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: