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High speed multibody dynamics _ sherman_Simbody _ 2006.ppt
- 1. SimTK Simbody™ High speedmultibody dynamics for biosimulation Michael Sherman (Sherm) OpenSim Science Advisor Workshop, 2 June 2006
- 2. 2 Topics • Motivation: DBPs •Multibody mechanics • Several S’s of SimTK Simulation • Where does Simbody fit in? • Simbody features • Simbody release schedule
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- 6. 6 What do thesehave in common? • Biological structures • Dynamics well described by classical physics — that is, F=ma • They’re “chunky”
- 7. 7 Multibody systems • Rigidparts … • … mobilized by Joints • … influenced by Forces • … and restricted by Constraints. • Key feature: motion is localized.
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- 9. 9 Abstractions for classicalphysics • Matter • Energy • Um …
- 10. 10 Matter • Mass • Spatialdistribution • Motion
- 11. 11 Abstract matter • Therigid body • What is a rigid body? Bi • Mass distribution: 10 constants • Decorate w/geom & other props • Ground is a (heavy) rigid body
- 12. 12 Abstract motion (1) •The joint. What is it? • Defines relative mobility btw 2 rigid bodies • Examples Ball Slider Pin • Defines topology & mechanical coordinates q
- 13. 13 Multibody system (1) •Tree of bodies interconnected by joints
- 14. 14 Abstract motion (2) •Oops! That’s a little too floppy … • Restrict with constraints. • Algebraic invariant relating q’s: g(q)=0 • Restricts allowable motion
- 15. 15 Multibody system (2) •Constraints permit loops g(q)= 0
- 16. 16 That’s it forrigid matter • Rigid bodies + mobility – restrictions • Solve for (response) • Integrate for trajectory • Still need forces … but that’s another matter.
- 17. 17 Summary • Rigid bodymechanics is a solved problem – Everything else is much harder and more interesting • But … the implementation is a bit tricky • Simbody addresses that problem.
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- 19. 19 Terminology: modeling createsa “System” System Physical world Modeling • We’re not talking about modeling today • A “System” is a computational embodiment of a mathematical model
- 20. 20 Properties of aSystem System • Defines its parametrization • But … it is stateless. • Given a State, performs useful computations State
- 21. 21 Studying a system •System+State+Study Simulation Study State System Results states
- 22. 22 Under the hood:subsystems • Interlocking computations • System provides the “edge pieces” Study State System subsystem
- 23. 23 What’s in amultibody system? • Matter and energy • Simbody deals with matter Forces (User supplied) Simbody Multibody System State
- 24. 24 Software stack Lapack/Blas Simmatrix Simbody Forces CVODE Rootfinding Spatial operators State Multibody system Time stepper Projection Study Results
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- 26. 26 Simbody™ a SimTK toolsetfor internal coordinate (torsion angle) mechanics
- 27. 27 SimTK Core GUI Tools| Documentation Tools | Installation Modeling Linear Algebra Multi Body Dynamics Simbody TAO • Integrator ODE DAE Contact Rigid Penalty • Optimize Sim Anneal Genetic SQ Prog Control PD PDEs Solids Fluids Meshing Monte Carlo Force Fields
- 28. 28 Ancestry IVM (Charles Schweiters, NIH) TAO(Chang, Ruspini, Arachi Corp.) Simbody Spatial operator algebra, Jain & Rodriguez, JPL Feathersone, Australian Natl. U. Khatib lab, Stanford Rosenthal & Sherman, Symbolic Dynamics, Protein Mechanics Linda Petzold, UCSB SD/FAST, Pro/Mechanica, Imagiro Thomas Kane, Stanford William Gear, UIUC
- 29. 29 Simbody vs. SD/FAST •All numerical – no compiler needed • No dealing with, um, (a company) – Completely open source, not GPL (MIT/BSD license) – But can I <whatever>? Yes. • O(N) all the time; targeted at 1000 bodies + 100 constraints – Goal: within 2X of sd/fast O(n) on small problems, faster on big ones • Structured as a set of O(N) operators – Including Operational Space Control • No internal state • Shareable subsystems, systems & studies – Mechanical models – Forces, constraints, custom joints – Numerical methods • Coordinate projection instead of Baumgarte stabilization • Advanced numerical integration (implicit/explicit/multistep)
- 30. 30 Simbody basic features •A linkable library with nice API • Solid theory; not a “game engine” • Technology is internal coordinate mechanics • Open & closed-chain models • Forward/inverse kinematics & dynamics • Parametrization/design studies • Easily extensible (e.g., new joint types) • Multiplatform binaries (Windows,Mac,Linux) & easily portable to more • Agnostic about forces & numerical methods, but … • … will come with some good ones.
- 31. 31 Schedule • Simbody 0.5available now – Very rough; brave souls only – Basic functions; C++ only; lightly tested – Everything will change before 1.0 • Simbody 1.0 (March 2007) – Complete functionality; SD/FAST replacement – Extensive testing & good documentation – Stable API: C++, C, Fortran, SD/FAST compatible – Integrated into several GUIs (OpenSim, ToRNAdo) • Simbody 2.0 (~ +1 year) – Analytic Jacobian in O(n2) – High level modeling support – Matlab/Java/Python wrapping – (Even) faster – Extensive libraries of force, constraint, joint, studies, reporters, etc. Simbody info & downloads: https://simtk.org/home/simbody
- 32. 32 Acknowledgments (software) • CharlesSchwieters • Arachi Corp. (K.C. Chang & D. Ruspini) • Paul Mitiguy • Bryan Keller • Jack Middleton Simbody info & downloads: https://simtk.org/home/simbody