This paper deals with frequency domain robust regulation of signals generated by an infinite-dimensional exosystem. The problem is formulated and the stability types are chosen so that one can generalize the existing finite-dimensional theory to more general classes of infinitedimensional systems and signals. The main results of this article are extensions of the internal model principle, of a necessary and sufficient solvability condition for the robust regulation problem, and of Davison's simple servo compensator for stable plants in the chosen algebraic framework.
Introduction.The topic of this article is robust regulation, which is a central problem in mathematical control theory. The robust regulation problem consists of two parts: robust stabilization, and robust asymptotic tracking and disturbance rejection. Asymptotic tracking and disturbance rejection means that the error between the reference and output signals vanishes as time goes to infinity despite some disturbance signals. The reference and disturbance signals are generated by an exogenous system called the exosystem. Robustness of regulation means that the output of the closed loop asymptotically tracks the reference signals even if the plant is perturbed. Robustness is of fundamental importance since small errors in mathematical models of real-world phenomena are unavoidable and stem from various sources such as model simplifications and erroneous parameter estimation.The robust regulation problem of finite-dimensional linear multi-input multioutput (MIMO) systems was solved in the 1970s. Francis, Wonham, and Davison had a central role in this work [5,6,7,9,10]. Many authors have generalized the finitedimensional results to infinite-dimensional and nonlinear systems in the time domain as well as in the frequency domain since then [1,13,15,17,30,32,35,36,37,41].Robust regulation of signals generated by an infinite-dimensional exosystem is a challenging problem [15,39]. For the recent development on this subject, see [14,18,31] and the references therein. One can generate very general classes of signals, e.g., general periodic functions, by using infinite-dimensional exosystems. Signals that are generated by an infinite-dimensional exosystem and their frequency domain counterparts are called infinite-dimensional signals in this paper. The main emphasis in recent research has been on the time domain, while the frequency domain theory has received only a little attention. The purpose of this paper is to develop theory for robust regulation in the frequency domain.There exist multiple different rings of stable transfer functions, each of which has its own special features and is suitable for certain purposes [24]. Thus, it is natural