Always distinguish clearly between a SIMUL8® Label (what some other software, simulation textbooks, and technical articles call an “attribute”) and a SIMUL8® Information Store (what some other software, simulation textbooks, and technical articles call a “variable”). A Label belongs to an Entity (Work Item); each Work Item has its own copy of the Label. For example, if refrigerators are being manufactured, and some have “freezer on top” and some have “freezer on bottom,” a Label may be used to distinguish the Work Items. An Information Store belongs to the model as a whole, and has only one value. For example, the number of refrigerators with “freezer on top” produced so far during the model run would be implemented as an Information Store.
Although changeover time and downtime are both non-value-added time, they should be modeled separately and differently. Downtime is related to quality and durability of equipment, and to the care with which the equipment is maintained. Changeover time is related to lot sizes – larger lot sizes reduce changeover time but increase inventory problems (overstock of model A and stockout of model B). Smaller lot sizes reverse this tradeoff, and simulation modeling and analysis can be highly effective at finding the “sweet spot” for lot size. In SIMUL8®, Activity Properties/Efficiency is the route to downtime specification, whereas Activity Properties/Routing In/Change Over is the route to changeover specification.
Continuing, consider the example of the two models of refrigerators. If a Label “Freezer” is used to specify the location of the freezer, and a machine needs changeover time whenever the incoming refrigerator has “freezer on bottom” whereas the previous refrigerator cycling through the machine had “freezer on top” (or vice versa), at Activity Properties/Routing In/Change Over, the modeler will click the When Label Changes option, click Detail and select “Freezer” from the list of Labels.
SIMUL8® can also distinguish between downtime (characteristically unplanned and occurring stochastically) and scheduled maintenance (planned for specific times). Clicking on the Data and Rules toolbar, then Scheduled Maintenance, allows specification of scheduled maintenance (start time, end time, repetition interval, and Resources (e.g., workers, special equipment) required. Only one Resource can be thus specified as required. However, Visual Logic can be set to run when the scheduled maintenance begins and when it ends; that Visual Logic could be used to require and free additional Resources. A seemingly open issue: If an Activity (e.g., a machine) is down when scheduled maintenance is to begin, how does SIMUL8® handle the situation?
The Resource Matrix is accessible from the Data and Rules toolbar. It appears as a matrix with Activities as the rows and Resources as the column. The modeler can then specify, for example, that the Xray Activity requires one Technician. The modeler is allowed to enter, for example, “1-3” in this matrix – either 1 or 2 Technicians will be required. Open question raised with SIMUL8® — if the modeler enters “1-3”, will 1, 2, or 3 Technicians be required, with probability ⅓ for each possibility? Or some other method of choosing?