8.2: Points Made in the Case Study

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This case study illustrates principle 2 of chapter 1. Modeler defined performance measures are extracted from standard simulation results to help assess system behavior. In this case, the performance measure of interest is the service level to customers, the percent of jobs completed on time. The shop has defined on time as a lead time of 8 hours or less.

This case study illustrates how analytic computations, such as those defined in principle 11 of chapter 1, can be used to set simulation experiment parameter values. The average number of busy machines of each type is determined using Little's law. Cycle time is a function of machine utilization, as seen in the VUT equation. Thus, increasing the number of machines of each type would lower utilization and reduce cycle time. Therefore additional machines may be necessary to achieve an acceptable service level.

The experimental design is sequential. The results of initial simulations are used to set the parameter values for subsequent simulations. Additional machines are added at the bottleneck station identified by initial simulations. Subsequent simulations are run to assess the effect on the service level of the additional machines.

The model adapts to the information that is available about the shop in accordance with principle 2 of chapter 1. Jobs are classified into three types with the arrival rate and distribution known for each type only. Each job within a type will be modeled as having the same route through the shop. Processing times are known only by station, independent of job type. Thus, processing times are modeled as random variables with a large variance.

Simulation results illustrate how relieving one bottleneck in a system can create and expose other bottlenecks in the system. As the number of machines of one type is increased, another type of machine becomes the bottleneck.

The job shop model includes several components. The arrival process for each of the three job types is modeled in the same manor. The operation process for each workstation is modeled in the same way. Routing of jobs is included.