C ONTROL S YSTEM O R G A N I Z AT I O N

C ONTROL S YSTEM O R G A N I Z AT I O N

With the advent of new, smarter programmable controllers, the decision about the type of control has become a very important consideration. Questions such as, What type of control should I use? are now asked more

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often when automating a process. Knowing process application and future automation requirements will help the user to decide what type of control, and thus PLC, is required. Possible control configurations include individual control, centralized control, and distributed control. Figure 21-9 illustrates these configurations.

PLC

Machine (a) Individual control (a) Individual control

(b) Centralized control (b) Centralized control

(c) Distributed control (c) Distributed control

Figure 21-9. (a) Individual, (b) centralized, and (c) distributed control configurations.

Individual Control. Individual control, or segregated control, is used when a PLC controls a single machine with only local I/O or with local and

a few remote I/O. This type of control does not normally require communi- cation with any other controllers or computers. Individual control is primarily applied to OEM and end-user equipment, such as injection-molding ma- chines, small machine tools, and small, dedicated batching processes. When deciding on this approach, the user should consider whether future intercontroller communication will be desired. If so, the user can choose the appropriate controller for the initial installation to avoid extra design ex- penses at a later date.

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Centralized Control. Centralized control is used when a central PLC controls several machines or processes. This type of control can have many subsystems spread throughout the factory. Each of these subsystems may interface with specific I/O devices that may or may not be related to the same control. A centralized programmable controller communicates only with its subsystems and/or peripherals; it does not exchange data with other PLCs.

The flexibility and potential advantages of a centralized application depend on the PLC used and the system designer’s design philosophy. For example, centralized control can be implemented as the large, individual control of a large process or the centralized control of a number of highly complex, small processes.

One distinct disadvantage of centralized control is that, if the main PLC fails, the whole process stops. Redundant systems can be used to overcome this problem in large, critical, central controls that require a backup. Several manufacturers offer this redundancy option.

Distributed Control. The need to have several main PLCs communicating with each other has brought about distributed control. This type of control employs local area networks (LANs), which allow several PLCs to control different stages or processes locally while constantly interchanging in- formation about the process. Communication among PLCs occurs at very high speeds (up to 1 megabaud) through single coaxial or fiber-optic cables. Despite this powerful configuration, communication between two different manufacturers’ LAN systems can be difficult. Therefore, the user should properly define the process application’s functional requirements from the beginning.