Improved Variable EWMA Controller for General ARIMA Processes
The exponentially weighted moving average (EWMA) controller is a popular run-to-run control scheme in semiconductor manufacturing because of its effectiveness and simplicity. In this paper, we propose an improved variable EWMA controller design method. The optimal discount factor of the EWMA controller is determined by minimizing the mean square error of process output at each run. The proposed method overcomes some potential problems of Tseng's method and can be applied to general autoregressive integrated moving average processes. A numerical study is performed to investigate the performance and sensitivity of the proposed method. It is shown that the proposed method is more effective and robust than the conventional EWMA controller when the process parameters are estimated with uncertainty.