Multihoist Cyclic Scheduling With Fixed Processing and Transfer Times

In this paper, we study the no-wait multihoist cyclic scheduling problem, in which the processing times in the tanks and the transfer times between tanks are constant parameters, and develop a polynomial optimal solution to minimize the production cycle length. We first analyze the problem with a fixed cycle length and identify a group of hoist assignment constraints based on the positions of and the relationships among the part moves in the cycle. We show that the feasibility of the hoist scheduling problem with fixed cycle length is consistent with the feasibility of this group of constraints which can be solved efficiently. We then identify all of the special values of the cycle length at which the feasibility property of the problem may change. Finally, the whole problem is solved optimally by considering the fixed-cycle-length problems at these special values. Note to Practitioners-Automated electroplating lines are commonly used in the production of many products, such as printed-circuit boards. The productivity of these systems depends heavily on effective scheduling of the material handling hoists that move the products between the processing tanks. This paper studies the hoist scheduling problem in systems where the processing times in the tanks and the intertank move times are fixed parameters. An efficient optimal algorithm is developed to solve the problem with any number of hoists. The resulting schedule can be programmed as programmable logic controller codes to directly control the hoist operations. The algorithm can also be used to develop heuristic solutions for multihoist scheduling in systems where the processing times may vary in given intervals.