| Æ¡Æ¡Exponentially weighted moving average (EWMA) feedback controller (with a fixed discount factor) is a popular run by run (RbR) control scheme which deals with the topic of adjusting the output of a process to a desired target. Although the EWMA controller with a small discount factor can always guarantee a long-term stability (under a fair regular condition), it usually requires a moderately large run to bring the output of a process to its target. This will lead to a very severe condition if fabrication is in small batches. The reason is that the output deviations are very large at the beginning of the first few runs and it will cause the output to be out of specification. In order to reduce a possibility of high rework rate, we propose a variable discount factor to overcome the difficulty. First, a motivating example is used to demonstrate the strength of EWMA controller with variable discount factor. Then we state the main results of this paper, both the stability condition and the optimal variable discount factor of this EWMA controller are derived. An example is also given to demonstrate the performance. Finally, we also proposed a heuristic method to simplify the computation of the variable discount factor. Comparing with the optimal variable discount factor, it is seen that this heuristic rule is quite robust. |