simulating multi-rigid-body dynamics with contact and friction
DESCRIPTION
Simulating Multi-Rigid-Body Dynamics with Contact and Friction. Mihai Anitescu SIAM OPTIMIZATION MEETING May 10, 1999. Rigid Multi-Body Dynamics Simulation Applications. Industrial simulation and design for aircraft, ships, heavy vehicles (product design -- production, 5 years). - PowerPoint PPT PresentationTRANSCRIPT
Simulating Multi-Rigid-Body Dynamics with Contact and Friction
Mihai Anitescu
SIAM OPTIMIZATION MEETING
May 10, 1999
Rigid Multi-Body Dynamics Simulation Applications
• Industrial simulation and design for aircraft, ships, heavy vehicles (product design -- production, 5 years).
• Flexible Manufacturing Systems design and redesign (commissioning+tuning, 3 years).
• Appropriate simulation could reduce lead times by up to 50%.
• Simulating virtual environments (+real time = interactive).
Virtual Environment Application Example
• Setting up the real installation for training is very expensive.
• The virtual environment can be easily modified for different scenarios, but simulating it in real-time is essential.
• Simplified model: rigid bodies with joints and contacts.
Insert PictureInsert Picture
Initial model requirements
• Acceleration based Newton laws.
• Rigid bodies.
• No inter penetration, nonnegative contact impulses and passive contact
• Joint constraints
• Coulomb friction or reasonable approximations.
• Impact resolution (Poisson hypothesis).
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01,0)(i ncniq
Classical Constrained Dynamics
(MDP)
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Inconsistency example
• State space representation
• Constraint , initially
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cos(
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Frictional Inconsistency
• What went wrong? Newton’s laws rather than Coulomb’s model.
• Configuration inconsistent, not only singular.
• P. Painleve: Governing equations of motion do not have a solution in general( Comptes Rendus Acad. Sci. Paris, 1895).
• Approaches for the continuous case.– Solutions in a distributional sense.
– Differential inclusions instead of differential equations (Moreau, 1986; Monteiro-Marques, 1993).
Avoiding Frictional Inconsistencyin time-stepping schemes
• Time-stepping: If a continuous model exists, how should the associated numerical integration scheme look like?
• Friction is estimated, and the frictionless problem is solved (Lotstedt,1982). Problems: starting configuration and the impact case.
• The inconsistent configuration is treated as a collision(Baraff,1993). Problems: arbitrary collisions and the impact case.
• Newton's law with impulses and velocities (Stewart & Trinkle 1995; Anitescu & Potra, 1996).
Revised model requirements
• Velocity based Newton laws, by combining time stepping with impulse resolution.
• Rigid bodies.
• No inter penetration, nonnegative contact impulses and passive contact
• Joint constraints
• Coulomb friction or reasonable approximations.
• Impact resolution (Poisson hypothesis).
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mjq 1,0)(j
Framework and notations used
• The rigid multi-body system is described by its generalized coordinates, q, and generalized velocities, v.
• The contacts (joints) are described by inequalities (equalities).
• The ``tangent plane'' for a contact constraint is defined by the cone generators
• M: mass matrix, symmetric positive definite; h: the time step k: external force.
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20),,( i iqt
Insert Model SlidesInsert Model Slides
Contact Model
• Contact configuration described by the (generalized) distance function , which allows for some interpenetration.
• Feasible set .
• Contact impulses are compressive
• The normal velocity for non inter penetration is velocity-based (as opposed to Stewart & Trinkle, where it is position-based):
• Complementarity guarantees energetic passivity of contact:
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0nc
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Contact Description
• is the normal impulse and is the tangential impulse
• is the total impulse in Newton Euler world coordinates .
• Classical Coulomb model requires the 3D contact impulse to lie in a circular cone.
• If relative motion exists, the frictional impulse has to be opposite to the relative velocity (Maximum Dissipation)
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1
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Polygonal approximation of Coulomb cone
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scoordinate NE : Each
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The Integration Step
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Matrix Form of The Integration Step
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Linear Complementarity Problems (LCPs)
• Linear and Quadratic Programming are LCPs, via primal-dual formulations.
• If , a solution exists, (by a Brouwer fixed point argument), and can be computed by Lemke’s algorithm (pivotal, similar to simplex).
• M has to be copositive
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0,)()( xcqMxxxf T
0,0 xMxxT
Interior Point Algorithms (IP)
• It takes corrected Newton steps to compute a solution if M is positive semi definite. The central path is well defined.
• In the convex case IP algorithms have polynomial complexity, but other extensions are unclear.
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..1)()(
0)(,0)(,)()(
ssxx
nisx
sxqMxs
ii
)1ln(( nO
Theorem
• Consider the mixed LCP above. If M is a symmetric positive definite matrix, N a copositive matrix and b a nonnegative vector, then the mixed LCP has a solution. Lemke’s algorithm (with anti cycling strategy) will find a solution of the LCP obtained by eliminating x,y.
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ss
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NH
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Properties of the formulation
• The model is solvable for any configuration.
• Discrete Maximum Dissipation Principle (MDP): The frictional impulse maximizes the dissipation over all feasible contact impulses, given
• If the mass matrix is constant, the kinetic energy at the new step cannot exceed the the kinetic energy of the system with no constraint enforced (contacts are passive).
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)1()()()1()1()()1()()1(
1)(1)()1()1(
2
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Energy/ stability properties
• Assumption: M constant.
• If the external force is of the form
•
• c depends only on M and d. h has to be sufficiently small but depending only on c, T.
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c
cTeeqMvvTMvTv
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2))0()0()0(()()(
then,)(,)(,),(
where),()(),()(),(
1)0()0()()(,0 If 32 cTMvvTMvTvkk TT
Constraint stability
• There are constants C,H depending on the problem data, but not on the time-step h, such that
• for any h < H (q(t) is obtained by linear interpolation).
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i
njtqCh
miChtqCh
1)),((
1,))((
Difficult Friction Configuration
• For any friction coefficient we obtain multiple solutions.
• Does this result in undesirable properties of the LCP matrix?
Column sufficiency?
0.any for ,sufficientcolumn not is Q
.0but ,0)( ,0any For
.0,0,0,0 If
0
011
11
nii
nnT
n
T
TT
TT
cQzz
ccn
c
e
eDMDnMD
DMnnMn
Q
Row sufficiency?
0.any for ,sufficient rownot is Q
.0but ,0)( ,0any For
.0,0,0,0 If
00
11
11
niT
i
nnT
n
T
TT
TT
T
czQz
ccn
c
e
eDMDnMD
DMnnMn
Q
Properties of the LCP matrix.
• The matrix Q is neither row, nor column sufficient.
• As a result, it cannot be in the class, which makes it difficult to use polynomial-time interior-point algorithms.
• Lemke’s algorithm appears to be the only one which is guaranteed to provide a solution to this model.
• However, further analysis may reveal good matrix properties for pointed friction cones.
• Since model needs to solve a succession of these LCPs anyway, other approaches may provide (computationally) better matrices.
*P
Differential Inclusions
cts.,0)(,)(co)(
)()(
|
ttKdtt
dvttK
dt
dv
tt
• v has bounded variation
• K(t) is closed and convex set valued
)]()()[lim 1 iiiTT tvtvtdv
Theorem
• For a pointed friction cone, there exists
• The quantities are obtained by linear interpolation. (D. Stewart)
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)cts. ,))((
*weak ),()(
a.e. ),()(
uniformly ),()(
dvtdvt
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Th
h
h
h
k
k
k
k
kk hh vq ,
Theorem(II)
• For one contact, maximal dissipation holds
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1
)(),(),(
0
0
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dtvkMvv
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dvM
tTT
t
a.e , nTT
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cvDvDdc
d
Accomplishments
• The time-stepping model to has a solution regardless of the configuration and dimension of the problem. Impulsive solutions are accommodated.
• Static and dynamic friction are treated in an unitary manner.• The constraints are guaranteed to be satisfied within any
given tolerance.• The numerical scheme is dissipative and, as a result the
velocities are uniformly bounded.• A differential inclusion model with energy dissipation
satisfied in the limit.
2-dimensional examples
• Long bar with multiple contacts:– low friction (mu=0.05, e=0.6)
– high friction.(mu=0.2, e=0.6)
• 2 blocks on table– low friction (mu=0.02, e=0.2)
– high friction (mu=0.1, e=0.2)
• Multiple collisions with high restitution, high friction (mu=0.3, e=0.9).
• Three blocks on table, low friction (mu=0.05, e=0.6).
• Several blocks, (mu=0.2, e=1.0)
VRML: to start animation click on the base object after browser is loaded.
Open Problems
• Can a consistent differential inclusion approach be formulated for general configurations?
• Under which conditions does the maximum dissipation principle apply in the multiple contact case?
• Can other properties of the time-stepping scheme be used in the continuous time context (such as the bound on the square of the variation of the velocities)?
• Is the solution set of the friction LCP convex? (it is not true for arbitrary copositive matrices),
• Can the LCP be recast as a convex optimization problem?
Future Research
• Treat the Coulomb cone directly rather than using a polygonal approximation (partly solved APS98 ).
• Design stable numerical algorithms to handle stiffness (implicit or linearly implicit, work in progress).
• Nonlinear formulations (NCP).
• Investigate Interior-Point Algorithms for solving the friction LCP or NCP.
• Are higher-order methods relevant?
• Formulate an efficient piecewise DAE strategy.