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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$facilities and human activities in an organi'ation
E*ective scheduling can yield+ost savings
,ncreases in productivity
ther benets
Instructor Slides 16-3
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Scheduling is constrained by multiplesystem design decisionsSystem capacity/roduct and0or service designE(uipment selectionorer selection and trainingggregate planning and master scheduling
Instructor Slides 16-4
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Instructor Slides 16-5
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Flow System4igh-volume system in which all obs follow
the same se(uenceFlow system schedulingScheduling for 5ow systems6he goal is to achieve a smooth rate of 5ow of
goods or customers through the system in order toget high utili'ation of labor and e(uipment
Instructor Slides 16-6
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Few fow systems are entirely dedicated toa single product or service Each product change re(uires
Slightly di*erent inputs of parts
Slightly di*erent materialsSlightly di*erent processing re(uirements that must
be scheduled into the line
7eed to avoid excessive inventory buildup
Disruptions may result in less-than-desired output
Instructor Slides 16-7
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The ollowing actors oten dictate thesuccess o high-volume systems:
8 /rocess and product design
8 /reventive maintenance
8 9apid repair when breadowns occur8 ptimal product mixes
8 :inimi'ation of (uality problems
8 9eliability and timing of supplies
Instructor Slides 16-8
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utputs fall between the standardi'ed type ofoutput of high-volume systems and the mae-to-order output of ob shops
utput rates are insu;cient to warrant
continuous production9ather$ it is more economical
to produce intermittentlyor centers periodically
shift from one product to
another
Instructor Slides 16-9
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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
Instructor Slides 16-10
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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
Instructor Slides 16-12
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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
Instructor Slides 16-13
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!oad chart #antt chart that shows the loading and idle
times for a group of machines or list ofdepartments
Instructor Slides 16-14
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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
Instructor Slides 16-15
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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?
$ac%ward scheduling Scheduling bacwards from some due date
"sed when the (uestion is)
>hen is the latest this ob can be started
and still be completed on time?@
Instructor Slides 16-16
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Schedule chart #antt chart that shows the orders or obs in
progress and whether they are on schedule
Instructor Slides 16-17
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Input&'utput (I&') control:anaging wor 5ow and (ueues at wor centersithout ,0 control) ,f demand exceeds processing capacity$ 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
Instructor Slides 16-18
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Instructor Slides 16-19
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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
16-20
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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
16-21
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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
16-23
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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
Instructor Slides 16-24
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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
16-25
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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
Instructor Slides 16-26
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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
16-27
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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
Instructor Slides 16-28
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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
16-29
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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
16-31
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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
Instructor Slides 16-32
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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
16-34
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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
16-35
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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
16-37
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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
16-38
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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
16-39
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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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