Considerations for low-inertia AC drives in machine tool axis servo applications

The important differences encountered in applying AC permanent magnet servomotors that might have one-tenth of the shaft inertias of their torque-equivalent predecessors to machine tool axis servo applications are examined. Experimental results demonstrating these differences and corroborating the conclusions drawn are provided. Experimental testing was accomplished on a motor test stand and in a machine tool axis servo application. The test stand had two AC servomotors coupled in a motor/dynamometer configuration. Analytical and experimental results demonstrate that in addition to affecting the acceleration or command-following capabilities of the servomotor, 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 servomotor for such axis servo applications is reviewed, and the factors that are affected by motor/magnet technology are summarized