USING LOOK-AHEAD TECHNIQUES IN JOB-SHOP SCHEDULING WITH RANDOM OPERATIONS

We consider a job-shop scheduling problem with n jobs (orders) and m m achines. Each job-operation O-il (the l-th operation of job i, 1 less than or equal to i less than or equal to n, 1 less than or equal to l less than or equal to m) has a random time duration t(il) with the ave rage value t(il) and the variance V-il. Each job has its priority inde x rho(i) and its due date D-i. The problem is to determine the startin g time value S-il for each job-operation O-il. In our recent paper we solved that problem by introducing, at each decision point, a competit ion among the jobs ready to be served on one and the same machine. Tha t competition is based on the idea of pairwise comparison. The main sh ortcoming of the developed model is that it does not deal with so-call ed ''tense jobs'', i.e., with jobs that, in reality, may cause a bottl eneck in the job-shop. This paper is a further extension of our previo us publication. The newly developed model to determine S-il values is based on two alternative decision-making procedures in the case of a n on-empty line for a certain machine: (A) To choose the winner of the c ompetition for that machine. (B) To keep the machine idle until the '' bottleneck'' job is ready to be served on that machine. Such a model i s, in essence, a combination of the pairwise comparison and the ''look ahead'' techniques which are modified for the case of random operatio ns. The model provides an essential refinement of the job-shop's deliv ery performance versus the previous model Extensive experimentation is undertaken to evaluate the efficiency of the model.