A Closed-Loop Actuated 3-(RRRPR)S Parallel Mechanism: Design, Modeling and Experimental Validation

Traditional 3-RRS and 3-RPS mechanisms adopt open-chain limbs, where R, P, and S denote revolute, prismatic, and spherical joints, respectively. Although these mechanisms are widely used, they suffer from insufficient stiffness and accuracy under heavy-load and high-precision operating conditions. By contrast, existing configurations equipped with closed-loop sub-chains can offer improved stiffness yet suffer from complex structures. To address these issues, a novel closed-loop actuated 3-(RRRPR)S parallel mechanism is proposed. The configuration consists of a moving platform, a fixed platform, and three identical (RRRPR)S closed-loop actuated limbs. Each limb contains a closed-loop sub-chain composed of a planar five-bar mechanism (RRRPR), in which the prismatic joint P is the actuated joint. The design of the closed-loop actuated limb effectively shortens the transmission chain length of the limb, reduces the overall height of the mechanism, and improves its stiffness and dynamic characteristics. To address the complex constraint relationship imposed by the closed-loop sub-chain (RRRPR) on the moving platform, the equivalent motion screw theory is used to systematically analyze the rationality of the actuated joint selection, thereby ensuring the correctness of the configuration design. Through the analysis of the motion characteristics of the closed-loop sub-chain, the motion mapping relationship between the serial open-chain limb and the closed-loop actuated limb is established. By combining the influence coefficient theory with the Newton–Euler equations in screw form, the kinematic and dynamic models of the mechanism are established in a concise and compact form. A prototype of the 3-(RRRPR)S closed-loop actuated parallel mechanism was developed. Experimental results show that the mechanism can achieve continuous and stable motion under the tested motion groups, providing a practical basis for the engineering application of similar parallel mechanisms.