Hybrid Type-1 and Type-2 Fuzzy Logic Control of Flexible Underwater Robots

Abstract

In this research paper, a mathematical analysis and dynamical comparison have been performed between the Type-1 and Interval Type-2 Fuzzy Sliding Mode Control (IT2-FSMC) frameworks for an underactuated flexible-link underwater robot. Although the Type-1 FSMC controller ensures stability and satisfactory trajectory tracking ability, the efficiency decreases tremendously whenever the underwater robot encounters severe disturbances of the hydrodynamic nature and uncertainties of up to 50% of the nominal values, causing high-frequency chattering issues and resonant control spillover problem in the flexible links. In order to overcome this basic limitation of the control framework, the nonlinear control policy based on the Interval Type-2 Fuzzy Logic (IT2-FSMC) scheme is formulated and designed, which exhibits better mathematical ability to cover the uncertainties with its Footprint of Uncertainty (FOU). Through numerical simulations under the effect of constant ocean currents, dry Coulomb friction force, and severe 50% parameter uncertainties, it is evident from the results that the proposed IT2-FSMC controller outperforms the Type-1 one in overcoming all kinds of problems and disturbances.