Multi-process capability plot and fuzzy inference evaluation

Abstract

Process capability indices C p, C pk, C pm and C pp fitting for nominal-the-best type quality characteristics, are effective tools to assess process capability since these indices can reflect a centering process capability and process yield adequately. The index C pp introduced by Greenwith and Jahr-Schaffrath [Greenwith, M., Jahr-Schaffrath, B.L., 1995. A process incapability index. International Journal of Quality and Reliability Management 12, 58–71] provides additional and individual information concerning the process accuracy and the process precision. Although C pp is useful to evaluate process capability for a single product in common situation, C pp cannot be applied to evaluate the multi-process capability. Referring to Vännman and Deleryd's ( C dr, C dp)-plot, a fuzzy inference approach is proposed in our study to evaluate the multi-process capability based on distance values of a confidence box. This method takes the advantages of fuzzy systems such that a grade instead of sharp evaluation result can be obtained. An illustrated example of color STN display demonstrates that the presented method is effective for assessment of multi-process capability.