510 IE BASIC SIMULATION TERMINOLOGY
PROMODEL BASICS
A system is an organized group of entities such as people, equipment, methods and parts, which work together toward a specific objective. A simulation model characterizes a system by mathematically describing the responses that can result from the interaction of entities. A system state is a collection of variables, stochastic (can change randomly) and deterministic (not influenced by probability), which contain all the information necessary to describe a system at any point of time. A static model is one, which is not influenced by time. There is no simulation clock involved. The state of the model does not change with respect to time. A simulation model that simulates the roll of a die is an example of static model. The output of the model is not affected by time. A dynamic model is representation, which is influenced by time. The state of the model evolves over simulated seconds, hours, days and months. Manufacturing and many service systems are generally modeled using a dynamic approach. A warm-up period is the amount of time that the model needs to run to remove the initialization bias before statistical data collection begins. The length of the period is dependent upon the type of model being used. Warm-up periods for steady state simulation can be found through experimentation with moving averages and other techniques. A model run involves operating a simulation for a specified period of time with a unique set of random values. The simulation run length is the amount of time simulated during the model run. An independent model replication (or simply "replication") entails operating the same model for the same period of time with one or more random seed values. Multiple model replications are essential when analyzing results. MANUFACTURING TERMINOLOGY In a Pull System everything is focused on the next stage of production and what is needed there (Customer Order Driven). That what is needed in the next stage of production is produced. The explanation of the name Pull System is: Raw materials and parts are pulled from the back of the factory towards the front where they become finished goods. So in a Pull system the ideal of producing at the same rate as customers are using the products is being realized. In a Push System the emphasis lies on using information about customers, suppliers and production to manage material flow. The name push system from results the way the system works:
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IE 510
PROMODEL BASICS
Materials and parts are made and after that they are send to the place where they are needed next (which is another stage in production or inventory), thus the system is pushing material through production. This pushing however, is done according to schedule. The push system is relying heavily on the accuracy of the schedules, which come from the MPS. These schedules, in turn, depends on the accuracy of information about the customers demand and lead times PROMODEL TERMINOLOGY Entities are items processed through the system. They may represent parts, people, and paperwork or almost anything that require system resources. Each entity may have different graphic icons to represent it in different states of processing or at various points in the system. Locations are fixed points through which entities move. These points may be anywhere that an entity is processed. They can also be queuing areas, storage locations or conveyors. Capacity, defined as the number of entities that can be at a location at one time. Processing logic defines the operations performed and onward routing for each entity type at each location in the system. Processing is defined for each type of quantity at each location where it either undergoes some action or simply awaits access to a subsequent location. Arrivals are introduction of entities into the system. An arrival location is specified, as is a quantity or batch size for each entity type. Both interarrival time and quantity of arrivals can be defined as a probability distribution. Resources are an individual person, piece of equipment or transporter that is required to perform some action, but is not a location as described above. In many cases a resource will be shared among several locations and must move along a pre-determined path network. Shifts are schedules that may be assigned to resources and locations. Hours of work are defined graphically and any breaks in the shift can also be specified. The model builder as a counter, a logic switch or for any one or more additional purposes can define variables. Variables are global to the model Attribute is very similar to the variable, but is unique to a specific location or entity. Attributes are used to capture information as an entity travels through the model Arrays is a matrix of values that may be referenced as a time saving feature in model construction.
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