Page 98 - ExtendSim User Guide
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Improving the accuracy of the model
Enter Calc overflow for the block’s label.
Close the Equation block’s dialog. ExtendSim dis- plays a message that it is compiling the equation; the message may appear too quickly for you to see it.
Run the simulation.
Note that the results haven’t changed - you’ve simply
substituted the Equation block for the deleted blocks.
For more information, see “Equation-based blocks” on page 695.
Improving the accuracy of the model
Because the contents of the Holding Tank block are
used to calculate the amount of water to remove from
the Holding Tank, the model incorporates feedback. When a model has feedback, the default delta time (dt) of 1.0 is too long and results won’t be accurate. (See “Feedback and delays” on page 90 for more information.)
Choose the command Run > Simulation Setup > Continuous tab and change the Time per step (dt) value to 0.1.
Close the Simulation Setup dialog.
Notice that the Holding Tank is properly already set to integrate its inputs, rather than sum them. Summation only gives the same results as integration when delta time is exactly 1.0, but integration works with any delta time. Learn more at “Integration vs. sum- mation in the Holding Tank block” on
page 709.
When delta time is small, such as for this
model, simulation results will be more realistic
if Lookup Table inputs are interpolated rather than stepped.
In the Lookup Table block (labeled Rainfall), change Output is: to Interpolated. This setting smooths the change between rows in the table.
Final simulation results
Simulation setup dialog

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