Type synthesis of symmetrical lower-mobility parallel mechanisms using the constraint-synthesis method

The lower-mobility parallel mechanism (PM) has fewer than six degrees of freedom (DoFs) and requires fewer links, joints and actuators than its 6-DoF counterpart. Hence, the use of the lower-nobility PM may save much cost in those applications requiring less mobility. In this paper; the tape synthesis of symmetrical lower-mobility My is performed systematically using the constraint-synthesis method. First, some important theoretical fundamentals of type synthesis of lower-mobility PMs are investigated, including a classification of symmetry of lower-mobility PMs, the identification of instantaneous mechanisms and input selection method. Moreover; the difficult problem of how to correctly apply the general Grubler-Kutzbach criterion to lower-mobility PMs is solved. A mobility analysis of PMs with a closed loop in the limbs is also presented to demonstrate the validity. The type synthesis of 5-DoF and 4-DoF PMs is then dealt with in detail. Enumerations of novel 5-DoF and 4-DoF PMs are presented. The characteristics of constraint and structure of 5-DoF and 4-DoF PMs are also proposed. In addition, some novel 3-DoF PMs are presented.