This paper proposes a new family of four degrees-of-freedom (dof) parallel mechanisms with two platforms and its application to a footpad device that can simulate the spatial motions of the human foot. The new mechanism consists of front and rear platforms, and three limbs. Two limbs with 6-dof serial joints (P-S-P-P) are attached to each platform and are perpendicular to the base plate, while the middle limb (Pe-Re-R or Pe-Pe-R) is attached to the revolute joint that connects the front and rear platforms. The middle limb is driven by the 2-dof driving mechanism that is equivalent active serial prismatic and revolute joints (Pe-Re), or prismatic and prismatic joints (Pe-Re) with two base-fixed prismatic actuators. Therefore, two new 4-dof parallel mechanisms with two platforms can generate pitch motion of each platform, and roll and heave motions (1T-3R) or pitch motion of each platform and two translational motions (2T-2R) at both platforms. Kinematic analyses of the 1T-3R mechanism were performed, including inverse and forward kinematics, and velocity analysis. Based on the 1T-3R mechanism, a footpad device was designed to generate foot trajectories for natural walking. Finally, simulations of the foot trajectories in the normal gait cycle were performed using the proposed footpad device.

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