基于改进弹簧振子模型的软组织形变仿真

研究虚拟手术中软组织形变的特定性优化问题。传统弹簧振子模型由于采用线弹力方程及显示欧拉积分法求解,导致的真实性低、计算复杂、实时性差的问题。为解决形变动特性问题,提出一种改进弹簧振子模型的形变方法,采用四面体网格弹簧振子模型进行建模,并通过定义质点间弹簧的非线性弹力方程以及作用于质点的体积力方程,来减弱超弹性现象的发生,提高真实性。模型求解时,采用一种改进显式欧拉积分的动态局部算法来求解形变方程。实验结果表明,所提算法提高了形变仿真的真实性、实时性和稳定性,可为软组织形变动态仿真系统的设计提供参考。     

虚拟肩关节镜手术中软组织形变的模拟

为实现虚拟肩关节镜手术中软组织的形变模拟,提出一种改进的质点-弹簧模型.通过自适应采样及K阶最邻近节点快速查询算法,建立表面质点密度高而中心质点密度低的拓扑结构模型,在传统质点-弹簧形变模型的基础上,增加防止弹簧翻转的复原力,采用计算统一设备构架实现图形处理器的加速.模拟结果表明,该模型能够实现较真实的形变模拟.     

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.     

基于局部动态模型的软组织形变建模与仿真

选择人体软组织模型作为研究对象,针对医学触诊训练的特点,提出了可变区域的局部质点-弹簧/阻尼器(ALMSDM)模型.该模型具有位置可变及区域可扩展的动态特性,改善了以往文献中局部建模方法静态限定的局限,解决了全局“面模型”形变恢复能力差及数据量大的问题.结合ALMSDM的特点,提出了顶点法向量局部更新与预计算策略,从而极大地提高了系统的实时性.从形变恢复能力、反馈力及实时性3方面对不同模型下的系统性能进行了评价,结果显示所提算法能够保证虚拟软组织形变仿真的精确性与实时性,具有可行性与通用性.     

一种基于弹簧振子模型的颅面软组织仿真算法

软组织仿真研究是利用计算机技术对真实软组织进行模拟以便研究,为医学工作者提供辅助和帮助科研工作者更好地进行虚拟手术。这方面的研究在国外已经较为普及,在国内尚处于起步阶段。软组织仿真算法已有多种成熟模型,其中较为代表性的有弹簧振子模型（MSS）和有限元模型（FEM）。在参阅了前人的研究基础上,选取了弹簧振子模型作为软组织仿真的物理形变模型,并对仿真算法进行编码和仿真实验,进而对实验效果进行分析,并提出改进之处。实验发现基于弹簧振子模型的软组织较好地实现了形变仿真的效果。     

一种基于弹簧振子模型的颅面软组织仿真算法

软组织仿真研究是利用计算机技术对真实软组织进行模拟以便研究,为医学工作者提供辅助和帮助科研工作者更好地进行虚拟手术.这方面的研究在国外已经较为普及,在国内尚处于起步阶段.软组织仿真算法已有多种成熟模型,其中较为代表性的有弹簧振子模型(MSS)和有限元模型(FEM).在参阅了前人的研究基础上,选取了弹簧振子模型作为软组织仿真的物理形变模型,并对仿真算法进行编码和仿真实验,进而对实验效果进行分析,并提出改进之处.实验发现基于弹簧振子模型的软组织较好地实现了形变仿真的效果.     

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.     

耦合无网格迦辽金与质点弹簧实现软组织形变仿真

软组织形变仿真是虚拟手术系统的关键技术.针对目前软组织形变仿真方法存在的问题,基于黏弹性力学模型提出了无网格伽辽金(EFG)与质点弹簧(MS)耦合的软组织形变仿真方法.在手术区域即大变形和拓扑改变区域采用EFG,其他区域采用MS方法,2个区域之间建立过渡单元;在过渡单元内建立过渡节点,根据过渡节点必须满足位移和力平衡条件来实现2个区域的无缝耦合.对人体肝脏的形变仿真实验结果表明,该方法是有效的.     

虚拟手术中模拟软组织切割的混合弹性模型

虚拟手术仿真系统应该同时满足真实性和实时性的要求.弹簧质点模型可以满足实时性的要求,但它无法模拟人体的粘弹性特征;而传统的有限元模型虽然可以模拟人体的粘弹性特征,但巨大的时间消耗使得这个方法几乎无法给出实时的反馈.混合弹性模型是一种基于有限元方法的优化模型,由于采用了简化措施,使得它既克服了弹簧质点模型的缺陷;同时也满足了虚拟手术的实时性要求.实验证明混合弹性模型在虚拟手术的真实性和实时性之间达到了一个较好的折中效果,它可以实时的提供视觉反馈,同时也使在虚拟手术系统中引入力反馈器成为可能.     

质点弹簧模型在牙龈软组织形变仿真上的改进与应用

牙龈软组织的形变仿真和建模是虚拟牙齿矫正系统的重要部分.针对外力过大时传统质点弹簧模型引起的牙龈脱落牙齿的"超弹性"问题,在传统"面模型"的基础上,通过对比形变效果和形变速度来设置形变弹簧最优阈值.当形变弹簧缩短或伸长超过阈值时,将弹簧长度恢复为原始长度以作为下一次单步形变的初始长度,从而使面模型具有一种"体"的特征.在形变结束后采用"光滑度之和"来度量牙龈表面的光滑度,该方法从数值上对光滑度进行合理定义.在VTK环境下实现改进模型及相关模型,在真实扫描的牙颌三维模型数据上进行牙龈软组织形变仿真对比实验.结果表明,改进模型形变速度提高28%以上且光滑度之和明显降低,说明改进模型在保证较快形变速度的前提下较好地解决了超弹性问题.     

一种基于物理意义的快速力反馈形变模型及实时力觉响应算法

虚拟物体在受力作用时的形变建模是虚拟环境中力/触觉人机交互的关键.文中提出了一种新的基于物理意义的形变建模方法,不仅计算速度快,满足力反馈的实时性要求,而且能够同时保证接触力和形变的计算具有较高的精度,适用于具有较大变形量的柔性物体的力反馈计算,满足精细作业对虚拟现实系统的要求.     

MIMO Output Estimation With Reduced Multirate Sampling for Real-Time Haptic Rendering

This paper presents an output-estimation method with reduced multirate sampling for real-time multi-input-multi-output (MIMO) haptic rendering. Haptic systems employ physics-based deformation models such as finite-element models and mass-spring models. These physics-based deformation models for high fidelity have to deal with complex geometries, material properties, and realistic behavior of virtual objects. This incurs heavy computational burden and time delays so that the reflective force often cannot be computed at 1 kHz which is a safe frequency for stability of the haptic systems. Lower update rates of the haptic loop and the computational time delay also deteriorate the realism of the haptic system. This problem is resolved by the proposed MIMO output-estimation method. The haptic system is designed to have two sampling times, T and JT, for the haptic loop and the graphic loop, respectively. Dynamics of the physics-based deformation is captured in a discrete and deterministic input-output model. The MIMO output estimation method is developed drawing on a least-squares algorithm and an output-error estimation model. The P-matrix resetting algorithm is also designed to deal with the changing input-output relationship of the deformation model. The parameters of the discrete input-output model are adjusted online. Intersample outputs are computed from the estimated input-output model at a high rate, and traces the correct output computed from the deformation model. This method enables graphics rendering at a lower update rate, and haptic rendering at a higher update rate. Convergence of the proposed method is proved, and performance is demonstrated through simulation with both a linear tensor-mass and a linear mass-spring models.     

Deformable part inspection using a spring–mass system

In order to inspect deformable parts, recent works use virtual deformation on a digitized version of a real-part to bring the part model back to its nominal shape. This simulation mimics the real process called fixturing, which is normally used by the manufacturer to bring back the part into its nominal shape once installed. To perform such virtual deformation Finite Element Methods (FEMs) are used in order to meet the precision requirements of the inspection process. This paper presents a method based on a spring–mass system, whose formulation is much simpler than the FEM, which allows the calculation of deformations of shell type parts with accuracy comparable to FEM. Furthermore, due to the simplicity in its formulation the algorithm can be implemented more easily than the FEM. The system is composed of two types of springs: one type models membrane behavior of the part’s mesh model and the second type models the flexion behavior between each mesh elements. We show that by applying the proposed mass-spring model, it is possible to reduce the calculation time by 80% over standard FEM calculation opening the door to real-time inspection.     

A fast parametric deformation mechanism for virtual reality applications

Virtual reality technologies have been adopted in a wide variety of applications for its interactive ability and realistic senses. Despite early implementations regard VR only as a medium for lively animation; a practical VR work must deliver precise deformation on virtual objects based on real-time interactions. The exact ability is especially important for users who utilize VR to do collaborative design, for it will greatly reduce the amount of on-line computations on operating substance-based interactions, and consequently facilitates the collaboration. Therefore, this research will employ neural networks to memorize the deformation behavior of solid objects, and then perform instant and accurate deformations in the virtual environment. The proposed method also allows design variations for parametric features, and uses feature parameters as variable switches to adjust the deformation mechanism. There are three steps in the method: (1) For a sample object, generate force-induced deformations using the finite-element method; (2) memorize the surface displacements with artificial neural networks; and (3) convert the parametric deformation matrices into Behavioral Modules for the virtual reality engine. In the implementations, ANSYS is used to generate model deformations, and MATLAB is used to perform neural training. Finally, a virtual environment is built using Virtools where customized Building Blocks are created to present interactive deformation behavior. Experiments were carried out on an Intel XEON workstation with nVIDIA Quadro4 750GL display device. Sample workparts are tested to examine the ability of the method. The results show that both training accuracy and real-time capability are more than satisfactory.     

柔软物体物理自由变形仿真

在虚拟现实和计算机动画技术中,基于几何模型和基于物理模型的方法已经得到广泛的研究。利用几何模型和物理模型相结合的方法,将质点弹簧模型加到Bezier网格上,通过控制Bezier网格上的控制点来达到物体的变形,该方法与直接在物体上建立质点弹簧模型的方法相比更加简单、快速,且更加容易达到物体的全局变形。质点的运动满足拉格朗日运动方程,采用了微分方程组来表示质点的运动规律,并给出了用数值求解法对系统进行了的实时计算的求解过程;同时还将一个简单的柔软物体碰撞处理方法应用在质点-弹簧模型中,实验表明该方法简单有效。     

在线VRFT数据驱动控制方法及其仿真研究*

虚拟参考反馈整定(VRFT)是一种离线的数据驱动控制器参数整定的方法,要求整定过程中对象特性保持不变。本文针对离线算法的不足,提出了一种在线VRFT数据驱动算法。首先利用滤波器改变了离线算法的时序,得到用于实时运算的有效数据。然后提出了基于带遗忘因子递推最小二乘法的VRFT控制器参数辨识方法,不依赖于对象模型,完全利用实时数据实现了在线控制器参数整定。仿真结果表明, 在对象特性变化较大的情况下,在线VRFT整定方法优于传统的离线VRFT方法,具有很好的自适应性。     

Comparison of FEM and BEM for interactive object simulation

There are two major approaches for real-time object simulation namely, the geometry (non-physically) based and the physically based approaches. Geometry based approaches such as free-form deformation (FFD) employ purely geometric techniques to model deformation. Physically based approaches usually adopt mass-spring system, finite element method (FEM) or boundary element method (BEM) for simulation. The mass-spring system is simple and only gives a coarse estimation of object deformation. Recently, FEM and BEM have been extensively used in object simulation because of the demand for more realistic simulation. However, a major drawback of FEM and BEM is their difficulty to achieve real-time deformation. In this article, we compare two different physically based approaches, FEM and BEM, according to their accuracy and computational complexity. Several experiments were conducted to compare the time required for the pre-computation process and the deformation process. In addition, the BEM with linear boundary elements is implemented and tested. At the current state of investigation, for the meshes with triangular elements, BEM with linear boundary elements is significantly faster than BEM with constant boundary elements under most of the circumstances. With the band matrix of FEM, the pre-computation process is faster than the BEM for a model with small mesh size. However if the mesh size of the model is large, the pre-computation process of BEM with linear boundary elements is the fastest.     

基于简化的质点弹簧模型的布料模拟

首先运用质点弹簧系统建立布料的面模型,针对布料模拟中经典的质点弹簧模型效率不高的问题,提出了简化的质点弹簧模型,通过减少弹簧的数量来简化模型提高效率;为提高数值求解的计算效率,用显式欧拉法对模型进行数值求解;针对模拟过程中存在过度拉伸的问题,通过构造约束变形进行了合理解决,选用层次包围盒法对布料进行碰撞检测;最后在确保模拟的稳定性的基础上,高效.逼真的模拟出了布料.     

基于改进质点弹簧模型下的布料实时仿真

布料模拟一直是计算机动画的研究热点.且对于虚拟现实技术有着重要意义.在以往的研究中,人们提出了许多模拟织物运动的方法,方法的一个主要缺点是复杂度高、计算效率低,无法满足交互设计和实时动画的要求.在经典质点掸簧模型的表示基础上结合Choi的布料模型,通过简化弹簧弯曲抗力的计算方法,合并了弹簧的结构力和剪切力,从而达到了简化了布料物理模型的目的,提高了模拟的速度.实验证明,采用经改进的质点弹簧模型可以得到实时稳定的布料仿真效果.