PERFORMANCE OF A HYSTERESIS BASED CONTROL STRATEGY FOR A FLEXIBLE MACHINE OPERATING UNDER A PERIODIC STATUS MONITORING POLICY

The role of on-line control strategies is crucial for exploiting the flexibility within discrete part manufacturing systems. When studying their impact on system performance, researchers have mostly assumed the presence of real-time information availability. Such assumption may not always be valid in real world manufacturing situations since on-line control strategies base their decisions on information that may entail significant time delays depending upon the extent of computer based automation and information integration employed. We refer to these delays as information delays. In the domain of discrete part manufacturing systems with defined levels of flexibility, it is essential explicitly to model and analyze the effect that information delays have on the performance of a given on-line control strategy. In this paper we conceptualize three basic modes of information delays and indicate the need to model them explicitly in order to assess their impact on the performance of on-line control strategies. One of these basic forms of information delay is caused due to a review period based status monitoring policy. We propose and characterize a hysteresis based on-line control strategy (HCS) to cope with information delays. The study system chosen represents the simplest possible manifestation of machine flexibility and resembles the two-queue single server case for which prior research results indicate the superiority of the alternating priority (AP) rule when average job flowtime is the measure of performance. We use computer simulation to compare the behaviour of HCS with the AP rule in a real-time as well as the information delay mode. The best HCS setting in the real-time mode fails to remain consistent in the information delay case, thus clearly indicating the need to explicitly model delays when developing suitable control strategies. Further, when compared with the AP rule, HCS shows consistently superior performance to AP. © 1997 Elsevier Science Ltd