1、Chapter 1What is Simulation?Simulation Is.Very broad term,set of problems/approachesGenerally,imitation of a system via computerInvolves a model validity?Dont even aspire to analytic solution Dont get exact results(bad)Allows for complex,realistic models(good)Approximate answer to exact problem is b
2、etter than exact answer to approximate problemConsistently ranked as most useful,powerful of mathematical-modeling approaches2Simulation with Arena Chapter 1 What is Simulation?Some Application AreasManufacturing scheduling,inventoryStaffing personal-service operations Banks,fast food,theme parks,Po
3、st Office,.Distribution and logisticsHealth care emergency,operating roomsComputer systemsTelecommunicationsMilitaryPublic policy Emergency planning Courts,prisons,probation/parole3Simulation with Arena Chapter 1 What is Simulation?SystemsPhysical facility/process,actual or plannedStudy its performa
4、nce Measure Improve Design(if it doesnt exist)Maybe control in real timeSometimes possible to“play”with the systemBut sometimes impossible to do so Doesnt exist Disruptive,expensive4Simulation with Arena Chapter 1 What is Simulation?ModelsAbstraction/simplification of the system used as a proxy for
5、the system itselfCan try wide-ranging ideas in the model Make your mistakes on the computer where they dont count,rather for real where they do countIssue of model validityTwo types of models Physical(iconic)Logical/Mathematical quantitative and logical assumptions,approximations5Simulation with Are
6、na Chapter 1 What is Simulation?What Do You Do with a Logical Model?If model is simple enough,use traditional mathematics(queueing theory,differential equations,linear programming)to get“answers”Nice in the sense that you get“exact”answers to the model But might involve many simplifying assumptions
7、to make the model analytically tractable validity?Many complex systems require complex models for validity simulation needed6Simulation with Arena Chapter 1 What is Simulation?Computer SimulationMethods for studying a wide variety of models of real-world systems Use numerical evaluation on computer
8、Use software to imitate the systems operations and characteristics,often over timeIn practice,is the process of designing and creating computerized model of system and doing numerical computer-based experimentsReal power application to complex systemsSimulation can tolerate complex models7Simulation
9、 with Arena Chapter 1 What is Simulation?PopularityM.S.grads,CWRU O.R.Department(1978)Asked about value after graduation;rankings:1.Statistical analysis,2.Forecasting,3.Systems analysis,4.Information systems5.Simulation137 large firms(1979)1.Statistical analysis(93%used it)2.Simulation(84%)Followed
10、by LP,PERT/CPM,inventory,NLP8Simulation with Arena Chapter 1 What is Simulation?Popularity(contd.)(A)IIE,O.R.division members(1980)First in utility and interest:Simulation But first in familiarity:LP(simulation was second)Longitudinal study of corporate practice(1983,1989,1993)1.Statistical analysis
11、2.SimulationSurvey of such surveys(1989)Consistent heavy use of simulation9Simulation with Arena Chapter 1 What is Simulation?Advantages of SimulationFlexibility to model things as they are(even if messy and complicated)Avoid“looking where the light is”(a morality play):Allows uncertainty,nonstation
12、arity in modeling The only thing thats for sure:nothing is for sure Danger of ignoring system variability Model validityYoure walking along in the dark and see someone on hands and knees searching the ground under a street light.You:“Whats wrong?Can I help you?”Other person:“I dropped my car keys an
13、d cant find them.”You:“Oh,so you dropped them around here,huh?”Other person:“No,I dropped them over there.”(Points into the darkness.)You:“Then why are you looking here?”Other person:“Because this is where the light is.”10Simulation with Arena Chapter 1 What is Simulation?Advantages of Simulation(co
14、ntd.)Advances in computing/cost ratios Estimated that 75%of computing power is used for various kinds of simulations Dedicated machines(e.g.,real-time shop-floor control)Advances in simulation software Far easier to use(GUIs)No longer as restrictive in modeling constructs(hierarchical,down to C)Stat
15、istical design&analysis capabilities11Simulation with Arena Chapter 1 What is Simulation?The Bad NewsDont get exact answers,only approximations,estimates Also true of many other modern methods Can bound errors by machine roundoffGet random output(RIRO)from stochastic simulations Statistical design,a
16、nalysis of simulation experiments Exploit:noise control,replicability,sequential sampling,variance-reduction techniques Catch:“standard”statistical methods seldom work 12Simulation with Arena Chapter 1 What is Simulation?Different Kinds of SimulationStatic vs.Dynamic Does time have a role in the mod
17、el?Continuous-change vs.Discrete-change Can the“state”change continuously or only at discrete points in time?Deterministic vs.Stochastic Is everything for sure or is there uncertainty?Most operational models:Dynamic,Discrete-change,Stochastic13Simulation with Arena Chapter 1 What is Simulation?Estim
18、ate p p (George Louis Leclerc,c.1733)Toss needle of length l onto table with stripes d(l)apartP(needle crosses a line)=Repeat;tally =proportion of times a line is crossedEstimate p p bySimulation by Hand:The Buffon Needle Problem14Simulation with Arena Chapter 1 What is Simulation?Why Toss Needles?B
19、uffon needle problem seems silly now,but it has important simulation features:Experiment to estimate something hard to compute exactly(in 1733)Randomness,so estimate will not be exact;estimate the error in the estimate Replication(the more the better)to reduce error Sequential sampling to control er
20、ror keep tossing until probable error in estimate is“small enough”Variance reduction(Buffon Cross)15Simulation with Arena Chapter 1 What is Simulation?Using Computers to SimulateGeneral-purpose languages(FORTRAN)Tedious,low-level,error-prone But,almost complete flexibilitySupport packages Subroutine
21、s for list processing,bookkeeping,time advance Widely distributed,widely modifiedSpreadsheets Usually static models Financial scenarios,distribution sampling,SQC16Simulation with Arena Chapter 1 What is Simulation?Using Computers to Simulate(contd.)Simulation languages GPSS,SIMSCRIPT,SLAM,SIMAN Popu
22、lar,in wide use today Learning curve for features,effective use,syntaxHigh-level simulators Very easy,graphical interface Domain-restricted(manufacturing,communications)Limited flexibility model validity?17Simulation with Arena Chapter 1 What is Simulation?Where Arena Fits InHierarchical structure M
23、ultiple levels of modeling Can mix different modeling levels together in the same model Often,start high then go lower as neededGet ease-of-use advantage of simulators without sacrificing modeling flexibilityArena TemplateStandard EditionProfessional EditionA singlegraphical userinterfaceconsistent
24、atany level ofmodelingHigherLevel ofModelingLowerSIMAN TemplateVertical SolutionsUser-Created Templates Commonly used constructs Company-specific processes Company-specific templates etc.Application Solution Templates Call$im BP$im etc.Common Panel Many common modeling constructs Very accessible,eas
25、y to use Reasonable flexibilitySupport,Transfer Panels Access to more detailed modeling for greater flexibilityBlocks,Elements Panels All the flexibility of the SIMAN simulation languageUser-Written Visual Basic,C/C+,FORTRANCode The ultimate in flexibility C/C+/FORTRAN requires compiler18Simulation
26、with Arena Chapter 1 What is Simulation?When Simulations are UsedUses of simulation have evolved with hardware,softwareThe early years(1950s-1960s)Very expensive,specialized tool to use Required big computers,special training Mostly in FORTRAN(or even Assembler)Processing cost as high as$1000/hour f
27、or a sub-286 level machine19Simulation with Arena Chapter 1 What is Simulation?When Simulations are Used(contd.)The formative years(1970s-early 1980s)Computers got faster,cheaper Value of simulation more widely recognized Simulation software improved,but they were still languages to be learned,typed
28、,batch processed Often used to clean up“disasters”in auto,aerospace industriesCar plant;heavy demand for certain modelLine underperformingSimulated,problem identifiedBut demand had dried up simulation was too late20Simulation with Arena Chapter 1 What is Simulation?When Simulations are Used(contd.)T
29、he recent past(late 1980s)Microcomputer power Software expanded into GUIs,animation Wider acceptance across more areasTraditional manufacturing applicationsServicesHealth care“Business processes”Still mostly in large firms Often a simulation is part of the“specs”21Simulation with Arena Chapter 1 Wha
30、t is Simulation?When Simulations are Used(contd.)The present Proliferating into smaller firms Becoming a standard tool Being used earlier in design phase Real-time controlThe future Exploiting interoperability of operating systems Specialized“templates”for industries,firms Automated statistical design,analysis22Simulation with Arena Chapter 1 What is Simulation?
