Manufacturing systems can be characterized by a variety of factors: the number of resources or machines, their characteristics and configuration, the level of automation, the type of material handling system, and so on. The differences in all these characteristics give rise to a large number of different planning and scheduling models. In a manufacturing model, a resource is usually referred to as a “machine”; a task that has to be done on a machine is typically referred to as a “job”. In a production process, a job may be a single operation or a collection of operations that have to be done on various different machines. Before describing the main characteristics of the planning and scheduling problems considered in Part II of this book, we give a brief overview of five classes of manufacturing models.
The first class of models are the project planning and scheduling models. Project planning and scheduling is important whenever a large project,that consists of many stages,h as to be carried out. A project,su ch as the construction of an aircraft carrier or a skyscraper,t ypically consists of a number of activities or jobs that may be subject to precedence constraints. A job that is subject to precedence constraints cannot be started until certain other jobs have been completed. In project scheduling,it is often assumed that there are an unlimited number of machines or resources,so that a job can start as soon as all its predecessors have been completed. The objective is to minimize the completion time of the last job,com monly referred to as the makespan. It is also important to find the set of jobs that determines the makespan,as these jobs are critical and cannot be delayed without delaying the completion of the entire project. Project scheduling models are also important in the planning and scheduling of services. Consider,fo r example, the planning and scheduling of a large consulting project.
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