A robot driven by McKibben pneumatic actuator (MPA) generates stable motions in spite of its simple control and simple actuator model. In this paper, we investigate a relation between the design of a robot driven by MPA and the joint stiffness, based on mechanical analysis and simulation. After deriving a singular point of the joint stiffness, we confirm that qualitative relation between input pressure and the joint stiffness is reversed where the hip position is higher than the singular point.

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... Moreover, we analyze the influence of MPA properties and geometric parameters on the singular point. From the results, we confirm that the significant posture disappear by increasing the radius of the knee-pulley. Finally, we verify the validity of these analytical results via numerical simulations.