Part scheduling and operation allocation in a flexible flowshop

In this study, we considered flowshops with 2 and 3 machines producing n different parts. As a requirement of the flowshop assumption, parts are processed on the machines in the same order. Each machine performs a specific operation on each of the parts. These operations are called as "fixed" operations. Apart from these fixed operations, there are some operations which can be performed by specific adjacent machines. These operations are named as "flexible" operations. For the 2-machine case, every part has three operations: The first operation can only be processed on the first machine and the third operation can only be processed on the second machine. Whereas, the second operation is the flexible one can be performed by both machines. For 3-machines systems different alternatives are considered regarding the number and the position of the flexible operations. Machines are assumed to be nonidentical meaning that the processing time of the flexible operation depends on the machine that is assigned to. The objective is to determine the part input sequence as well as the assignment of the flexible operations in order to maximize the throughput rate. For each problem alternative, mixed integer programming formulations are developed. However, since the solution times appeared to be very large, a number of polynomial time heuristic algorithms are developed. The performances of these heuristics are tested through experimental studies. Keywords: Flowshops, scheduling, flexible operations, heuristics