In this paper, we study the problem of adaptive trajectory tracking control for a class of nonlinear systems with structured parametric uncertainties. We propose to use an iterative modular approach: we first design a robust nonlinear state feedback that renders the closed-loop input-to-state stable (ISS). Here, the input is considered to be the estimation error of the uncertain parameters, and the state is considered to be the closed-loop output tracking error. Next, we propose an iterative adaptive algorithm, where we augment this robust ISS controller with an iterative data-driven learning algorithm to estimate online the parametric uncertainties of the model. We implement this method with two different learning approaches. The first one is a data-driven multiparametric extremum seeking method, which guarantees local convergence results, and the second is a Bayesian optimization-based method called Gaussian Process Upper Confidence Bound, which guarantees global results in a compact search set. The combination of the ISS feedback and the data-driven learning algorithms gives a learning-based modular indirect adaptive controller. We show the efficiency of this approach on a two-link robot manipulator numerical example.
INTRODUCTIONClassical adaptive methods can be classified into two main approaches: "direct" approaches, where the controller is updated to adapt to the process, and "indirect" approaches, where the model is updated to better reflect the actual process. Many adaptive methods have been proposed over the years for linear and nonlinear systems; we cannot possibly cite them all. Instead, we refer the reader to, eg, other works 1-4 and the references therein for more details. Of particular interest to us is the indirect modular approach to adaptive nonlinear control, eg, see the work of Krstić et al. 3 In this approach, first the controller is designed by assuming that all the parameters are known, and then, an identifier is used to guarantee boundedness or asymptotic convergence of the estimation error. When the identifier is based on a data-driven learning algorithm, which is independent of the designed controller, the approach is called "learning-based," eg, see the work of Benosman, 5 In this line of research in adaptive control we, can cite the following references, eg, see other works. For example, in the neural network (NN)-based modular adaptive control design, the idea is to write the model of the system as a combination of a known part and an unknown part (a.k.a. the disturbance part). The NN is then used to Int J Adapt Control Signal Process. 2019;33:335-355. wileyonlinelibrary.com/journal/acs © 2018 John Wiley & Sons, Ltd. 335 336 BENOSMAN ET AL.approximate the unknown part of the model. Finally, a controller based on both the known and the NN-estimate of the unknown part is determined to realize some desired regulation or tracking performance, eg, see other works. 8,10,13 In this work, we build upon this type of modular learning-based adaptive design and provide a framework that combines...