Workspace analysis of positioning discontinuities due to clearances in parallel manipulators

Clearances at joints produce a loss of accuracy when positioning a mechanism. The end-effector pose error due to clearances depends on the mechanism configuration, the magnitude of the clearance itself and applied external wrenches. Sudden changes which occur in the actual posture of the mechanism owing to a change of contact mode at joints can be detected in the neighbourhood of some configuration. These sudden changes lead to positioning discontinuities on certain trajectories, or on the workspace. In this paper, a methodology for analysing the location of the discontinuities by means of a dynamic or kinetostatic analysis is presented. The advantages of choosing either the dynamic or the kinetostatic approach are analysed, making use of the 5R planar mechanism.