Kineto-static optimisation of PKMs

One of the main drawbacks of PKMs is the non uniformity of force and velocity transmission and stiffness characteristics in the workspace. This non uniformity can be dramatically reduced and in some cases, but the case of 6 dof PKMs, even eliminated using efficient kineto-static optimization criteria in the design phase. In this paper new performance indexes for the optimisation of 6 dof fully parallel PKMs, developed by the authors, are presented. These indexes overcome the fact that the Jacobian matrix J is not consistent in units and allow an independent study of the linear and the angular behaviour of the machine. The well known octahedron proposed by Duffy is analysed using the new indexes. The first Italian PKM for 5 axis milling operations - Celerius - developed by ITIA-CNR using an optimisation criteria based on the presented indexes is introduced. This machine exhibits an isotropic behaviour in the middle of its workspace and is almost isotropic in all the other points of the useful workspace.