Minimizing total tardiness of orders with re-entrant lots on a hybrid flowshop

Abstract

This thesis focuses on a scheduling problem in a hybrid flow shop, in which here are serial stages, each with identical parallel machines. We suggest heuristic algorithms for the problem with the objective of minimizing total tardiness of a given set of orders. Each order is composed of multiple lots and each lot is processed on any one of parallel machines at each stage. In this shop, there are reentrant flows, since products for certain orders should visit the processing stages twice. The suggested heuristic algorithms are composed of three phases. In the first phase, an initial sequence is made using priority dispatching rules, and then a construction algorithm is used to obtain sequences for each stage in the second phase. Finally, in the third phase, lots that visit the stages for the second time are scheduled using priority dispatching rules. To evaluate performance of the suggested algorithms, computational experiments are performed on randomly generated test problems. Results show that the suggested algorithms perform better than existing algorithms and an algorithm that is used in practice.