Nonpermutation flow line scheduling by ant colony optimization

A flow line is a conventional manufacturing system where all jobs must be processed on all machines with the same operation sequence. Line buffers allow non-permutation flowshop scheduling (NPFS) and job sequences to be changed on different machines. A mixed-integer linear programming model for non-permutation flowshop scheduling and the buffer requirement along with manufacturing implication is proposed. Ant Colony Optimization (ACO) based heuristic is evaluated against Taillard's (1993) well-known flowshop benchmark instances, with 20 to 500 jobs to be processed on 5 to 20 machines (stages). Computation experiments show that the proposed algorithm is incumbent to the state-of-the-art ACO for flowshop with higher job to machine ratios, using the makespan as the optimization criterion.