OM NOTES CHAPTER 16

7/23/2019 OM NOTES CHAPTER 16

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Scheduling

McGraw-Hill/Irwin Copyright 2012 by The McGraw-Hill Companies, Inc !ll rights reser"e#


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You should be able to:

1. Explain what scheduling involves and the importance of goodscheduling

2. Describe scheduling needs in high-volume and intermediate-

volume systems

3. Describe scheduling needs in ob shops!. "se and interpret #antt charts$ and use the assignment

method for loading

%. #ive examples of commonly used priority rules

&. Summari'e some of the uni(ue problems encountered in

service systems$ and describe some of the approaches usedfor scheduling service systems

Instructor Slides 16-2


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Scheduling)

Establishing the timing of the use of e(uipment$facilities and human activities in an organi'ation

E*ective scheduling can yield+ost savings

,ncreases in productivity

ther benets



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Scheduling is constrained by multiplesystem design decisionsSystem capacity/roduct and0or service designE(uipment selectionorer selection and trainingggregate planning and master scheduling



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Flow System4igh-volume system in which all obs follow

the same se(uenceFlow system schedulingScheduling for 5ow systems6he goal is to achieve a smooth rate of 5ow of

goods or customers through the system in order toget high utili'ation of labor and e(uipment



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Few fow systems are entirely dedicated toa single product or service Each product change re(uires

Slightly di*erent inputs of parts

Slightly di*erent materialsSlightly di*erent processing re(uirements that must

be scheduled into the line

7eed to avoid excessive inventory buildup

Disruptions may result in less-than-desired output



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The ollowing actors oten dictate thesuccess o high-volume systems:

8 /rocess and product design

8 /reventive maintenance

8 9apid repair when breadowns occur8 ptimal product mixes

8 :inimi'ation of (uality problems

8 9eliability and timing of supplies



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utputs fall between the standardi'ed type ofoutput of high-volume systems and the mae-to-order output of ob shops

utput rates are insu;cient to warrant

continuous production9ather$ it is more economical

to produce intermittentlyor centers periodically

shift from one product to

another



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Three basic issues:9un si'e of obs6he timing of obs6he se(uence in which obs will be produced

up

p

H

DSQ

=

2

O



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Important considerationsSetup cost"sage is not always as smooth as assumed in

the economic lot si'e model

Alternative scheduling approach


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ob shop schedulingScheduling for low-volume systems with many

variations in re(uirements:ae-to-order products/rocessing re(uirements:aterial re(uirements/rocessing time/rocessing se(uence and steps

complex scheduling environment ,t is impossible to establish rm schedules until actual

ob orders are received



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!oading the assignment of obs to processing centers

"antt chart

"sed as a visual aid for loading and scheduling

purposes/urpose of the #antt chart is to organi'e and

visually display the actual or intended use ofresources in a time framewor

:anagers may use the charts for trial-and-error

schedule development to get an idea of whatdi*erent arrangements would involve



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!oad chart #antt chart that shows the loading and idle

times for a group of machines or list ofdepartments



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In#nite loading

=obs are assigned to worstations without regard to the capacityof the wor center

Finite loading

=obs are assigned to wor centers taing into account the worcenter capacity and ob processing times

1 2 3 4 5 6

over over Capacity

Infinite loading

1 2 3 4 5 6

Capacity

Finite loading



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Forward scheduling Scheduling ahead from some point in time.

"sed when the (uestion is)

>4ow long will it tae to complete this ob?

$ac%ward scheduling Scheduling bacwards from some due date

"sed when the (uestion is)

>hen is the latest this ob can be started

and still be completed on time?@



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Schedule chart #antt chart that shows the orders or obs in

progress and whether they are on schedule



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Input&'utput (I&') control:anaging wor 5ow and (ueues at wor centersithout ,0 control) ,f demand exceeds processing capacity$ a wor center

overload is created ,f wor arrives more slowly than a wor center can

handle$ wor center underutili'ation results

6he goal is to strie a balance between input andoutput rates in order to minimi'e (ueues andmaximi'e utili'ation



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Assignment model linear programming model for optimal

assignment of tass and resources

*ungarian method:ethod of assigning obs by a one-for-one

matching to identify the lowest cost solution

Instructor Slides

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+, ow reduction: subtract the smallest number ineach row rom every number in the row

a. Enter the result in a new table

., /olumn reduction: subtract the smallest number ineach column rom every number in the column

a. Enter the result in a new table

0, Test whether an optimum assignment can be madea. Determine the minimum number of lines needed to cross out all 'eros

b. ,f the number of lines e(uals the number of rows$ an optimumassignment is possible. #o to step &

c. Else$ go to step !

Instructor Slides

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1, I the number o lines is less than the number o rows2modiy the table:

a. Subtract the smallest number from every uncovered number in the table

b. dd the smallest uncovered number to the numbers at intersections ofcross-out lines

c. 7umbers crossed out but not at intersections of cross-out lines carry overunchanged to the next table

3, epeat steps 0 and 1 until an optimal table isobtained

4, 5a%e the assignmentsa.


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Se6uencingDetermine the orderin which obs at a wor center

will be processed

7riority rules

Simple heuristics used to select the order in whichobs will be processed

6he rules generally assume that ob setup cost andtime are independent of processing se(uenceob time

6ime needed for setup and processing of a ob

Instructor Slides

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F/FS- rst come$ rst served

S7T - shortest processing time

899- earliest due date

/- critical ratio

S&'- slac per operation

ush- emergency



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6he set of obs is nownA no new orders arriveafter processing begins and no obs are canceled

Setup time is independent of processing time

Setup time is deterministic

/rocessing times are deterministic

6here will be no interruptions in processing suchas machine breadowns or accidents

Instructor Slides

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!ocal priority rules:Bocus on information pertaining to a single

worstation when establishing a ob se(uence

"lobal priority rules:,ncorporate information from multiple

worstations when establishing a ob se(uence



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/ommon perormance metrics:ob fow time

6his is the amount of time it taes from when a ob arrives until it iscomplete

,t includes not only processing time but also any time waiting to beprocessed

ob lateness6his is the amount of time the ob completion time is expected to exceed

the date the ob was due or promised to a customer

5a%espan6he total time needed to complete a groupof obs from the beginning of

the rst ob to the completion of the last ob

Average number o obs=obs that are in a shop are considered to be ,/ inventory

Instructor Slides

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ohnson;s ule6echni(ue for minimi'ing maespan for a group

of obs to be processed on two machines or attwo wor centers.

:inimi'es total idle time

Several conditions must be satised



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=ob time must be nown and constant foreach ob at the wor center

=ob times must be independent ofse(uence

=obs must follow same two-step se(uencell obs must be completed at the rst

wor center before moving to secondwor center

Instructor Slides

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1. Cist the obs and their times at each wor center

2. Select the ob with the shortest time

a. ,f the shortest time is at the rst wor center$ schedule thatob rst

b. ,f the shortest time is at the second wor center$ schedule the

ob last.c.


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ariability in Setup times

/rocessing times

,nterruptions

+hanges in the set of obs

Except for small ob sets$ there is no method foridentifying an optimal schedule

Scheduling is not an exact science

,t is an ongoing tas for a manager

Instructor Slides

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Set realistic due datesBocus on bottlenec operations Birst$ try to increase the capacity of the operations

,f that is not possible

Schedule bottlenec operations rst

6hen$ schedule non-bottlenec operations around thebottlenec operations

+onsider lot splitting of large obs

ften wors best when there are large di*erences in obtimes



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Theory o constraints/roduction planning approach that emphasi'es balancing

5ow throughout a system$ and pursues a perpetual ve-step improvement process centered around the systemscurrently most restrictive constraint.


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arying batch si'es to achieve greatest output ofbottlenec operations 7rocess batch

6he economical (uantity to produce upon the activation ofa given operation

Transer batch6he (uantity to be transported from one operation to

another$ assumed to be smaller than the rst operationsprocess batch

Instructor Slides

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Improving bottlenec% operations:1. Determine what is constraining the operation

2. Exploit the constraint Gi.e.$ mae sure the constrainingresource is used to its maximumH

3. Subordinate everything to the constraint Gi.e.$ focus onthe constraintH

!. Determine how to overcome GeliminateH the constraint

%. 9epeat the process for the next highest constraint

Instructor Slides

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Three important theory o constraintsmetrics:Throughput6he rate at which the system generates money through sales

Inventory,nventory represents money tied up in goods and materials

used in a process

'perating e


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Service scheduling often presentschallenges not found in manufacturing6hese are primarily related to)

1. 6he inability to store or inventory services

2. 6he random nature of service re(uestsService scheduling may involve

scheduling)

1. +ustomers

2. orforce3. E(uipment

Instructor Slides

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Scheduling customers: 9emand5anagementAppointment systems

+ontrols customer arrivals for service

eservation systemsEnable service systems to formulate a fairly

accurate estimate demand on the system for agiven time period

Scheduling the wor%orce: /apacity

5anagement /yclical Scheduling

Employees are assigned to wor shifts or time slots$and have days o*$ on a repeating basis

Instructor Slides

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In many environments2 the schedulingre6uirements are airly similar e.g.$ hospitals$ police0re departments$ restaurants$

supermarets

otating schedules Set a scheduling hori'on ,dentify the wor pattern

Develop a basic employee schedule

ssign employees to the schedule

Instructor Slides

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I scheduling is done well:#oods and services can be made or delivered in a

timely manner

9esources can be used to best advantage

+ustomers will be satised

It is important to not overloo% theimportance o scheduling to strategy andcompetitive advantage

Instructor Slides

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OM NOTES CHAPTER 16