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Robust tracking, error feedback, and two-degree-of-freedom controllers

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2 Author(s)
J. W. Howze ; Dept. of Electr. Eng., Texas A&M Univ., College Station, TX, USA ; S. P. Bhattacharyya

The purpose of this paper is to establish first, that in the absence of error feedback, asymptotic tracking is a property that is fragile with respect to the controller parameters. This implies that arbitrary small perturbation of the parameters of such a controller will cause asymptotic tracking to fail. Second, we propose a general 2-DOF controller architecture. We prove that in this structure, asymptotic tracking is, in general, fragile with respect to the controller parameters Fr, Fy and C, while it is robust with respect to the plant parameters as well as the components H and L of the 2-DOF controller. Moreover, the only way to realize a 2-DOF controller in a nonfragile way is to generate the error signal exactly, which amounts to setting Fr=Fy=1. The results reconcile the requirements of the internal model principle, namely the necessity of error feedback, with the additional design freedom potentially offered by 2-DOF controllers. It specifies the controller architecture necessary for robustness and identifies the blocks whose parameters are available for tuning in the design process

Published in:

IEEE Transactions on Automatic Control  (Volume:42 ,  Issue: 7 )