Page 354 - ExtendSim User Guide
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328 Introduction
Terminology and architecture
and the structure of the model. After all the rules for storage capacity and movement have been declared in the model, ExtendSim uses the LP calculation to cause as much flow as possible to move through the system. This calculation is handled automatically and internally. For more information, see the advanced topic “LP technology” on page 448.
Layout of a discrete rate model
A discrete rate model can combine continuous blocks (such as those in the Value library), and discrete event blocks (typically from the Item library), with discrete rate blocks from the Rate library. If you use any discrete rate blocks in a model, the model will require the Executive block (Item library).
Other than the Executive block, you can place the blocks in a model anywhere you want, remembering that ExtendSim evaluates discrete rate blocks along the path of the connections.
☞ Since ExtendSim will always try to maximize the flow, causing the rate of flow to approach infinity in the absence of any constraints, it is important to place upper limits on the flow at strategic locations throughout the model. Otherwise, the flow would move instantaneously from one part of the model to another; this would be a modeling error.
Executive block
The Executive block (Item library) does event scheduling and makes the LP calcu- lation for rate-based models. It must be present in every discrete rate model and it must be placed to the left of all the other blocks in the model.
In addition to the information discussed on page 313, the Executive plays a special
role in discrete rate simulations. The block’s Discrete Rate tab allows you to set
global options for discrete rate models, manage quantity units, and select advanced
options for specific Merge and Diverge modes. Its LP Solver tab has information about the lin- ear program (LP) that provides global oversight for discrete rate models.
The settings in the Discrete Rate tab are explained fully on page 436.
☞ For most purposes you will not need to change the settings in the Executive block.
Connectors and connections
The Rate library provides blocks for simulating rate-based flows. Most of the blocks in the Rate library have flow connectors and value connectors; the Interchange block also has item connectors.:
Connector type Line type Flow
Value
Item
• In a discrete rate model, flow connectors report the effective rate of the flow at each event. The flow moves in one direction, from one block’s flow output (“outflow”) connector to another block’s flow input (“inflow”) connector.
• Value connectors provide information about the quantity of flow and a block’s capacity, as well as information about the effects that the flow has in the model.
Executive
Discrete Rate


































































































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