An implicit time-stepping method for multibody systems with intermittent contact

In this paper we present an implicit time-stepping scheme for multibody systems with intermittent contact by incorporating the contact constraints as a set of complementarity and algebraic equations within the dynamics model. Two primary sources of stability and accuracy problems in prior time stepping schemes for differential complementarity models of multibody systems are the use of polyhedral representations of smooth bodies and the approximation of the distance function (arising from the decoupling of collision detection from the solution of the dynamic time-stepping subproblem). Even the simple example of a disc rolling on a table without slip encounters these problems. We assume each object to be a convex object described by an intersection of convex inequalities. We write the contact constraints as complementarity constraints between the contact force and a distance function dependent on the closest points on the objects. The closest points satisfy a set of algebraic constraints obtained from the KKT conditions of the minimum distance problem. These algebraic equations and the complementarity constraints taken together ensure satisfaction of the contact constraints. This enables us to formulate a geometrically implicit time-stepping scheme (i.e., we do not need to approximate the distance function) as a nonlinear complementarity problem (NCP). The resulting time-stepper is therefore more accurate; further it is the first geometrically implicit time-stepper that does not rely on a closed form expression for the distance function. We demonstrate through example simulations the fidelity of this approach to analytical solutions and previously described simulation results.