ESTIMATION AND CONTROL OF NONLINEAR SYSTEMS: MODEL-BASED AND MODEL-FREE APPROACHES

Abstract

State estimation and subsequent controller design for a general nonlinear system is an

important problem that have been studied over the past decades. Many applications,

e.g., atmospheric and oceanic sampling or lift control of an airfoil, display strongly nonlinear

dynamics with very high dimensionality. Some of these applications use smaller

underwater or aerial sensing platforms with insufficient on-board computation power to

use a Monte-Carlo approach of particle filters. Hence, they need a computationally efficient

filtering method for state-estimation without a severe penalty on the performance.

On the other hand, the difficulty of obtaining a reliable model of the underlying system,

e.g., a high-dimensional fluid dynamical environment or vehicle flow in a complex

traffic network, calls for the design of a data-driven estimation and controller when abundant

measurements are present from a variety of sensors. This dissertation places these

problems in two broad categories: model-based and model-free estimation and output

feedback.

In the first part of the dissertation, a semi-parametric method with Gaussian mixture

model (GMM) is used to approximate the unknown density of states. Then a Kalman

filter and its nonlinear variants are employed to propagate and update each Gaussian

mode with a Bayesian update rule. The linear observation model permits a Kalman

filter covariance update for each Gaussian mode. The estimation error is shown to be

stochastically bounded and this is illustrated numerically. The estimate is used in an

observer-based feedback control to stabilize a general closed-loop system. A transferoperator-

based approach is then proposed for the motion update for Bayesian filtering

of a nonlinear system. A finite-dimensional approximation of the Perron-Frobenius (PF)

operator yields a method called constrained Ulam dynamic mode decomposition (CUDMD).

This algorithm is applied for output feedback of a pitching airfoil in unsteady

flow.

For the second part, an echo-state network (ESN) based approach equipped with an

ensemble Kalman filter is proposed for data-driven estimation of a nonlinear system from

a time series. A random reservoir of recurrent neural connections with the echo-state

property (ESP) is trained from a time-series data. It is then used as a model-predictor for

an ensemble Kalman filter for sparse estimation. The proposed data-driven estimation

method is applied to predict the traffic flow from a set of mobility data of the UMD

campus. A data-driven model-identification and controller design is also developed for

control-affine nonlinear systems that are ubiquitous in several aerospace applications. We

seek to find an approximate linear/bilinear representation of these nonlinear systems from

data using the extended dynamic mode decomposition algorithm (EDMD) and apply Liealgebraic

methods to analyze the controllability and design a controller. The proposed

method utilizes the Koopman canonical transform (KCT) to approximate the dynamics

into a bilinear system (Koopman bilinear form) under certain assumptions. The accuracy

of this approximation is then analytically justified with the universal approximation

property of the Koopman eigenfunctions. The resulting bilinear system is then subjected

to controllability analysis using the Myhill semigroup and Lie algebraic structures, and a

fixed endpoint optimal controller is designed using the Pontryagin’s principle.