The promise of legged robots over standard wheeled robots is to provide improved mobility over rough terrain. This promise builds on the decoupling between the environment and the main body of the robot that the presence of articulated legs allows, with two consequences. First, the motion of the main body of the robot can be made largely independent from the roughness of the terrain, within the kinematic limits of the legs: legs provide an active suspension system. Indeed, one of the most advanced hexapod robots of the 1980s was aptly called the Adaptive Suspension Vehicle. Second, this decoupling allows legs to temporarily leave their contact with the ground: isolated footholds on a discontinuous terrain can be overcome, allowing to visit places absolutely out of reach otherwise. Note that having feet firmly planted on the ground is not mandatory here: skating is an equally interesting option, although rarely approached so far in robotics.
Unfortunately, this promise comes at the cost of a hindering increase in complexity. It is only with the unveiling of the Honda P2 humanoid robot in 1996, and later of the Boston Dynamics BigDog quadruped robot in 2005 that legged robots finally began to deliver real-life capacities that are just beginning to match the long sought animal-like mobility over rough terrain. In fact, work in legged robotics has even contributed to the understanding of human and animal locomotion, as evidenced by the many fruitful collaborations between robotics and biomechanics researchers over legged locomotion.