Abstract:The parallel manipulators with n<6spherical joint-prismatic joint-spherical joint (SPS) type active limbs and one passive constrained limb have attracted more attention and been used in many practical applications.The 4-SPS/S parallel manipulator was proposed and used to simulate ship swaying motion in the sea.Real mobility property of the 4-SPS/S parallel manipulator was analyzed by using screw theory. The inverse position kinematics model and the differential motion characteristics of the SPS active limbs and moving platform were established by coordinating rotation matrix transformation and vector derivative.The analytical solution of forward position kinematics was established by rod length constraint equations and Sylvester resultant elimination.The result of simulation and analysis showed that the parallel manipulator has four forward position solutions including two real solutions and two imaginary solutions,and the algorithm of forward position solution is correct.