Optimal transient multiple hoist scheduling for processes with loops

This paper addresses the transient multi-hoist scheduling problem in flexible manufacturing facilities and focuses on Lot Switching. The problem arises in electroplating lines with multiple hoists operating on shared tracks. Due to a flexible production different products need to be manufactured which differ in the routing and processing times. The considered two optimal cycles have to be combined and each may contain loops, i.e. multiple usages of single machines. In addition the electroplating line may be equipped with parallel resources. A mixed integer linear programming (MILP) model for optimizing the transient schedule is presented considering collisions of hoists. This paper offers a new formulation for the collision avoidance by introducing spacial relations for the movement and improving the handling with loops and parallel resources. The fixed allocation of the hoists to the transports of the consecutive cyclic schedules is suspended for the transient schedule resulting in an optimal solution.