  In this letter, we propose a whole-body control strategy for humanoid robots
in multi-contact settings that enables switching between fixed and sliding
contacts under active balance. We compute, in real-time, a safe center-of-mass
position and wrench distribution of the contact points based on the Chebyshev
center. Our solution is formulated as a quadratic programming problem without a
priori computation of balance regions. We assess our approach with experiments
highlighting switches between fixed and sliding contact modes in multi-contact
configurations. A humanoid robot demonstrates such contact interchanges from
fully-fixed to multi-sliding and also shuffling of the foot. The scenarios
illustrate the performance of our control scheme in achieving the desired
forces, CoM position attractor, and planned trajectories while actively
maintaining balance.
