Experimental Validation of the Dynamic Models of a High Performance Workpiece Table Under Preloaded Hertzian Contact

In order to achieve active control for improved machining accuracy and efficiency, the control commands will include a substantial amount of high frequency signals. The high frequency command signals may not be sufficiently implemented by the single actuating unit that is the wheel infeed system. To solve this problem, an additional actuating unit will be required to support the workpiece in order to implement small but fast control actions. A high performance workpiece micro-positioning table utilizing a piezoelectric translator under preloaded Hertzian contact has been developed and experimentally tested. The table has shown to have a high resolution of better than 20 nm and a high natural frequency of approximately 579.2 Hz. When implementing a 43.587 mum step motion, the acceleration could be up to 33.51g, where g is acceleration due to gravity. It is found that when large step commands are applied, the workpiece table would exhibit excessive overshoot due to the separation between the piezoelectric translator and the moving part of the table. This would cause large shock accelerations, seriously affecting the machining accuracy. Dynamic models have been established to describe the table-top motions and the occurrence of the separation, which should be significant due to the wide use of the design of preloaded Hertzian contact. The models have not been examined systematically using an experimental approach. The micro positioning table is briefly introduced and the velocity and acceleration models of the table are presented. Experimental results are analyzed to verify the established models and to validate the findings about the separation. The average modeling error was found to be less than 3\%. This gives confidence in using the system for the fast active control required in high speed machining processes. (C) 2002 Elsevier Science B.V. All rights reserved