Dispatching strategies for managing uncertainties in automated manufacturing systems

Manufacturers in the western world need to exploit and perfect all their strengths to reduce the flight of manufacturing to global outsourcing destinations. Use of automated manufacturing systems (AMSs) is one such strength that needs to be improved to perfection. One area for improvement is the management of uncertainties on the production floor. This paper explores strategies for modifying detailed event list schedules following the occurrence of an interruption. Advanced planning and scheduling (APS) software packages provide a detailed advance plan of production events. However, the execution of this advance plan is disrupted by a myriad of unanticipated interruptions, such as machine breakdowns, yield variations, and hot jobs. The alternatives available to respond to such interruptions can be classified in four groups: regenerating the complete schedule using APS, switching to dispatching mode, modifying the existing schedule, and continuing to follow the schedule and letting the production system gradually absorb the impact of the interruption. Regeneration of the complete schedule using APS requires a large computation effort, may result in large changes in the schedule, and hence is not recommended. This paper reports on an experimental study for evaluating 10 strategies for responding to machine failures in AMSs that broadly fall in the latter three groups. The strategies are evaluated using simulation under an experimental design with manufacturing scenario, load level, severity and duration of interruptions as factors. The results are analyzed to understand the strengths and weaknesses of the considered strategies and develop recommendations.