Vibration Reduction in Passively-Isolated Ultra-Precision Manufacturing Machines using Mode Coupling.

Abstract

Ultra-precision manufacturing (UPM) machines are designed to fabricate and measure complex parts having micrometer-level features and nanometer-level tolerances/surface finishes. Therefore, they must be isolated from deleterious effects of vibration to enable them to meet stringent precision requirements. UPM machine builders often prefer passive isolators for reducing vibration because they are easy to use, cost effective, energy neutral and reliable. A long-standing rule-of-thumb in passive isolation system design, recommended in academic literature and industrial practice, is to decouple all the vibration modes of an isolated machine by aligning the isolator mounting locations with the center of gravity (CG) of the machine. However, there is anecdotal evidence scattered in the literature that suggests that locating isolators such that vibration modes are coupled could help reduce vibrations of passively-isolated machines. What is lacking, however, is a proper understanding of when and how to use mode coupling to achieve superior vibration reduction. This doctoral dissertation research seeks to provide a theoretical foundation as well as analysis-based design guidelines and tools for reducing unwanted vibration in passively-isolated UPM machines using mode coupling. Its primary contributions are threefold. Firstly, it demonstrates that the drastic reduction of vibration caused by mode coupling is primarily linked to so-called “critical configurations” induced by curve veering and mode localization. It therefore clears the misconception purported in academic literature and industrial practice that the vibration-reduction effects of mode coupling on UPM machines are simply linked to damping. Secondly, it proves analytically that mode coupling is almost always better than the recommended practice of modal decoupling with regard to vibration reduction in passively-isolated UPM machines. Thirdly, it proposes a framework for reformulating the generalized UPM isolator placement problem as a linear feedback controller design problem whose gains represent isolator locations. All work is backed up by simulations and experiments conducted on prototypes of UPM machines. The results demonstrate that, when properly exploited, mode coupling could bring about huge reductions in UPM machine vibration compared to modal decoupling; Its methods and findings are also applicable to the placement of passive isolators/suspensions/dampers in automotive, aerospace, civil, and other applications.