A linguistic fuzzy model with a monotone rule base is not always monotone

In this paper experiments are described with three linguistic fuzzy models sharing the same monotone rule base and the same membership functions for the two input variables, but applying different membership functions in the output domain. We investigated which inference methods result in a monotonic input-output behaviour. Apart from the conventional Mamdani–Assilian inference method with Center of Gravity (COG) defuzzification, three implicator-based inference methods combined with COG-like defuzzification are discussed.