EXAMPLE: SETS VERSION OF THE GEAR MANUFACTURING JOB SHOP MODEL

13.6 EXAMPLE: SETS VERSION OF THE GEAR MANUFACTURING JOB SHOP MODEL

This section exemplifies the use of Arena sets via a more advanced version of the previous job shop example, by taking advantage of the repetitive structure of the model to reduce its size (number of modules). To wit, note that the model logic for each

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Figure 13.64 Arena model with sets for the gear manufacturing job shop example.

individual operation in Figure 13.48 is essentially the same. Consequently, it is possible in this model to create sets of queues and resources, effectively vectorizing the entire model. The resultant (smaller) vectorized model is depicted in Figure 13.64.

The logic until a job is transferred to the first operation location (first row of modules in Figure 13.64) and the logic of shop exit (last row of modules in Figure

13.64) are not part of the repetitive pattern, and therefore they carry over unchanged from Figure 13.48. However, the middle set of modules in Figure 13.64 has been redefined to incorporate sets, and is considerably smaller in size than its counterpart in Figure 13.48.

Observe that the logic of each operation location in Figure 13.48 consists of freeing

a truck, processing, requesting a truck, and transporting to the next operation location. The middle part in Figure 13.64 captures the logic of all operation locations using the concepts of set and sequence. In effect, a job traverses (iterates) the middle set of modules multiple times according to its operations sequence. In each iteration, the middle set of blocks plays the role of the current operation location in the sequence, as explained later.

The first module in each iteration is the Station module, called Machining Stations, whose dialog box is displayed in Figure 13.65. The dialog box specifies a station set, called Process Stations, whose members consist of all operation locations (Milling

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Figure 13.65 Dialog box of the Station module Machining Stations.

Station, Drilling Station, Paint Shop, and Polishing Area). Recall that as soon as a gear entity enters the Transport module, called Transport to Shop Floor, the gear entity is assigned the ID of the destination operation location, according to its operations sequence as specified in the Sequence module spreadsheet (see Figure 13.51). On arrival at the Machining Stations module, the destination ID is saved in the incoming

, gear entity s attribute Entity.Station for use in the current iteration.

Next, the gear entity proceeds to free its truck in module Free Truck, and then enters the Seize module, called Seize Process. Figure 13.66 displays the dialog box of this module (bottom) and the details of the machine resource to be seized (top). The Seize Process module plays the role of the Seize module of each workstation resource (one particular resource per iteration). On entry of a gear entity into this module, it selects a specific member of the resource set called Machines, based on its Entity.Station attribute. The Machines set consists of the resources Milling Machine, Drilling Machine, Paint Booth, and Polishing Worker. Note that the incoming job entity is queued in a corresponding member of the queue set called Machine Queues, again based on its Entity.Station attribute. The actual operation takes place in the Delay module, called Processing, whose dialog box is displayed in Figure 13.67.

Recall that in the previous version (without sets), individual operation times were assigned in the Sequence module spreadsheet and associated dialog boxes of Figure

13.51, and stored in separate gear entity s attributes (Milling Time, Drilling Time, Spray Time, and Polishing Time). In the current version (with sets), however, all these attributes are replaced by a single attribute, called Processing Time, of the incoming gear entity that is assigned the processing time of the current operation.

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Figure 13.66 Dialog box of the Seize module Seize Process (bottom) and the Resources dialog box of the seized machine resource (top).

Figure 13.67 Dialog box of the Delay module Processing.

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Figure 13.68 Dialog box of the Release module Release Process (bottom) and the Resources dialog box of the released machine resource (top).

Once the current operation is completed, the gear entity proceeds to the module, called Release Process, where it releases the specific resource it was holding. Figure

13.68 displayed the dialog box of this module and the released resource details. The gear entity next proceeds to the Request module, called Request Truck, to be transported to the next destination in its operations sequence via the Transport module, called Transport to Next Station, precisely as in the previous model (see Figures 13.52 and 13.53). Finally, when all operations are completed, the gear entity proceeds to the Station module, called Shop Exit, as was done in the previous model.

The current model with sets adds three main elements to the previous model. First, the Stations element from the Elements template panel is used to enforce an explicit numbering of stations (operation locations), as shown in its dialog box depicted in Figure 13.69. Using the Stations Element dialog box to ensure a preferred numbering is recommended. This in turn ensures the correct referencing of set members (e.g., queues and resources) in model logic (e.g., the aforementioned iteration in the middle modules of Figure 13.64).

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Figure 13.69 The Stations Element dialog box.

Figure 13.70 Dialog spreadsheet of the Set module (bottom) and the Members dialog spreadsheet for the Machines set (top).

Second, the Set module spreadsheet from the Basic Process template panel now includes the Machines resource set in addition to the Flow Times set. Figure 13.70 displays this spreadsheet (bottom) and the members of the Machines resource set (top).

Lastly, the current model also includes a new Advanced Set module spreadsheet from the Advanced Process template. Figure 13.71 displays this spreadsheet and the members of its sets.

Of course, running the gear manufacturing job shop model with sets produces identical results to those of its counterpart without sets. However, models with sets are more parsimonious in that they reduce the number of modules as evidenced by

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Figure 13.71 Dialog spreadsheet of the Advanced Set module (bottom) and the Members dialog spreadsheet (top).

Figure 13.48 versus Figure 13.64. Finally, using sets renders models more intelligible and easier to modify.