This paper describes an approach to control of nonzero set points of uncertain linear systems based on the theory of ultimate boundedness. We derive a nonlinear control which guarantees that every system response is ultimately bounded within a certain neighborhood of the desired nonzero set point. Analytical expressions for the bounds on variations in parameters for ultimate boundedness are obtained. These results are applied to control of a VTOL aircraft whose time-varying parameters are uncertain. Unlike previous works, arbitrary and fast variations in aircraft parameters are allowed. It is shown that the nonlinear controller is effective for larger variation in parameters than the reduced linear controller. Digital simulation results show that the aircraft can be precisely maneuvered in spite of uncertain parameter variations.

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