Machine Scheduling and Job Shop Scheduling

Abstract

This chapter focuses on job shops. There are n jobs and each job visitsa number of machines following a predetermined route. In some modelsa job may visit any given machine at most once and in other modelsa job may visit each machine more than once. In the latter case itis said that the job shop is subject to recirculation. A generalizationof the basic job shop is a so-called flexible job shop. A flexiblejob shop consists of a collection of workcenters and each workcenterconsists of a number of identical machines in parallel. Each jobfollows a predetermined route visiting a number of workcenters; whena job visits a workcenter, it may be processed on any one of themachines at that workcenter.Job shops are prevalent in industries where each customer order isunique and has its own parameters. Wafer fabs in the semiconductorindustry often function as job shops; an order usually implies abatch of a certain type of item and the batch has to go through thefacility following a certain route with given processing times. Anotherclassical example of a job shop is a hospital. The patients in ahospital are the jobs. Each patient has to follow a given route andhas to be treated at a number of different stations while going throughthe system.