Manufacturing process plans are usually defined by the sequence of operations a job has to go through to transform raw materials to a finished product. Restricting process plans are a sequence of operations that often over constrain the process plan beyond what the processing technology would require. If the strict ordering format in which operations are prescribed in conventional process plans are relaxed, then it can result in substantial reduction of lead time in manufacturing. Machine scheduling can take advantage of this fact by optimizing the job schedule and the order of operations in each job simultaneously. Although there is a strong relation between process planning and scheduling, conventionally the two functions have been studied independently (Lee and Kim, 2001).
In the most current Advanced Planning and Scheduling (APS) systems, the planning and scheduling components are implemented separately in different modules. The planning module is responsible for preparing plans, i.e., the sequences of activities to satisfy orders. The scheduling module schedules these activities, i.e., it assigns the activities to the resources and it determines the exact start and end times of the activities as well as other parameters of the activities. If the scheduler finds that it is not possible to schedule all the activities, then it backtracks to the planner to find another plan. Thus, the process plan and schedule are prepared for a static production environment with readily available manufacturing resources. This is an oversimplification, as manufacturing is dynamic in nature. Once a part is loaded to the manufacturing environment, it is inevitably subjected to unexpected production disturbances such as machine breakdowns, rush order arrivals and order cancellations. A process plan may become inefficient or infeasible due to changes (Wong et al., 2006). To cope with the unexpected disturbances, it is necessary to revise the process plan and schedule dynamically. |
