MULTI-OBJECTIVE LEAD TIME CONTROL IN MULTISTAGE ASSEMBLY SYSTEMS

In this paper \ve develop a multi-objective model to optimally control the lead tinle of a lTIultistage assembly system. The multistage assembly system is 1110deled as an open queueing network, whose service stations represent manufacturing or asselTIbly operations. [he arrival processes of the individual parts of the product, which should be assembled to each other in assembly stations, are assull1ed to be independent Poisson processes with equal rates. In each service station, there is one machine with exponentiaJly distributed processing time, such that the service rate is controllable. The transport times bet\veen the service stations are independent random variables with exponential distributions. By applying the longest path analysis in queueing networks, we obtain the distribution function of time spend by a product in the system or the manufacturing lead time. The decision variables of the lTIodel are the number of servers in the service stations. l~he problem is formulated as a multi-objective optimal control problelTI that involves three conflicting objective functions. The objective functions are the total operating costs of the systelTI per period (to be minitnized), the average lead time (min). and the probability that the lnanufacturing lead tilne does not exceed a certain threshold (nlax). The goal progranlming method is used to solve a discrete-time approximation of the original probleln.