Computation and Graphical Characterization of Robust Multiple-Contact Postures in 2D Gravitational Environments

This paper is concerned with the problem of identifying robust equilibrium postures of a planar mechanism supported by fixed frictional contacts in a two-dimensional gravitational field. The complex kinematic structure of the mechanism is lumped into a single rigid body, B, with a variable center of mass. Inertial forces generated by moving parts of the mechanism are lumped into a neighborhood of wrenches centered at the nominal gravitational wrench. The identification of the robust equilibrium postures associated with a given set of contacts is reduced to the identification of center-of-mass locations that maintain equilibrium of B with respect to any wrench in the given neighborhood. The static response of B to an external wrench involves static indeterminacy and frictional constraints. The region of center-of-mass locations that generate equilibrium with respect to a particular external wrench is formulated as a linear programming problem, and a full graphical characterization is provided. The result is then generalized to robust equilibrium postures that resist a neighborhood of external wrenches. Finally, we present experimental results that validate the criteria for feasible equilibrium postures.