Optimal cyclic multiple hoist scheduling for processes with loops and parallel resources

This paper addresses the cyclic multi-hoist scheduling problem. The problem arises in automated electroplating lines where multiple hoists are operating on a shared track. The considered processing sequences may be different from the location sequence of the tanks and may contain loops, i.e. multiple usages of single tanks within a sequence. In addition the electroplating line may be equipped with identical tanks used as parallel resources. A mixed integer linear programming (MILP) formulation for optimizing the cycle time is developed considering in particular collisions of hoists, loops in the process sequence and parallel tanks. The developed model is tested and evaluated using a real world example indicating that the presented solution finds optimal cyclic schedules in reasonable time.