio of maximum principal stress to strength was chosen as the objective function for optimization. Using non-dimensional analysis of the 2-D elasticity equations, we identified three parameters that govern the optimum design and are applicable to a wide range of materials, loading, and geometries. The first is a material parameter that describes the relationship between the strength and elastic modulus, the second is the geometric parameter given by the ratio of the optimized field to the hole radius, and the third is the biaxial load ratio. The effect of failure criterion choice on the optimum elastic modulus distribution is also investigated. Optimum elastic modulus distributions for materials whose strength increases faster than the stiffness, as density and/or composition is varied, completely eliminated the effect of the hole by locally stiffening areas that experience high stresses. When the strength lagged behind the stiffness, optimum designs were similar to those found in bones, and relied on modulus distributions that direct the loads away from the hole.     

Addendum to: An energy model for the optimal design of linear continuum structures

The original paper of the above title presents an analytical model for problems in the optimal design of linearly elastic continuum structures, where the material modulus tensor has the role of design variable. Both internal (strain) energy and the expression of generalized cost are represented conveniently there, in a form where the modulus tensor is transformed into vector coordinates. The general design of linear continuum structures is stated as a max-min problem. Optimality conditions for the transformed design problem have particularly simple form.¶Both local properties, represented by the relative values of components of the modulus tensor, and the global distribution of structural resource (material) are variable in the design.With some modification to the original formulation, these separate aspects of design can be represented explicitly in the model. This modified form, which directly facilitates study of the role of local properties in the prediction of optimal design, and which ultimately serves as the basis for schemes to perform computational solution, is described and substantiated here.     

Developing an advanced ANN-based approach to estimate compaction characteristics of highway subgrade

The current methodology of assessing the compaction characteristic of subgrade fillers is time-consuming. The spot-sampling method and the following laboratory tests bring bias to engineering practice. This study proposes an advanced approach to determine the maximum dry density, optimum moisture content, and compactness of subgrade based on ANN algorithms. A large number of compaction tests are performed to assess the compaction quality of various types of subgrade fillers. The particle gradation is specifically considered. Wave propagations of the compacted specimen are assessed based on the ultrasonic test, and the anisotropic feature is demonstrated. The dynamic elastic parameters are derived to further reveal the interaction mechanism. A dataset is proposed by combining these laboratory data and the literature data. The PSO-BP-NN model is developed to automatically predict the compaction parameters of different filling materials. A positive correlation is demonstrated between the elastic modulus and the total density, and Poisson’s ratio illustrates an inverse trend. The compaction energy is influential to the maximum dry density while the moisture content has the greatest impact on the compactness. This study aims to develop an accurate model to replace the extensive Proctor test and efficiently evaluate the compaction quality of subgrade for highway constructions.     

An artificial neural network-based multiscale method for hybrid atomistic-continuum simulations

This paper presents an artificial neural network-based multiscale method for coupling continuum and molecular simulations. Molecular dynamics modelling is employed as a local “high resolution” refinement of computational data required by the continuum computational fluid dynamics solver. The coupling between atomistic and continuum simulations is obtained by an artificial neural network (ANN) methodology. The ANN aims to optimise the transfer of information through minimisation of (1) the computational cost by avoiding repetitive atomistic simulations of nearly identical states, and (2) the fluctuation strength of the atomistic outputs that are fed back to the continuum solver. Results are presented for prototype flows such as the isothermal Couette flow with slip boundary conditions and the slip Couette flow with heat transfer.     

Systematic coarse-graining of spectrin-level red blood cell models

We present a rigorous procedure to derive coarse-grained red blood cell (RBC) models, which yield accurate mechanical response. Based on a semi-analytic theory the linear and nonlinear elastic properties of healthy and infected RBCs in malaria can be matched with those obtained in optical tweezers stretching experiments. The present analysis predicts correctly the membrane Young's modulus in contrast to about 50% error in predictions by previous models. In addition, we develop a stress-free model which avoids a number of pitfalls of existing RBC models, such as non-smooth or poorly controlled equilibrium shape and dependence of the mechanical properties on the initial triangulation quality. Here we employ dissipative particle dynamics for the implementation but the proposed model is general and suitable for use in many existing continuum and particle-based numerical methods.     

一种太阳能集热系统集热量实时预测的混合建模方法

太阳能集热器的集热量受光照度、环境温度、风速等多种因素的影响,其预测模型很难从预测精度和实时性上同时满足用户需求。本文提出一种实时预测太阳能集热系统集热量的混合建模方法。该方法首先从能量守恒出发,根据热管式太阳能集热系统传热机理推导出集热量的理论模型,并把理论模型中的散热系数、透射率、吸收率等经验参数以及采光面积、散热面积等几何参数集总为模型的未知参数,进而提出混合模型的结构。然后,利用TRNSYS仿真软件搭建太阳能集热器模拟仿真系统,对仿真系统不同的运行工况进行仿真实验,获取用于辨识混合模型未知参数的稳态数据。最后,选用粒子群优化算法（PSO）作为模型参数的辨识方法,利用所获得的稳态数据辨识模型的未知参数。模型预测值与仿真实验结果的比较表明,预测模型简单而精确,能够在各种工况下实时地、高精度地预测太阳能集热器的集热量,其平均相对误差可达到2.02%。该模型在太阳能热泵、太阳能热水器等系统的优化控制领域得以广泛应用。     

Optimal bone density distributions: Numerical analysis of the osteocyte spatial influence in bone remodeling

In this paper a control and optimization procedure for bone remodeling simulations was adopted to study the effect of the osteocyte influence range on the predicted density distribution. In order to reach this goal, the osteocyte network regulating bone remodeling process in a 2-D bone sample was numerically simulated. The assumed proportional-integral-derivative (PID) bone remodeling rule was related to the error signal between the strain energy density and a selected target. Furthermore the control parameters and the target were optimally determined minimizing a suitable cost index: the goal was to minimize the final mass and the energy thus maximizing the stiffness. The continuum model results show that the developed and adapted trabecular structure was consistent with the applied loads and only depended on the external forces, the value of the cost index, the maximum attainable elastic modulus value (hence, the maximum density value) and the value of the energy target. The remodeling phenomenon determined the number and thickness of the trabeculae which are formed from a uniform distribution of mass density in the considered domain; this number and these thicknesses are controlled by the values assigned to the parameters of the model. In particular, the osteocyte decay distance (D) of the influence range affected the trabecular patterns formation, showing an important effect in the adaptive capacity of the optimization numerical model.     

基于欧拉 伯努利梁和铁木辛柯梁理论的功能梯度材料模量测定

为测定功能梯度材料的弹性模量和剪切模量,引入梁理论并将梁沿长度方向离散,建立单元平衡方程后可得到弹性模量和剪切模量分布;假设弹性模量为沿长度方向的线性函数或指数函数,用有限元软件仿真计算功能梯度材料梁单元节点处的挠度和转角,然后用插值法构造变形特征函数,并计算得出弹性模量和剪切模量,且计算值与理论值的误差较小.计算结果还表明,采用铁木辛柯梁理论不仅可以得到弹性模量,还可以计算剪切模量,且弹性模量计算结果比用欧拉-伯努利梁计算结果更接近真实值,但铁木辛柯梁理论中需测定转角,对测定过程的要求会更加严格。     

基于快速多极边界元法的沥青混凝土弹性模量预测

由于对沥青混凝土材料的研究无论是试验法还是经验法均建立在宏观层面上,无法与其细观结构建立本质的联系,因此利用快速多极边界元法(Fast Multipole Boundary Element Method,FMBEM),结合数字图像处理技术,实现沥青混凝土二维几何建模及弹性模量预测.通过数字图像处理技术识别拍摄得到的原始...     

Continuum-based sensitivity analysis for coupled atomistic and continuum simulations for 2-D applications using bridging scale decomposition

The concept of linking simulations at multiple length and time scales is found useful for studying local physical phenomena such as crack propagation. Many multi-scale methods, which couple molecular dynamics models with continuum models, have been proposed over the last decade. One of the most advanced methods developed recently is the bridging scale method, in which the total displacement is decomposed into orthogonal coarse and fine scales. This paper presents the continuum-based sensitivity analysis for two-dimensional coupled atomistic and continuum problems using the bridging scale method. A variational formulation for the bridging scale decomposition is developed based on the Hamilton’s principle. The continuum-based variational formulation provides a uniform and generalized system of equations from which the differential equations can be obtained naturally. The sensitivity expressions for both direct differentiation method (DDM) and adjoint variable method (AVM) are derived in a continuum setting. Due to its efficiency for crack problems, the direct differentiation method is chosen to be implemented numerically and applied to two examples, including a crack propagation problem. Both material and sizing design variables are included to reveal the impact of design changes at the macroscopic level to the responses at the atomistic level. Also demonstrated is the feasibility of achieving the variation of the time history kernel computed using numerical procedures. The sensitivity coefficients calculated are shown to be accurate compared with overall finite difference method. The physical implications of the sensitivity results are also discussed, which accurately predict the behavior of the structural responses.     

混凝土浇筑过程中温度场和应力场仿真分析

在混凝土浇筑过程中,其弹性模量随时间变化.这一独特现象给混凝土结构的残余热应力应变的理论求解带来极大困难.即使使用有限元法对混凝土进行热力耦合分析,也必须解决材料弹性模量徐变的问题.采用分时间段方法对弹性模量的时变特性进行等效近似处理,在一个时间段内近似认为弹性模量为一个稳定值,并在各个时间段对混凝土进行热力耦合计算.每一时间段仿真分析时读取上个时间段的结果作为初始条件.最后提取整个龄期各测试点处的仿真温度数据,并与实测温度数据进行对比,验证温度场仿真方法的正确性.     

氧化锌纳米线振动问题研究

采用连续介质理论与分子动力学模拟相结合的方法,研究了氧化锌纳米线的振动问题.建立了氧化锌纳米线核壳模型,解释其等效杨氏模量及压电常数的尺寸效应.通过连续介质理论求得氧化锌纳米线振动固有频率,并与分子动力学模拟得到的结果进行对比.研究表明,氧化锌纳米线在极化方向的等效拉伸杨氏模量随着横截面尺寸的增加而逐渐增大,且通过核壳模型分别求得核、壳拉伸杨氏模量.拟合得到的等效拉伸杨氏模量与分子动力学方法获得的等效拉伸杨氏模量符合得很好.根据连续介质理论得到等效弯曲杨氏模量,发现等效弯曲杨氏模量也随着横截面尺寸的增加而增大.氧化锌纳米线极化方向的压电耦合能力比一般压电陶瓷好,压电常数随着横截面尺寸的增加逐渐减小.氧化锌纳米线在不同温度条件下的振动频率没有明显变化,在不同外电场条件下的振动频率有显著变化.分子动力学模拟得到不同横截面尺寸的氧化锌纳米线振动频率不同.根据连续介质理论,求得悬臂Timoshenko梁模型相应尺寸的振动频率,发现横截面的尺寸越大,连续介质理论与分子动力学模拟得到的振动频率越接近.     

Experimental determination of mechanical properties of Ni and Ni-Fe microbars

 In this study, nickel and nickel-iron alloy microsamples produced using the LIGA technique are investigated. With vibration tests and tensile tests, design parameters such as Young’s modulus, elastic limit, and failure stress are determined. A new microsample tensile test apparatus is presented, in which forces and elongation are separately measured with very high accuracy. The comparison between the Young’s modulus which is determined with vibration experiments and the Young’s modulus derived from tensile tests ensures the reliability of the measurements presented. The error sources are analysed with the help of finite element models and the uncertainty in determining the Young’s modulus is calculated. The strength values measured for LIGA specimens are about 3 times higher than for comparable macroscopic specimens. Received 27 February 1996 / Accepted: 11 March 1996     

Comprehensive analysis of the position error and vibration characteristics of Delta robot

Taking Delta robot as research object, its characteristics of position error and vibration are analyzed synthetically. Using the position characteristic of driven arms, based on the method of geometry space vector, establish the mechanism error model; Taking use the principle of mathematical statistics and space vector as the basic, deriving the joint clearance error model; Based on finite element theory, on the basic of elastic dynamic model, establishing the flexible error model. Then, the kinematics and dynamics of branched chain with joint clearance generalized collision force are studied. Based on the definition of rod virtual length, combining the generalized collision force with the elastic dynamics model which includes mechanism error, joint clearance error and flexible error, establishing the comprehensive elastic dynamic model. Finally, by use of simulation software, numerical calculation and experiment, verified the correctness of comprehensive elastic dynamic model, and the characteristics of position error and vibration are analyzed.