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Rates, Constraints, and Movement
Defining a critical constraint
Polling constraints
The checkbox to Poll constraint every... can be used when a Valve’s maximum rate is con- figured to change dynamically. This option directs the Valve to request a new maximum rate value at fixed intervals during the run. This causes the Valve to periodically query the block connected to its R (maximum rate) input connector or the cell linked to its maximum rate field for the new values. Any values received between the queries will be ignored.
This checkbox is optional when the maximum rate field is connected to a fixed number in a linked cell in a data source. It is required if the linked cell contains a random number or if the R input connector is connected to a passive block like the Random Number (Value library), since a passive block won’t independently generate a new value for the maximum on its own. (The checkbox is not needed if the R connector is connected to block that actively generates values, such as the Lookup Table block set to output values at regular time intervals in the dis- crete rate tutorial.)
☞ Each time the maximum rate in a Valve changes, effective rates must be re-calculated across multiple sections. If you are using the poll constraint feature in several Valves, consider having them update at the same time. This will dramatically reduce the number of recalculations.
☞ While the polling feature can be handy during the early stages of the model building process, flow rates in real world systems rarely change at fixed intervals. Use this feature judiciously and with caution.
Controlling how and when the Valve applies its maximum rate
The Valve’s Control Flow tab has advanced options that allow you to manage how and when that block applies its maximum rate. By setting a goal or using hysteresis, you can explicitly control when the Valve’s constraining rate will be observed, when it will be ignored, and for how long either of those will happen. These topics are discussed in the “Delaying Flow” chap- ter.
Tank and Interchange
The Tank and Interchange blocks have dialog options for explicitly defining their maximum inflow and outflow rates. Unlike the maximum rate in a Valve, these constraints do not change dynamically during the simulation.
Default maximum rates for Tank
You can enter either a maximum inflow rate or a maximum outflow rate, or both of these.
Discrete Rate


































































































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