Optimal Loop Shaping of Position Controls of Feed Axes in Machine Tools with Position-Dependent Behavior

Feed drives in machine tools have the task to position quickly tools and work pieces with high accuracy. For rising demands of work piece quality and productivity, industry demands for rising bandwidths of position controls to be able to increase further the process speed (especially limits for jerk, acceleration and dynamic limits for process forces). Main problem of feed axes are typically position-dependent mechanical eigenfrequencies between slides, ball-screw drives, gearboxes, guideways and permanent synchronous motors that have to be damped. This paper presents a new automated procedure to replace the mainly used P-control in the position control loop by a robust optimal position control based on H
_∞
optimal loop shaping theory. The design process includes four steps: system identification, control optimization, simulative check on robust performance and practical check on the feed drive. Simulations and practical experiments show for a pilot machine that a better performance can be reached for each optimized bandwidth in comparison to the conventional control. The deduced methodology also copes with the challenge on how to get all axes of a machine tool to a similar dynamic behavior in order to decrease further positioning errors in multi axes movements.