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1. A processing system.
2. A material handling and storage system.
3. A computer control system.
Computer control systems and automated equipment are the major differences between FMSs and traditional manufactur- ing systems.
Different authors classify different types of FMS. Browne et al. [8] classified FMSs into four types: flexible machining
cell; flexible machining system; flexible transfer line; and flexible transfer multi-line. This classification was based on process attributes and captures the principal attitudes of system design and operation such as the equipment selection, layout, capacity decisions, and other issues. Later, Stecke and Browne [9] extended the classification scheme to include the type of material handling system as a further descriptor. Their classi- fication scheme was based on the flow pattern of parts through the system and emphasises routeing flexibility.
Kusiak [10] discussed FMS in its broadest sense to include fabrication, machining and assembly, and gave a brief structural taxonomy of FMSs. The author listed five classes of FMSs, namely: flexible manufacturing module, flexible manufacturing cell, flexible manufacturing group, flexible production system, and flexible manufacturing line. The author also showed an approximate graphical relationship between the classes with respect to the number of different parts per system per year and also the annual production rate.
MacCarthy and Liu [11] classified FMSs into four types: a single flexible machine, a flexible manufacturing cell, a multi- machine FMS, and a multi-cell FMS. Then they discussed the relationships and boundaries between these four types of FMS. The approach considered the number of characteristics of the material handling devices as well as the configuration of the processing elements.
Based on the mode of operation, Rachamadugu and Stecke [12] classified FMSs into two levels. The classification of the first level was based on the physical flow and the second level was based on the number of part types.
1.2 Introduction to Scheduling of FMS
Production management and scheduling problems in an FMS are more complicated than in job shops and transfer lines. Several reasons can be advanced to support the above state- ment:
Each machine is versatile and capable of holding different tools to perform different operations. Therefore, different part types can be manufactured at any given time.
In addition to machines, material handling systems such as automated guided vehicles (AGVs), jigs, fixtures, and pallets must also be scheduled. In other words, the number of decision points, where scheduling or operation rules can be varied, is greater in FMSs than in job shops.
There may be a rapid change of demand, or random entering of the new products with high priority.
An operation is capable of being performed on a number of alternative machines with possibly different process times.
Production is continuous even during unexpected events such as breakdown of machines. Because of the large set-up times required for an alternative operation in job shops, if a break- down occurs, production would be interrupted, but in automated manufacturing systems such as FMSs, programmability of the instructions for operations dramatically reduces set-up times.
Generally, when an FMS is being planned, the objective is to design a system that will be most efficient in the production
of the entire range of parts. This cannot be achieved unless all of the following four stages work well: