Stiffness evaluation and experimental test of a novel redundantly actuated parallel machining robot

Parallel machining robot is a new type of robotized equipment for high-efficiency machining structural components with complex geometries. Terminal rigidity is of great importance index for such type of equipment, which affects their load capacity and working accuracy. Before a parallel machining robot can be used for heavy-load and high-efficiency machining, its terminal rigidity should be evaluated systematically. The present study is to quantitatively reveal the stiffness properties of a previously invented Z4 redundantly actuated parallel machining robot (RAPMR). For this purpose, two critical issues, i.e., stiffness modelling and index construction, are clarified to carry out stiffness evaluation of the Z4 RAPMR. Firstly, drawing on the screw theory, a semi-analytic stiffness model of the proposed RAPMR is established at a component level. Secondly, a set of virtual work-based stiffness indices is constructed to evaluate the terminal rigidity of parallel robots. Those indices have a consistent physical unit in describing linear and angular terminal rigidity. With these indices, the local and the global stiffness performance of the Z4 RAPMR are predicted. Thirdly, a laboratory prototype of the proposed RAPMR is fabricated. And the experimental test is performed to verify the correctness of the established stiffness model. The present work is expected to provide fundamental information for further light-weight design and rigidity enhancement.