Passive walker can walk down gentle slope without actuator, sensor and control. This motion is attractive because its gait is natural. Passive walking may give us an insight into understanding human locomotion and developing biped robots. Passive walking with upper body has been studied by many researchers. The upper body is just a pendulum jointed around hip. In general, the upper body is kept by PD control or bisecting hip mechanism. While, we focus on a hip spring-damper mechanism for keeping upper body upright. In this paper, first, we model a passive walker that added upper body by means of a hip spring-damper mechanism. Secondly, we demonstrate that the passive walker with upper body can generate a stable limit cycle. Finally, the walking performance is evaluated from the stability of fixed point and energy efficiency.
