Passive and active gravity compensation of horizontally-mounted 3-RPS parallel kinematic machine

Taking a horizontally-mounted 3-RPS parallel kinematic machine (PKM) as an example, this paper investigates passive and active gravity compensation strategies for improving PKM precision. Herein, R, P and S denote revolute, actuated prismatic and spherical joint, respectively. Based on inverse position kinematic and force analysis, passive gravity compensation is firstly implemented by three extension springs. Geometric parameters of these springs are optimized by minimizing potential energy fluctuation within a prescribed workspace. Then deformation caused by gravity is adopted to evaluate compensation strategies. The deviations of an end reference point demonstrate that passive gravity compensation is capable of substantially balancing gravity. In order to further eliminate the effect of gravity, active gravity compensation is proposed to adjust spring displacements by several one degree-of-freedom (DoF) translational mechanisms. Deformation of the 3-RPS PKM is directly regarded as the objective function for determining the active translations. Overall, both passive and active gravity compensations are able to improve mechanism precision due to the partial and complete gravity balancing capability, respectively, which makes the 3-RPS PKM suitable for the application of rough and fine machining.