A new method of cyclic hoist scheduling for multi-recipe and multi-stage material handling processes

Multi-recipe and multi-stage material handling (M3H) processes are widely employed by various industries where complex multi-product manufacturing and assembly tasks are required. Cyclic hoist scheduling (CHS) could significantly improve the productivity of an M3H process. In this paper, a novel CHS methodology has been developed, which considers employing multiple sub-cycles to efficiently deal with job duality issues associated with multi-capacity processing units. The methodology contains three modeling and solving stages. In Stage I, a mixed-integer linear programing (MILP) model is developed to obtain the optimal solution of a sub-cycle CHS problem with the tolerance of job duality. In Stage II, the obtained solution will be examined to see if the job duality exists. Once a job duality issue is identified, another MILP model in Stage III will be performed to schedule an additional sub-cycle CHS problem, which targets the minimum slot usage discrepancy caused by the identified job duality. After that, the combined CHS solutions from the previous and additional sub-cycles will be fed back to Stage II for the job duality examination again. Iteratively checking and rescheduling between Stages II and III, job duality issues will be eventually eliminated and a full scheduling cycle coupling multiple sub-cycles will be accomplished. The methodology can significantly increase the CHS optimality for M3H processes, which are demonstrated by two case studies.