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Short – Term Scheduling

Short – Term Scheduling. Professor Ahmadi. The Goals of Short-Term Scheduling. Minimize completion time Maximize utilization of facilities Minimize work-in-process (WIP) inventory Minimize customer waiting time. Optimize the use of resources so that production objectives are met.

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Short – Term Scheduling

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  1. Short – Term Scheduling Professor Ahmadi

  2. The Goals of Short-Term Scheduling Minimize completion time Maximize utilization of facilities Minimize work-in-process (WIP) inventory Minimize customer waiting time Optimize the use of resources so that production objectives are met

  3. Strategic Implications of Short-Term Scheduling • By scheduling effectively, companies use assets more effectively and create greater capacity per dollar invested, which, in turn, lowers cost • This added capacity and related flexibility provides faster delivery and therefore better customer service • Good scheduling is a competitive advantage which contributes to dependable delivery

  4. Scheduling IssuesForward and Backward Scheduling • Forward scheduling: begins the schedule as soon as the requirements are known • jobs performed to customer order • schedule can be accomplished even if due date is missed • often causes buildup of WIP • Backward scheduling: begins with the due date of the final operation; schedules jobs in reverse order • used in many manufacturing environments, catering, scheduling surgery

  5. Loading Jobs in Work Centers • Assigning jobs to work centers • Considerations • Job priority (e.g., due date) • Capacity • Work center hours available • Hours needed for job • Approaches • Input-Output Control • Gantt charts (load & scheduling) - capacity • Assignment method - job to specific work center

  6. Input-Output Control Example(See page 595 of your textbook)

  7. Start of an activity Day End of an activity Job Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 S T W T F S Scheduled activity time allowed Job A Actual work progress Job B Maintenance Non-production time Job C Point in time when chart is reviewed Now Gantt Scheduling Chart(See page 596 of your textbook)

  8. Assignment Method • Assigns tasks or jobs to resources • Type of linear programming model • Objective • Minimize total cost, time etc. • Constraints • 1 job per resource (e.g., machine) • 1 resource (e.g., machine) per job

  9. Assignment Problem • Four persons are to be assigned to four different jobs. The following table represents the cost of assigning various people to different jobs. Assign each person to a job so that the costs will be minimized. What is the minimum cost of assignment? Indicate the assignment. • JOBS PERSON I II III IV A 11 7 4 8 B 3 14 12 6 C 9 7 13 10 D 6 11 5 14

  10. Sequencing Jobs • Specifies the order in which jobs should be performed at work centers • Priority rules are used to dispatch or sequence jobs • Sequencing (Priority) rules • First come, first served (FCFS) • Shortest processing time (SPT) • Earliest due date (EDD) • Longest processing time (LPT) • Critical ratio (CR) • Johnson’s rule

  11. Priority Rules for Dispatching Jobs • First come, first served (FCFS) • The first job to arrive at a work center is processed first • Earliest due date (EDD) • The job with the earliest due date is processed first • Shortest processing time (SPT) • The job with the shortest processing time is processed first • Longest processing time (LPT) • The job with the longest processing time is processed first • Critical ratio (CR) • The ratio of time remaining to required work time remaining is calculated, and jobs are scheduled in order of increasing ratio.

  12. Criteria to Evaluate Priority Rules

  13. Job Sequencing Example(See page 600 of your textbook)

  14. FCFS (first come, first served) Sequence A-B-C-D-E

  15. Total flow time Number of jobs Average completion time = = 77/5 = 15.4 days Utilization = = 28/77 = 36.4% Total job work time Total flow time Total late days Number of jobs Average job lateness = = 11/5 = 2.2 days Average number of jobs in the system = = 77/28 = 2.75 jobs Total flow time Total job work time FCFS (first come, first served) Sequence A-B-C-D-E

  16. Shortest Processing Time (SPT) Sequence B-D-A-C-E

  17. Total flow time Number of jobs Average completion time = = 65/5 = 13 days Utilization = = 28/65 = 43.1% Total job work time Total flow time Total late days Number of jobs Average job lateness = = 9/5 = 1.8 days Average number of jobs in the system = = 65/28 = 2.32 jobs Total flow time Total job work time Shortest Processing Time (SPT) Sequence B-D-A-C-E

  18. Earliest Due Date (EDD) Sequence B-A-D-C-E

  19. Total flow time Number of jobs Average completion time = = 68/5 = 13.6 days Utilization = = 28/68 = 41.2% Total job work time Total flow time Total late days Number of jobs Average job lateness = = 6/5 = 1.2 days Average number of jobs in the system = = 68/28 = 2.43 jobs Total flow time Total job work time Earliest Due Date (EDD) Sequence B-A-D-C-E

  20. Longest Processing Time (LPT) Sequence E-C-A-D-B

  21. Total flow time Number of jobs Average completion time = = 103/5 = 20.6 days Utilization = = 28/103 = 27.2% Total job work time Total flow time Total late days Number of jobs Average job lateness = = 48/5 = 9.6 days Average number of jobs in the system = = 103/28 = 3.68 jobs Total flow time Total job work time Longest Processing Time (LPT) Sequence E-C-A-D-B

  22. Sequencing Example Summary of Rules

  23. Comparison of Sequencing Rules • No one sequencing rule excels on all criteria • SPT does well on minimizing flow time and number of jobs in the system • But SPT moves long jobs to the end which may result in dissatisfied customers • FCFS does not do especially well (or poorly) on any criteria but is perceived as fair by customers • EDD minimizes lateness

  24. Critical Ratio (CR) • Process job with smallest CR first • Performs well on average lateness • Ratio of time remaining to work time remaining Time remaining = CR Work days remaining Due date - Today' s date = Work (lead ) time remaining

  25. Johnson's Rule - Scheduling N Jobs on Two Machines • All jobs are to be listed, and the time each requires on a machine shown. • Select the job with the shortest activity time. If the shortest time lies with the first machine, the job is scheduled first; if with the second machine, the job is scheduled last. • Once a job is scheduled, eliminate it. • Apply steps 2-3 to the remaining jobs, working toward the center of the sequence.

  26. 2 List jobs & Schedule Machine? activity times LAST 1 Select job with Schedule shortest time FIRST No Eliminate job Ties? from list Yes No Break Jobs left? Stop arbitrarily Yes Johnson’s Rule Steps

  27. Johnson’s Rule Example(See Page 603 0f your textbook)

  28. Johnson’s Rule Example B E D C A

  29. Techniques for Dealing With Bottlenecks • Increase the capacity of the constraint • Ensure well-trained and cross-trained employees are available to operate and maintain the work center causing the constraint • Develop alternate routings, processing procedures, or subcontractors • Move inspections and tests to a position just before the constraint • Schedule throughput to match the capacity of the bottleneck

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