  Forthcoming applications concerning humanoid robots may involve physical
interaction between the robot and a dynamic environment. In such scenario,
classical balancing and walking controllers that neglect the environment
dynamics may not be sufficient for achieving a stable robot behavior. This
paper presents a modeling and control framework for balancing humanoid robots
in contact with a dynamic environment. We first model the robot and environment
dynamics, together with the contact constraints. Then, a control strategy for
stabilizing the full system is proposed. Theoretical results are verified in
simulation with robot iCub balancing on a seesaw.
