ON THE OPTIMUM DESIGN OF STEWART PLATFORM TYPE PARALLEL MANIPULATORS

This paper studies the static rigidity behaviour of a parallel manipul ator with legs modelled as elastic members under axial loading. Struct urally, a parallel module is more rigid compared to a serial module an d is expected to take heavier payloads. Therefore, a guidance for desi gn of such parallel manipulators is needed which leads to maximum rigi dity over the workspace. In the present work, the authors propose the concept of the flexibility ellipsoid for a parallel system. Various sc alar measures of rigidity are formulated on the basis of the proposed ellipsoid. An algorithm, involving multiple objective nonlinear progra mming technique, is implemented to decide upon some important design p arameters of a generalised six degrees of freedom Stewart platform typ e parallel manipulator. It is observed that irrespective of the other parameters, parallel manipulators with the legs pairwise joined at the top platform possess the highest rigidity. Moreover, there exists cer tain kinematic dimensions for which the designed parallel system is co mpletely free from all sorts of singularity.