Page 149 - ExtendSim User Guide
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As seen in the dialog of the Create block, items arrive exponentially. Notice that the exponen- tial distribution has a “Mean” parameter. Connecting the output of the Lookup Table block to the Mean input connector of the Create block causes the mean of the distribution to come from the Lookup Table during the simulation run, overriding any entry in the dialog. This dynami- cally changes the average interarrival time.
☞ The distribution determines the interarrival time. A smaller mean value indicates that there is less time between arrivals and items arrive more frequently.
In the Lookup Table’s dialog, the mean is smallest from hour 10 until hour 12, causing items to arrive more frequently during that period.
Choosing time units for the columns
When the block is set to “Lookup the: time”, the Lookup Table block looks at the current simulation time and outputs a corresponding value. Its table is used to determine the value that is output (by default the Output column; in this model, the Mean column) at a given simulation time (by default the Time column; in this model the Hour column). Its time units popup menu (in this model “hours”) rep- resents the unit of time for the values in the Time/ Hour column. However, that does not control what the output of the Output/Mean column represents.
☞ The time unit for the Lookup Table block’s output column is determined by the block that receives its output values.
Scheduling interarrival time
Items, Properties, and Values 123
Item generation
Specifying the dynamic parameters
When a value passed to a block’s input connector is
used to set a time parameter (as in this case), the value sent must be defined in the time unit that the receiving block expects. In this model, the values from the Mean column are sent to the Create block and are used to calculate the average time between arrivals. Since the Create block is using minutes as its local time unit, the values in the Mean column of the Lookup Table block also represent minutes. For instance, the value of 6 in the Mean column represents an average of 6 minutes between arrivals, even though the event time in the Hour column is in hours.
☞ Do not set the mean of a distribution to 0. The Create block will warn you if you make this modeling error.
Making sure the arrival occurs when expected
To avoid unexpected results, it is important to understand what happens in the Create block when you vary the mean of the arrival intervals over time. The block’s default behavior is to generate an arrival time, called “nextTime”, for the next item based on the current input param- eters. When simulation time reaches nextTime, the Create block releases an item and generates a new nextTime based on the current values of the input parameters. For the period of time between releasing items, the Create block will not react to changes in the input parameters. If
Discrete Event

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