Contact Mechanics

1. Contact Mechanics B659: Principles of Intelligent Robot Motion Spring 2013 Kris Hauser

2. Agenda • Modeling contacts, friction • Form closure, force closure • Equilibrium, support polygons

3. Contact modeling • Contact is a complex phenomenon involving deformation and molecular forces… simpler abstractions are used to make sense of it • We will consider a rigid object against a static fixture in this class • Common contact models: • Frictionless point contact • Point contact with Coulomb friction • Soft-finger contact

4. Point contact justification • Consider rigid objects A and B that make contact over region R • Contact pressures (x)  0 for all x  R • If R is a planar region, with uniform friction and uniform normal, then all pressure distributions over R are equivalent to • A combination of forces on convex hull of R • If R is polygonal, a combination of forces on the verticesof the convex hull of R • [“Equivalent”: one-to-one mapping between span of forces/torques caused by pressure distribution over R and the span of forces/torques caused by forces at point contacts] B R A

5. Frictionless contact points • Contact point ci, normal ni for i=1,…,N • Non-penetration constraint on object’s motion: • Here is measured with respect to the motion of the object • Unilateral constraint object fixture

6. Frictionless dynamics • Assume body at rest • Consider pre-contact acceleration a, angular accel • Nonpenetration must be satisfied post-contact • Solve for nonnegative contact forces fithat alter acceleration to satisfy constraints object fixture a

7. Post impact velocity • Post impact velocities • Post-contact acceleration at contact: • Formulating nonpenetration constraints: Forces at COM Torques about COM

8. Matrix formulation • Note that the terms can be written • With , , element-wise inequality • G is the grasp matrix(Jacobianof contact points w.r.t. rigid body transform) • Each of these linear inequalities in the fk’s must be satisfied for all i. • Write out • (symmetric positive semi-definite) • (vector of initial contact accelerations in normal dir.)

9. Complementarity constraints • Nonpenetration constraints • Positivity constraints • Underconstrainedsystem – how to prevent arbitrarily large forces? • Extra complementarity constraint: fi must be 0 whenever • Meaning: a contact force is allowed only if the contact remains after the application of forces • Expressed as • More compactly formulated as • Result: linear complementarity problem (LCP) that can be solved as a convex quadratic program (QP) or using more specialized solvers (Lemke’s algorithm) Note relationship to virtual work!

10. Frictional contact • Coulomb friction model • Normal force • Tangential force • Coefficient of friction μ • Constraint: • Space of possible contact forces described by a friction cone n n

11. Quadratic constraint model • Cone specified exactly using following two constraints • (quadratic nonconvex constraint) • (linear) • Constraint 1 is relatively hard to deal with numerically

12. Frictional contact approximations • In the plane, frictional contacts can be treated as two frictionless contacts • The 3D analogue is the common pyramidal approximation to the friction cone • Caveats: • In formulation Af + b >= 0, A is no longer a symmetric matrix, which means solution is nonunique and QP is no longer convex • Complementarity conditions require consideration of sticking, slipping, and separating contact modes

13. High level issues • Zero, one, or multiple solutions? (Painlevé paradox) • Rest forces (acceleration variables) vsdynamic impacts (velocity variables) • Active research in improved friction models • Most modern rigid body simulators use specialized algorithms for speed and numerical stability • Often sacrificing some degree of physical accuracy • Suitable for games, CGI, most robot manipulation tasks where microscopic precision is not needed

14. Other Tasks • Determine whether a fixture resists disturbances (form closure) • Determine whether a disturbance can be nullified by active forces applied by a robot (force closure) • Determine whether an object is stable against gravity (static equilibrium) • Quality metrics for each of the above tasks

15. Form Closure • A fixture is in form closure if any possible movement of the object is resisted by a non-penetration constraint • Useful for fixturingworkpieces for manufacturing operations (drilling, polishing, machining) • Depends only on contact geometry Form closure Not form closure

16. Testing Form Closure • Normal matrix N and grasp matrix G • Condition 1: A grasp is notin form closure if there exists a nonzero vector x such that NTGTx > 0 • x represents a rigid body translation and rotation • Definition: If the only x that satisfies NTGTx >= 0 is the zero vector, then the grasp is in first-order form closure • Linear programming formulation • How many contact points needed? • In 2D, need 4 points • In 3D, need 7 points • Nondegeneracy of NTGT must be satisfied

17. Higher-order form closure • This doesn’t always work… sometimes there are nonzero vectors x with NTGTx= 0 but are still form closure! • Need to look at second derivatives (or higher) Form closure Not form closure

18. Force Closure • Force closure: any disturbance force can be nullified by active forces applied by the robot • This requires consideration of robot kinematics and actuation properties • Form closure => force closure • Converse doesn’t hold in case of frictional contact Force closure but not form closure Not force closure

19. f2 mg f1 Static Equilibrium • Need forces at contacts to support object against gravity Force balance Torque balance Friction constraint

20. Equilibrium vs form closure • Consider augmenting set of contacts with a “gravity contact”: a frictionless contact at COM pointing straight downward • Form closure of augmented system => equilibrium

21. Support Polygon Side Top Doesn’t correspond to convex hull of contacts projected onto plane

22. Strong vs. weak stability • Weak stability: there exist a set of equilibrium forces that satisfy friction constraints • Strong stability: all forces that satisfy friction constraints and complementarity conditions yield equilibrium (multiple solutions) • Notions are equivalent without friction A situation that is weakly, but not strongly stable

23. Some robotics researchers that work in contact mechanics • Antonio Bicchi (Pisa) • Jeff Trinkle (RPI) • Matt Mason (CMU) • ElonRimon (Technion) • Mark Cutkosky (Stanford) • Joel Burdick (Caltech) • (many others)

24. Recap • Contact mechanics: contact models, simulation • Form/force closure formulation and testing • Static equilibrium