Control of a Non-Orthogonal Reconfigurable Machine Tool

Abstract

Computerized control systems for machine tools must generate coordinated movements of the separately driven axes of motion in order to trace accurately a predetermined path of the cutting tool relative to the workpiece. However, since the dynamic properties of the individual machine axes are not exactly equal, undesired contour errors are generated. The contour error is defined as the distance between the predetermined and actual path of the cutting tool. The cross-coupling controller (CCC) strategy was introduced to effectively decrease the contour errors in conventional, orthogonal machine tools. This paper, however, deals with a new class of machines that have non-orthogonal axes of motion and called reconfigurable machine tools (RMTs). These machines may be included in large-scale reconfigurable machining systems (RMSs). When the axes of the machine are non-orthogonal, the movement between the axes is tightly coupled and the importance of coordinated movement among the axes becomes even greater. In the case of a non-orthogonal RMT, in addition to the contour error, another machining error called in-depth error is also generated due to the non-orthogonal nature of the machine. The focus of this study is on the conceptual design of a new type of cross-coupling controller for a non-orthogonal machine tool that decreases both the contour and the in-depth machining errors. Various types of cross-coupling controllers, symmetric and non-symmetric, with and without feedforward, are suggested and studied. The stability of the control system is investigated, and simulation is used to compare the different types of controllers. We show that by using cross-coupling controllers the reduction of machining errors are significantly reduced in comparison with the conventional de-coupled controller. Furthermore, it is shown that the non-symmetric cross-coupling feedforward (NS-CC-FF) controller demonstrates the best results and is the leading concept for non-orthogonal machine tools.