Gain-scheduled robust control of a novel 3-DOF micro parallel positioning platform via a dual stage servo system

This paper presents a gain-scheduled robust control of a micro positioning platform using a dual stage servo system, which recently has been developed to achieve the 3-DOF motions (x- and y-translations and α-tilting) with the accuracy to the sub-micrometer. The proposed platform consists of three sets of two-stage actuators: AC servo motors with ball screw for rough positioning and piezo actuators for fine positioning. Unlike existing parallel mechanisms using dual servo systems, the proposed platform is unique due to the fact that the actuation directions of the coarse and fine actuators are vertical with respect to each other, which enables them to effectively increase the mobility and resolution of the platform. While a well-known PI controller was adopted to establish the coarse control system due to its simplicity and reasonable performance, a gain-scheduled H∞ controller was also employed to robustly ensure the desired accuracy of the fine control system. Since the coarse actuator is relatively slow and the mass of the proposed platform is negligible, the PI controller design for the coarse system is based on the kinematics of the proposed platform. Conversely, a physics-based dynamic model for the fine system is derived from a non-parametric system identification in order to match the experimental data well. Based on the combination of coarse and fine controllers, a dual servo control system is proposed, which sequentially selects the controller according to the pose of the proposed platform such as rotational angle, vertical and horizontal position. The successful performance of the synthesized controllers was verified through extensive experiments.