Page 44 - Continuous Process Modeling Quick Start Guide
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Continuous Process Quick Start Guide
Simulation order
• You would generally use the integrated (delay) choice when there is only one integrating block in the model, when the integrating blocks are not interdependent or cross-coupled, or when there is no feedback.
• Inmodelswithmorethanoneintegratingblock,wheretheintegratingblocksareinterdepen- dent or cross-coupled, the feedback between the blocks is usually a correction factor. If this feedback is delayed, the system may correct too late or over correct, causing the model results to become unstable. This is often observed as a graph where the traces oscillate with increasing magnitude as time progresses. The integrated (no delay) choice compensates for the feedback delays by outputting results one step earlier. For example, the Predator_Prey model is an example of interdependent Holding Tank blocks; the model is located in the folder Examples/Continuous/Standard Block Models.
Simulation order
The Run > Simulation Setup > Continuous tab allows you to choose the order in which ExtendSim executes block data for continuous models. The choices are Flow order (the default), Left to right, and Custom. To see the order in which blocks are executing, select the command Model > Show Simulation Order before the model is run.
☞ It would be unusual to change the simulation order from the default choice, Flow order.
Flow order
During a simulation, the blocks that compose an ExtendSim model perform calculations that generally depend on their inputs. After doing their calculations, the blocks set their output con- nectors to the results of that calculation so that other blocks may use their results.
In this type of system, there has to be a “first” block: a block that calculates before all of the others that depend on its results. After the first block calculates, the other blocks should calcu- late in the order and direction of their connections. This order is repeated for every time step of the simulation. To see this order, choose Model > Show Simulation Order.
The following are the rules that ExtendSim uses to derive the order of the block calculations in continuous models:
• Blocksthatgenerateinputstothesimulationgofirst.Forexample,LookupTableorCon- stant blocks with only their outputs connected to inputs of other blocks would be put first.
• Next, ExtendSim executes blocks that are connected to those first blocks, in the order and direction of their connections.
• Unconnected blocks and bi-directional network blocks (that have only inputs connected to inputs) are executed in left-to-right order.
☞ The Feedback block (Utilities library) is useful when there are flow-order issues due to feed- back in a continuous model.
Left to right order
‘Use caution when changing from Flow order.
If you choose this option, ExtendSim looks at the left/top corner of each block on the work- sheet. The left-most block gets executed first, and the next left-most block gets executed sec- ond, and so on. Blocks with equal left edges get executed in top to bottom order. If your model flows to the right, and then continues at the left below that flow, this choice will still calculate

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