Scientific Articles, Reports & Theses
List of Publications
Select tag “Algoryx” for publications affiliated with Algoryx, or “External” for external publications that use or cite Algoryx software.
2007 |
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 | Lacoursière, Claude: Ghosts and machines: regularized variational methods for interactive simulations of multibodies with dry frictional contacts. Department of Computing Science, Umeå University, Sweden, 2007. (Type: PhD Thesis | Abstract | Links | BibTeX | Tags: Algoryx)@phdthesis{lacoursiere2007ghosts,A time-discrete formulation of the variational principle of mechanics is used to provide a consistent theoretical framework for the construction and analysis of low order integration methods. These are applied to mechanical systems subject to mixed constraints and dry frictional contacts and impacts|machines. The framework includes physics motivated constraint regularization and stabilization schemes. This is done by adding potential energy and Rayleigh dissipation terms in the Lagrangian formulation used throughout. These terms explicitly depend on the value of the Lagrange multipliers enforcing constraints. Having finite energy, the multipliers are thus massless ghost particles. The main numerical stepping method produced with the framework is called SPOOK. Variational integrators preserve physical invariants globally, exactly in some cases, approximately but withinfixed global bounds for others. This allows to product realistic physical trajectories even with the low order methods. These are needed in the solution of nonsmooth problems such as dry frictional contacts and in addition, they are computationally inexpensive. The combination of strong stability, low order, and the global preservation of invariants allows for large integration time steps, but without loosing accuracy on the important and visible physical quantities. SPOOK is thus well-suited for interactive simulations, such as those commonly used in virtual environment applications, because it is fast, stable, and faithful to the physics. New results include a stable discretization of highly oscillatory terms of constraint regularization; a linearly stable constraint stabilization scheme based on ghost potential and Rayleigh dissipation terms; a single-step, strictly dissipative, approximate impact model; a quasi-linear complementarity formulation of dry friction that is isotropic and solvable for any nonnegative value of friction coefficients; an analysis of a splitting scheme to solve frictional contact complementarity problems; a stable, quaternion-based rigid body stepping scheme and a stable linear approximation thereof. SPOOK includes all these elements. It is linearly implicit and linearly stable, it requires the solution of either one linear system of equations of one mixed linear complementarity problem per regular time step, and two of the same when an impact condition is detected. The changes in energy caused by constraints, impacts, and dry friction, are all shown to be strictly dissipative in comparison with the free system. Since all regularization and stabilization parameters are introduced in the physics, they map directly onto physical properties and thus allow modeling of a variety of phenomena, such as constraint compliance, for instance. Tutorial material is included for continuous and discrete-time analytic mechanics, quaternion algebra, complementarity problems, rigid body dynamics, constraint kinematics, and special topics in numerical linear algebra needed in the solution of the stepping equations of SPOOK. The qualitative and quantitative aspects of SPOOK are demonstrated by comparison with a variety of standard techniques on well known test cases which are analyzed in details. SPOOK compares favorably for all these examples. In particular, it handles ill-posed and degenerate problems seamlessly and systematically. An implementation suitable for large scale performance and accuracy testing is left for future work. |
 | Lacoursière, Claude: A Parallel Block Iterative Method for Interactive Contacting Rigid Multibody Simulations on Multicore PCs. In: Kågström, Bo; Elmroth, Erik; Dongarra, Jack; Wasniewski, Jerzy (Ed.): Applied Parallel Computing. State of the Art in Scientific Computing, PARA 2006. Lecture Notes in Computer Science, pp. 956–965, Springer Berlin Heidelberg, Berlin, Heidelberg, 2007, ISBN: 978-3-540-75755-9. (Type: Proceedings Article | Abstract | Links | BibTeX | Tags: )@inproceedings{parallell_block_lacoursiere2007,A hybrid, asynchronous, block parallel method to approximately solve complementarity problems (CPs) in real-time on multicore CPUs is described. These problems arise from interactive real-time simulations of systems of constrained, contacting rigid bodies, which are useful in virtual operator training systems for instance. A graph analysis phase identifies components which are weakly coupled using simple heuristics. Each component is then solved in parallel using either a block principal pivot or a projected block Gauss-Seidel method running in separate threads. Couplings which generate forces between the subsystems are handled iteratively using a Gauss-Seidel process which communicates updates between the interacting subsystems asynchronously. Preliminary results show that this approach delivers good performance while keeping overhead small. |
2006 |
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Lacoursière, Claude: A regularized time stepper for multibody systems. Department of Computing Science, Umeå University, 2006. (Type: Book | BibTeX | Tags: Algoryx)@book{lacoursiere2006regularized, | |
 | Servin, Martin; Lacoursière, Claude; Melin, Niklas: Interactive simulation of elastic deformable materials. In: SIGRAD 2006. The Annual SIGRAD Conference; Special Theme: Computer Games, Linköping University Electronic Press 2006. (Type: Proceedings Article | Abstract | Links | BibTeX | Tags: Algoryx)@inproceedings{servin2006interactive,A novel; fast; stable; physics-based numerical method for interactive simulation of elastically deformable objects is presented. Starting from elasticity theory; the deformation energy is modeled in terms of the positions of point masses using the linear shape functions of finite element analysis; providing for an exact correspondence between the known physical properties of deformable bodies such as Young’s modulus; and the simulation parameter. By treating the infinitely stiff case as a kinematic constraint on a system of point particles and using a regularization technique; a stable first order stepping algorithm is constructed which allows the simulation of materials over the entire range of stiffness values; including incompressibility. The main cost of this method is the solution of a linear system of equations which is large but sparse. Commonly available sparse matrix packages can process this problem with linear complexity in the number of elements for many cases. This method is contrasted with other well-known point mass models of deformable solids which rely on penalty forces constructed from simple local geometric quantities; e.g.; spring-and-damper models. For these; the mapping between the simulation parameters and the physical observables is not well defined and they are either strongly limited to the low stiffness case when using explicit integration methods; or produce grossly inaccurate results when using simple linearly implicit method. Validation and timing tests on the new method show that it produces very good physical behavior at a moderate computational cost; and it is usable in the context of real-time interactive simulations. |
2005 |
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 | Backman, Anders: Colosseum3D-Authoring framework for Virtual Environments.. In: IPT/EGVE, pp. 225–226, Citeseer 2005. (Type: Proceedings Article | Abstract | Links | BibTeX | Tags: Algoryx)@inproceedings{backman2005colosseum3d,This paper describes an authoring environment for real time 3D environments, Colosseum3D. The framework makes it possible to easily create rich virtual environments with rigid-body dynamics, 3D rendering using OpenGL Shaders, 3D sound and human avatars. The creative process of building com- plex simulators is supported by allowing several authoring paths such as a low level C++ API, an ex- pressive high level file format and a scripting layer. To exemplify the use of the framework, an immersive wheelchair simulator application is presented. A natural and intuitive interaction method is implemented using dynamic simulation. |
2003 |
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 | Lacoursière, Claude: Splitting methods for dry frictional contact problems in rigid multibody systems: Preliminary performance results. In: The Annual SIGRAD Conference. Special Theme-Real-Time Simulations. Conference Proceedings from SIGRAD2003, pp. 11–16, Linköping University Electronic Press 2003. (Type: Proceedings Article | Abstract | Links | BibTeX | Tags: Algoryx)@inproceedings{lacoursiere2003splitting,A splitting method for solving LCP based models of dry frictional contact problems in rigid multibody systems based on box MLCP solver is presented. Since such methods rely on fast and robust box MLCP solvers; several methods are reviewed and their performance is compared both on random problems and on simulation data. We provide data illustrating the convergence rate of the splitting method which demonstrates that they present a viable alternative to currently available methods. |