Wheel-Based Stair Climbing Robot with Hopping Mechanism - Fast Stair Climbing and Soft Landing Using Vibration of 2-DOF System -. - Keisuke Sakaguchi; Takayuki Sudo; Naoki Bushida; Yasuhiro Chiba; Yuji Asai; Koki Kikuchi (J. Robotics Mechatronics, 2007)