  Hybrid wheeled-legged locomotion is a navigation paradigm only recently
opened up by novel robotic designs,e.g. the centaur-type humanoid CENTAURO [1]
or the quadruped ANYmal [2] in its configuration featuring non-steerable
wheels. The term Hybrid Locomotion is hereafter used to indicate a particular
type of locomotion, achieved with simultaneous and coordinate use of legs and
wheels,see Fig. 1. Such choice stems at the intersection between legged
locomotion and the simpler wheeled navigation, in order to get the best from
both techniques: agility and ability to traverse uneven terrains from the
first, speed and stability from the second. As a consequence, the problem of
planning feasible trajectories for a hybrid robot shares many similarities with
the legged locomotion problem: also in the hybrid case the motion of the base
is reached through contact of the feet with the environment, taking into
account that the wheeled feet can just push on the ground and not pull it.
Forces compatible with friction cones have to be considered, while the contacts
can slide just along the direction prescribed by the orientation of the wheels.
