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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Mechanical Engineering

Year Degree Awarded

2016

Month Degree Awarded

September

First Advisor

Kourosh Danai

Second Advisor

Lee Spector

Third Advisor

Matthew Lackner

Subject Categories

Acoustics, Dynamics, and Controls

Abstract

Mathematical descriptions of natural and man-made processes are the bedrock of science, used by humans to understand, estimate, predict and control the natural and built world around them. The goal of system identification is to enable the inference of mathematical descriptions of the true behavior and dynamics of processes from their measured observations. The crux of this task is the identification of the dynamic model form (topology) in addition to its parameters. Model structures must be concise to offer insight to the user about the process in question. To that end, this dissertation proposes three methods to improve the ability of system identification to identify succinct nonlinear model structures.

The first is a model structure adaptation method (MSAM) that modifies first principles models to increase their predictive ability while maintaining intelligibility. Model structure identification is achieved by this method despite the presence of parametric error through a novel means of estimating the gradient of model structure perturbations. I demonstrate MSAM's ability to identify underlying nonlinear dynamic models starting from linear models in the presence of parametric uncertainty. The main contribution of this method is the ability to adapt the structure of existing models of processes such that they more closely match the process observations.

The second method, known as epigenetic linear genetic programming (ELGP), conducts symbolic regression without a priori knowledge of the form of the model or its parameters. ELGP incorporates a layer of genetic regulation into genetic programming (GP) and adapts it by local search to tune the resultant model structures for accuracy and conciseness. The introduction of epigenetics is made simple by the use of a stack-based program representation. This method, tested on hundreds of dynamics problems, demonstrates the ability of epigenetic local search to improve GP by producing simpler and more accurate models.

The third method relies on a multidimensional GP approach (M4GP) for solving multiclass classification problems. The proposed method uses stack-based GP to conduct nonlinear feature transformations to optimize the clustering of data according to their classes. In comparison to several state-of-the-art methods, M4GP is able to classify test data better on several real-world problems. The main contribution of M4GP is its demonstrated ability to combine the strengths of GP (e.g. nonlinear feature transformations and feature selection) with the strengths of distance-based classification.

MSAM, ELGP and M4GP improve the identification of succinct nonlinear model structures for continuous dynamic processes with starting models, continuous dynamic processes without starting models, and multiclass dynamic processes without starting models, respectively. A considerable portion of this dissertation is devoted to the application of these methods to these three classes of real-world dynamic modeling problems. MSAM is applied to the restructuring of controllers to improve the closed-loop system response of nonlinear plants. ELGP is used to identify the closed-loop dynamics of an industrial scale wind turbine and to define a reduced-order model of fluid-structure interaction. Lastly, M4GP is used to identify a dynamic behavioral model of bald eagles from collected data. The methods are analyzed alongside many other state-of-the-art system identification methods in the context of model accuracy and conciseness.

Comments

revised according to revisions received 8/02/16

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