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Resources and Shifts 233
The Shift block
ing repeated shift patterns, e.g., an eight-hour workday each day of the week or breaks that occur every four hours.
☞ It would be quite easy to define a complex shift schedule that includes all breaks, holidays, weekends, and so forth. However, before adding such complexity to a model, carefully con- sider whether such detail adds to the validity of the model. If, for example, nothing at all hap- pens during the weekend, a better solution would be to simply assume one week is 5 days long (or specify that in the Simulation Setup dialog) rather than adding a Shift block to model the weekends. Shifts should only be used when they will make a significant difference in the results of the simulation.
Shift types and what they control
A Shift can schedule capacity in a number of Item and Rate library blocks. For example, it can turn an Activity on or off, or specify a maximum number of items the block can process at a time. The schedule depends on whether the selected Shift type is On/Off or Number.
• AnOn/Offtypeofshiftactslikeabinaryswitchthatturnsassociatedblocksonoroffatspe- cific points in time. For example, an On/Off shift might be used to shut down an Activity block during lunch and evening hours, to open or close a Gate block, or to turn a Valve (Rate library) on and off according to a schedule.
• The Number shift type explicitly defines the size of a block’s capacity over time. For instance, it might set the size of a Resource Pool to 3 for the morning shift, 0 over lunch, 5 for the afternoon shift, and 0 overnight.
The following table shows which Item library blocks can be controlled by a Shift block, which types of shifts those blocks support, and what aspect of a block, if any, the Number type of shift controls:
Item Blocks That Can Use Shifts
Activity
Convey Items Create
Gate
Resource Item Resource Pool
Transport Workstation
Shift Type: On/ Off or Number
Both
On/Off On/Off Both
Both Both
Both Both
Number Type Setting on Shift Controls This Aspect of Item Library Block
Maximum number of items in the Activity at a time.
N/A N/A
In “area gating” mode, the number of items allowed in the gated area at a time. In “schedule gating with Shift”, the Gate is always open when the Shift value is >0.
Total number of resources available, per the TR (Total resources) connector.
The total count of resources available, per the TR (Total resources) connector.
Block capacity.
Maximum number of items in process.
In the Rate library, the Convey Flow, Interchange, Tanker, and Valve blocks can use a Shift set to On/Off type. See the Rate module for more information about using the Rate library.
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