Tabu Search for the Flexible-Routing Job Shop Problem

In the classical job shop scheduling problem (JSSP), n jobs are processed to completion on m unrelated machines. Each job requires processing on each machine exactly once. For each job, technology constraints specify a complete, distinct routing which is fixed and known in advance. Processing times are sequence-independent, fixed, and known in advance. Each machine is continuously available from time zero, and operations are processed without preemption. The objective is to minimize the maximum completion time (makespan). The flexible-routing job shop (FRJS) scheduling problem, or job shop with multipurpose machines, extends JSSP by assuming that a machine may be capable of performing more than one type of operation. (For a given operation, there must exist at least one machine capable of performing it.) FRJS approximates a flexible manufacturing environment with numerically controlled work centers equipped with interchangeable tool magazines. This report extends a dynamic, adaptive tabu search (TS) strategy previously described for job shops with single and multiple instances of single-purpose machines, and applies it to FRJS. We present “proof-of-concept” results for three problems constructed from difficult JSSP instances.