Type synthesis of the 2R1T parallel mechanism with two continuous rotational axes and study on the principle of its motion decoupling

The two-rotational-degrees-of-freedom and one-translational-degree-of-freedom (2R1T) parallel mechanism (PM) with two continuous rotational axes (CRAs) has a simple kinematic model, and it is therefore easy to implement trajectory planning, parameter calibration, and motion control. However, no systematic analysis on the structural constraints of the 2R1T-PM with two CRAs has been performed. Furthermore, there are few types of 2R1T-PM with two CRAs. Firstly, combining the theories of screw theory and space geometry, we explore the spatial geometrical relationships of the constraint forces supplied by the branches that give the 2R1T-PM two CRAs. Then, type synthesis of the 2R1T-PM with two CRAs is carried out based on the constraint forces meeting special arrangements. In addition, based on the positional relationship between the actuation force and the rotational axe, we analyze the decoupling principle between two rotational degrees of freedom for the 2R1T-PM, and put forward the criteria for distinguishing whether the two rotational degrees of freedom are partially decoupled or completely decoupled. At last, aiming at the different spatial arrangements of the two CRAs for the 2R1T-PM, we propose different configuration modes to construct 5-DOF hybrid serial-parallel manipulator, and get a 5-DOF hybrid serial-parallel manipulator on the basis of one 2R1T-PM.