Identification and robust control for regenerative chatter in internal turning with simultaneous compensation of machining error

Abstract

The motivation behind this study is the problem of regenerative chatter vibration in internal turning. Regenerative chatter is an extremely undesired phenomenon in machining. This is since, in addition to causing surface finish degradation and cutting tool wear, regenerative chatter imposes a fundamental limitation on the material removal rate. In internal turning, due to the relatively low internal damping of the boring bar, regenerative chatter is especially undesired. Another factor, which reduces the material removal rate, as well as surface accuracy, is machining error. Machining error is very common in internal turning due to low boring bar stiffness. At the same time, machining error compensation has received little attention in the literature hitherto. This study proposes a control methodology for chatter vibration suppression with simultaneous compensation of the machining error in internal turning. The proposed methodology consists of three components. The first component is a newly developed method for the identification of the cutting force model parameters. The second component, which utilizes the cutting force model parameters identified in the previous step, is a robust stabilizing controller synthesis developed to address the key issues arising in the context of regenerative chatter control in the internal turning process. In contrast to the corresponding control methods reported in the literature, the control approach developed in this study accounts explicitly for the time-delayed term associated with the regenerative feature of chatter, as well as uncertainty in both cutting force and the flexible boring bar models. The last third component of the presented control methodology is a novel approach for machining error compensation, which is based on a feedforward term designed to eliminate the bar tip deflection. Every single component of the proposed control methodology can be utilized separately in the context of active control in internal turning. The potential applicability of the control methodology presented in this study is demonstrated experimentally, using an active slender bar demonstrator imitating the internal turning process. In particular, it is demonstrated that: (i) given the identified model of the boring bar, the proposed method for cutting parameters estimation allows to obtain reasonable estimations of the cutting coefficient and the overlap factor, (ii) the presented robust control system synthesis procedure allows to obtain controllers which increases the chatter immunity of the actively controlled cutting process, and (iii) the presented method for machining error compensation counteracts successfully the error due to flexible bar deflection caused by the DC component of the cutting force model.