PLCs and Fuzzy Logic (9)

GUIDELINES FOR CONTROLLING COMPLEX SYSTEMS

All fuzzy logic applications are different. However, certain design guidelines apply to all fuzzy logic systems. These guidelines provide the basic elements for the successful implementation of a
fuzzy logic control system.

They include:
• control objectives
• control system configuration
• input/output determination
• fuzzy inference engine design

Control Objectives
Fuzzy logic can be applied to virtually any type of control system, but it is especially suited for applications that rely heavily on human intuition and experience. The primary objective of applying fuzzy logic to an existing process is to improve the overall process and to automate tasks that previously required human judgment. In a new system, the primary objective of using fuzzy logic is to implement control that cannot be implemented using standard control methods. A system designer should not use fuzzy logic control just because it is available. Rather, the designer should use it because it will enhance the system. Otherwise, the outcome may not be enhanced; it may just become confusing.
Typical applications of fuzzy logic involve batching systems and temperature control loops, where process control involves “tweaking” the output based on judgments about input conditions. For example, a temperature control loop application typically requires a knowledgeable operator who can regulate the control element based on decisions such as “if the temperature is a little high but all other inputs are OK, then turn the steam valve a little clockwise.” This rationale lends itself to fuzzy logic control.

Control System Configuration
The control objective leads to the selection of the fuzzy logic system configuration. There are several types of fuzzy configurations. Fuzzy logic does not have to be applied only in dedicated fuzzy control applications. It can also be used as a complementary system that supports another more conventional control method, such as a PID control loop. When used in this manner, the system is said to be a conventional fuzzy hybrid control system.

to be continued..................

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