Chapter 7 Modeling Systems with Entity Movement and Material Handling Constructs
To be able to model constrained entity transfer with resources
To be able to model constrained entity transfer with transporters
To be able to model systems involving conveyors
To be able to model systems involving automatic guided vehicles
To be able to perform basic animation for entity transfer situations
Chapter 4 introduced the concept of unconnected entity movement through the use of the ROUTE and STATION modules. The ROUTE module models entity movement between stations as a simple time delay. The entities can be imagined as “having little feet” that allow them to move from one station to another. That is, the entity is able to move itself. If the entity is a person (e.g. a patient in a clinic), this representation makes sense; however, if the entity is a part in a manufacturing system, this representation begins to breakdown. For example, in the test and repair situation of Chapter 4, the parts moved between stations with a time delay. But how did they physically move? One way to think about this is that there were workers always available to move the parts between the stations. If there is always a worker available, then it is as if there is an infinite supply of workers. Thus, whenever a part must be moved from one station to another the part uses one of the workers to make the movement. Since there is an infinite supply of workers, this is the same as the part moving itself (i.e. having little feet) and only the time delay for moving between the stations is relevant.
In many situations, modeling transfers with a delay is perfectly reasonable, especially if you are not interested in how the entities moved (only that they moved). However, in many situations, the movement of entities can become constrained by the lack of availability of the transfer mechanism. For example, the movement of parts may require that the parts be placed on a pallet and that a fork lift be used to move the parts. At any point in time, there may not be enough fork lifts available and thus the parts may have to wait for a fork lift to become available. When the potential for waiting for transport is significant, you might want to model with more detail the “how” behind the entity transfer. In addition, since movement can be a significant part of an operation, the design of the material movement system may be the main focus of the modeling effort.
This chapter explores the various constructs available within to facilitate the modeling of the physical movement of entities between stations. The chapter begins by describing how to model transfers using resources. In this case, the transfer delay is accompanied by the use of a resource. Then, Section 7.2 presents how facilitates resource constrained movement using the TRANSPORTER module and its accompanying constructs. A transporter is a special kind of resource in that can move. Since not all movement is as freely moving through space as people walking or fork trucks moving, provides constructs for modeling entity movement when the space between the locations becomes an important aspect of the modeling. Section 7.3 indicates how conveyors can be represented within and how they represent the space between stations. Then, in Section 7.4, the modeling of transporters will be revisited to understand how to model the situation where the transporters may compete for space while moving. This will involve the modeling of the space between stations as a fixed path (i.e. like a road network with intersections, etc.) As usual, these concepts will be illustrated with example models.