12 INTRODUCTION TO OPERATIONS SCHEDULING：12引言作业调度.pptVIP

LESSON 12: INTRODUCTION TO OPERATIONS SCHEDULING Outline Hierarchy of Production Decisions Operations Scheduling Production Systems Sequencing As it is discussed in Lesson 1, scheduling and sequencing are similar terms. Scheduling provides a detail plan over time. Sequencing does not refer to time at all. Sometimes, a unique schedule follows from a given sequence. In such a case, it’s enough to solve the sequencing problem and not worry about the detail scheduling problem. For example, in Lesson 2, Example 1, Schedule 1 follows from sequencing Job 1 before Job 2 and Schedule 2 follows from sequencing Job 2 before Job 1. Sequencing Rules The first sequencing rule, that comes naturally to everyone’s mind is the first-come first-served (FCFS) rule. We observe this rule several times a day when we visit a bank for a teller’s service, wait in a grocery store check-out, or cross the Ambassador bridge. The rule is simple, but not always the best. It’s certainly not desirable that a customer who has to pay for just one bag of bread must wait in a queue behind another customer with a cart full of groceries. So, the express lines are established. This is some sort of implementation of the shortest processing time (SPT) rule. Sequencing Rules You get the most value of money if you pay the bills on the due dates. The simplest rule that comes to mind in presence of due dates is the earliest due date (EDD) rule which requires that the jobs be done in the order in which the jobs are due. Often in manufacturing, items are put in a stack. The last item arriving is put on the top and processed first. The last-come first-served (LCFS) rule is also observed in elevators. The person arriving last must step out first. The critical ratio (CR) rule combines SPT and EDD rule. The CR rule is explained on the next slide. Sequencing Rules: Critical Ratio CR = time remaining / work remaining Each time a job is scheduled, CR is recalculated for every unscheduled job. CR selection criteri