A quality monitoring and decision-making scheme for automated production processes

Abstract

Traditional attribute control charts are based on the monitoring of the number of nonconforming items in a sample of fixed size. In modern manufacturing processes, since items can be checked automatically, use of samples of a fixed subjective size with traditional charts is not suitable for on-line continuous inspection. Furthermore, the sample size usually has to be large when the process fraction nonconforming is not reasonably high. If a decision is to be made only when a sufficient number of items are manufactured and inspected, many nonconforming items might have been produced. In this paper, a control scheme is presented based on the monitoring of cumulative counts of items inspected. This procedure will limit the number of consecutive nonconforming items to a small value when the process has suddenly deteriorated, can detect a sudden process shift quickly and is suitable for continuous inspection processes. The exact probability control limits are derived and the implementation procedure is described. © MCB UP Ltd.