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Mixing Flow and Items
Controlling flow with items and items with flow
It is common for systems to exhibit a mixture of behaviors, where items from the discrete event arena intermingle with discrete rate processes. In these “mixed mode” cases, a proper understanding of both the Rate and Item libraries and how they can interact with each other is required.
There are two general techniques for integrating discrete event blocks from the Item library with discrete rate blocks from the Rate library:
1) Sendingsignalsandsharinginformationviavalueconnections.Valueconnectionscanbe very useful for triggering some type of action as the system moves from one state to another. For example:
• Value connections can trigger the generation of an item when the level of flow in a Tank reaches a certain level.
• Thevalueofanattributeonanitempassingthroughsomepartofadiscreteratemodel could trigger a change in the constraining rate in a Valve block.
2) MixingitemswithflowusingtheInterchangeblock,asyousawintheDiscreteRatetuto- rial on page 342. The Interchange block (Rate library) provides the ability for items and flow to interface with each other. For example, an empty tanker truck (an item) might arrive at a refinery and fill with gas (flow) at a continuous rate using an Interchange block. Once full, the truck might be routed through a series of Item library blocks until it reached its unloading destination. At that point the item would again interface with an Interchange block to discharge its load.
This chapter focuses on the techniques used when the need for “mixed mode models” arises. It will show how to:
• Control flow using blocks from the Item library • ControlitemswithblocksfromtheRatelibrary • Mix flow with items using the Interchange block
☞ The models for this chapter are located in the folder \Examples\Discrete Rate\Flow and Items.
Controlling flow with items and items with flow
Rate library blocks can control the movement of items and Item library blocks can control the movement of flow. In either case, value connectors communicate state changes to the con- trolling blocks.
Items controlling flow
The Item Controls Flow model shows one of many ways blocks from the Item library can be used to control the movement of flow in a model. It uses one item to open a valve, allowing a tank to fill, then uses the same item to trigger the emptying of that tank.
Discrete Rate

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