1 1 slide short – term scheduling professor ahmadi

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Short – Term SchedulingShort – Term Scheduling

Professor Ahmadi

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1. Minimize completion time

2. Maximize utilization of facilities

3. Minimize work-in-process (WIP) inventory

4. Minimize customer waiting time

Optimize the use of resources so that Optimize the use of resources so that production objectives are metproduction objectives are met

The Goals of Short-Term Scheduling

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Strategic Implications of Strategic Implications of Short-Term Scheduling 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

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Scheduling IssuesForward and Backward Scheduling

Forward scheduling: begins the schedule as Forward scheduling: begins the schedule as soon as the requirements are knownsoon as the requirements are known• jobs performed to customer orderjobs performed to customer order• schedule can be accomplished even if due schedule can be accomplished even if due

date is misseddate is missed• often causes buildup of WIPoften causes buildup of WIP

Backward scheduling: begins with the due date Backward scheduling: begins with the due date of the final operation; schedules jobs in reverse of the final operation; schedules jobs in reverse orderorder• used in many manufacturing environments, used in many manufacturing environments,

catering, scheduling surgerycatering, scheduling surgery

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Assigning jobs to work centersAssigning jobs to work centers ConsiderationsConsiderations

• Job priority (e.g., due date)Job priority (e.g., due date)• CapacityCapacity

Work center hours availableWork center hours available Hours needed for jobHours needed for job

ApproachesApproaches• Input-Output ControlInput-Output Control• Gantt charts (load & scheduling) - capacityGantt charts (load & scheduling) - capacity• Assignment method - job to specific work centerAssignment method - job to specific work center

Loading Jobs in Work CentersLoading Jobs in Work Centers

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Week EndingWeek Ending 6/66/6 6/136/13 6/206/20 6/276/27 7/47/4 7/117/11

Planned inputPlanned input 280280 280280 280280 280280 280280

Actual inputActual input 270270 250250 280280 285285 280280

Cumulative Cumulative DeviationDeviation

-10-10 -40-40 -40-40 -35-35

Planned OutputPlanned Output 320320 320320 320320 320320

Actual OutputActual Output 270270 270270 270270 270270

Cumulative Cumulative DeviationDeviation

-50-50 -100-100 -150-150 -200-200

Cumulative Change Cumulative Change in Backlogin Backlog

00 -20-20 -10-10 +5+5

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

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Gantt Scheduling ChartGantt Scheduling Chart(See page 596 of your textbook)(See page 596 of your textbook)

Start of an activity

Scheduled activity time allowed

Point in time when chart is reviewed

S T W T F SDay

Job

Job A

Job B

Job C

Maintenance

Now

End of an activity

Actual work progress

Non-production time

Day 1

Day 2

Day 3

Day 4

Day 5

Day 6

Day 7

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Assigns tasks or jobs to resourcesAssigns tasks or jobs to resources Type of linear programming modelType of linear programming model

• ObjectiveObjective Minimize total cost, time etc.Minimize total cost, time etc.

• ConstraintsConstraints 1 job per resource (e.g., machine)1 job per resource (e.g., machine) 1 resource (e.g., machine) per job1 resource (e.g., machine) per job

Assignment MethodAssignment Method

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Assignment ProblemAssignment Problem

Four persons are to be assigned to four different Four persons are to be assigned to four different jobs. The following table represents the cost of jobs. The following table represents the cost of assigning various people to different jobs. assigning various people to different jobs. Assign each person to a job so that the costs will Assign each person to a job so that the costs will be minimized. What is the minimum cost of be minimized. What is the minimum cost of assignment? Indicate the assignment.assignment? Indicate the assignment.

JOBSJOBS

PERSONPERSON I I IIII IIIIII IVIV

AA 1111 77 44 88

BB 33 1414 1212 66

CC 99 77 1313 1010

DD 66 1111 55 1414

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Specifies the order in which jobs should Specifies the order in which jobs should be performed at work centersbe performed at work centers

Priority rules are used to dispatch or Priority rules are used to dispatch or sequence jobssequence jobs

Sequencing (Priority) rulesSequencing (Priority) rules• First come, first served (First come, first served (FCFSFCFS))• Shortest processing time (Shortest processing time (SPTSPT))• Earliest due date (Earliest due date (EDDEDD))• Longest processing time (Longest processing time (LPTLPT))

Critical ratio (Critical ratio (CRCR)) Johnson’s ruleJohnson’s rule

Sequencing JobsSequencing Jobs

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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.

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Criteria to Evaluate Priority RulesCriteria to Evaluate Priority Rules

Jobs#

timesΣFlow timecompletion Average

timesFlow

timesProcess n Utilizatio

timesProcess

timesFlow system in the jobs ofnumber Average

jobs of Number

timesLate lateness job Average

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Job Sequencing ExampleJob Sequencing Example(See page 600 of your textbook) (See page 600 of your textbook)

JobJob Job WorkJob Work

Processing Processing time in daystime in days

Job Due Date Job Due Date (day)(day)

AA 66 88

BB 22 66

CC 88 1818

DD 33 1515

EE 99 2323

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FCFS (first come, first served)FCFS (first come, first served)

Job Job SequenceSequence

Job Work Job Work (Processing) (Processing)

TimeTimeFlow Flow TimeTime

Job Due Job Due DateDate

Job Job LatenessLateness

AA 66 66 88 00

BB 22 88 66 22

CC 88 1616 1818 00

DD 33 1919 1515 44

EE 99 2828 2323 55

2828 7777 1111

Sequence A-B-C-D-ESequence A-B-C-D-E

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FCFS (first come, first served)FCFS (first come, first served)

Sequence A-B-C-D-ESequence A-B-C-D-E

Average completion time = = 77/5 = 15.4 daysTotal flow timeNumber of jobs

Utilization = = 28/77 = 36.4%Total job work time

Total flow time

Average number of jobs in the system = = 77/28 = 2.75 jobs

Total flow timeTotal job work time

Average job lateness = = 11/5 = 2.2 daysTotal late daysNumber of jobs

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Shortest Processing Time (SPT)Shortest Processing Time (SPT)






BB 22 22 66 00

DD 33 55 1515 00

AA 66 1111 88 33

CC 88 1919 1818 11

EE 99 2828 2323 55

2828 6565 99

Sequence B-D-A-C-ESequence B-D-A-C-E

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Shortest Processing Time (SPT)Shortest Processing Time (SPT)

Sequence B-D-A-C-ESequence B-D-A-C-E

Average completion time = = 65/5 = 13 daysTotal flow timeNumber of jobs


Total flow time




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Earliest Due Date (EDD)Earliest Due Date (EDD)






BB 22 22 66 00

AA 66 88 88 00

DD 33 1111 1515 00

CC 88 1919 1818 11

EE 99 2828 2323 55

2828 6868 66

Sequence B-A-D-C-ESequence B-A-D-C-E

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Earliest Due Date (EDD)Earliest Due Date (EDD)

Sequence B-A-D-C-ESequence B-A-D-C-E



Total flow time




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Longest Processing Time (LPT)Longest Processing Time (LPT)






EE 99 99 2323 00

CC 88 1717 1818 00

AA 66 2323 88 1515

DD 33 2626 1515 1111

BB 22 2828 66 2222

2828 103103 4848

Sequence E-C-A-D-BSequence E-C-A-D-B

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Longest Processing Time (LPT)Longest Processing Time (LPT)

Sequence E-C-A-D-BSequence E-C-A-D-B



Total flow time




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Sequencing ExampleSequencing Example

RuleRule

Average Average Completion Completion Time (Days)Time (Days)

Utilization Utilization (%)(%)

Average Average Number of Jobs Number of Jobs

in Systemin System

Average Average Lateness Lateness (Days)(Days)

FCFSFCFS 15.415.4 36.436.4 2.752.75 2.22.2

SPTSPT 13.013.0 43.143.1 2.322.32 1.81.8

EDDEDD 13.613.6 41.241.2 2.432.43 1.21.2

LPTLPT 20.620.6 27.227.2 3.683.68 9.69.6

Summary of RulesSummary of Rules

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Comparison of Comparison of Sequencing RulesSequencing Rules

No one sequencing rule excels on all criteriaNo one sequencing rule excels on all criteria SPT does well on minimizing flow time and SPT does well on minimizing flow time and

number of jobs in the systemnumber of jobs in the system But SPT moves long jobs to the end which But SPT moves long jobs to the end which

may result in dissatisfied customersmay result in dissatisfied customers FCFS does not do especially well (or poorly) FCFS does not do especially well (or poorly)

on any criteria but is perceived as fair by on any criteria but is perceived as fair by customerscustomers

EDD minimizes latenessEDD minimizes lateness

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Ratio of time remaining to work time Ratio of time remaining to work time remainingremaining

Process job with smallest CR firstProcess job with smallest CR first Performs well on average latenessPerforms well on average lateness

CR Time remainingWork days remaining

Due date - Today' s dateWork (lead ) time remaining

=

=

Critical Ratio (CR)Critical Ratio (CR)

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Johnson's Rule - Scheduling N Jobs on Two Johnson's Rule - Scheduling N Jobs on Two MachinesMachines

1.1. All jobs are to be listed, and the time each All jobs are to be listed, and the time each requires on a machine shown.requires on a machine shown.

2.2. Select the job with the shortest activity time. If Select the job with the shortest activity time. If the shortest time lies with the first machine, the the shortest time lies with the first machine, the job is scheduled first; if with the second job is scheduled first; if with the second machine, the job is scheduled last.machine, the job is scheduled last.

3.3. Once a job is scheduled, eliminate it.Once a job is scheduled, eliminate it.4.4. Apply steps 2-3 to the remaining jobs, working Apply steps 2-3 to the remaining jobs, working

toward the center of the sequence.toward the center of the sequence.

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List jobs &activity times

Select job withshortest time

Machine?

ScheduleFIRST

ScheduleLAST

Eliminate jobfrom list

Jobs left?Breakarbitrarily

Ties?

YesYes

11

22

YesYesStop

NoNo

NoNo

Johnson’s Rule StepsJohnson’s Rule Steps

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

JobJobWork Center 1 Work Center 1

(Drill Press)(Drill Press)Work Center 2 Work Center 2

(Lathe)(Lathe)

AA 55 22

BB 33 66

CC 88 44

DD 1010 77

EE 77 1212

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Johnson’s Rule ExampleJohnson’s Rule Example

JobJobWork Center 1 Work Center 1

(Drill Press)(Drill Press)Work Center 2 Work Center 2

(Lathe)(Lathe)

AA 55 22

BB 33 66

CC 88 44

DD 1010 77

EE 77 1212

DD AABB CCEE

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Techniques for Dealing With BottlenecksTechniques for Dealing With Bottlenecks

1.1. Increase the capacity of the constraintIncrease the capacity of the constraint2.2. Ensure well-trained and cross-trained employees Ensure well-trained and cross-trained employees

are available to operate and maintain the work are available to operate and maintain the work center causing the constraintcenter causing the constraint

3.3. Develop alternate routings, processing Develop alternate routings, processing procedures, or subcontractorsprocedures, or subcontractors

4.4. Move inspections and tests to a position just Move inspections and tests to a position just before the constraintbefore the constraint

5.5. Schedule throughput to match the capacity of Schedule throughput to match the capacity of the bottleneckthe bottleneck

1 1 slide short – term scheduling professor ahmadi

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