  Maintaining balance while walking is not a simple task for a humanoid robot
because of its complex dynamics. The presence of a persistent disturbance makes
this task even more challenging, as it can cause a loss of balance and
ultimately lead the the robot to a fall. In this paper, we extend our
previously proposed Intrinsically Stable MPC (IS-MPC), which guarantees
boundedness of the CoM with respect to the ZMP, to the case of persistent
disturbances. This is achieved by designing a disturbance observer whose
estimate is used to compute a modified stability constraint included in the QP
problem formulation. The method is validated by MATLAB simulations for the LIP
as well as dynamic simulations for a NAO humanoid in DART.
