Tracking control of a motor-piezo XY gantry using a dual servo loop based on ILC and GA

The main objective of this paper is to improve the tracking accuracy of a XY gantry stage by a biaxial piezoelectric-actuated micro stage. A combined positioning system including a coarse stage and a micro stage is designed in this study. Two servo motors are used to actuate the coarse stage (the XY gantry stage) via ball screws. Two piezo-electric actuators (PEA) are used to actuate the micro stage to compensate the residual tracking error of the coarse stage. To compensate the undesired backlash of the gantry stage, an iterative learning control (ILC) is applied to achieve a permissible accuracy, and then the micro stage compensates the residue tracking error. However, the positioning accuracy of the PEA is limited due to the hysteretic nonlinearity; to compensate this hysteresis problem, the dynamics of the hysteresis is formulated by the Bouc-Wen model. Besides, genetic algorithm (GA) is studied to identify the optimal parameters of the hysteresis model. To verify the consistency, a novel dual servo loop controller is proposed by a DSP architecture and the improved positional accuracy has been readily achieved within 0.1 micro meter.