Process control using VSI cause selecting control charts |
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Authors: | Yang Su-Fen |
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Affiliation: | (1) Department of Industrial Engineering, Da-Yeh University, 112 Shan-Jiau Road, Da-Tsuen, Chang-Hua 51505, Taiwan R.O.C;(2) Department of Business Administration, National Changhua University of Education, Changhua, 500, Taiwan R.O.C. |
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Abstract: | The article considers the variable process control scheme for two dependent process steps with incorrect adjustment. Incorrect
adjustment of a process may result in shifts in process mean, process variance, or both, ultimately affecting the quality
of products. We construct the variable sampling interval (VSI) Z`(X)]-ZSX2{Z_{\overline{X}}-Z_{S_X^2}} and Z`(e)]-ZSe2{Z_{\bar{{e}}}-Z_{S_e^2}} control charts to effectively monitor the quality variable produced by the first process step with incorrect adjustment and
the quality variable produced by the second process step with incorrect adjustment, respectively. The performance of the proposed
VSI control charts is measured by the adjusted average time to signal derived using a Markov chain approach. An example of
the cotton yarn producing system shows the application and performance of the proposed joint VSI Z`(X)] -ZSX2 {Z_{\overline{X}} -Z_{S_X^2 }} and Z`(e)] -ZSe2 {Z_{\bar{{e}}} -Z_{S_e^2 }} control charts in detecting shifts in mean and variance for the two dependent process steps with incorrect adjustment. Furthermore,
the performance of the VSI Z`(X)]-ZSX2 {Z_{\overline{X}}-Z_{S_X^2 }} and Z`(e)] -ZSe2 {Z_{\bar{{e}}} -Z_{S_e^2 }} control charts and the fixed sampling interval Z`(X)] -ZSX2 {Z_{\overline{X}} -Z_{S_X^2 }} and Z`(e)] -ZSe2 {Z_{\bar{{e}}} -Z_{S_e^2 }} control charts are compared by numerical analysis results. These demonstrate that the former is much faster in detecting
small and median shifts in mean and variance. When quality engineers cannot specify the values of variable sampling intervals,
the optimum VSI Z`(X)]-ZSX2 {Z_{\overline{X}}-Z_{S_X^2 }} and Z`(e)] -ZSe2 {Z_{\bar{{e}}} -Z_{S_e^2 }} control charts are also proposed by using the Quasi-Newton optimization technique. |
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