ON-LINE PROCESS CONTROL OF THE NUMBER OF NON-CONFORMITIES IN THE INSPECTED ITEM

Abstract

Generally, production systems that use automatic control as automatic welding process, production of ceramic products, making clothes use on-line process control to evaluate the quality of their production processes. The control system consists of a periodic inspection of one item after every m produced items. If the number of non-conformities meets the control limit then it is decided that the process is in-control, otherwise the process is stopped for adjustment. The process starts in-control with a non-conformities rate and, after an assignable cause, this rate increases leading the system to operate out of control. The process remains in these conditions until the change is detected and the process adjusted. After adjustment, the process returns to operate in-control. The aim of this paper is to present an economic approach to monitor the rate of non-conformities in a production by on-line process control. To design such type of process, an average cost per item produced is achieved through the properties of an ergodic Markov chain and the two required parameters: the inspection interval and the upper control limit are obtained by minimizing the average cost per produced item.  A numerical example illustrates the proposed procedure.