Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Generalized Synchronization Trees(2019) Ferlez, James Robert; Marcus, Steven I; Cleaveland, Walter R; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We propose a novel framework for modeling cyber-physical systems (CPSs) that we call Generalized Synchronization Trees (GSTs). GSTs provide a rich framework for modeling systems that contain both discrete and continuous behavior in any combination, as well as enabling novel methods for analyzing such systems. GSTs were inspired by Milner's successful use of Synchronization Trees (STs) to model interconnected computing processes, and GSTs afford a means of applying those same perspectives to CPSs. In particular, STs -- and thus GSTs -- provide a very natural setting for studying bisimulation and composition. In this thesis, we study both matters from a number of different perspectives: different notions of bisimulation over GSTs are defined and their (unexpected) semantic differences are established; the relationship of GSTs to behavioral, state-based systems is demonstrated; a simple modal logic for GSTs is defined and its relationship to bisimulation is established, in particular with respect to Hennessy-Milner classes of GSTs; and finally, bisimulation is demonstrated to be a congruence for several different composition operators. First, we contribute a novel counterexample to the semantic equivalence of two common notions of bisimulation, as they are naturally adapted to GSTs; this is in contrast to the situation for discrete processes, where these two notions of bisimulation coincide. This example -- and the definitions of bisimulation on which it is based -- thus provides crucial guiding intuition for further study. Second, we demonstrate how GSTs may be regarded as "unrollings" of conventional state-based behavioral systems, in direct analog to the way STs may be regarded as "unrollings" of ordinary labeled transitions systems. Crucially, conditions are established under which this unrolling procedure is shown to preserve a notion of bisimulation, as it does in the discrete case. Third, we study the relationship between bisimulation for GSTs and a generalization of Hennessy-Milner Logic (HML) that we call Generalized Hennessy-Milner Logic (GHML). In particular, we establish a relationship between maximal Hennessy-Milner classes of discrete structures and maximal Hennessy-Milner classes of GSTs; a Hennessy-Milner class is a class of models for which modal equivalence implies bisimulation equivalence, and this direction of implication is seldom studied in the context of CPSs. Moreover, we translate Linear, Time-Invariant Systems -- regarded as behavioral systems -- into GSTs, and study the relationship between these GSTs and maximal Hennessy-Milner classes. Finally, we study the congruence properties of bisimulation with respect to several composition operators over GSTs. One such composition operator mirrors a synchronous composition operator defined over behavioral systems, so the relationship between GSTs and state-based systems leads to a natural congruence result. Another composition operator we consider is a novel generalization of the CSP parallel composition operator for discrete systems; for this operator, too, we establish that bisimulation is a congruence, although the operator itself has subtle, implicit restrictions that make this possible. We also show that discrete GSTs can capture the bisimulation semantics of HyPA, a hybrid process algebra; consequently, this is implicitly a congruence result over compositions obtained using HyPA terms for which bisimulation is a congruence.Item Nonlinear Optics Quantum Computation and Quantum Simulation with Circuit-QED(2014) Adhikari, Prabin; Taylor, Jacob M.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Superconducting quantum circuits are a promising approach for realizations of large scale quantum information processing and quantum simulations. The Josephson junction, which forms the basis of superconducting circuits, is the only known nonlinear non-dissipative circuit element, and its inherent nonlinearities have found many different applications. In this thesis I discuss specific implementations of these circuits. I show that strong two-photon nonlinearities can be induced by coupling photons in the microwave domain to Josephson nonlinearities. I then propose a method to simulate a parent Hamiltonian that can potentially be used to observe fractional quantum Hall states of light. I will also explore how superconducting circuits can be used to modify system-bath couplings to emulate a chemical potential for photons. Finally, I consider the limitations of devising a scheme to couple superconducting circuits to trapped ions, and consider the challenges for such hybrid approaches.Item Utility Driven Sampled Data Control Under Imperfect Information(2013) Tallapragada, Pavankumar; Chopra, Nikhil; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Computer based control systems, which are ubiquitous today, are essentially sampled data control systems. In the traditional time-triggered control systems, the sampling period is conservatively chosen, based on a worst case analysis. However, in many control systems, such as those implemented on embedded computers or over a network, parsimonious sampling and computation is helpful. In this context, state/data based aperiodic utility driven sampled data control systems are a promising alternative. This dissertation is concerned with the design of utility driven event-triggers in certain classes of problems where the information available to the triggering mechanisms is imperfect. In the first part, the problem of utility driven event-triggering under partial state information is considered - specifically in the context of (i) decentralized sensing and (ii) dynamic output feedback control. In the case of full state feedback, albeit with decentralized sensing, methods are developed for designing local and asynchronous event-triggers for asymptotic stabilization of an equilibrium point of a general nonlinear system. In the special case of Linear Time Invariant (LTI) systems, the developed method also holds for dynamic output feedback control, which extends naturally to control over Sensor-Controller-Actuator Networks (SCAN), wherein even the controller is decentralized. The second direction that is pursued in this dissertation is that of parsimonious utility driven sampling not only in time but also in space. A methodology of co-designing an event-trigger and a quantizer of the sampled data controller is developed. Effectively, the proposed methodology provides a discrete-event controller for asymptotic stabilization of an equilibrium point of a general continuous-time nonlinear system. In the last part, a method is proposed for designing utility driven event-triggers for the problem of trajectory tracking in general nonlinear systems, where the source of imperfect information is the exogenous reference inputs. Then, specifically in the context of robotic manipulators we develop utility driven sampled data implementation of an adaptive controller for trajectory tracking, wherein imperfect knowledge of system parameters is an added complication.Item Optimal Control of Hybrid Systems, with Application to Vehicle Dynamics(2011) Kefauver, Kevin; Levine, William; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hybrid dynamical systems are common throughout the physical and computer world, and they consist of dynamical systems that contain both continuous time and discrete time dynamics. Examples of this type of system include thermostat controlled systems, multi-geared transmission based systems, and embedded computer systems. Sometimes, complicated non-linear continuous time systems can be simplified by breaking them up into a set of less complicated continuous systems connected through discrete interactions (referred to as system hybridization). One example is modeling of vehicle dynamics with complicated tire-to-ground interaction by using a tire slipping or no slip model. When the hybrid system is to be a controlled dynamical system, a limited number of tools exist in the literature to synthesize feedback control solutions in an optimal way. The purpose of this dissertation is to develop necessary and sufficient conditions for finding optimal feedback control solutions for a class of hybrid problems that applies to a variety of engineering problems. The necessary and sufficient conditions are developed by decomposing the hybrid problem into a series of non-hybrid optimal feedback control problems that are coupled together with the appropriate boundary conditions. The conditions are developed by using a method similar to Bellman's Dynamic Programming Principle. The solution for the non-hybrid optimal control problem that contains the final state is found and then propagated backwards in time until the solution is generated for every node of the hybrid problem. In order to demonstrate the application of the necessary and sufficient conditions, two hybrid optimal control problems are analyzed. The first problem is a theoretical problem that demonstrates the complexity associated with hybrid systems and the application of the hybrid analysis tools. Through the control problem, a solution is found for the feedback control that minimizes the time to the origin problem for a hybrid system that is a combination of two standard optimal control problems found in the literature; the double integrator system and a harmonic oscillator. Through the second problem, optimal feedback control is found for the drag racing and hot-rodding control problems for any initial reachable state of the system and a hybrid model of a vehicle system with tire-to-ground interaction.