Page 130 - ExtendSim User Guide
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104 Introduction
Application areas
Closed and open systems
Blocks that provide a finite number of resources can be part of closed or open systems. How blocks are connected in the model determines whether the system is considered open or closed.
Closed systems
In a closed system, resources are routed from a resource block and used in the model. Once they are no longer being used, the resources are recycled back to the resource block and become available for further use. For example, assume a technician (the resource) is required to assemble parts of a television. While the technician is assembling the parts, he/she will be busy and will not be available to perform work elsewhere. In a closed system, the technician will return to the technician pool after assembling the parts and will become available for other assignments.
In a partially closed system, only a portion of the resources are returned to the resource block for re-use. For example, consider a case were there are different shifts of laborers (the resource). Suppose three laborers are assigned to a task. Upon completion, the shift for one of the laborers is finished and he does not return to the labor pool to be assigned to a new task.
Open systems
Resource blocks may also be part of open systems when the block’s resources are not recycled. In an open system, resources at the end of the line are not passed back to the Resource block. The most common example of an open system is stock. Normally, stock passes out of the model at the end of the line. Another example of an open system is a consumable resource such as a disposable fixture that makes only one pass through the manufacturing process.
Types of item handling blocks
Each Item library is identified in its dialog as being a residence, passing, or decision type of block, as follows:
• Residenceblocksareabletostoreanitemforsomeamountoftime.Examplesofresi- dence-type blocks are the Queue and Activity.
• Passing blocks must pass the item along before any simulation time elapses. Example blocks include the Set, which sets item properties, and the Equation (I) which performs a calculation as an item passes through.
• Decisionblocksconditionallyallowanitemtopassthrough.ExamplesincludetheGate and Select Item In blocks.
Knowing these categories of blocks and how they relate to the processing of items will help you to build better models. For complete information, see “Block types” on page 314.
Application areas
Simulation is indispensable for understanding, analyzing, and predicting the behavior of com- plex and large-scale systems. It is used to gain an understanding of the functioning of existing systems and to help design new systems by predicting their behavior before they are actually
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