Design and analysis of a cable-driven rigid–flexible coupling parallel mechanism with variable stiffness

• A novel rigid–flexible coupling parallel mechanism with variable stiffness. • It consists of a 3-5R parallel mechanism with a spring and four cables, based on a flexible anti-parallelogram mechanism. • The spring and cables constraint translation motion and thus provides flexibility. • The mechanism offers a wide range of stiffness controllability. This paper proposes a rigid–flexible coupling parallel mechanism with variable stiffness based on a novel 3-5R parallel mechanism, which is derived from a special form of anti-parallelogram mechanism. A spring is used as the fourth limb of the 3-5R parallel mechanism to endow the mechanism with elasticity, and then the motion of the 3-5R parallel mechanism is accompanied by the spring's length and curvature. Four driving cables are installed to actuate the mechanism, and the elastic spring limb makes the stiffness of the mechanism adjustable. The working process, kinematics, and stiffness of the proposed mechanism are analyzed, the lateral flexion of the spring and rigid–flexible coupling of the mechanism are also studied. Finally, a physical prototype is fabricated, the motion trajectory and stiffness of the moving platform are tested to show the feasibility of the proposed mechanism.