Statistical and engineering process control integration strategies for constrained controllers

Statistical process control (SPC) and engineering process control (EPC) are two distinct techniques utilized in the industry for variability reduction. Even though these techniques were developed in isolation they have a common objective of variability reduction and they accomplish it using different approaches. In recent years, there has been a growing interest in integrating these techniques.; This research aims to develop strategies for integrating SPC and EPC schemes under different drifts and shifts in the process mean. Specifically, feedback control of constrained integral and proportional-integral (PI) controllers in combination with SPC techniques is evaluated. Constrained controllers become a necessity when unacceptably large manipulations are required that may be beyond the scope of adjustment. This research also evaluated the: (1) run length distribution of the standard controllers, (2) adjusted output series subjected to constrained control scheme and, (3) application of these strategies to a hybrid industry.; The research indicates that the integrated SPC/EPC schemes reduce process variance when compared to either the SPC or the EPC schemes. The constrained schemes significantly reduce the adjustment variance at the expense of a slight increase in the output variance. The run length distribution of the SPC/EPC schemes follows a geometric distribution. However, the average run length (ARL) of these schemes is normally higher than the SPC schemes. In order to apply integrated SPC/EPC system and at the same time maintain sensitivity to drifts and shifts in the process mean, a set of exponentially weighted moving average (EWMA) control chart parameters were developed to yield an in-control ARL of less than 1000. It is also discovered that the constrained adjustment outputs exhibits autoregressive [AR(1)] characteristics. A new constrained scheme is proposed which is based on applying control limits to the manipulated variable. It enables the use of a standard controller as a constrained controller. The performance of this controller is comparable to the constrained controllers.; The integrated SPC/EPC schemes were successfully demonstrated on a hybrid industry application. The relevance and implementation aspects of SPC/EPC systems are provided with detailed benefits of an integrated SPC/EPC system for the semiconductor industry.