Flexible job shop scheduling with preventive maintenance consideration

In highly automated manufacturing systems running 24/7, preventive maintenance activities need to be executed during production times. Flexible job shops with several identical machines generally bear the potential to compensate temporary machine unavailability times caused by preventive maintenance without a considerable increase of the makespan due to machine paralleling. However, this requires sophisticated scheduling of manufacturing jobs and the assignment of maintenance activities over the scheduling horizon. This work, therefore, introduces a mixed-integer program that models both job scheduling and maintenance activity assignment. A local search algorithm is developed to solve both problems in an integrated way. Numerical studies are carried out based on data from a real flexible job shop in the automotive industry with 78 machines and four products. The results show that joint job scheduling and maintenance activity assignment is required to obtain a makespan similar to the makespan without maintenance consideration. Non-sophisticated scheduling of maintenance windows can increase the makespan by more than 20 percentage points compared to sophisticated scheduling. Besides, we show that only a limited amount of maintenance activities can be compensated and that higher unavailability times will inevitably lead to detrimental effects on the makespan unless maintenance worker capacity is increased.