Page 351 - ExtendSim User Guide

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Introduction 325
Simulating discrete rate systems
• Mining
• WaterTreatment
• Pharmaceutical
• Metallurgy
• Electric power transmissions
• Any other industry that processes commodities in bulk or batches
• To model “things” that are so numerous that it would be inconvenient or overwhelming to model them individually:
• Food and beverages (tea bags, cereals, soda cans, cheese)
• Drugs,cosmetics,andbiotech(pills,bottlesoflotion)
• Milling (carpet and paper)
• Data storage and manipulation (samples, messages, packets)
• Any other industry that mixes, fills, or packages products on high-volume or high- speed lines
Thus discrete rate models simulate flows that are either homogeneous (identical goods that are the same throughout and do not vary in essential characteristics) or heterogeneous (numerous items that are clearly distinct, but cannot be easily sorted or separated).
☞ As will be seen in the Discrete Rate Tutorial, it is common for discrete rate models to also include portions that are discrete event processes.
Simulating discrete rate systems
Discrete rate modeling takes a very different approach compared to continuous or discrete event modeling.
Comparison to discrete event and continuous modeling
The Value library blocks in continuous models and the Item library blocks in discrete event models act individually and independently to calculate values or move items. They may send messages and communicate with each other, but there is no overall global connection between the blocks in those types of models.
By comparison, Rate library blocks are dependent on each other and have an effect on one another. Discrete rate models are divided into areas where the included blocks are not indepen- dent but instead are part of a global system. The blocks within each area communicate through an internal linear program (LP) that provides the global oversight for that area. Each block in an LP area contributes a part of the LP equation for the area; the result of the LP calculation is the effective rate for that part of the model. This system is optimized such that, if a particular area does not need to recalculate, it won’t.
Another major difference is how items move in a discrete event model compared to how flow moves in a discrete rate model.
• Inadiscreteeventmodel,itemsmovefromoneblocktoanotherinstantaneously.AnItem library block might hold an item for some simulated period of time, but there is no constraint on the movement of items between blocks and that movement is instantaneous.
• By contrast, the movement of flow in a discrete rate model must take some time. In the absence of any constraints, the rate of flow would approach infinity and the flow would
Discrete Rate
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