This paper investigates the problem of reference tracking for systems defined by a Wiener model. The proposed method does not suppose that the full state vector is measured and it is based on a dynamic inversion approach with error feedback. It is assumed however,that the inverse of the nonlinear part can be constructed. As an example a tracking control of a pressurizer is also provided.
I. PROBLEM FORMULATIONWiener models have the capability of approximating, with arbitrary accuracy, any fading memory nonlinear time invariant system. For Wiener models, see Figure I, the system has a linear part Σ, followed by a nonlinear memoryless subsystem, i.e.,ẋ(1)They have been successfully used to model several nonlinear systems encountered in the process industry, in chemistry, pH control systems, distillation columns or processes where the measurement device has a nonlinear characteristics.Fig. 1. Wiener model Several methods have been proposed in the literature for the identification of Wiener models (see for instance [5], [32], [9], [28], [29], [20], [1], [10], [12], for some classical identification methods for Wiener models). The problem consists in estimating the nonlinear characteristic of the memoryless subsystem and recovering the impulse response of the linear one from input-output observations of the whole system. The main difficulty is caused by the fact that the inner system signal is not measured.By a particular choice of parametrization of the linear subsystem and the inverse of the nonlinearity, it is possible to formulate an error criterion where the parameters enter quadratically. This error criterion may be minimized using linear regression, quadratic programming or the total least squares method. This initial estimate may then be used in the numerical minimization of the prediction error criterion.There are a lot of papers dealing with the control design based on Wiener model structure, e.g. [21], [13], [14], [30], [31]. Usually, for controller design purposes the static nonlinearities are removed by an inversion, however, the inverse of the nonlinearity can be delivered by the identification algorithm, too. The special way in which the nonlinearity enters the Wiener model can be exploited by transforming it into uncertainty. The result will be an uncertain linear model, which enables to use for example robust linear MPC techniques, see [8], [25], [6].In the present paper, we study the reference tracking problem of a Wiener system. The problem of (asymptotic) tracking consists in finding a compensator such that the closed-loop system is internally stable and for any desired trajectory y d the output of the closed-loop system (asymptotically) approaches y d .Different type of problems can be distinguished based on the structure imposed to the set of the desired trajectories. The most highly structured situation is when this set is finite [15] . If the class of desired trajectories can be described by an exosystem, i.e., an autonomous, noninitialized set of differential equations with an output, then the...