A Fuzzy Logic Based Adaptive Feedforward PI Controller for Nanometer Positioning

摘要

A technology driver in high precision manufacturing is a position measurement and control system which allows manufacturers to control the movement of their equipment at extremely high levels of precision. Performances of controllers play important roles in nanometer level positioning. A robust, disturbance resistant controller is necessary for nanometer positioning. However, the time varying and nonlinear properties of plants frequently result in the performance degradation of widely used PID controllers. The performance of feedback system can be improved considerably by combining PI feedback control with feedforward control together. Feedback controllers are very capable of compensating disturbances because feedback controllers are basically error driven. However, they often suffer from a trade-off between high performance and robust stability. With good dynamic property knowledge available, a feedforward controller may be able to prevent control errors, because feedforward controller output is based on the reference, instead of the error signal. Feedforward controllers improve the control performance by including a feedforward term to the PI output so that the controller can react to the command more quickly in order to bring the plant to the desired setpoint. A fuzzy logic controller will adjust the feedforward and PI feedback gains (K_(ffv), K_(ffa), K_(ffd), K_p and K_I) to adapt the change of dynamic behavior caused by the time varying and nonlinear properties inherent in the positioning systems. The developed control algorithm is simulated in MATLAB, and is implemented with a TMS320M67 DSP in an experimental nanolithography stage. A X-Y grating based metrology is used to measure the position of stage, which is more robust to environmental change and more suitable with high performance servomechanisms. Experiments show that stage will follow 10 nm step input with nm level steady state error, which is mainly caused by floor and acoustic vibrations.