The present interest in fuzzy theory is mainly due to successful fuzzy logic controller applications in industrially important systems. Using a priori information elicited from the process operator fuzzy logic controllers can control process disturbances and uncertainties that would normally be difficult for conventional controllers. Application of fuzzy set theory on the proportional-integral (PI) control law introduces gain scheduling ability into the PI controller. By modifying the rule base, it is possible to link the controller gain values to the process variable value.