On-line reference trajectory adaptation for the control of a planar biped

Assuming that a reference trajectory has been defined for a walking robot, a control law must be defined to follow this reference trajectory. But for a walking robot, in the presence of perturbation, the prevention of the fall down is of primary importance. This means that the forces exerted by the ground must be directed upwards to avoid unexpected take-off of a foot, the ratio between vertical and horizontal components must be such that sliding does not occur and the zero moment point must be on the support surface (or line). All these requirements are more important than a good convergence to the desired trajectory. In the studied strategy the tracking of a prescribed reference trajectory is replaced by the tracking of a trajectory belonging to a set of reference trajectories described by some parameters. And an on line adaptation of these parameters is allowed to preserve the equilibrium of the robot. When it is possible, without effect on the robot stability, the parameters go back to their desired value.