A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Inertial Parameter Identification of a Captured Payload Attached to a Robotic Manipulator on a Free-Flying Spacecraft(2024) Limparis, Nicholas Michael; Akin, David L; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The groundwork for the dynamics of a free-flyer with a manipulator has been laid out by Yoshida, Vafa and Dubowsky, and Papadopoulos and Moosovian with the Generalized Jacobian Matrix, Virtual Manipulator, and Barycentric Vector Approach respectively. The identification of parameters for a robot manipulator has also been approached for industrial robots as well as through adaptive control theory. What is proposed is a method for inertial parameter identification and verification for a spacecraft with an attached manipulator that is an extension of the ground-fixed Inverse Direct Dynamic Model to function for a free-flying spacecraft. This method for inertial parameter identification for a spacecraft-manipulator system with an attached client spacecraft, debris, or other grappled payload is developed in this thesis and is experimentally tested using results for a servicer and an "unknown" grappled payload using three separate test beds. The results of the experiments show that the proposed method is capable of identifying the inertial parameters of the servicer and the grappled payload.Item Influence of Noise on Response Localizations in Mechanical Oscillator Arrays(2022) Cilenti, Lautaro Daniel; Balachandran, Balakumar; Cameron, Maria; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The dynamics of mechanical systems such as turbomachinery and vibration energy harvesting systems (VEH) consisting of one or multiple cantilever structures is often modeled by arrays of periodically driven coupled nonlinear oscillators. It is known that such systems may have multiple stable vibration steady states. Some of these steady states are localized vibrations that are characterized by high amplitude vibrations of a subset of the system, with the rest of the system being in a state of either low amplitude vibrations or no vibrations. On one hand, these localized vibrations can be detrimental to mechanical integrity of turbomachinery, while on the other hand, the vibrations can be potentially desirable for increasing energy yield in VEHs. Transitions into or out of localized vibrations may occur under the influence of random factors. A combination of experimental and numerical studies has been performed in this dissertation to study the associated transition times and probability of transitions in these mechanical systems. The developments reported here include the following: (i) a numerical methodology based on the Path Integral Method to quantify the probability of transitions due to noise, (ii) a numerical methodology based on the Action Plot Method to quantify the quasipotential and most probable transition paths in nonlinear systems with periodic external excitations, and (iii) experimental evidence and stochastic simulations of the influence of noise on response localizations of rotating macro-scale cantilever structures. The methodology and results discussed in this dissertation provide insights relevant to the stochastic nonlinear dynamics community, and more broadly, designers of mechanical systems to avoid potentially undesirable stochastic nonlinear behavior.Item INVESTIGATION OF COMPOUND ROTORCRAFT AEROMECHANICS THROUGH WIND-TUNNEL TESTING AND ANALYSIS(2022) Maurya, Shashank; Datta, Anubhav; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The aeromechanics of a slowed-rotor compound rotorcraft is investigated through wind-tunnel testing and comprehensive analysis. The emphasis is on a lift-offset wing compound with a hingeless rotor configuration. A new Maryland Compound Rig is developed and instrumented for wind-tunnel testing and an in-house rotor comprehensive code is modified and expanded for compound rotorcraft analysis. The compound rig consists of a lift compound model and a propeller model. The lift compound model consists of an interchangeable hub (articulated or hingeless), a fuselage, a half-wing of 70% rotor radius on the retreating side. The wing has a dedicated load cell and multiple attachment points relative to the rotor hub (16%R, 24%R, and 32%R and 5%R aft of the hub). The rotor diameter is 5.7-ft. The rotor has four blades with NACA 0012 airfoils with no twist and no taper. The wing incidence angle is variable between 0 to 12 degrees. The wing has a linearly varying thickness with symmetric airfoils NACA 0015 at the tip and NACA 0020 at the root. Sensors can measure rotor hub forces and moments, wing root forces and moments, blade pitch angles, structural loads (flap bending moment, lagbending moment, and torsional moment) at 25%R, pitch link loads, and hub vibratory loads. Wind tunnel tests are conducted up to advance ratio 0.7 for lift compound with half-wing at wing incidence angles of 4 and 8 degrees and compared with an isolated rotor. Hover tests are conducted up to tip Mach number of 0.5 to measure download penalty with the wing at various positions. The University of Maryland Advanced Rotorcraft Code (UMARC) is modified for compound rotorcraft analysis code. Aerodynamic models for the wing and the propeller are integrated. A recently developed Maryland Free Wake model is integrated, which can model the wake interaction between unequal and inharmonic speed rotor, wing, and propeller. The analysis is then validated with the test data. The validated analysis is used to analyze the US Army hypothetical full-scale aircraft. The compound rotorcraft is categorized into multiple configurations in a systematic manner to find the extreme limits of speed and efficiency of each. The key conclusions are: 1) slowing the rotor or compounding the configuration provide no benefit individually; they must be accomplished together, 2) Half-Wing is more beneficial if a lift-offset hingeless rotor is used, 3) hover download penalty is only 3% of net thrust, and this penalty can be predicted satisfactorily by free wake, 4) the main rotor wake interaction is more pronounced on the wing and less on the propeller, 5) the validated analysis indicates a speed of 240 knots may be possible with 20% RPM reduction along with a wing and propeller, if structural weights allow, and 6) the oscillatory and vibratory lag moments and in-plane hub loads may be significantly reduced by compounding.Item Modeling Viscoelastic Behavior Using Flexible Multibody Dynamics Formulations(2020) Nemani, Nishant; Bauchau, Olivier Prof.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Viscoelastic behavior is frequently observed in dynamical flexible multibody systems. In the simplest form it is manifested in one dimensional revolute and prismatic joints. Beyond which more complex force elements such as six degree of freedom flexible joints can also be found. Finally, beams, plates and shells are found to exhibit viscoelastic behavior too. In the past extensive work has been done on analyzing the dynamic response of three dimensional beams by performing cross-sectional analysis through finite element methods and subsequently solving the reduced beam problem. The approach is particularly relevant for the analysis of complex cross sections and helps improve computational efficiency significantly. A formulation which incorporates a viscoelastic model of the generalized Maxwell type with a solution of the three dimensional beam theory which gives an exact solution of static three dimensional elasticity problems is presented. Multiple examples incorporating the use of the aforementioned model in the context of viscoelastic beams and joints are presented. Shortcomings of the Kelvin-Voigt model, which is often used for flexible multibody systems, are underlined.Item PHYSICS-BASED AND DATA-DRIVEN MODELING OF HYBRID ROBOT MOVEMENT ON SOFT TERRAIN(2020) Wang, Guanjin; Balachandran, Balakumar; Riaz, Amir; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Navigating an unmapped environment is one of the ten biggest challenges facing the robotics community. Wheeled robots can move fast on flat surfaces but suffer from loss of traction and immobility on soft ground. However, legged machines have superior mobility over wheeled locomotion when they are in motion over flowable ground or a terrain with obstacles but can only move at relatively low speeds on flat surfaces. A question to answer is as follows: If legged and wheeled locomotion are combined, can the resulting hybrid leg-wheel locomotion enable fast movement in any terrain condition? To investigate the rich physics during dynamic interactions between a robot and a granular terrain, a physics-based computational framework based on the smoothed particle hydrodynamics (SPH) method has been developed and validated by using experimental results for single robot appendage interaction with the granular system. This framework has been extended and coupled with a multi-body simulator to model different robot configurations. Encouraging agreement is found amongst the numerical, theoretical, and experimental results, for a wide range of robot leg configurations, such as curvature and shape. Real-time navigation in a challenging terrain requires fast prediction of the dynamic response of the robot, which is useful for terrain identification and robot gait adaption. Therefore, a data-driven modeling framework has also been developed for the fast estimation of the slippage and sinkage of robots. The data-driven model leverages the high-quality data generated from the offline physics-based simulation for the training of a deep neural network constructed from long short-term memory (LSTM) cells. The results are expected to form a good basis for online robot navigation and exploration in unknown and complex terrains.Item Dynamics in Metal Halide Perovskites for Optoelectronics(2020) Howard, John Michael; Leite, Marina S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A diverse portfolio of renewable energy technologies is required to limit global warming to less than 2 ◦C. Of the possible emissions-free options, photovoltaic (PV) technologies can be most widely deployed, given the abundance of the solar resource compared. As with all power generation sources, PV adoption is predicated on the availability of technology solutions that are both inexpensive and highly efficient. One solar cell material, the metal halide perovskites (MHP), may provide the ideal combination, with > 25% efficiency devices within the first decade since their invention fabricated through simple spin coating. Despite the unprecedented rise in MHP performance, stability remains a critical challenge with the most stable devices at the 1-year benchmark compared to the >25-year lifetime of Si-based PV. Further progress concerning enduring power output will require a fundamental understanding of the impact of environmental stressors (light, temperature, bias, oxygen, and water) on the basic physical processes governing solar cell operation. Therefore, my dissertation elucidates the interplay between the ambient environment and MHP composition on both the optical and electrical behavior using in situ methods. The first part of my thesis elucidates the time-dependent optical and elec- tronic response of different MHP compositions using different in situ microscopy techniques. I capture the transient photovoltage of both Br- and I-containing per- ovskites for different photon energies using heterodyne Kelvin probe measurements. My measurements demonstrate that the voltage rise (light ON) is 104× faster than the subsequent decay (light OFF). Uniquely, the decay time for the residual voltage depends on the excitation wavelength, but only for the MAPbBr3 thin film. Next, I spatially and temporally resolve the relationship between radiative recombination and relative humidity (rH) for multi-cation films. The time-dependent photolumi- nescence (PL) indicates that the Cs-Br ratio impacts the magnitude of light emission hysteresis across an rH cycle. Further, I establish the existence of a repeatable and reversible ≈25× PL gain for multiple moisture cycles up to 70% rH. The second part of my thesis establishes the ability of machine learning (ML) models to predict the time-dependent behavior of perovskite material properties. I collect a comprehensive set of humidity-dependent PL data for both MAPbBr3 and MAPbI3 perovskites. I then use that data to train recurrent neural networks to forecast light emission based on only the recorded rH values. Using Echo State Networks, I achieve a normalized root-mean-squared error of <11% for both compositions for a 12+ h prediction win- dow. Further, I use a Long Short Term Memory network to predict the PL from a degrading sample, achieving <5% error. My in situ measurements and predictive ML models provide a powerful framework for identifying structure-property rela- tionships and can help accelerate the development of long-term stable perovskite materials.Item Wind Tunnel Test on Slowed Rotor Aeroechanics at High Advance Ratios(2020) Wang, Xing; Chopra, Inderjit; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In forward flight, slowing down a rotor alleviates compressibility effects on the advancing side, extending the cruise speed limit and inducing high advance ratio flight regime. To investigate the aerodynamic phenomena at high advance ratios and provide data for the validation of analysis tools, a series of wind tunnel tests were conducted progressively with a 33.5-in radius, 4-bladed Mach-scaled rotor in the Glenn L. Martin Wind Tunnel. In the first stage of the research, a wind tunnel test was carried out at high advance ratios with highly similar, non-instrumented blades and on-hub control angle measurements, in order to gain a baseline performance and control dataset with minimum error due to blade structural dissimilarity and pitch angle discrepancy. The tests were conducted at advance ratios of 0.3 to 0.9, and a parametric study on shaft tilt was conducted at $0^\circ$ and $\pm 4^\circ$ shaft tilt angles. The test data were then compared with those of previous tests and with the predictions of the in-house comprehensive analysis UMARC. The airload results were investigated using comprehensive analysis to gain insights on the influences of advance ratio and shaft tilt angle on rotor performance and hub vibratory loads. Results indicate that the thrust benefit from backward shaft tilt is dependent on the change in the inflow condition and the induced angle of attack increment, and the reverse flow region at high advance ratios is the major contributor to changes in shaft torque and horizontal force. In the second stage of the research, the rotor blades were instrumented with pressure sensors and strain gauges at 30\% radius, and pressure data were acquired to calculate the sectional airloads by surface integration up to an advance ratio of 0.8. The test results of blade airloads and structural loads were compared with the predictions of comprehensive analysis (UMARC and PrasadUM) and CFD/CSD coupled analysis (PrasadUM/HAMSTR). The focus was on the data correlation between experimental pressure, airload and structural load data and the CFD/CSD predicted results at various collective and shaft tilt settings. Overall, the data correlation was found satisfactory, and the study provided some insights into the aerodynamic mechanisms that affect the rotor airload and performance, in particular the mechanisms of backward shaft tilt, hub/shaft wake and the formation of dynamic stall in the reverse flow region. The next stage focused on hingeless rotor with lift offset. Previous wind tunnel tests have shown that an articulated rotor trimmed to zero hub moment generates limited thrust at high advance ratios, because the advancing side needs to be trimmed against the retreating side with significant reverse flow, in which the rotor is ineffective in generating thrust. Therefore, a hingeless rotor that allows the advancing side to generate more thrust can be rewarding in overall thrust potential. Wind tunnel tests were conducted up to an advance ratio of 0.7 to investigate the behavior of hingeless rotors at high advance ratios with lift offsets. Performance, control angles, hub vibratory loads and blade structural loads were compared with comprehensive analysis predictions from UMARC, plus the wing performance predictions from AVL. The results demonstrate that a hingeless rotor with lift offset is more efficient in generating thrust and exhibits higher lift-to-drag ratio at high advance ratios. The blade structural load level is significantly higher compared to an articulated rotor, especially for 2/rev flap bending moment, which can pose a critical structural constraint on the rotor.Item Dynamics and Control of a Hovering Quadrotor in Wind(2019) Craig, William Stephen; Paley, Derek A; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quadrotor helicopters show great promise for a variety of missions in commercial, military, and recreational domains. Many of these missions require flight outdoors where quadrotors struggle, partially due to their high susceptibility to wind gusts. This dissertation addresses the problem of quadrotor flight in wind with (1) a physics-based analysis of the interaction between the wind and the quadrotor, (2) the addition of flow sensing onboard the quadrotor to measure external wind, and (3) both linear and nonlinear control development incorporating flow sensing and taking aerodynamic interactions into account. Using flow measurements in addition to traditional IMU sensing enables the quadrotor to react to the wind directly, rather than delaying until the wind affects the rigid-body dynamics as with IMU sensing alone. The aerodynamic response of a quadrotor to wind is modeled using blade flapping, which characterizes the tilt of the rotor plane a result of uneven lift on the blades. The model is validated by mounting a motor and propeller to a spherical pendulum and subjecting it to a wind gust. The blade-flapping model is utilized in a nonlinear geometric feedback-linearization controller that is built in a cascaded framework, first developing the inner-loop attitude controller, then the outer-loop position controller. The controller directly cancels the forces and moments resulting from aerodynamic disturbances using measurements from onboard flow probes, and also includes a variable-gain algorithm to address the inherent thrust limitations on the motors. A linear model and controller is also developed, using frequency-domain system-identification techniques to characterize the model, and handling-qualities-criteria based optimization to select gains. A linear model of the aerodynamic interactions, based on the blade-flapping work, provides flow-feedback capability similar to the nonlinear controller. Experimental testing is performed for each of the developed controllers, all of which show improvement through the use of flow feedback. Attitude is tested independently by mounting the quadrotor on a ball-joint, allowing for both gust and saturation testing. Gust rejection is also tested for both linear and nonlinear controllers in free flight, showing further benefits than considering attitude alone.Item MICRO AIR VEHICLE SCALE GUST-WING INTERACTION IN A WIND TUNNEL(2018) Smith, Zachary Francis; Jones, Anya R; Hrynuk, John T; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Studying isolated gust effects on simple airfoil models in a controlled environment is a necessity to further the development of MAV gust response and control laws. This work describes the creation of a vertical gust generator in a low speed, low turbulence wind tunnel through the use of an actuated fan placed below the tunnel and ducted through its floor. Gusts of up to 40% of the freestream velocity were created. Characterization of the gust generator is shown, and its interaction with a stationary wing at several angles of attack is evaluated. The actuated gust profile is also compared to that of a pitched wing in a gust-less environment with many visible similarities.Item Analysis of Helicopter Flight Dynamics through Modeling and Simulation of Primary Flight Control Actuation System(2015) Nelson, Hunter Barton; Celi, Robertro; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A simplified second-order transfer function actuator model used in most flight dynamics applications cannot easily capture the effects of different actuator parameters. The present work integrates a nonlinear actuator model into a nonlinear state space rotorcraft model to determine the effect of actuator parameters on key flight dynamics. The completed actuator model was integrated with a swashplate kinematics where step responses were generated over a range of key hydraulic parameters. The actuator-swashplate system was then introduced into a nonlinear state space rotorcraft simulation where flight dynamics quantities such as bandwidth and phase delay analyzed. Frequency sweeps were simulated for unique actuator configurations using the coupled nonlinear actuator-rotorcraft system. The software package CIFER was used for system identification and compared directly to the linearized models. As the actuator became rate saturated, the effects on bandwidth and phase delay were apparent on the predicted handling qualities specifications.