PURPOSE:To obtain a control output equivalent to normal PI control by executing control operation by inference method based upon a rule extended by an arithmetic control means. CONSTITUTION:A deviation arithmetic means 13 subtracts a controlled variable PV obtained by a detector 12 from a set value SV to determore a deviation (e) and sends the deviation (e) to a deviation change arithmetic means 14 and a fuzzy operation control part 15. The means 14 finds out a deviation change e from the preceding and current deviations and then sends the obtained value e similarly to the control part 15. In the control part 15, an operator sets up I/O ranges such as the deviation (e), the deviation change e and an operation output change MV, the language label (the name of a fuzzy set) of the deviation and its repersentative point, and the language label of the deviation change e and its representative point in a membership function forming means 151. The means 151 forms a membership function. A rule expanding means 152 determines an idle rule and an arithmetic control means 153 finds out a control output from a weighted average value by means of the inference method.
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