虚拟手术仿真中人体软组织形变技术的研究

对虚拟手术仿真中人体软组织形变技术进行深入研究,利用OpenGL三维图形标准建立了基于质点-弹簧物理模型的虚拟人体软组织形变系统。围绕虚拟手术仿真中人体软组织形变的逼真度和实时性两大要素展开研究,通过对比四边形网格结构提出了改进的基于质点-弹簧模型的正六边形几何拓扑结构,并对软组织形变动力学模型及其数值积分算法、软组织形变力反馈计算模型进行了讨论,针对以往的虚拟手术器械与软组织表面接触时作用点的最近邻质点求取算法存在的不足,提出了改进的求取算法。实验结果表明,改进的算法在模拟软组织形变时具有较好的稳定性和实时性。     

虚拟三维树叶变形的可视化研究

植物器官的真实感模拟和可视化表达,对于虚拟植物的研究和应用具有重要作用,针对植物的真实生长过程,为了仿真树叶在枯萎、老化过程中几何形态的变化,引入一种质点--弹簧模型的三维变形方法.对质点--弹簧模型的树叶网格模型的建立,确定了模型的关键因素:质点和弹簧;对模型中的质点确定,根据树叶在形态变化过程中受到的诸多外力因素和树叶本身的物理属性,进行了受力分析,建立了三维变形模型的微分方程,并进行仿真,并以点的位置变化来表现树叶形态的变化.实验结果与观察结果相近,说明质点--弹簧模型用于树叶的几何形态仿真具有可行性和有效性.     

基于改进弹簧-质点模型的柔性绳索仿真

针对虚拟吊装训练系统中柔性绳索模拟仿真时存在的实时性、逼真程度较差等问题,基于物理建模提出了一种蜂窝状弹簧-质点模型应用于绳索形变仿真.该模型依据绳索细长、柔韧等特点,将传统正方形弹簧质点网格改进为蜂窝状弹簧-质点模型,其由多簇六边形立柱状弹簧-质点包围而成,绳索各部分质量均匀分布,质点间设置4种弹簧模拟绳索内部弯曲、扭转、拉伸等形变特性;为加快求解速度同时保证精度要求,使用改进的Verlet-梯形预测-校正法对数值计算进行求解.仿真结果表明,文中模型是可行的;数值计算结果表明,Verlet-梯形预测-校正法在效率、精度方面均优于其他数值计算方法.     

数值稳定的可展网格仿真方法

动态可展曲面是采用哈密尔顿原理的一类新型动态模型。详细论述了这类动态模型的数值计算引擎。大量实验结果表明:(1)光滑和褶皱的可展曲面都可用动态网格模拟;(2)传统的挑战性问题如纸张褶皱仿真和塑性服装建模等都可用动态网格以稳定、快速的方式进行计算。     

真实软组织特性的肝脏组织物理建模及受力分析优化

为解决人软组织系统物理建模过程中存在的精确性与实时性之间的矛盾,选择典型的人体软组织——肝脏为实验对象,建立了一套满足实时性的复杂软组织物理建模方法.首先在基于表面模型的质点-弹簧系统基础上引入了体弹簧,采用能够满足体行为模拟的质点-弹簧模型;在质点质量分配方面,采用体离散化的方法完成了质点质量的确定,同时提出了基于层次包围盒的重心修正方法;在弹簧参数设定方面,通过真实软组织生物力学实验和参数分析,提出分段线性化弹簧参数设定方法;最后提出了作用力影响范围的受力优化分析方法,以提升变形仿真的速度.实验结果表明,该方法能够更好地模拟软组织的形变,并提高模拟仿真的实时性.     

物理与凸凹贴图结合的实时布料褶皱模拟方法

介绍了一种物理计算与凸凹贴图相结合实时模拟布料褶皱的方法.基于质点一弹簧模型建模,根据模型的空间状态通过几何插值的方法为布料表面实时计算凸凹贴图高度纹理.在此过程中,通过对高度纹理进行平滑或噪声处理,以获得更佳效果,同时给出试验条件与结果,作为参考.     

融合QPSO算法的多精度布料仿真建模方法

在布料建模领域，如何快速模拟布料形变之后的褶皱细节是研究的热点。通过使用多精度布料建模方法，在布料的不同形变区域使用不同精度的网格，可以有效平衡建模的精度和速度，已有的工作主要是在布料形变过程中，动态计算出布料质点邻域的曲率，依据人为设定的阈值，划分出布料的多精度区域，而在大部分场景中，布料的变形模式没有规律，固定不变的阈值可能会影响布料的仿真效果。针对该问题，首先将基于量子行为的粒子群算法引入建模过程，通过粒子群算法对布料表面的搜索，提高了布料弯曲部位的搜索效率，优化了多精度布料的建模速度和精度，其次针对布料仿真运动过程进行研究，参考布料受空气阻力的数学模型，以及粒子动力学中的数值积分方法，优化布料运动的仿真计算方法。实验证明，与现有布料多精度方法相比，该方法能较快检测到布料褶皱区域并判断是否需要细化，且能较好地表现出布料仿真过程中空气阻力对布料造成的形变。     

改进的采用表面网格的弹簧振子模型

提出一种用于实时弹性形变模拟的弹簧振子模型.通过定义与模型对应的刚体核,并且向模型中添加一种新的广义弹簧,保持了与表面网格模型对应的几何体体积和形状等几何特征;采用形状匹配技术实现了整体性形变效果的模拟;最后使用逆向动力学方法对线弹性模型的"超弹性"现象进行修正.该模型可以对表面网格模型的弹性形变进行快速、逼真的模拟.     

基于移动最小二乘法的无网格肝脏形变研究

在手术虚拟仿真对软组织形变研究中,采用传统的网格形变方法,由于出现网格畸变、网格重构会影响形变效果的问题.针对手术虚拟仿真要求尽可能逼真地模拟形变以达到良好的真实感,提出了一种基于移动最小二乘法的无网格方法对以肝脏为对象的软组织进行形变研究,考虑肝脏软组织生物力学特性,采用Voigt模型对肝脏软组织进行物理建模.同时,针对无网格肝脏模型的碰撞检测,提出AABB包围盒与八叉树空间分割相结合的碰撞检测算法,在肝脏模型体节点的合理布局基础上可以对碰撞进行有效准确的检测.实验结果表明,无网络方法实现简单且具有较好的形变真实感,对虚拟手术中软组织形变研究具有重要的指导意义.     

等几何连续介质弹性模型的带厚度布料仿真方法

目的 布料仿真是计算机动画领域的一个研究热点和难点,基于粒子系统的模型快速高效却难以准确描述布料真实的物理特性,这些物理属性可以由连续介质的弹性模型利用有限元方法来实现,但所需网格单元较多,求解复杂且耗时。现有方法通常将布料建模为曲面和壳模型,对较厚布料的仿真能力较弱。针对这些常用模型在几何建模、物理真实性和仿真速度上难以兼顾的问题,提出了一种带厚度的布料仿真模型,能够更真实地表达不同厚度布料的形变行为,并利用等几何分析方法进行基于物理的快速动态仿真模拟。方法 将有厚度的布料建模为三变量B样条体表示的薄板模型,利用等几何—伽辽金方法,推导出在给定边界条件下三维连续介质线弹性力学方程的弱形式,将B样条体的控制网格作为计算网格进行仿真分析,最后在隐式动力系统框架下对线性方程组直接求解。结果 对本文布料模型进行了多方面的讨论和分析。首先,与几种主流的离散模型下的模拟效果进行了光滑度的对比,本文方法的NURBS(nonuniform rational B-splines)建模有着明显的光滑性优势;并在不同自由度下比较了与经典有限元连续介质模型的计算时间,结果表明模拟结果的平方根误差（root ...     

Real-Time Simulation of Thin Shells

This paper proposes a real-time simulation technique for thin shells undergoing large deformation. Shells are thin objects such as leaves and papers that can be abstracted as 2D structures. Development of a satisfactory physical model that runs in real-time but produces visually convincing animation of thin shells has been remaining a challenge in computer graphics. Rather than resorting to shell theory which involves the most complex formulations in continuum mechanics, we adopt the energy functions from the discrete shells proposed by Grinspun et al. [ [GHDS03] ]. For real-time integration of the governing equation, we develop a modal warping technique for shells. This new simulation framework results from making extensions to the original modal warping technique [ [CK05] ] which was developed for the simulation of 3D solids. We report experimental results, which show that the proposed method runs in real-time even for large meshes, and that it can simulate large bending and/or twisting deformations with acceptable realism.     

Host-based intrusion detection using dynamic and static behavioral models

Intrusion detection has emerged as an important approach to network security. In this paper, we adopt an anomaly detection approach by detecting possible intrusions based on program or user profiles built from normal usage data. In particular, program profiles based on Unix system calls and user profiles based on Unix shell commands are modeled using two different types of behavioral models for data mining. The dynamic modeling approach is based on hidden Markov models (HMM) and the principle of maximum likelihood, while the static modeling approach is based on event occurrence frequency distributions and the principle of minimum cross entropy. The novelty detection approach is adopted to estimate the model parameters using normal training data only, as opposed to the classification approach which has to use both normal and intrusion data for training. To determine whether or not a certain behavior is similar enough to the normal model and hence should be classified as normal, we use a scheme that can be justified from the perspective of hypothesis testing. Our experimental results show that the dynamic modeling approach is better than the static modeling approach for the system call datasets, while the dynamic modeling approach is worse for the shell command datasets. Moreover, the static modeling approach is similar in performance to instance-based learning reported previously by others for the same shell command database but with much higher computational and storage requirements than our method.     

Subdivision elements for large deformation of liquid capsules enclosed by thin shells

This paper presents a numerical method for studying the large deformation of a liquid capsule enclosed by a thin shell in a simple shear flow. An implicit immersed boundary method is employed for calculating the hydrodynamics and fluid–structure interaction effects. A thin-shell model for computing the forces acting on the shell middle surface during the deformation is described within the framework of the Kirchhoff–Love theory of thin shells. This thin-shell model takes full account of finite-deformation kinematics which allows thickness stretching as well as large deflections and bending strains. The interpolation of the reference and deformed surfaces of the shell is accomplished through the use of Loop's subdivision surfaces. The resulting limit surface is C¹-continuous which significantly improves the ability of the method in simulating capsules enclosed by hyperelastic thin shells with different physical properties. The present numerical technique has been validated by several examples including an inflation of a spherical shell and deformations of spherical, oblate spheroidal and biconcave capsules in the shear flow. In addition, different types of motion such as tank-treading, tumbling and transition from tumbling to tank-treading have been studied over a range of shear rates and viscosity ratios.     

Geometrically based potential energy for simulating deformable objects

This paper presents a fast and stable technique for simulating deformable objects. Unlike in previous physically based methods, our potential energy of deformation is purely geometrically based. It is defined as the L ² norm of the change of the differential coordinates. A key feature of this energy formulation is that the corresponding stiffness matrix is approximately constant, which enables fast and stable implicit integration and large deformations. Our algorithm can simulate various effects including solid, thin shell and plasticity. We also adopt two schemes to accelerate the simulation process: dimensionality reduction in frequency domain and adaptive rotation computation in spatial domain.     

船用蒸汽轮机键合图建模与仿真研究

从基本物理规律出发,应用键合图理论研究了船用蒸汽轮机的仿真建模问题,以规范化的方式统一分析了汽轮机装置中同时存在的热功转换、热传递和质量迁移3种能量作用,建立了一类具有通用性的船用汽轮机装置全工况键合图模型。结合具体算例对模型进行验证,通过仿真结果与实测值比较证明模型能够真实地反映船用汽轮机装置的静态与动态工作特性,具有较好的精确性和实时性。对键合图理论在热工对象仿真建模领域内的应用作了一些探讨,所得结论具有一定的理论和工程实用价值。     

Contact and Deformation Modeling for Interactive Environments

Contact and deformation modeling for interactive environments has seen many applications, from surgical simulation and training, to virtual prototyping, to teleoperation, etc., where both visual feedback and haptic feedback are needed. High-quality feedback demands a high level of physical realism as well as a high update rate in rendering, which are often conflicting requirements. In this paper, we present a unique approach to modeling force and deformation between a rigid body and an elastic object under complex contacts, which achieves a good compromise of reasonable physical realism and real-time update rate (at least 1 kHz). We simulate contact forces based on a nonlinear physical model. We further introduce a novel approximation of material deformation suitable for interactive environments based on applying Bernoulli-Euler bending beam theory to the simulation of elastic shape deformation. Our approach is able to simulate the contact forces exerted upon the rigid body (that can be virtually held by a user via a haptic device) not only when it forms one or more than one contact with the elastic object, but also when it moves compliantly on the surface of the elastic object, taking friction into account. Our approach is also able to simulate the global and local shape deformation of the elastic object due to contact. All the simulations can be performed in a combined update rate of over 1 kHz, which we demonstrate in several examples.     

Dynamic hybridization of MILES and RANS for predicting airfoil stall

Hybridization comprised of an algebraic turbulence model based on the Reynolds average Navier-Stokes (RANS) equations with a monotonically integrated large eddy simulation (MILES) is proposed to simulate transient fluid motion during separation and vortex shedding at high Reynolds numbers. The proposed hybridization utilizes the Baldwin-Lomax model with the Degani-Schiff modification as the RANS model in the near-wall region and a MILES far from the wall. Although the hybridization is assumed to be a MILES with wall modeling, the transition line between the RANS and the MILES modes is determined by the turbulent intensity that is dominated by the large eddies in the grid-scale. This hybrid model is applied to the flows past three different types of airfoils (NACA63₃-018, NACA63₁-012 and NACA64A-006) near stall, at a chord Reynolds number of Re = 5.8 × 10⁶. These airfoils are classified as trailing-edge-stall, leading-edge-stall and thin-airfoil-stall airfoils, respectively. The computed results are compared with wind tunnel experiments. The hybrid model successfully demonstrates accurate stall angle and surface pressure distribution predictions near the stall for each type of airfoil. The airfoil simulation results confirmed that the hybrid model provides a better prediction than the RANS model for unsteady turbulent flows with separation and vortex shedding simulations.     

A robust least squares based approach to min‐max model predictive control

This article deals with the model predictive control (MPC) of linear, time‐invariant discrete‐time polytopic (LTIDP) systems. The 2‐fold aim is to simplify the treatment of complex issues like stability and feasibility analysis of MPC in the presence of parametric uncertainty as well as to reduce the complexity of the relative optimization procedure. The new approach is based on a two degrees of freedom (2DOF) control scheme, where the output r(k) of the feedforward input estimator (IE) is used as input forcing the closed‐loop system ∑_f. ∑_f is the feedback connection of an LTIDP plant ∑_p with an LTI feedback controller ∑_g. Both cases of plants with measurable and unmeasurable state are considered. The task of ∑_g is to guarantee the quadratic stability of ∑_f, as well as the fulfillment of hard constraints on some physical variables for any input r(k) satisfying an “a priori” determined admissibility condition. The input r(k) is computed by the feedforward IE through the on‐line minimization of a worst‐case finite‐horizon quadratic cost functional and is applied to ∑_f according to the usual receding horizon strategy. The on‐line constrained optimization problem is here simplified, reducing the number of the involved constraints and decision variables. This is obtained modeling r(k) as a B‐spline function, which is known to admit a parsimonious parametric representation. This allows us to reformulate the minimization of the worst‐case cost functional as a box‐constrained robust least squares estimation problem, which can be efficiently solved using second‐order cone programming.     

基于SHML的CPS行为建模及仿真

信息物理融合系统（cyber-physical systems，简称CPS）是深度融合了计算进程和物理进程的统一体，是集计算、通信与控制于一体的下一代智能系统，具有广阔的应用前景.CPS的行为具有混成性、随机性等特征，建模及仿真CPS的动态行为对于开发高质量的CPS系统至关重要.但是目前缺乏面向CPS的领域建模方法及建模CPS的领域建模语言，也迫切需要支持仿真CPS领域模型的仿真工具.针对以上问题，提出一种面向CPS领域的随机混成建模语言（stochastic hybrid modeling language，简称SHML）以支持建模CPS系统的行为.首先，根据CPS的领域特征定义了SHML的元模型作为其抽象语法，并定义了SHML的具体语法和操作语义；其次，基于GEMOC框架实现了SHML的可视化建模工具.此外，集成GEMOC的序列化执行引擎和Scilab的连续行为仿真引擎，实现仿真CPS的混成行为.提出了一种面向CPS领域的建模及仿真方法，设计并实现了一个集成的面向CPS行为的建模与仿真平台，为CPS的建模及仿真提供了一种有效的方法及工具支撑.