1. World-Views of Simulation
Chapter 2
World-Views of Simulation

2. Chapter objectives Understand discrete event simulation
Be familiar with the theory behind the simulation process
Understand the effective mechanism of discrete event simulation
Explain the characteristics of simulation software tools

3. Chapter Content Chapter Overview System Modeling with DES
Elements of DES
DES Functionality
DES and Animation Mechanism
Monte Carlo Simulation
Continuous Simulation
WITNESS world-views of simulation
Chapter Summary

4. Computer Simulation
A computer program to imitate the operation of a real-world system as it evolves over time
Process simulation, which is used to model production and business processes, is referred to as Discrete Event Simulation (DES).
Discrete event systems are dynamic and potentially complex systems that evolve in time by the occurrence of events at random time intervals in a stochastic manner

5. System Modeling with DES
By utilizing computer capabilities in logical programming, random number generation, fast computations, and animation, DES provides numerous powerful capabilities that facilitate realistic system representation and accurate performance estimation.
DES models the structure, layout, data, logic, and statistics of the real-world system

6. Elements of System Modeling with DES

7. System Structure
System “elements” which model relevant physical components such as resources, materials, flow lines, and infrastructure:
Elements of a manufacturing system may include machines, labor, work pieces, conveyors, etc.
Elements of a business system may include staff, customers, information flow, and operations
The level of details put into model elements is dependent mainly on system nature and modeling objective:
Basic and advanced graphical representations of model elements are used for animation purposes.

8. Examples of structural elements in a plant DES
Modeled structural element
Impacted model performance
Conveyor length
Conveyor capacity
Unit load dimensions
Number of stacked units
Buffer size
Buffer capacity
Length of aisles and walkways
Walking distance and time
Size of AGV
Number of AGV carriers
Length of monorail
Carrier traveling time
Dog spacing of power and free
Power and free throughput
Dimensions of storage units
AS/RS storage and retrieval time

9. System Layout
A system layout is simply the configuration of system structural elements.
The system layout imported from a CAD file shows the modeler where to locate pieces of equipment, operators, pathways, repair units, storage units, loading/unloading docks, and so on.
Interdepartmental distances and flow rates are of particular importance in simulating material handling systems such as forklifts, AGVs, and Conveyors.

10. System Data
DES models of real-world systems are data-driven and pertinent system data is collected and imported into the model to mimic the operation of real-world systems.
Developing a DES model requires the simulation analyst to precisely define what data elements are needed, the method of collecting such data, and how it will be represented and used in the DES model.
Data collection systems (manual or automatic) are typically used to collect data, monitor and control operations, and generate management reports.

11. Data Collected at Various DES Elements
Modeled element
Pertinent simulation data
Machine
Load, cycle, unload times, downtime data
Conveyor
Speed, capacity, type of accumulation
Operator
Walk speed, work sequence, walk path
Buffer
Capacity, discipline, input/output rules
AGV
Speed, acceleration/deceleration, route
Power & free system
Speed, dog spacing, chain length
Part/load
Attributes of size, color, route, mix

12. System Logic
System logic is the set of rules, procedures, and controls that govern the model behavior and determine the flow of entities and the interaction of model elements.
Designing model logic is, however, a relatively difficult and challenging task.
Developing model logic is highly dependent on the programming capability of the used simulation language and the complexity of the underlying system

13. Examples of model logic
System Activity
Model Logic
Parts arriving to loading dock
Sorting and inspecting scheme
Requesting components
Model mix rules
Producing an order
Machine scheduling rules
Material handling
Carrier routing rules
SPC (statistical process control)
Accept/reject/rework rules
Machining a part
Sequence of operation
Forklift floor operation
Driver dispatching rules
AS/RS storage system
AS/RS vehicle movement rules

14. System Statistics
System statistics are means of collecting run time data from the model, aggregating collected values, and summarizing results at the end of simulation.
At simulation end, these statistics are accumulated and generated as simulation outputs to provide a summary of model performance.
Some of these statistics are automatically generated from simulation elements such as resources, buffers, and entities. Others can be user-defined using variables, tallies, and counters.

15. Examples of model Statistics
Measured Value
Jobs produced per hour
System throughput
Percent of machine busy time
Machine utilization
Number of units in system
Work-In-Progress level
Time units spent in system
Manufacturing lead time
Number of defectives
Process quality
Number of machine failures
Maintenance plan
Number of units on a conveyor
Conveyor utilization
Number of units in a buffer
Buffer utilization

16. System Resources
A system resource represents the tool or the means by which model activities are carried out.
Resources consume capital and should be utilized wisely. The utilization, which measures the percentage of resource usage, is optimized through better scheduling and resource allocation.
Key factors that impact the performance of resources include capacity (limit), speed (productivity), and reliability (availability)

17. Examples of system resources
Manufacturing resources
Service resources
Machines and Machine Centers
Doctors
Operators and general labor
Bank tellers
Inspectors and quality controllers
ATM machines
Repairmen and maintenance crew
Drivers
Assembly stations and tools
Operating rooms

18. DES system elements

19. DES Functionality

20. Example: Execution of a car arrival event