A robust nonlinear composite controller, which is simple in structure and has the characteristics of good tracking performance and guaranteed system stability, is designed in this study. A pre-compensator is proposed to eliminate uncertainties or disturbances so that the tracking error is bounded in a pre-specified ball. The composite control law proposed in this study guarantees system tracking precision and stability by the aid of a third-order differential equation that has been derived in this research. The control parameters in the proposed controller can be adjusted systematically. For unknown serious variation in system surrounding, a composite control law with fuzzy-type pre-compensator is proposed to guarantee the system stability and the tracking precision. The compensating terms of the proposed controller use the mean-values of uncertainties to represent the system time-varying uncertainties at the first trial and then instead of sticking to the mean values, the proposed controller utilizes the fuzzy logics to update its nonlinear compensating terms so that good tracking results can be maintained under the presence of disturbances or plant uncertainties. Finally a submarine is used as an example to demonstrate the feasibility of the proposed controller.

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