Algorithms for scheduling problems in re-entrant flowshops

Abstract

This dissertation focuses on scheduling problems in re-entrant flowshops, flowshops with reentrant flows of jobs. In a typical flowshop composed of m machines, jobs are composed of m operations at most and each job visits each machine at most once. On the other hand, in re-entrant flowshops, jobs should visit the machines multiple times. In other words, jobs should go through multiple passes of serial manufacturing processes, that is, routes of all jobs are identical as in ordinary flowshop, but the jobs must be processed multiple times on the machines. We consider three different problems for re-entrant flowshop scheduling, and develop algorithms for the problems. First, we consider a two-machine re-entrant flowshop scheduling problem with the objective of minimizing makespan. In the re-entrant flowshop, all jobs must be processed twice on each machine, that is, each job should be processed on machine 1, machine 2 and then machine 1 and machine 2. We develop dominance properties, lower bounds and upper bounds on the makespan for the problem, and suggest a branch and bound algorithm that is developed using these properties and bounds. Secondly, we focus on a two-machine re-entrant flowshop scheduling problem with the objective of minimizing total tardiness. We develop dominance properties, lower bounds and upper bounds on the total tardiness of a given set of jobs for the problem, and suggest a branch and bound algorithm that is developed using these properties and bounds. Finally, we consider an m-machine re-entrant flowshop scheduling problems with the objective of minimizing makespan and total tardiness, respectively. In the m-machine (m≥2) re-entrant flowshop considered here, all jobs should be processed L (≥2) times on each machine, and hence all jobs are composed of Lm operations with zero or positive processing times. We suggest heuristic algorithms, which are modified from well-known existing algorithms for the general m-machine flowshop problem o...