With the development of modern acquisition techniques, data with several correlated quality characteristics are increasingly accessible. Thus, multivariate control charts can be employed to detect changes in the process. This study proposes two multivariate control charts for monitoring process variability (MPVC) using a progressive approach. First, when the process parameters are known, the performance of the MPVC charts is compared with some multivariate dispersion schemes. The results showed that the proposed MPVC charts outperform their counterparts irrespective of the shifts in the process dispersion. The effects of the Phase I estimated covariance matrix on the efficiency of the MPVC charts were also evaluated. The performances of the proposed methods and their counterparts are evaluated by calculating some useful run length properties. An application of the proposed chart is also considered for the monitoring of a carbon fiber tubing process.
K E Y W O R D Sdispersion monitoring, estimation effects, multivariate control chart, phase I, progressive setup
INTRODUCTIONMany manufacturing processes have multiple correlated quality characteristics; multivariate control chart is very effective in monitoring such processes. 1 For example, monitoring multivariate quality variables like the diameter and length of the manufacturing process of a dowel pin. Hotelling 2 introduced a 2 -control chart; this chart is very efficient at detecting large shifts in the process mean vector, which is also mentioned by Mahmood et al. 3 Two multivariate cumulative sum (MCUSUM) charts were proposed by Healy, 4 where the first chart monitors the process mean vector, and the other monitors the covariance matrix. Crosier 5 also proposed a multivariate CUSUM chart. Pignatiello Jr and Runger 6 introduced another multivariate CUSUM control charts for detecting shifts in the process mean vector. Lowry et al 7 provided a Multivariate exponentially weighted moving average (EWMA) chart for detecting small and intermediate shifts in the process parameter. This chart is a direct extension of the univariate EWMA control chart by Roberts. 8 The multivariate CUSUM and EWMA control charts are very efficient in detecting small and moderate shifts in the process parameters. Ajadi and Riaz 9 proposed two multivariate charts which combine the effects of MEWMA and MCUSUM charts. Their proposed charts are more effective in detecting only small shifts in the proposed than the MEWMA and MCUSUM charts. Next, multivariate dispersion charts are employed to detecting changes in the covariance matrices. For example, Alt 10 proposed generalized variance chart (GVC), |S|. The determinant of the sample covariance matrix was employed as the summary statistics. Chen et al 11 worked on a Max-MEWMA chart in monitoring the mean vector and covariance matrix 2724