تغییر زمان بدون بن بست در سیستم های تولید انعطاف پذیر

The dynamic nature of manufacturing makes rescheduling essential in today's complex production environment, particularly in flexible and re-configurable systems. Research on optimizing schedules, that includes deadlock avoidance, is rather limited. Furthermore, the deadlock problem is mostly ignored in research on rescheduling. A rescheduling algorithm, that uses time Petri-Nets and the minimal siphons concept, was developed to deal with sources of disturbance such as machine breakdowns in real-time. It guarantees a deadlock-free new schedule. The existence of alternative routes, availability of material handling facilities, and the limitation of buffer capacities were taken into consideration. The developed algorithm modifies only the affected portion of the original schedule, rather than rescheduling all jobs, in order to limit changes to the original schedule and reduce the impact on the response time.

The continuous customer demands for larger variety of products, faster production rates, and higher delivery commitments are a challenge for most modern industries. The performance of a production system greatly depends on the availability of efficient rescheduling capability in order to deal with Changes in real-time. The re-configuration of manufacturing systems requires not only hardware changes but also software adaptation including replanning and rescheduling. A variety of conditions, demands and constraints necessitate revisions to existing schedules. Some of the common reasons for rescheduling are [Li et al., 19931: machine breakdown; rush order arrival, shortage of materials, quality problems, over- or under-estimation of process time, order cancellation, due date changes, and being behind or ahead of the current schedule. Manual rescheduling is not practical for complex and automated manufacturing systems. It can be done using computer tools such as electronic Gantt chart, simulation, expert systems, algorithmic methods as well as hybrid approaches. Early surveys of real-time scheduling include Harmonosky and Robohn (1991), Basnet and Mize (1994), Shukla and Chen (1996), Harmonosky (1995), Szelke and Kerr (1994) and Gonzalez (1995).