Masters Thesis: Model-free motion control of positioning stage

Driven by the rise of market demand and development of technology, high-precision motion systems must meet the increasing accuracy requirement. Currently, High-precision positioning stage is widely used in many different application areas, such as hard disk drives, wafer steppers and electron and atomic force microscopes for nano-scale imaging. Classical PID control which occupied 95% of precision mechanical industry is no longer sufficient to satisfy the future demand for higher accuracy and performance. Therefore, the application of advanced control method on positioning stage to improve the system performance and tracking accuracy is of great importance for both academia and industry. The challenge for the control of positioning stage lies in the strong nonlinearity such as friction or hysteresis and difficulty in modeling and identification. A novel model-free control method which has strong ability in handling nonlinearities is applied to the nano-positioning stage. This method has relatively low computational load and does not need a model. The experimental validation shows the average tracking error of model-free control reduces by 83.5% as compared with PID for the ramp tracking with maximum 50nm/s. Both the numerical simulation and experiment results show its superiority in coping with nonlinearities and disturbance rejection. For the sake of comparison, another control method called active disturbance rejection control which has a similar structure as model-free control is also developed and applied to the positioning stage. In terms of the drawbacks of the current model-free control method, some modifications, such as the higher order derivative estimator, the new model estimation method and nonlinear tracking differentiator, are successively applied to model-free control method to improve the tracking performance. Besides, as positioning stage often executes repetitive tasks, iterative learning control is added to compensate for repetitive error and disturbances. Moreover, a feedforward model which can predict the required force for pre-sliding motion is added to cancel the nonlinear pre-sliding friction and reduce the control effort. The numerical simulation and experiment validations illustrate, with this simple control structure, the excellent tracking accuracy and insensitivity to disturbance are ensured. Master of Science Thesis Xiao Zhang