Thursday, November 13, 2008



Scheduling concerns the allocation of limited resources to tasks over time. Bitran [1]explained  “Production scheduling is concerned with the allocation of resources and the sequencing of tasks to produce goods and services. Although allocation and sequencing decisions are closely related, it is very difficult to model mathematically the interaction between them. However, by using a hierarchical approach, the allocation and the sequencing problems can be solved separately. The allocation problem is solved first and its results are supplied as inputs to the sequencing problem. The resource allocation problem can sometimes be solved using aggregate production planning techniques. To specify completely the input to the sequencing problem, the resulting detailed or item plan (also referred to as the master schedule) has to be disaggregated. A breakdown by component parts can be obtained in a straightforward way by using Material Requirements Planning (MRP) systems. Although MRP continues to be popular in practice, many issues still need to be resolved to make it an effective production planning tool.

Because of complexity of production scheduling there are different views of it [2].

Problem Solving Perspective views the scheduling as an optimization problem. It is the formulation of scheduling as a combinatorial optimization problem isolated form the manufacturing planning and control system place.

Decision making Perspective is the view that scheduling is a decision that a human must make. Schedulers perform a variety of tasks and use both formal and informal information to accomplish these. Schedulers must address uncertainty, manage bottlenecks, and anticipate the problems that people cause


Organizational Perspective: is a systems-level view that scheduling is part of the complex flow of information and decision-making that forms the manufacturing planning and control system. Such systems are typically divided into modules that perform different functions such as aggregate planning and material requirements planning


Production scheduling can be classified according to the following criteria [3]:

1. Flow patterns

(a) Flow shop: All the jobs have identical process flows and require the same sequence of


(b) Job shop: Jobs have different process flows, and may require significantly different sequence

of operations.

2. Processing mode

(a) Unit processing: Jobs are processed one by one.

(b) Batch processing: A number of jobs are processed together as a batch.

3. Job release pattern (job release time is the earliest time at which processing can start)

(a) Static: Jobs are (or assumed to be) released to the shop floor at time zero.

(b) Dynamic: Jobs are (or assumed to be) released to the shop floor over time.

4. Work center configuration

(a) Single machine

(b) Identical parallel machines:

(c) Uniform parallel machines:

(d) Unrelated parallel machines:



Barták[4] stated that “the main difference is in the resolution of the resulting plan or schedule. While the industrial planning deals with the task of finding “rough” plans for longer period of time where activities are assigned to departments etc., the industrial scheduling deals with the task of finding detail schedules for individual machines for shorter period of time. From this point of view, scheduling can be seen as a high-resolution short-term planning.”


     Planning and scheduling in industry


Hierarchical Planning


Barták[4]  also defines a new  mixed planning and scheduling approach in his paper.