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(1 - 7 of 7)
- Title
- VISION BASED TRAJECTORY TRACKING OF SPACE DEBRIS IN CLOSE PROXIMITY VIA INTEGRATED ESTIMATION AND CONTROL.
- Creator
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Li, Ni, Xu, Yunjun, University of Central Florida
- Abstract / Description
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The increasingly cluttered environment in space is placing a premium on techniques capable of tracking and estimating the trajectory of space debris. Unlike the debris smaller than 1 cm or larger than 10 cm, it is always a challenge for spacecraft or satellite mission designers to consider explicitly the ones ranged from 1 cm to 10 cm a priori. To tackle this challenge, a vision based debris' trajectory tracking method in close proximity using two cameras onboard satellites in a formation is...
Show moreThe increasingly cluttered environment in space is placing a premium on techniques capable of tracking and estimating the trajectory of space debris. Unlike the debris smaller than 1 cm or larger than 10 cm, it is always a challenge for spacecraft or satellite mission designers to consider explicitly the ones ranged from 1 cm to 10 cm a priori. To tackle this challenge, a vision based debris' trajectory tracking method in close proximity using two cameras onboard satellites in a formation is presented in the thesis. Also to differentiate the target debris from other clutters, data association is investigated. A two-stage nonlinear robust controller is developed to adjust the attitude of the satellites such that the desired field of view can be achieved for the target debris. Capabilities of the proposed integrated estimation and control methods are validated in the simulations.
Show less - Date Issued
- 2011
- Identifier
- CFE0003731, ucf:48775
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0003731
- Title
- In Quest of Bernstein Inequalities Rational Functions, Askey-Wilson Operator, and Summation Identities for Entire Functions.
- Creator
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Puwakgolle Gedara, Rajitha, Li, Xin, Mohapatra, Ram, Ismail, Mourad, Xu, Mengyu, University of Central Florida
- Abstract / Description
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The title of the dissertation gives an indication of the material involved with the connecting thread throughout being the classical Bernstein inequality (and its variants), which provides an estimate to the size of the derivative of a given polynomial on a prescribed set in the complex plane, relative to the size of the polynomial itself on the same set. Chapters 1 and 2 lay the foundation for the dissertation. In Chapter 1, we introduce the notations and terminology that will be used...
Show moreThe title of the dissertation gives an indication of the material involved with the connecting thread throughout being the classical Bernstein inequality (and its variants), which provides an estimate to the size of the derivative of a given polynomial on a prescribed set in the complex plane, relative to the size of the polynomial itself on the same set. Chapters 1 and 2 lay the foundation for the dissertation. In Chapter 1, we introduce the notations and terminology that will be used throughout. Also a brief historical recount is given on the origin of the Bernstein inequality, which dated back to the days of the discovery of the Periodic table by the Russian Chemist Dmitri Mendeleev. In Chapter 2, we narrow down the contents stated in Chapter 1 to the problems we were interested in working during the course of this dissertation. Henceforth, we present a problem formulation mainly for those results for which solutions or partial solutions are provided in the subsequent chapters.Over the years Bernstein inequality has been generalized and extended in several directions. In Chapter \ref{Bern-ineq}, we establish rational analogues to some Bernstein-type inequalities for restricted zeros and prescribed poles. Our inequalities extend the results for polynomials, especially which are themselves improved versions of the classical Erd\"{o}s-Lax and Tur\'{a}n inequalities. In working towards proving our results, we establish some auxiliary results, which may be of interest on their own. Chapters \ref{AW-on-polynomials} and \ref{AW-on-entire} focus on the research carried out with the Askey-Wilson operator applied on polynomials and entire functions (of exponential type) respectively.In Chapter 4, we first establish a Riesz-type interpolation formula on the interval $[-1,1]$ for the Askey-Wilson operator. In consequence, a sharp Bernstein inequality and a Markov inequality are obtained when differentiation is replaced by the Askey-Wilson operator. Moreover, an inverse approximation theorem is proved using a Bernstein-type inequality in $L^2-$space. We conclude this chapter with an overconvergence result which is applied to characterize all $q$-differentiable functions of Brown and Ismail. Chapter \ref{AW-on-entire} is devoted to an intriguing application of the Askey-Wilson operator. By applying it on the Sampling Theorem on entire functions of exponential type, we obtain a series representation formula, which is what we called an extended Boas' formula. Its power in discovering interesting summation formulas, some known and some new will be demonstrated. As another application, we are able to obtain a couple of Bernstein-type inequalities.In the concluding chapter, we state some avenues where this research can progress.
Show less - Date Issued
- 2018
- Identifier
- CFE0007237, ucf:52220
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007237
- Title
- Optical Fluid-based Photonic and Display Devices.
- Creator
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Xu, Su, Wu, Shintson, Li, Guifang, Moharam, M., Wu, Xinzhang, University of Central Florida
- Abstract / Description
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Conventional solid-state photonic devices exhibit an ultra-high optical performance and durability, but minimal adaptability. Recently, optical fluid-based photonic and display devices are emerging. By dynamically manipulating the optical interface formed by liquids, the optical output can be reconfigured or adaptively tuned in real time. Such devices exhibit some unique characteristics that are not achievable in conventional solid-state photonic devices. Therefore, they open a gateway for...
Show moreConventional solid-state photonic devices exhibit an ultra-high optical performance and durability, but minimal adaptability. Recently, optical fluid-based photonic and display devices are emerging. By dynamically manipulating the optical interface formed by liquids, the optical output can be reconfigured or adaptively tuned in real time. Such devices exhibit some unique characteristics that are not achievable in conventional solid-state photonic devices. Therefore, they open a gateway for new applications, such as image and signal processing, optical communication, sensing, and lab-on-a-chip, etc. Different operation principles of optical fluid-based photonic devices have been proposed, for instance fluidic pressure, electrochemistry, thermal effect, environmentally adaptive hydrogel, electro-wetting and dielectrophoresis. In this dissertation, several novel optical fluid-based photonic and display devices are demonstrated. Their working principles are described and electro-optic properties investigated.The first part involves photonic devices based on fluidic pressure. Here, we present a membrane-encapsulated liquid lens actuated by a photo-activated polymer. This approach paves a way to achieve non-mechanical driving and easy integration with other photonic devices. Next, we develop a mechanical-wetting lens for visible and short-wavelength infrared applications. Such a device concept can be extended to longer wavelength if proper liquids are employed.In the second part, we reveal some new photonic and display devices based on dielectrophoretic effects. We conceive a dielectric liquid microlens with well-shaped electrode for fixing the droplet position and lowering the operating voltage. To widen the dynamic range, we demonstrate an approach to enable focus tuning from negative to positive or vice versa in a single dielectric lens without any moving part. The possibility of fabricating microlens arrays with different aperture and density using a simple method is also proposed. Furthermore, the fundamental electro-optic characteristics of dielectric liquid droplets are studied from the aspects of operating voltage, frequency and droplet size. In addition to dielectric liquid lenses, we also demonstrate some new optical switches based on dielectrophoretic effect, e.g., optical switch based on voltage-stretchable liquid crystal droplet, variable aperture or position-shifting droplet. These devices work well in the visible and near infrared spectral ranges. We also extend this approach to display and show a polarizer-free and color filter-free display. Simple fabrication, low power consumption, polarization independence, relatively low operating voltage as well as reasonably fast switching time are their key features.
Show less - Date Issued
- 2012
- Identifier
- CFE0004620, ucf:49943
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004620
- Title
- Sampling and Reconstruction of Spatial Signals.
- Creator
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Cheng, Cheng, Li, Xin, Sun, Qiyu, Yong, Jiongmin, Liu, Zhe, Xu, Mengyu, University of Central Florida
- Abstract / Description
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Digital processing of signals f may start from sampling on a discrete set ?, f ?? f(?_n), ?_n ??.The sampling theory is one of the most basic and fascinating topics in applied mathematics and in engineering sciences. The most well known form is the uniform sampling theorem for band-limited/wavelet signals, that gives a framework for converting analog signals into sequences of numbers. Over the past decade, the sampling theory has undergone a strong revival and the standard sampling paradigm...
Show moreDigital processing of signals f may start from sampling on a discrete set ?, f ?? f(?_n), ?_n ??.The sampling theory is one of the most basic and fascinating topics in applied mathematics and in engineering sciences. The most well known form is the uniform sampling theorem for band-limited/wavelet signals, that gives a framework for converting analog signals into sequences of numbers. Over the past decade, the sampling theory has undergone a strong revival and the standard sampling paradigm is extended to non-bandlimited signals including signals in reproducing kernel spaces (RKSs), signals with finite rate of innovation (FRI) and sparse signals, and to nontraditional sampling methods, such as phaseless sampling.In this dissertation, we first consider the sampling and Galerkin reconstruction in a reproducing kernel space. The fidelity measure of perceptual signals, such as acoustic and visual signals, might not be well measured by least squares. In the first part of this dissertation, we introduce a fidelity measure depending on a given sampling scheme and propose a Galerkin method in Banach space setting for signal reconstruction. We show that the proposed Galerkin method provides a quasi-optimal approximation, and the corresponding Galerkin equations could be solved by an iterative approximation-projection algorithm in a reproducing kernel subspace of Lp.A spatially distributed network contains a large amount of agents with limited sensing, data processing, and communication capabilities. Recent technological advances have opened up possibilities to deploy spatially distributed networks for signal sampling and reconstruction. We introduce a graph structure for a distributed sampling and reconstruction system by coupling agents in a spatially distributed network with innovative positions of signals. We split a distributed sampling and reconstruction system into a family of overlapping smaller subsystems, and we show that the stability of the sensing matrix holds if and only if its quasi-restrictions to those subsystems have l_2 uniform stability. This new stability criterion could be pivotal for the design of a robust distributed sampling and reconstruction system against supplement, replacement and impairment of agents, as we only need to check the uniform stability of affected subsystems. We also propose an exponentially convergent distributed algorithm for signal reconstruction, that provides a suboptimal approximation to the original signal in the presence of bounded sampling noises.Phase retrieval (Phaseless Sampling and Reconstruction) arises in various fields of science and engineering. It consists of reconstructing a signal of interest from its magnitude measurements. Sampling in shift-invariant spaces is a realistic model for signals with smooth spectrum. We consider phaseless sampling and reconstruction of real-valued signals in a shift-invariant space from their magnitude measurements on the whole Euclidean space and from their phaseless samples taken on a discrete set with finite sampling density. We find an equivalence between nonseparability of signals in a shift-invariant space and their phase retrievability with phaseless samples taken on the whole Euclidean space. We also introduce an undirected graph to a signal and use connectivity of the graph to characterize the nonseparability of high-dimensional signals. Under the local complement property assumption on a shift-invariant space, we find a discrete set with finite sampling density such that signals in shift-invariant spaces, that are determined by their magnitude measurements on the whole Euclidean space, can be reconstructed in a stable way from their phaseless samples taken on that discrete set. We also propose a reconstruction algorithm which provides a suboptimal approximation to the original signal when its noisy phaseless samples are available only.
Show less - Date Issued
- 2017
- Identifier
- CFE0006726, ucf:51889
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006726
- Title
- Bio-inspired, Varying Manifold Based Method with Enhanced Initial Guess Strategies for Single Vehicle's Optimal Trajectory Planning.
- Creator
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Li, Ni, Xu, Yunjun, Lin, Kuo-Chi, Bai, Yuanli, Behal, Aman, University of Central Florida
- Abstract / Description
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Trajectory planning is important in many applications involving unmanned aerial vehicles, underwater vehicles, spacecraft, and industrial manipulators. It is still a challenging task to rapidly find an optimal trajectory while taking into account dynamic and environmental constraints. In this dissertation, a unified, varying manifold based optimal trajectory planning method inspired by several predator-prey relationships is investigated to tackle this challenging problem. Biological species,...
Show moreTrajectory planning is important in many applications involving unmanned aerial vehicles, underwater vehicles, spacecraft, and industrial manipulators. It is still a challenging task to rapidly find an optimal trajectory while taking into account dynamic and environmental constraints. In this dissertation, a unified, varying manifold based optimal trajectory planning method inspired by several predator-prey relationships is investigated to tackle this challenging problem. Biological species, such as hoverflies, ants, and bats, have developed many efficient hunting strategies. It is hypothesized that these types of predators only move along paths in a carefully selected manifold based on the prey's motion in some of their hunting activities. Inspired by these studies, the predator-prey relationships are organized into a unified form and incorporated into the trajectory optimization formulation, which can reduce the computational cost in solving nonlinear constrained optimal trajectory planning problems. Specifically, three motion strategies are studied in this dissertation: motion camouflage, constant absolute target direction, and local pursuit. Necessary conditions based on the speed and obstacle avoidance constraints are derived. Strategies to tune initial guesses are proposed based on these necessary conditions to enhance the convergence rate and reduce the computational cost of the motion camouflage inspired strategy. The following simulations have been conducted to show the advantages of the proposed methods: a supersonic aircraft minimum-time-to-climb problem, a ground robot obstacle avoidance problem, and a micro air vehicle minimum time trajectory problem. The results show that the proposed methods can find the optimal solution with higher success rate and faster convergent speed as compared with some other popular methods. Among these three motion strategies, the method based on the local pursuit strategy has a relatively higher success rate when compared to the other two.In addition, the optimal trajectory planning method is embedded into a receding horizon framework with unknown parameters updated in each planning horizon using an Extended Kalman Filter.
Show less - Date Issued
- 2013
- Identifier
- CFE0005023, ucf:49986
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005023
- Title
- Consumer Engagement in Travel-related Social Media.
- Creator
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Li, Xu, Wang, Youcheng, Robinson, Edward, Kwun, David, Nusair, Khaldoon, He, Xin, University of Central Florida
- Abstract / Description
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The term of (")consumer engagement(") is extensively used in the digital era. It is believed that engaged consumers play an important role in products/services referral and recommendation, new product/service development and experience/value co-creation. Although the notion of consumer engagement sounds compelling, it is not fully developed in theory. Different interpretations coexist, resulting in confusion and misuse of the concept. This study attempts to define consumer engagement and...
Show moreThe term of (")consumer engagement(") is extensively used in the digital era. It is believed that engaged consumers play an important role in products/services referral and recommendation, new product/service development and experience/value co-creation. Although the notion of consumer engagement sounds compelling, it is not fully developed in theory. Different interpretations coexist, resulting in confusion and misuse of the concept. This study attempts to define consumer engagement and develop a conceptual framework of consumer engagement, addressing antecedents of consumer engagement in online context. Moreover, some situational and social media usage-related factors are incorporated into the framework. A set of propositions are presented based on literature review and the conceptual framework to illustrate the relationship between consumer engagement and related factors. To provide empirical evidence for the conceptual model, an online survey is conducted. Participants complete the self-administered survey by answering questions concerning their online experience with the travel-related social media website they visit most. Two-step structural equation modeling is employed to analyze the data. The results show that both community experience and community identification have significant and positive relationship with consumer engagement. Community experience is also a strong predictor of community identification. Attitude toward using social media and travel involvement influence the relationship between consumer engagement and its antecedents.With focus on the interactive and experiential nature of consumer engagement, this study expands current understanding of consumer engagement and provides insights for hospitality and tourism businesses regarding how to engage consumers through travel-related social media.
Show less - Date Issued
- 2013
- Identifier
- CFE0004878, ucf:49657
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004878
- Title
- Synthesis and Properties of Self-Assembled C/SiCN Nanocomposite Derived from Polymer Precursors.
- Creator
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Li, Cheng, An, Linan, Fang, Jiyu, Coffey, Kevin, Liao, Yi, Xu, Chengying, Zhai, Lei, University of Central Florida
- Abstract / Description
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The properties of C/SiCN nanocomposites synthesized by thermal decomposition of polymer precursors were studied in this work. The novel polymer-to-ceramic process enables us to tailor the ceramic structure in atomic level by designing the starting chemicals and pyrolysis procedures. It is of both fundamental and practical significance to investigate the properties and structures relationship of the nanocomposites. In this work, we explored their application potential in using as anode of...
Show moreThe properties of C/SiCN nanocomposites synthesized by thermal decomposition of polymer precursors were studied in this work. The novel polymer-to-ceramic process enables us to tailor the ceramic structure in atomic level by designing the starting chemicals and pyrolysis procedures. It is of both fundamental and practical significance to investigate the properties and structures relationship of the nanocomposites. In this work, we explored their application potential in using as anode of lithium-ion secondary batteries. The structure and structural evolution of C/SiCN nanocomposite were investigated by using XRD, FTIR, SEM, TEM, Solid state NMR and Raman spectroscopy. The results revealed the nanocomposites consisted of amorphous SiCxNx-4 matrix and carbon nanoclusters distributed within it. The size of the carbon was measured by Raman spectroscopy, varied with starting chemicals and pyrolysis temperature. The electronic properties of the C/SiCN nanocomposite were studied by measuring the I-V curves and a.c. impedance. The d.c. conductivity increased with carbon content and pyrolysis temperatures. The impedance spectra and fitted equivalent circuit results confirmed the existence of two phases in the nanocomposite. The possibility of using C/SiCN as anode in lithium-ion secondary batteries was investigated by electrochemical measurements, namely cyclic voltammetry, galvanostatic cyclic test and electrochemical impedance spectroscopy. The galvanostatic measurements showed that the nanocomposite with 26% of carbon nanoclusters exhibited a specific capacity of 480 mAh/g, which is 30% higher than that of commercial graphite anode. The high capacity of the nanocomposites is attributed to the formation of a novel structure around C/SiCN interface. The excellent electrochemical properties, together with the simple, low-cost processing, make the nanocomposites very promising for Li-ion battery applications.
Show less - Date Issued
- 2012
- Identifier
- CFE0004194, ucf:49028
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004194
