Passive limbs can be introduced into the design of parallel mechanisms to provide constraints, generate decoupled configuration, bear full or partial weights and/or payloads, separate constraints from the actuations, decrease the required actuating forces of active limbs, and even increase the stiffness and accuracy of the whole system. Currently, there is still no systematic study on the utilization of passive limbs. The available parallel mechanisms having one passive limb were developed mainly based on intuition and ingenuity. In this paper 4-DOF and 5-DOF parallel mechanisms 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 4 & 5-DOF parallel mechanisms with one or more passive limbs. All 9 categories of the 4-DOF and 2 categories of 5-DOF parallel mechanisms with passive limbs are synthesized and exemplified through kinematic diagrams merely using independent constraints. Several nonoverconstrained 4 or 5-DOF parallel mechanisms with one or two constraining passive limb are presented.

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