This article describes the features of two contrastive mechanisms in biped robot systems. One of the compared models is the serial mechanism in which each link is connected in series, and each joint is driven by its own actuator independently. Another model is the parallel mechanism and there is a single close linked chain. One of the important merits of using the parallel mechanism to the biped robot is that all the actuators can be placed on the upper leg, thus the inertia of the leg can be smaller, even if its kinematics and dynamics becomes more complicated. In the biped robot with parallel mechanisms, all the joint angles are set as a variable as well as a serial mechanism, and the geometric constraint function can be defined by the kinematic relation of each closed linked chain. From the second order differential of this function, the angular acceleration of the passive joint is represented by the angular acceleration of the active joint. Thus, the dynamic equation is derived. In this paper, the theoretical analysis of the biped robot motion with parallel and serial mechanisms is shown. The numerical results obtained are useful to the mechanical design.
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