Implementation of Self-Tuning Control to Surface Accuracy Control in End Milling

Abstract

This paper discusses the necessity and possibility of the application of the self-tuning principle to surface accuracy control in end milling. When the conventional numerical control is used, the actual tool position, which represents the finished work-piece geometry, often deviates from the desired position while milling thin webs. The necessary compensation for deflections is difficult because (1) on-line assessment of work-piece geometry is not realizable in practice, since only the cutting force is available for measurement, and (2) the parameters of the milling processes are partially unknown due to the fact that the chip thickness, depth of cut, cutting stiffness and work-piece stiffness vary with dfferent cutting conditions. An algorithm for self-tuning control of an immeasurable state of systems is developed in this paper. The suggested algorithm can be viewed as a concatenation of the Recursive Prediction Error (RPE) method for joint estimation of states and parameters of a state-space innovations model and the state fedback control law with pole-zero placement. The proposed algorithm is used for surface accuracy control in the end milling operation. The control objective is given by a external reference trajectory. The desired control is achieved by manipulating the position command for the servo-loop in the milling process. Results of simulations are obtained. These demonstrate that the proposed algorithm works quite satisfactorily.