Topic 7B-Short Term Scheduling

8/10/2019 Topic 7B-Short Term Scheduling

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Operat ionsManagement

Top ic 7B Sho rt TermSchedul ing


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Scheduling

Schedul ing : Establ ish ing the t iming of

the use of equ ipment, faci l i t ies and

human act iv i t ies in an o rganizat ion

Effect ive schedu l ing can y ield

Cost sav ings

Inc reases in product iv i ty


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Sequencing

Sequenc ing : Determ ine the order in

which jobs at a work center w i ll be

processed.

Works tat ion: An area where one

person w orks , usually w i th special

equ ipment, on a special ized job .


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Sequencing Jobs

Speci fies the order in which jobs

shou ld be performed at work centers

Prior i ty ru les are used to dispatch o rsequence jobs

FCFS: First come, f irst served

SPT: Sho rtest process ing t im eEDD: Earl iest due date

LPT: Longest processing t ime


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Average com pletion t ime =Total f low time

Number of jobs

Uti l ization =Total job wo rk t im e

Total f low time

Averagenumber of jobsin the sys tem

= Total f low time

Total job wo rk t im e

Average job lateness =Total late days

Number of jobs


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Sequencing Example

Job

Job Work (Processing)

Time(Days)

Job Due

Date(Days)

A 6 8

B 2 6

C 8 18

D 3 15

E 9 23

Apply the four popular sequencing rules

to these f ive jobs


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Sequencing Example

Job

Sequence

Job Work

(Processing)

Time

Flow

Time

Job Due

Date

Job

Lateness

A 6 6 8 0

B 2 8 6 2

C 8 16 18 0

D 3 19 15 4

E 9 28 23 5

28 77 11

FCFS: Sequence A-B -C-D-E


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Sequencing Example

Job

Sequence

Job Work

(Processing)

Time

Flow

Time

Job Due

Date

Job

Lateness

A 6 6 8 0

B 2 8 6 2

C 8 16 18 0

D 3 19 15 4

E 9 28 23 5

28 77 11

FCFS: Sequence A-B -C-D-E

Average com plet ion t ime = = 77/5 = 15.4daysSum of tota l flow t ime

Number of jobs

Uti l ization = = 28/77 = 36.4%Tota l job work t ime

Sum o f total f low t ime

Average number of

jobs in the sys tem

= = 77/28 = 2.75jobsSum of tota l f low t ime

Tota l job w ork t ime

Average job lateness = = 11/5 = 2.2daysTotal late days

Number of jobs


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Sequencing N Jobs on Two

Machines: Johnsons RuleJohnsons Rule N 2 (N jobs; more

than 2 jobs)

Min p rocessing t ime for sequencing a

group of jobs through 2 wo rk centerswhi le m in tota l id le t ime in the wo rk

centers


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Johnsons Rule

1. L is t al l jobs and t imes for each work

center

2. Choose the job wi th the shor tes t act iv i ty

t ime. If that t ime is in the f irst work center,

schedule the job f i rs t . If i t is in the second

work center, schedu le the job last.

3. Once a job is scheduled, i t is el im inatedfrom the lis t

4. Repeat s teps 2 and 3 wo rk ing toward the

center of the sequence


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Johnsons Rule Example

JobWork Center 1

(Drill Press)Work Center 2

(Lathe)

A 5 2

B 3 6

C 8 4

D 10 7

E 7 12


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B E D C A


JobWork Center 1


(Lathe)

A 5 2

B 3 6

C 8 4

D 10 7

E 7 12


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35333328A

33292820C

29222010D

2210103E

9330B

Time OutTime InTime OutTime InJob

WORK CENTER 2WORK CENTER 1

Idle Time M1 = 2 Idle Time M2 = 3 + 1 = 4


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JobWork Center 1


(Lathe)

A 5 2

B 3 6

C 8 4

D 10 7

E 7 12

Tim e 0 3 10 20 28 33

B ACDEWC1

WC2

B ACDE


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JobWork Center 1


(Lathe)

A 5 2

B 3 6

C 8 4

D 10 7

E 7 12

Tim e 0 3 10 20 28 33

Time

0 1 3 5 7 9 10 11 12 13 17 19 21 22 2325 27 29 31 33 35

B ACDE

B ACDE

WC1

WC2

B E D C A

B ACDE


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Id le Time WC 1 = 2

Id le Time WC 2 = 3 + 1 = 4

The five jobs are completed in 35

hours. The WC 2 w il l have to wait 3

hours before f i rst job at WC 1 is

completed and w i ll also wai t for

ano ther 1 hour after complet ing job B .



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Limitations

Rules do not look beyond due dates.

May be two work centers at the

same due date. Wh ich one is

important

Rules do no t look upstream or

downs tream . Idle resources &bo t t leneck resou rces may be not

recognized.


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Finite Capacity Scheduling

MRP Data

Masterschedule

BOM

Inventory

Pr ior i tyrules

Expertsys tems

Simulat ionmodels

Rout ing f i les

Work centerin format ion

Tool ingand otherresources

Setups and

run t ime

Figure 15.5

Interact ive Finite Capacity Sch edulin g


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Bottlenecks

Bo tt leneck work centers are cons traintsthat l im i t outpu t

Common occu rrence due to frequent ch anges

Management techniques include: Inc reasing the capaci ty of th e constra int

Cross -trained employees and maintenance

Alternat ive rout ings

Moving ins pect ion and test

Schedul ing throughpu t to m atch bot t leneck

capaci ty


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Service Sys tems Dif fer From

Manufactur ing

Manufactur ing Services

Schedules machines

and mater ia ls

Schedule s taff

Inventor ies used to

smooth demand

Seldom mainta in inventor ies

Mach ine-intensiv e and

demand may be smooth

Labor -intensive and demand

may be variable

Schedu l ing may be bound by

un ion cont rac ts

Legal issues may constra in

f lex ib le sch edul ing

Few soc ial or behavior al

issues

Social and behavioral issues

may be qui te impo rtant


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Figure 15.1

Summary

Topic 7B-Short Term Scheduling

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