Design of EWMA Control Charts for Assuring Predetermined Production Process Quality

In this study, we establish the relationship model in process capability indices, the coefficient of process shift and quality target utilizing statistical tolerance technology. A new systematic method for concurrent design of process quality, Statistical Tolerance (ST) and control chart is presented. A set of standardized Process Quality Indices (PQIs) for variables is introduced for meeting the measurement and evaluation to process yield, process centering and quality loss. To develop a quality-oriented SPC approach, a standardized interface between process quality indices system and the parameter design of EWMA control charts is needed. The two-dimensional array form ST (Cp*, &delta*) is presented for selecting a set of parameters (&lambda, L) with suitable ARL0 and ARL1 to realize the optimal control for a process toward a predetermined quality target. The ST (Cp*, k*) will be the guideline to form the restriction of the centerlines of the control charts for assuring the preset PQIs. If the process statistical parameters are unknown, (Cp*, k*) will be converted into the ST form for estimated values based on a required confidence probability. A case study shows the detail procedure of this concurrent design for PQIs, ST and SPC parameters.