Robustly stable fuzzy controller for uncertain nonlinear systems with unknown input gain sign

We propose and analyze a robust adaptive fuzzy controller for uncertain nonlinear systems without information on the input gain sign. The proposed scheme completely overcomes the singularity problem which occurs in the indirect adaptive feedback linearizing control. No projection in the estimated parameters and no switching in the control input are needed. The stability of the closed-loop system is guaranteed in the Lyapunov viewpoint.