ISA Certified Automation Professional (CAP) program

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Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.

CAP question

Which of the following statements about fuzzy logic controllers is true?

A. The rules for the fuzzy logic replacement for a proportional-integral (PI) controller have two antecedents and two consequents.
B. If-then statements are developed as backup rules in case of system failure.
C. A fuzzy logic controller is tuned by adjusting the scale factors.
D. A fuzzy logic controller cannot replace a proportional-integral-derivative (PID) controller unless the fuzzy controller is linear.

CAP Answer

The correct answer is C, “A fuzzy logic controller is tuned by adjusting the scale factors.”

A PI controller works to keep an output from the process, termed the controlled variable (CV), at a desired operating point, called the set point (SP), by adjusting an input to the process, known as the manipulated variable (MV). The control error (E) is the controlled variable minus the set point.

The CV, SP, and E in a PI control algorithm are converted to a percent of measurement scale, and the MV is the percent of the scale of whatever is manipulated, which could be a valve, speed, or set point.

In a fuzzy logic algorithm, these variables are converted to a fractional value from –1 to +1 based on scale factors that the user must enter for each variable. A PI controller is tuned by adjusting the gain or proportional band and integral time settings. A fuzzy logic controller is tuned by adjusting the scale factors.

Reference: Trevathan, Vernon L., A Guide to the Automation Body of Knowledge, Second Edition, ISA, 2006.


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