Hybrid feed drive (HFD) is a precise and energy efficient solution for motion delivery in machining applications. The hybrid concept arises from two basic machining operations: rapid positioning and cutting. The rapid positioning desires high speed; while in cutting the speed is relatively slower, however stringent tolerance is required under significant cutting force. To address the drastic different needs of the two operations, actuators of different types are employed. As shown in the figure, a fast, precise, but energy costly linear motor drive (LMD), is combined with an energy efficient screw drive (SD) with lower speed and precision. This combination of SD and LMD is reconfigurable based on operations. For rapid motion, the LMD drives the table alone to achieve high speed; for cutting motion, both LMD and SD are synergistically employed–SD provides the holding force to reduce energy consumption while the LMD fine-tunes the tracking performance for precision. The (dis)engagement of the rotary motor in the SD is achieved through a reconfigurable friction drive device Roh’lix. The Roh’lix translates the rotation to linear motion like traditional screw drive, however (dis)engages freely across the shaft with the help of pneumatic actuated toggle mechanism.
This dual-input-single-output over-actuated system requires dedicate control design for the two actuators to collaborate desirably. A computational-efficient energy optimal control allocation scheme is proposed. The optimal control ratio between the actuators is derived and an energy efficiency metric (proxy) is defined accordingly. The proxy is theoretically shown to measure the deviation from the optimal ratio, and thus the regulation of the proxy allocates the control efforts. The proxy-based control allocation is compatible with various regulator design methodologies and saturation-type input constraints. This method has been further extended to multi-input multi-output system; a robust control allocation scheme which considers the model uncertainty is currently under investigation.