Chen Qiaohong

Review on 2R1T 3-DOF parallel mechanisms

Mechanisms are the skeletons and end-effectors of manufacturing equipments. The innovation of mechanism is indispensable to the independent innovation of manufacturing equipments. Compared with serial industrial manipulators, parallel mechanisms offer high rigidity, good dynamic response, potential high accuracy and large payload capability with the price of less dexterity, complex distribution of singular configurations and limited workspace. Over the last two decades, lower-mobility parallel mechanisms whose degrees of freedom (DOF) are fewer than six have been successfully implemented in many applications like positioning, orientation, and axis-symmetrical machining, which do not reqiure all six DOFs. With the well optimized architecture of a lower-mobility parallel mechanism, the reduced cost of fabrication, actuation, control and maintenance can be obtained compared with their 6-DOF counterparts. The 2R1T (R: rotation, T: translation) 3-DOF parallel mechanisms are most typical examples of successful applications in advanced manufacturing equipments such as parallel kinematic machine, industrial robot, and articulated tool head. Based on the relative geometrical relations between the two axes of rotation, the 2R1T 3-DOF parallel mechanism is classified into four categories: UP, PU, RPR and P*U*. The motion of the moving platform of a UP (U: universal joint, P: prismatic joint) parallel mechanism can be generated by a UP serial chain, including a rotation about a fixed axis, a rotation about a moving axis perpendicular to the fixed one, and a translation along the normal of the two axes. The motion of the moving platform of a PU parallel mechanism can be generated by a PU serial chain, including a rotation about an orientation-fixed axis, a rotation about a moving axis perpendicular to the orientation-fixed one, and a translation perpendicular to the orientation-fixed one. The motion of the moving platform of a RPR parallel mechanism can be generated by a RPR serial chain, including a rotation about a fixed axis near the base, a translation perpendicular to the fixed revolute axis and a rotation about a moving axis perpendicular to the orientation-fixed one. When the orientation and position of the two rotational axes of a 2R1T parallel mechanism are time varying, the motion of the moving platform cannot be modeled by a 3-DOF serial chain and is denoted by a virtual serial chain P*U*. The current research status of study on type synthesis, performance analysis and dimensional synthesis of the UP, PU, RPR and P*U* parallel mechanisms are reviewed and discussed. It is worth noting that despite the fact that many well-acknowledged progresses have been obtained by Chinese researchers in type synthesis, kinematic analysis and synthesis of lower-mobility parallel mechanisms, there is still a lack of basic principles and methods for selecting the architectures with engineering potentials from the massive outputs of type synthesis. This is an important challenge that the Chinese researchers in the fundamental research area of parallel mechanisms must face to support the self-innovation of manufacturing equipments. In addition, there are many open problems concerning rigid-flexible coupled dynamic design, accuracy calibration, and control of 2R1T 3-DOF parallel mechanisms, which require an immediate deployment of systematic and thorough research.