On the Monitoring of Linear Profiles in Multistage Processes

In most modern manufacturing systems, products are often the output of several correlated stages. Nevertheless, quality of a product or process in both single and multistage processes is usually expressed by a single quality characteristic, two or more characteristics, or profiles. Although there are many studies in univariate and multivariate‐multistage process monitoring, fewer works focus on profile monitoring of multistage processes. This paper addresses the problem of monitoring a simple linear profile that is going through a multistage process in phase II. Using a first‐order autoregressive correlation model, the relationship between the stages is first modeled. Then, the cascade effect of multistage processes on the performance of T² control chart is studied. We show that the cascade property has a significant impact on the performances of the chart in downstream stages. Next, a U statistic is used to eliminate the cascade effect, and the control scheme is modified accordingly. Simulation studies reveal that the modified control chart performs well.