Design of Insensitive Multirate Aircraft Control Using Optimized Eigenstructure Assignment

This paper presents a new method for the design of constant-gain controllers for digital aircraft systems monitored and updated at multiple frequencies. It addresses two main problems associated with such a scheme: the adverse effects of intersample cross coupling and the high-amplitude control signals that normally result from a multirate feedback structure. An examination of the problem indicates that the two objectives present conflicting design criteria. Thus, a solution must seek to achieve an acceptable compromise between the minimization of control effort and the assignment of a prescribed modal structure. An unconstrained numerical optimization procedure, based on eigenstructur e assignment using full-state feedback, has been developed to provide for this compromise. A suitable choice of weighting factors, introduced into the scalar cost function defined for the optimization, allows for the computation of two alternative control laws. They differ only in the emphasis placed on the two conflicting design objectives. The lateral motion of a light aircraft is the test system employed to illustrate the application of the method. The controllers are evaluated using a fully nonlinear simulation of the aircraft. The results fully demonstrate the eigeninsensitivity of the multirate feedback solution and the flexibility offered by the eigenstructure-assignment optimization approach in both cases.

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