  Stability of equilibrium states in mechanical systems with multiple
unilateral frictional contacts is an important practical requirement, with high
relevance for robotic applications. In our previous work, we theoretically
analyzed finite-time Lyapunov stability for a minimal model of planar rigid
body with two frictional point contacts. Assuming inelastic impacts and Coulomb
friction, conditions for stability and instability of an equilibrium
configuration have been derived. In this work, we present for the first time an
experimental demonstration of this stability theory, using a variable-structure
rigid ''biped'' with frictional footpads on an inclined plane. By changing the
biped's center-of-mass location, we attain different equilibrium states, which
respond to small perturbations by divergence or convergence, showing remarkable
agreement with the predictions of the stability theory. Using high-speed
recording of video movies, good quantitative agreement between experiments and
numerical simulations is obtained, and limitations of the rigid-body model and
inelastic impact assumptions are also studied. The results prove the utility
and practical value of our stability theory.
