Structural design of a novel family of 2-DOF translational parallel robots to enhance the normal-direction stiffness using passive limbs

Many planar 2-DOF translational parallel robots which were invented for the simple industrial tasks are easy to suffer the problem of the poor intrinsic stiffness along the normal direction to the plane of motion. To solve this problem, the passive limbs can be introduced into the design of parallel mechanisms to increase the stiffness and stability of the robots. Besides the capability of stiffness increasing, the passive limbs can also provide constraints, generate decoupled configuration, bear full or partial weights and/or payloads, liberate constraints from the actuations, and even decrease the required actuating forces of active limbs. However, there is still no systematic study on the utilization of passive limbs to date. In this paper, the stiffness–robust 2-DOF translation parallel robots with passive limbs are investigated in terms of type synthesis. Based on the distribution of wrench system among the active limbs and passive limbs, a full-scale criterion is developed for effectively and efficiently synthesizing all kinds of 2-DOF translational parallel mechanisms with one or more passive limbs. All 14 types of the 2-DOF translational parallel mechanisms with passive limbs are synthesized and exemplified through kinematic diagrams. A qualitative stiffness index is purposed to evaluate the stiffness performance of all of the synthesized configurations directly and rapidly. Finally, an optimized configuration of stiffness-enhanced 2T PM is derived and exhibits the best stiffness performance.