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
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Applied Aerial Autonomy for Reliable Indoor Flight and 3D Mapping(2024) Shastry, Animesh Kumar; Paley, Derek; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Uncrewed Aerial Systems (UAS) are essential for safely exploring indoor environments damaged by shelling, fire, floods, and structural collapse. These systems can gather critical visual and locational data, aiding in hazard assessment and rescue planning without risking human lives. Reliable UAS deployments requires advanced sensors and robust algorithms for real-time data processing and safe navigation, even in GPS-denied and windy conditions. This dissertation details three research projects to improve UAS performance: (1) in-flight calibration to improve estimation and control, (2) system identification for wind rejection, and (3) indoor aerial 3D mapping. The dissertation begins with introducing a comprehensive nonlinear filtering framework for UAV parameter estimation, which considers factors such as external wind, drag coefficients, IMU bias, and center of pressure. Additionally, it establishes optimized flight trajectories for parameter estimation through empirical observability. Moreover, an estimation and control framework is implemented, utilizing the mean of state and parameter estimates to generate suitable control inputs for vehicle actuators. By employing a square-root unscented Kalman filter (sq-UKF), this framework can derive a 23-dimensional state vector from 9-dimensional sensor data and 4-dimensional control inputs. Numerical results demonstrate enhanced tracking performance through the integration of the estimation framework with a conventional model-based controller. The estimation of unsteady winds results in improved gust rejection capabilities of the onboard controller as well. Closely related to parameter estimation is system identification. Combining with the previous work a comprehensive system identification framework with both linear offline and nonlinear online methods is introduced. Inertial parameters are estimated using frequency-domain linear system identification, incorporating control data from motor-speed sensing and state estimates from automated frequency sweep maneuvers. Additionally, drag-force coefficients and external wind are recursively estimated during flight using a sq-UKF. A custom flight controller is developed to manage the computational demands of online estimation and control. Flight experiments demonstrate the tracking performance of the nonlinear controller and its improved capability in rejecting gust disturbances. Aside from wind rejection, aerial indoor 3D mapping is also required for indoor navigation, and therefore, the dissertation introduces a comprehensive pipeline for real-time mapping and target detection in indoor environments with limited network access. Seeking a best-in-class UAS design, it provides detailed analysis and evaluation of both hardware and software components. Experimental testing across various indoor settings demonstrates the system's efficacy in producing high-quality maps and detecting targets.Item SYSTEM IDENTIFICATION OF A MULTI-ROTOR VEHICLE WITH ACTIVE FEEDBACK CONTROL(2020) Cunningham, Michael A; Hubbard Jr., James E; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As multi-rotor vehicles become integrated into the national airspace for applications such as package delivery and videography, it is important that the inner-loop control system be robust and able to meet ever-demanding performance constraints. To achieve high bandwidth control designs, it is necessary to have accurate and high bandwidth open-loop models. The material covered in this dissertation was aimed towards the flight-based identification and comprehensive understanding of the open-loop dynamics and aerodynamics of a multi-rotor vehicle despite the active feedback control system. The analytical first principles modeling and the discussion of system identification techniques informed the process for identifying the unstable multi-rotor dynamics. The inherently unstable nature of the vehicle, combined with the available measurements, necessitated a methodical approach towards identification from flight experiments. The non-negligible issue of unstable system identification was addressed by both the experiment design and the applied estimation technique. An individual propulsion system was tested on a hover stand to understand both the static and dynamic behavior of the combined propeller, motor, and electronic speed controller system. The results of the ground experiments influenced the design of the flight experiments, the postulation of the models, and the understanding of the estimated model parameters. The methodology for flight-based multi-rotor system identification involved the combined application of manual pilot inputs and automated multi-sine inputs added to the output of the controller. A series of five main flight tests were conducted for the identification of a high bandwidth open-loop linear rigid body model and two aerodynamic models of a multi-rotor vehicle. The comparison of the linear rigid body model with the two aerodynamics models emphasized the clear and substantial importance of the nonlinear rigid body coupling terms even in operations near hover. The relative improvement from the linear model to the application of the aerodynamics models to the nonlinear rigid body dynamics was considerable for the translational dynamics, yet the linear model was similarly applicable to the rotational dynamics in aggressive flight near hover.Item Modeling, Estimation, and Control of Actuator Dynamics for Remotely Operated Underwater Vehicles(2019) Boehm, Jordan; Paley, Derek A; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Modeling and control of remotely operated underwater vehicles is a challenging problem that depends greatly on how the dynamics of their thrusters are compensated. In this thesis a novel method for characterizing thruster dynamics using a six-axis load cell is presented. Multiple dynamic models are characterized with this test setup. Model-based control design strategies are used to compensate for nonlinearities in the dynamics, which include input dead zones and coupling with fluid dynamics. Multiple estimation methods are presented to construct an estimate of fluid velocity which is handled as an unmeasured state. The different models, controllers, and estimators are comparatively evaluated in closed-loop experiments using the six-axis load cell to measure thrust tracking performance. Full vehicle simulations using the experimentally characterized models provide additional opportunities for comparison of control and estimation strategies. The potential tracking control benefits from the variety of presented thruster dynamics compensation strategies are evaluated for a remotely operated underwater vehicle with multiple thrusters.Item Lower-Body Mechanical Perturbation of Gait to Identify Neural Control(2017) Rafiee, Shakiba; Kiemel, Tim; Kinesiology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Neural feedback plays a key role in maintaining locomotor stability in face of perturbations. In this study, we systematically identified properties of neural feedback that contribute to stabilizing human walking by examining how the nervous system responds to small kinematic deviations away from the desired gait pattern. We applied small continuous mechanical perturbation, forces at the ankles, as well as small continuous sensory perturbation, movement of a virtual visual scene, in order to compare how neural feedback responds to actual and illusory kinematic deviations. Computing phase-dependent impulse response functions (φIRFs) that describe kinematic and muscular responses to small brief perturbations (impulses), enabled us to identify critical phases of the gait cycle when the nervous system modulates muscle activity. In particular, our results suggest that an early-stance modulation of anterior leg-muscles is a general control mechanism that serves multiple functions, including controlling walking speed and compensating for errors in foot placement.Item AN ACTIVE NON-INTRUSIVE SYSTEM IDENTIFICATION APPROACH FOR CARDIOVASCULAR HEALTH MONITORING(2014) Fazeli, Nima; Hahn, Jin-Oh; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this study a novel active non-intrusive system identification paradigm is developed for the purpose of cardiovascular health monitoring. The proposed approach seeks to utilize a collocated actuator sensor unit devised from the common blood pressure cuff to simultaneously 1) produce rich transmural blood pressure waves that propagate through the cardiovascular system and 2) to make measurements of these rich peripheral transmural blood pressures utilizing the pressure oscillations produced within the cuffs bladder in order to reproduce the central aortic blood pressure accurately. To achieve this end a mathematical model of the cardiovascular system is developed to model the wave propagation dynamics of the external (excitation applied by the cuff) and internal (excitation produced by the heart) blood pressure waveforms through the cardiovascular system. Next a system identification protocol is developed in which rich transmural blood pressures are recorded and used to identify the parameters characterizing the model. The peripheral blood pressures are used in tandem with the characterized model to reconstruct the central aortic blood pressure waveform. The results of this study indicate the developed protocol can reliably and accurately reproduced the central aortic blood pressure and that it can outperform its intrusive passive counterpart (the Individualized Transfer Function methodology). The root-mean-square error in waveform reproduction, pulse pressure error and systolic pressure errors were evaluated to be 3.31 mmHg, 1.36 mmHg and 0.06 mmHg respectively for the active nonintrusive methodology while for the passive intrusive counterpart the same errors were evaluated to be 4.12 mmHg, 1.59 mmHg and 2.67 mmHg indicating the superiority of the proposed approach.Item A Comparative Framework for Maneuverability and Gust Tolerance of Aerial Microsystems(2012) Campbell, Renee; Humbert, James S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Aerial microsystems have the potential of navigating low-altitude, cluttered environments such as urban corridors and building interiors. Reliable systems require both agility and tolerance to gusts. While many platform designs are under development, no framework currently exists to quantitatively assess these inherent bare airframe characteristics which are independent of closed loop controllers. This research develops a method to quantify the maneuverability and gust tolerance of vehicles using reachability and disturbance sensitivity sets. The method is applied to a stable flybar helicopter and an unstable flybarless helicopter, whose state space models were formed through system identification. Model-based static H-infinity controllers were also implemented on the vehicles and tested in the lab using fan-generated gusts. It is shown that the flybar restricts the bare airframe's ability to maneuver in translational velocity directions. As such, the flybarless helicopter proved more maneuverable and gust tolerant than the flybar helicopter. This approach was specifically applied here to compare stable and unstable helicopter platforms; however, the framework may be used to assess a broad range of aerial microsystems.Item Open Loop System Identificaiton of a Micro Quadrotor Helicopter from Closed Loop Data(2011) Miller, Derek; Humbert, James S; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quadrotors are a favorite platform amongst academic researchers. Yet there is limited work present on the identification of Linear Time Invariant (LTI) state space models for quadrotors. This thesis focuses on the development of a 70 gram quadrotor with a maximum dimension of less than 6 inches. An MAV this small is extremely agile and in this case, dynamically unstable. Therefore it requires an active control scheme to stabilize the vehicle's attitude. To develop an effective controller using the vast array of available linear control tools, model knowledge is required. The scope of this thesis is to identify an LTI state space model of the 70 gram quadrotor mentioned about hover. Because the quadrotor is unstable, it cannot be flown without some form of feedback control present. To determine the bare airframe dynamics of a closed loop system, the feedback gains must be turned down as low as possible so as to not distort the natural response. Then the bare airframe dynamics can be calculated from the identified closed loop dynamics if the feedback gains and control law are known. Using the derived open loop system this thesis presents options for controller development for both a stabilizing inner loop, and a station keeping outer loop controller.Item Development of a Forced Oscillation Test Technique for Determination of MAV Stability Characteristics(2005-12-08) Everson, Daniel Peter; Pines, Darryll; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents the development and validation of a forced oscillation test technique for the determination of Micro Air Vehicle (MAV) stability characteristics. The test setup utilizes a scotch yoke mechanism to oscillate a MAV along a single axis at a fixed amplitude and frequency. The aerodynamic reaction forces to this sinusoidal perturbation are measured and converted into meaningful stability parameters. The purpose of this research is to demonstrate that forced oscillation testing is an effective means of measuring the stability parameters of a MAV. Initial tests show that the forced oscillation test process is returning results which match the expected trends. Comparison of the results to an analytical model of blade flapping shows that the experimental results are of the proper magnitude. It can be concluded from this research that forced oscillation testing is a feasible method for determining the stability parameters of MAVs.