Ali Moradi - MSc. Student, School of Mechanical Engineering, Iran University of Science and Technology, Tehran, Iran
Mohammad Reza Sabaapour - Postdoctoral Researcher, School of Integrated Technology, Gwangju Institute of Science and Technology, South Korea
Borhan Beigzadeh - Assistant Professor, Biomechatronics and Cognitive Engineering Research Lab, School of Mechanical Engineering, IranUniversity of Science and Technology, Iran
Mohammad Reza Hairi Yazdi - Professor, School of Mechanical Engineering, University of Tehran, Tehran, Iran

Passive biped walkers who walk naturally on a slope surface without any control input, just due to gravity,are very popular nowadays. Actually, they are an excellent platform to explore dynamically stable biped robotscontrolled based on the passive dynamic walking concept. In this research, a novel 3D passive biped walker withcompliant ankle and flat foot has been designed and manufactured. It is a straight-legged compass-like biped modelhaving four Degrees-of-Freedom (DOF) in the spatial space totally; three DOFs related to the ankle joint (pitch-rollyaw)and one DOF related to the hip joint (relative pitch of two legs with respect to each other). Some experiment hasbeen also conducted to verify the design. The walker successfully exhibits several asymptotically stable periodic gaitcycles on a gentle slope ramp resembles to human walking. The experimental results show a rational sensitivity of thewalker to the initial position and velocity parameters that should be adjusted carefully.

Biped robot; passive walker; 3D walking; Asymptotical stability; Compass like model