Monitoring Process Steps Using Vssi Ewma Control Charts
نویسندگان
چکیده
1. 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, ultimately affecting the quality of products. We construct variable sample size and variable sampling interval (VSSI) 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. The performance of the proposed VSSI control charts is measured by the adjusted average time to signal (AATS) derived using a Markov chain approach. An example of the automobile braking system with incorrect adjustment shows the application and performance of the proposed VSSI control charts in detecting shifts in process mean. Furthermore, the performance of the specified VSSI control charts, the optimum VSSI and the fixed sample size and sampling interval (FSSI) control charts are compared by numerical analysis results. It has been found that the optimum VSSI control charts work better than the specified VSSI control charts, and the specified VSSI control charts outperform the FSSI control charts. When quality engineers cannot specify the variable sample sizes and sampling intervals, the optimum VSSI control charts are suggested. Moreover, the impacts of misusing charts to monitoring the process mean and variance in the second step are also investigated.
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