CONSIDERATIONS FOR LOW-INERTIA AC DRIVES IN MACHINE-TOOL AXIS SERVO APPLICATIONS

The recent advances in permanent magnet materials have allowed construction of ac permanent magnet servo motors that might have one tenth of the shaft inertias of their torque-equivalent predecessors. This paper examines and quantifies the important differences encountered in applying these new generation motors to machine tool axis servo applications. Experimental results demonstrating these differences and corroborating the conclusions drawn are provided. Experimental testing was accomplished on a motor test stand and on a machine tool axis servo application. The test stand had two ac servo motors coupled in a motor/dynamometer configuration. Analytical and experimental results demonstrate that in addition to affecting the acceleration or command-following capabilities of the servo motor, reduction of shaft inertia affects the controller tuning robustness, bandwidths, and dynamic stiffness characteristics and also has implications for the drive/controller sampling rate. The process of selecting an appropriate servo motor for such axis servo applications is reviewed, and the factors that are affected by motor/magnet technology are summarized.