Introduction to simulation.pdf
- 1. Chapter 1 Introduction to Simulation Banks, Carson, Nelson & Nicol Discrete-Event System Simulation
- 2. 2 Outline „ When Simulation Is the Appropriate Tool „ When Simulation Is Not Appropriate „ Advantages and Disadvantages of Simulation „ Areas of Application „ Systems and System Environment „ Components of a System „ Discrete and Continuous Systems „ Model of a System „ Types of Models „ Discrete-Event System Simulation „ Steps in a Simulation Study
- 3. 3 Definition „ A simulation is the imitation of the operation of real-world process or system over time. Generation of artificial history and observation of that observation history „ A model construct a conceptual framework that describes a system „ The behavior of a system that evolves over time is studied by developing a simulation model. „ The model takes a set of expressed assumptions: Mathematical, logical Symbolic relationship between the entities
- 4. 4 Goal of modeling and simulation „ A model can be used to investigate a wide verity of “what if” questions about real-world system. Potential changes to the system can be simulated and predicate their impact on the system. Find adequate parameters before implementation „ So simulation can be used as Analysis tool for predicating the effect of changes Design tool to predicate the performance of new system „ It is better to do simulation before Implementation.
- 5. 5 How a model can be developed? „ Mathematical Methods Probability theory, algebraic method ,… Their results are accurate They have a few Number of parameters It is impossible for complex systems „ Numerical computer-based simulation It is simple It is useful for complex system
- 6. 6 When Simulation Is the Appropriate Tool „ Simulation enable the study of internal interaction of a subsystem with complex system „ Informational, organizational and environmental changes can be simulated and find their effects „ A simulation model help us to gain knowledge about improvement of system „ Finding important input parameters with changing simulation inputs „ Simulation can be used with new design and policies before implementation „ Simulating different capabilities for a machine can help determine the requirement „ Simulation models designed for training make learning possible without the cost disruption „ A plan can be visualized with animated simulation „ The modern system (factory, wafer fabrication plant, service organization) is too complex that its internal interaction can be treated only by simulation
- 7. 7 When Simulation Is Not Appropriate „ When the problem can be solved by common sense. „ When the problem can be solved analytically. „ If it is easier to perform direct experiments. „ If cost exceed savings. „ If resource or time are not available. „ If system behavior is too complex. Like human behavior
- 8. 8 Advantages and disadvantages of simulation „ In contrast to optimization models, simulation models are “run” rather than solved. Given as a set of inputs and model characteristics the model is run and the simulated behavior is observed
- 9. 9 Advantages of simulation „ New policies, operating procedures, information flows and son on can be explored without disrupting ongoing operation of the real system. „ New hardware designs, physical layouts, transportation systems and … can be tested without committing resources for their acquisition. „ Time can be compressed or expanded to allow for a speed-up or slow-down of the phenomenon( clock is self-control). „ Insight can be obtained about interaction of variables and important variables to the performance. „ Bottleneck analysis can be performed to discover where work in process, the system is delayed. „ A simulation study can help in understanding how the system operates. „ “What if” questions can be answered.
- 10. 10 Disadvantages of simulation „ Model building requires special training. Vendors of simulation software have been actively developing packages that contain models that only need input (templates). „ Simulation results can be difficult to interpret. „ Simulation modeling and analysis can be time consuming and expensive. Many simulation software have output-analysis.
- 11. 11 Areas of application „ Manufacturing Applications „ Semiconductor Manufacturing „ Construction Engineering and project management „ Military application „ Logistics, Supply chain and distribution application „ Transportation modes and Traffic „ Business Process Simulation „ Health Care „ Automated Material Handling System (AMHS) Test beds for functional testing of control-system software „ Risk analysis Insurance, portfolio,... „ Computer Simulation CPU, Memory,… „ Network simulation Internet backbone, LAN (Switch/Router), Wireless, PSTN (call center),...
- 12. 12 Systems and System Environment „ A system is defined as a groups of objects that are joined together in some regular interaction toward the accomplishment of some purpose. An automobile factory: Machines, components parts and workers operate jointly along assembly line „ A system is often affected by changes occurring outside the system: system environment. Factory : Arrival orders „ Effect of supply on demand : relationship between factory output and arrival (activity of system) Banks : arrival of customers
- 13. 13 Components of system „ Entity An object of interest in the system : Machines in factory „ Attribute The property of an entity : speed, capacity „ Activity A time period of specified length :welding, stamping „ State A collection of variables that describe the system in any time : status of machine (busy, idle, down,…) „ Event A instantaneous occurrence that might change the state of the system: breakdown „ Endogenous Activities and events occurring with the system „ Exogenous Activities and events occurring with the environment
- 14. 14 Discrete and Continues Systems „ A discrete system is one in which the state variables change only at a discrete set of points in time : Bank example
- 15. 15 Discrete and Continues Systems (cont.) „ A continues system is one in which the state variables change continuously over time: Head of water behind the dam
- 16. 16 Model of a System „ To study the system it is sometimes possible to experiments with system „ This is not always possible (bank, factory,…) „ A new system may not yet exist „ Model: construct a conceptual framework that describes a system It is necessary to consider those accepts of systems that affect the problem under investigation (unnecessary details must remove)
- 18. 18 Characterizing a Simulation Model Characterizing a Simulation Model „ Deterministic or Stochastic Does the model contain stochastic components? Randomness is easy to add to a DES „ Static or Dynamic Is time a significant variable? „ Continuous or Discrete Does the system state evolve continuously or only at discrete points in time? Continuous: classical mechanics Discrete: queuing, inventory, machine shop models
- 19. 19 Discrete-Event Simulation Model „ Stochastic: some state variables are random „ Dynamic: time evolution is important „ Discrete-Event: significant changes occur at discrete time instances
- 21. 21 DES Model Development DES Model Development How to develop a model: 1) Determine the goals and objectives 2) Build a conceptual model 3) Convert into a specification model 4) Convert into a computational model 5) Verify 6) Validate Typically an iterative process
- 22. 22 Three Model Levels Three Model Levels „ Conceptual Very high level How comprehensive should the model be? What are the state variables, which are dynamic, and which are important? „ Specification On paper May involve equations, pseudocode, etc. How will the model receive input? „ Computational A computer program General-purpose PL or simulation language?
- 23. 23 Verification vs. Validation Verification vs. Validation „ Verification Computational model should be consistent with specification model Did we build the model right? „ Validation Computational model should be consistent with the system being analyzed Did we build the right model? Can an expert distinguish simulation output from system output? „ Interactive graphics can prove valuable