Implementation of high bandwidth control for rotary and linear motor drives of milling machines

This study compares ballscrew drives and linear motor drives for milling machine feed drives. In high-speed milling, feed drives with high accelerations are necessary to use the full capacity of the new high-speed spindles. Acceleration is limited by the lack of bandwidth in the position loop.; Two feed drives are investigated: a ballscrew drive and a linear motor drive. Descriptions of the mechanical and electrical components of each feed drive with emphasis on advantages and disadvantages is given. Proportional-derivative control schemes are implemented on each drive, and for the ballscrew drive a sliding mode controller is applied.; A ballscrew drive under PD control was found to have a 30 Hz bandwidth. This low bandwidth limits the maximum desired acceleration. The bandwidth was limited by structural resonances in the ballscrew. Use of digital filtering techniques increased the bandwidth to 100 Hz; thus the ballscrew was able to reach a maximum acceleration of 2 g. The complexity in designing digital filters, their long calculation time and their lack of robustness make them undesirable in a machine tool controller. Therefore, a sliding mode controller was implemented as a substitute for the PD controller with digital filters to acquire a high bandwidth. This robust controller permitted a bandwidth of 65 Hz to achieve an acceleration of 1 g.; A linear motor drive under PD control was found to have a 70 Hz bandwidth. No additional filters were required to achieve this bandwidth. A major drawback of this drive is its sensitivity to changes of mass and to external disturbances like cutting forces and friction.; Feedforward filters were applied on both drives to decrease tracking errors during high acceleration moves. These feedforward filters reduced the tracking error to four counts on the ballscrew drive and to two counts on the linear motor drive.