Iterative tuning of feedforward controller with force ripple compensation for wafer stage

Iterative controller tuning is a method to fine-tune controllers in a repetitive process using only data collected in experiment runs. The controller parameters are chosen so as to minimize a certain cost function. The values of the parameters are updated iteratively based on tracking error in a gradient search. Iterative controller tuning is an advantageous method of controller tuning because no prior plant model is necessary. In this paper, we iteratively tune the feedforward controller of a wafer stage of a photolithography machine. A fixed-structure feedforward controller consisting of the inverted plant model was tuned. In addition, an additive feedforward term for compensation of nonlinear force ripple disturbance was tuned. The feedforward controller is tuned with the objective of minimizing a cost function that is a quadratic function of tracking error. Simulation and experimental results are presented which show that the iterative tuning method effectively reduced tracking error.