(1994-). At present, there are many studies about leg-wheel hybrid mobile robot because walking robot has high terrain adaptability on irregular ground but wheeled robot takes advantage of moving speed on smooth terrain. In the past, active wheels were often used for wheeled locomotion. However installation of active wheels restricted walking machine's ability very much. Because active wheels need actuators, brake mechanism and steering mechanism. This equipment is so heavy and bulky that it's not practical solution for walking robot which has many degrees of freedom. Proposed hybrid mobile robot named "Roller-Walker" is a vehicle with a special foot mechanism which changes between feet soles for the walking mode and passive wheels for the wheel. (Photo 2(a),(b)) Roller-Walker can utilize the installed actuators for walking, so additional weight is very light. The wheeled locomotion is based on the same principle of roller-skating.
Photo.1 shows prototype machine model. The height is 0.25 m; the left/right width is 0.6 m; the front/back length is 0.5 m; and the total weight is 24 kg. Passive wheels were equipped on the tips of each TITAN VIII leg and also installed small actuators for the ankle change. The additional weight is only 7% of TITAN VIII. The camber angle of passive wheel against the ground is always kept 90[deg] by parallel link mechanism using wire and pulley.
Fig.1 indicates Straightforward propulsion in the wheel mode. Fig.2 shows an example of the leg trajectory to maximize the constant velocity. Both right and left legs are moved symmetrically to cancel the lateral reaction force. It was demonstrated by experiments that wheeled locomotion can generate the velocity of 0.8 m/s over twice as large as the walking speed. (Photo.3)

Photo.1 Wheeled locomotion and trajectory	Photo.2a Walking mode Photo.2b Skating mode
Fig.1 Leg trajectory refered from the body coordinate	Photo.3 Figure 8 propulsion
Photo.4 Rotational motion

References:

1. Shigeo HIROSE, Hiroki TAKEUCHI; Roller-Walker: A Proposal of New Leg-Wheel Hybrid Mobile Robot, Proc. Int. Conf. on Advanced Robotics, pp.917-922 (1995)
2. Shigeo HIROSE, Hiroki TAKEUCHI; Study on Roller-Walk (Basic Characteristics and its Control), Proc. IEEE Int. Conf. on Robotics and Automation, pp.3265-3270 (1996)