Forecasting compensatory control of an piezo-actuated micro-boring bar for dynamic error reduction

Abstract

In recent years, although significant volume of research work has been devoted to active error compensation in the field of external machining such as turning, milling and grinding, there were only a few attempts that were targeted at overhang boring. In this thesis, the development of a piezoelectric actuated micro-boring bar system with the function of on-line machining error measurement and compensation will be reported. The sensors used for on-line measuring and compensating the machining error can be easily installed in the tool-post of the machine tool where space is not restricted. Unlike other processes, the implementation of active error compensation for overhang boring poses some real challenge in modern day machine control technology because the conventional sensors which are successful for on-line measurement of the machining errors in external machining are of little use when internal surfaces are encountered. The outer diameter of the boring bar is strictly limited by the size of the bored hole. In order to solve this problem, a new boring bar consists of two coaxial bars. As a result, the micro-boring bar can be made with smaller outer diameter even after the measuring and compensating sensors have been incorporated. There are many types of errors effecting the accuracy of the workpiece during the boring operation. In order to compensate the machining errors on-line, a relatively new control strategy, Forecasting Compensatory control (FCC), is adopted for this boring bar system. The advantage of FCC is its ability to predict the future values of the machining errors without the necessity of solving the complex cause-and-effect relationships between various errors and their source. Through the off-line simulation and on-line cutting experiments, the improvement in the roundness accuracy was found to be 39%, and cutting depth accuracy to 60%, which confirms the effectiveness of this proposed strategy.