DSpace Repository :: Browsing by Author "Williams, Keith A."

Browsing by Author "Williams, Keith A."

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Acoustic methods for regionalizing an impact force acting on a helmet structure
(University of Alabama Libraries, 2018) Davis, Jacob; Shepard, W. Steve; University of Alabama Tuscaloosa
It is often desired to know the location and magnitude of a force acting on a structure. Unfortunately, it is not always possible or desirable to install a sensor at the force location, such as when the force location is unknown or when the application of a force sensor would change the force transmission characteristics. A structure subjected to an impact has many different vibrational modes that are excited to different levels based on the excitation location. These vibrations decay with time depending on their different rates of modal damping and the associated acoustic radiation characteristics. This response of the structure can be measured and used to inversely reconstruct the input force. It is theoretically possible to use acoustic measurements for force reconstruction, but the method involved would be extremely difficult. In this study, approaches that are much simpler and easier to implement were considered. Acoustic signatures for several structure impact locations were measured, normalized relative to the force magnitude, and processed to examine the ability to correlate the acoustic signal to the force impact location. Various processing techniques, such as the Short Time Fourier Transform, were considered. A primary interest focused on the ability of using single-number metrics that describe features of the acoustic signature to aid in identifying the force location. For the experiments, a football helmet structure was used and multiple impact locations on the helmet were tested. The ability of these acoustic signatures, including those processed into single-number metrics, to aid in identifying the impact location was assessed.
Analytical modeling and design optimization of piezoelectric bimorph energy harvester
(University of Alabama Libraries, 2010) Zhang, Long; Williams, Keith A.; University of Alabama Tuscaloosa
As wireless sensor networks continue to grow in size and scope, the limited life span of batteries produces an increasingly challenging economic problem, in terms of not only the capital cost of replacing so many batteries, but also the labor costs incurred in performing battery replacement, particularly with sensor nodes in remote or limited-access locations. This growing problem has motivated the development of new technologies for harvesting energy from the ambient environment. Piezoelectric energy harvesters (PEH) are under consideration as a means for converting mechanical energy, specifically vibration energy, to electrical energy, with the goal of realizing completely self-powered sensor systems. There are three primary goals with regards to this study. The first goal is to develop an analytical model for the resonant frequency of a piezoelectric cantilever bimorph (PCB) energy harvester, aiming to study the geometric effects of both the piezoelectric bimorph and the proof mass on the resonant frequency of a PEH. The analytical model is developed using the Rayleigh-Ritz method and Lagrange's equation of motion and is validated by finite element analysis (FEA) and laboratory experiments. It is shown that this analytical model is better at predicting resonant frequencies than a model currently available in the literature. The second goal is the development of an enhanced analytical model for the voltage and power output of the PCB. The modified analytical model is realized using the conservation of energy method and Euler-Bernoulli beam theory. It is compared with a general equivalent spring-mass-damper model and an equivalent electrical circuit model, and validated by the laboratory prototype experiments. The results show that the modified model provides improved prediction of PCB voltage and power output. Simultaneously, finite element analysis on piezoelectric structures using the commercially available software package ANSYS® Multiphysics is also carried out to study the dynamic response of the PCB in terms of both tip displacements and the electrical potentials of the top and bottom electrodes. It is shown that the simulations are quite close to the experimental results, in terms of both peak frequencies and peak amplitudes. The third goal is the design optimization of the PCB energy harvester in order to maximize the power harvesting from the ambient vibration. Three design optimization approaches are carried out, including multi-parameter optimization of the single PCB generator using a genetic algorithm (GA), a band-pass generator design with a group of the PCB generators based on the system transfer function, and the new design features of the PCB generator for consideration of the improvements of the strain energy and the lifetime. The results of the optimized designs are validated through FEA, and the discrepancies between the theoretical derivation and FEA are also analyzed. Other optimal design considerations are also discussed.
Battery charge and discharge control for energy management in EDV and utility integration
(University of Alabama Libraries, 2012) Bao, Ke; Li, Shuhui; University of Alabama Tuscaloosa
Electric drive vehicles (EDVs) have many benefits as compared to normal petrol or gas cars. Moreover, the electrification of transportation systems would enable increased electricity generation from carbon-free and renewable energy sources, such as wind, solar, and hydro. However, due to highly distributed and mobile nature as well as high charge and discharge power demand of EDVs, it is important to investigate how to manage EDV charge and discharge to enhance the usage of renewable enough resources in the future smart grid framework. For this purpose, this thesis first investigates typical battery electrochemical properties which are important concerns for the design of EDV charge and discharge. In this section, mathematical and circuit-oriented battery models are investigated to reflect typical battery electrochemical properties. Meanwhile, the relation between mathematical and circuit-oriented battery models is analyzed. Then, this thesis presents an energy control study in a charging station, a typical integrated EDV and utility system. The charging station consists of an AC/DC converter for grid interface and multiple dc/dc converters for EDV battery management. For the grid-side converter, a direct-current control mechanism is employed for reactive power, ac system bus voltage, and DC-link voltage control. For the EDV-side converters, constant-current and constant-voltage control mechanisms are investigated for charging and discharging control. The thesis considers energy management need for charge and discharge of multiple EDVs simultaneously as well as energy transferring from vehicle to grid and grid to vehicle requirements. A real-time simulation model is investigated and the performance of the integrated EDV and utility system is investigated.
A computational investigation of impulsive and pulsed starting annular jets
(University of Alabama Libraries, 2011) Abdel-Raouf, Emad; Baker, John; University of Alabama Tuscaloosa
A computational study is carried out on low Reynolds number impulsive and pulsating annular jets. This work is inspired by the biological flow of marine life that uses jet propulsion for self maneuver. Marine life such as squids and jellyfish propel themselves by discharging a water jet followed by a refilling phase. The discharging portion is a starting jet, i.e. the releasing of a moving fluid into a quiescent fluid, while the refilling phase can be viewed as an inflow jet. The combined jets will be called fully oscillating jets. Although fully oscillating jets have been indirectly examined experimentally, they have never been studied computationally. This dissertation is divided into three investigations that examine the starting jet, inflow jet, and fully oscillating jet based on the resultant force (i.e. either thrust or suction force) at the annulus exit plane, jet efficiency, and vortex dynamics. Furthermore, each of the following three performance criterion is examined under various velocity imposed boundaries (i.e. impulsive, unit pulsed, and sinusoidal pulsed jets), ambient pressure, and blocking ratios. An axisymmetric, incompressible and unsteady Navier Stokes numerical model was used to implement the analysis. The model was validated against theoretical and experimental results, where both result types bounded the computational results of this endeavor. In addition, numerical verification was carried out on each of the three investigations ensuring grid and time independent results. Several substantial outcomes were drawn from the results of the three investigations. The numerical results confirmed previously published experimental data regarding the universal dimensionless time scale (i.e. vortex formation number) of optimal vortex ring development triggered by starting jets. Moreover, the computational results showed evidence that the vortex formation number was not affected by ambient pressure nor blocking ratio. The computational results also confirmed earlier experimental findings that pulsed jet inflows trigger a standing vortex ring. Furthermore, the current study showed that impulsive jet inflows do not trigger vortex ring formation. In addition, unlike the expected net thrust of zero due to mass flux, fully oscillating jets showed evidence of thrust augmentation due to the enhanced entrainment caused by the vortex formation.
Control and power management schemes for distributed and battery powered systems
(University of Alabama Libraries, 2016) Huang, Wangxin; Abu Qahouq, Jaber A.; University of Alabama Tuscaloosa
Battery systems are widely used in many applications including portable electronics, EVs/HEVs, and distributed smart power grids. In addition to battery technologies, the battery management system (BMS) plays a critical role in enabling the widespread adoption of battery-powered applications. This dissertation work focuses on addressing several issues and improving performance of several aspects of battery powered applications. These focused topics include online monitoring of battery impedance, charge balancing between battery cells during both discharging and charging operation, and power electronic topologies and control in order to improve reliability, efficiency, and density of the battery-powered applications. In chapter 2, a practical method is presented in order to achieve accurate online battery impedance measurement while maintaining output voltage regulation of the power converter. The proposed method is based on converter duty cycle control and perturbation. As a result, all the external signal injection circuitries are eliminated. In chapter 3 and 4, the charge balancing issue is addressed from the root by automatically adjusting the discharge/charge rate of each cell based on a new distributed battery system architecture with energy sharing control. The proposed energy sharing controller does not require any charge/energy transfer between the cells, thus eliminating the power losses during energy transfer process. To gain insights into the dynamics of the energy sharing controlled distributed battery system, the state-space averaging small-signal modeling and controller design is performed in Chapter 5. Simulation and experimental results are presented for verification. Single-inductor multiple-output DC-DC converter has gained increased popularity in the portable applications where a battery is used to power multiple loads. However, a common issue facing the SIMO converter design is the cross regulation between the multiple outputs during steady-state and dynamic operations. To address this issue, a power-multiplexed controller is presented in Chapter 6 which eliminates the cross regulation between the outputs by multiplexing the conduction of each output channels. Each output is independently regulated under steady-state and dynamic operations regardless of the operating mode, i.e., continuous or discontinuous conduction mode. Chapter 7 summarizes this work and provides conclusions before discussing some possible future research directions related to this dissertation work.
Control of power converter for grid integration of renewable energy conversion and STATCOM systems
(University of Alabama Libraries, 2009) Xu, Ling; Li, Shuhui; University of Alabama Tuscaloosa
Investment in renewable energy is rapidly increasing worldwide. This is in response to a number of global challenges and concerns, including climate change, increasing energy demand, and energy security. The investment is widely spread over the leading renewable energy technology sectors: wind, solar, biofuels, biomass, and fuel cells. Among those, wind, solar photovoltaic, and fuel cells require power electronic converters for grid integration. This thesis investigates advanced control technology for grid integration control of renewable energy sources and STATCOM systems. First, the conventional control mechanism of power converters applied in renewable energy conversion and STATCOM systems is studied. Through both theoretical and simulation studies, a deficiency of the conventional control mechanism is identified. It is found that malfunctions of traditional power converter control techniques may occur when the controller output voltage exceeds the converter linear modulation limit. Then, the thesis proposes a novel control mechanism consisting of a current control loop and a voltage control loop. The proposed control mechanism integrates PID, adaptive, and fuzzy control techniques. An optimal control strategy is developed to ensure effective active power delivery and to improve system stability. The behaviors of conventional and proposed control techniques are compared and evaluated on both simulation and laboratory hardware testing systems, which demonstrates that the proposed control mechanism is effective for grid integration control over a wide range of system operating conditions while the conventional control mechanism may behave improperly, especially when the converter operates beyond its linear modulation limit and under variable system conditions.
Coordination of systems of mobile robots with social potential functions
(University of Alabama Libraries, 2009) Samples, Richard Patrick; Leland, R. P.; University of Alabama Tuscaloosa
The research in this dissertation is concerned with developing a method to produce flocking behavior in a swarm of mobile robots. The dissertation itself is divided into three articles. The research in the first article is a proof-of-concept that demonstrates that one can create a swarm of mobile robots that engage in flocking behavior by the use of a position tracking controller in concert with a method for defining the desired trajectory for each object. The research in the second article develops a method for implementing both attractive and repulsive artificial potential functions with a position tracking controller. The research in the third article builds on that in the first two articles in order to construct a large swarm of mobile robots that engages in flocking behavior using a social potential function that is contructed from the attractive and repulsive functions of the second article. Each article contains both control-theoretic analysis of robot behavior and demonstrations of the behavior using Matlab simulations. The overall result is a successful, yet relatively simple, method for creating a swarm of mobile robots in which each robot has a degree of freedom of action.
Counter-balancing mechanism for improving independence when using an exoskeleton
(University of Alabama Libraries, 2013) Lathan, Drew; Todd, Beth A.; University of Alabama Tuscaloosa
An exoskeleton is a robotic device used in assisting paraplegics with standing and walking. Existing designs use a series of DC motors and brakes to move the different parts of the device. Some exoskeletons mimic the musculoskeletal system by sending signals to a computer that tells the motors to rotate the knee, ankle, and hip joints appropriately for correct forward movement of the device. Users of the devices have a walker or crutches with controls to aid in balance. The goal of this project is to provide complete independence by removing the need for these walking aids. A new leg orthotic has been designed that may be implemented on any exoskeleton device to maintain balance in the fore-aft direction. A series of fast-acting electric actuators respond to the individual's movements. If at any point the device begins to tip, the actuators engage in such a way that the user's leg is brought back to an up-right position allowing balance to be recovered. As this movement takes place, the normal actions of the device's DC motors and brakes are also engaged to avoid falling (the reactions from the motors and brakes are already a feature of current exoskeleton designs.) This is a counter-balancing mechanism and could provide more independence to paraplegics in the future.
Design and control of powered lower limb prostheses
(University of Alabama Libraries, 2016) Wu, Molei; Shen, Xiangrong; University of Alabama Tuscaloosa
In the development of powered lower-limb prostheses, providing sufficient power and torque to support amputees’ locomotion is a major challenge, considering prostheses’ weight and size limits. Furthermore, regulating the power delivery during locomotion is equally important that gives amputees safe and natural movements. This dissertation aims to address these challenges by investigating new approaches in the actuation and control of powered lower-limb prostheses, with the overarching objective to obtain compact, powerful lower-limb prostheses that interact with amputees and the environment in a coordinated manner. The initial efforts were focused on the design and control of transfemoral (TF, also known as above-knee) prostheses powered by pneumatic muscles, an extraordinary actuator with superb power-to-weight ratio. The first prototype incorporates powered knee and ankle joints in a volumetric profile similar to that of human leg. The unique feature is a single-acting-spring-return mechanism, in which a single pneumatic muscle drives the motion in the torque-demanding direction, while a set of mechanical springs drives the motion in the opposite direction. A finite-state impedance controller has been developed for this prosthesis, which was demonstrated to provide a natural gait. Based on previous success, a novel type of pneumatic muscle, namely double-acting sleeve muscle (DASM), was examined to replace traditional pneumatic muscle. Incorporating a second chamber, the DASM is able to provide additional extensional force without using return springs. Therefore, the prosthesis can be significantly simplified into a more compact and lightweight device. Compared with pneumatic muscles, traditional cylinder-type actuators are more technologically mature. Therefore, the subsequent efforts were to develop a pneumatic cylinder-actuated TF prosthesis, which has great potential for real-world applications. All peripheral components were integrated, including a carbon fiber air tank as the energy source, and the prosthesis’ capability of independent, untethered operation was demonstrated in human walking test. In addition to the improvement of prosthetic design, control methods were also investigated. The results include an integrated walking – stair climbing controller and a sit-to-stand controller. Both were developed based on biomechanical analysis of the knee dynamics in human locomotion. In the walking – stair climbing control system, an improved finite state impedance controller was constructed, which incorporates a unique time function to enable gradual energy injection during weight acceptance phase. An intuitive thigh position-based switching condition was introduced to merge the walking and stair climbing controllers into one system. In the sit-to-stand controller, a similar controller was established, which eliminates the need for a state machine and significantly simplifies the controller tuning and implementation. The human testing was conducted with results demonstrating the effectiveness of both control systems.
Design and control of powered transtibial prostheses
(University of Alabama Libraries, 2017) Zheng, Hao; Shen, Xiangrong; University of Alabama Tuscaloosa
The primary purpose of lower extremity prostheses is to restore the locomotive functions of amputees’ lost sections and joints. The objective of this dissertation is to develop energetically active transtibial (TT, also known as below-knee) prostheses with powered ankle joints to generate sufficient power and torque with a compact form factor. Firstly, pneumatic sleeve muscle actuators have been designed and tested to investigate the feasibility of application of such type of actuator in TT prosthesis. Experimental results obtained on the prototypes validated and proved that sleeve muscle is a good fit for robotic systems with asymmetric force/torque requirements. Although the pneumatic sleeve muscle can provide sound force/torque capacities, it is challenging to build a transtibial prosthesis with this type of actuator that can match the size and weight of the human ankle. Hence, the subsequent efforts are directed towards utilizing a more compact actuation unit – pneumatic cylinder, which is well known for its capability in generating large force/power output with light weight and compact volumetric profile. Thus, a transtibial prosthesis has been designed using pneumatic cylinder. A finite-state impedance controller (FSIC) has been developed as a walking controller, and the parameters are tuned in walking experiments. The results from the experiments proved that the prosthesis is able to provide an improved gait compared with the traditional passive prosthesis. Additionally, a model that characterizes the stiffness and equilibrium point as functions of the chamber air masses in the pneumatic cylinder has been developed and a predictive pressure control algorithm was used to improve pressure control performance while minimizing valve actions. This enables the pneumatic actuator to be used as a variable series elastic actuator (VSEA). Experimental results showed that the proposed approach is able to provide the desired elastic characteristics of an artificial spring in stiffness control and demonstrated the advantages of this new approach for potential prosthetic applications. Lastly, the dissertation presents VSEA-powered TT prosthesis with direct implementation of the finite-state impedance control (FSIC). The human subject walking experiments were also conducted, and the results demonstrated the effectiveness of the direct FSIC prosthesis control approach.
Design Innovations for Rehabilitation and Assistive Robotics
(University of Alabama Libraries, 2021) Afsar, Md Rayhan; Shen, Xiangrong; University of Alabama Tuscaloosa
When accelerated aging becomes a major trend all over the world, mobility impairment and the resulted disability are affecting an increasing number of individuals. People’s mobility can be impaired due to a variety of reasons (limb loss, age-related functional decline, neuromuscular pathologies, etc.), but they all face a common set of difficulties in daily life (slow walking speed, difficulty in stair climbing, difficulty in standing up, etc.). Motivated by this increasingly important and challenging problem, research in this dissertation aims at developing multiple robotic assistive and rehabilitation devices with the common purpose of enhancing people’s mobility and physical capacity and enabling them to live a higher-quality, more productive life. The first type of assistive robot in this dissertation, a novel robotic lower-limb prosthesis is presented. This new robotic prosthesis adopts a unique design framework of Common Core Components Knee-Ankle Prosthesis (C3KAP), and thus is able to provide the desired dynamic performance to support healthy human-like walking while significantly reducing the complexity and cost. The second type of assistive device in this dissertation is the Quadrupedal Human-Assistive Robotic Platform (Q-HARP). As a legged assistive robot functioning as a smart walker, the Q-HARP provides an unprecedented potential to adapt to a wide variety of challenging terrains, many of which are common in people’s daily life (e.g., roadside curbs and the few steps leading to a front door). The third type of assistive robot is RailBot, addresses the difficulty in stair climbing. It is a novel rail-sliding robot providing powered assistance in a user’s stair climbing process. With its unique mode of operation and assistance, the compact RailBot system can be installed in almost any homes with stairs and help the large number of elderly residents to age in place. As the final part of this dissertation, Semi-Wearable Sit-to-Stand Assist (SW-SiStA) which provides powered joint assistance to a mobility-challenged individual during the sit-to-stand process. Unlike traditional exoskeletons, this semi-wearable assistive device can be easily detached after the completion of the sit-to-stand process, and thus allows the user to continue the subsequent ambulation without carrying extra weight.
Design of a powered above knee prosthesis using pneumatic artificial muscles
(University of Alabama Libraries, 2010) Waycaster, Garrett Clinton; Shen, Xiangrong; University of Alabama Tuscaloosa
This paper describes the mechanical design for both a one and two degree of freedom above-knee (AK) prosthesis actuated by pneumatic artificial muscles. Powered prosthetics aim to improve the quality of life of the 50% of AK amputees who never regain the ability to walk. Pneumatic artificial muscle (PAM) provides great potential in prosthetics, since this type of actuator features a high power density and similar characteristics to human muscles. Currently, commercially available AK prosthetics are largely passive devices, and no research has been conducted on PAM actuators in AK prosthetics. In this thesis, the design requirements of an above knee prosthesis using PAM are discussed and a prototype one degree of freedom prosthesis with a PAM actuated knee joint is constructed. This prototype is then tested, and based on the results a new actuator is developed. This new actuator uses a flexible tendon and an elliptical pulley to improve torque, adding more functionality and increasing the maximum mass of a user by 25 kilograms. This actuator is also tested and compared to the initial prototype design. Finally, this new actuator is incorporated into the design of a two degree of freedom prosthesis with an actuated ankle as well as the knee joint.
Design, instrumentation, and control of a heliogyro solar sail ground test demonstrator
(University of Alabama Libraries, 2016) Jenkins, Arik T.; Williams, Keith A.; University of Alabama Tuscaloosa
Jenkins, Arik Thomas (M.S., Mechanical Engineering) Design, Instrumentation, and Control of a Heliogyro Solar Sail Ground Test Demonstrator Thesis directed by Dr. Keith Williams The goal of this research is to establish a means for terrestrial testing and model verification for heliogyro solar sail technology. A simplified testbed was in place but utilized unrealistic flight hardware and software, including a low resolution stepper motor with incomplete range of motion, a camera located at an unrealistic tracking angle, and a LabView-based control system. The work presented here is meant to enable macro-scale controls and dynamic testing for blade specimens, whereas previous experiments have been primarily theoretical and have used small-scale physical apparatus solely for dynamic property validation. Additionally, two control methods showcasing the use of this system are to be developed and the results reported. The hardware and software systems were replaced with a realistic flight motor, a camera for target tracking using a realistic location, and light Python control program capable of being used on a micro-computer platform. Several methods for target tracking were investigated, and hue, saturation, value filtering was chosen as the most reliable and accurate method under the current sensing conditions. A test blade specimen was designed and built with design variables chosen such that its dynamic properties matched that of a portion of the NASA heliogyro concept, HELIOS. System identification with several pitch profiles yielded useful dynamic information about the system. The system remains in the linear range for all collective profiles, while cyclic profiles show significant nonlinear behavior. Open loop input shaping was chosen as the control method for collective profiles and showed up to 93% residual vibration reduction in pitch maneuvers of up to 45 degrees. A model-referenced feedback approach was chosen for cyclic control and was shown to be effective for reference tracking of cyclic maneuvers of up to 60 degrees. In all cases the model was capable of driving the magnitude of oscillation to at least 90% of the desired magnitude, easing the burden on the feedback control to the final 10%.
Development of a high-fidelity engine modeling framework in Simulink with automated combustion parameter tuning
(University of Alabama Libraries, 2017) Thompson, Bradley Adam; Yoon, Hwan-Sik; University of Alabama Tuscaloosa
The automotive industry continually seeks to improve performance and fuel efficiency due to increasing fuel costs, consumer demands, and greenhouse gas regulations. With advancements in computer-aided design, engine simulation has become a vital tool for product development and design innovation, and as computation power improves, the ability to optimize designs improves as well. Among the simulation software packages currently available, Matlab/Simulink is widely used for automotive system simulations but does not contain a detailed engine modeling toolbox. To leverage Matlab/Simulink’s capabilities, a Simulink-based 1D flow engine modeling architecture is proposed. The architecture allows engine component blocks to be connected in a physically representative manner in the Simulink environment, therefore reducing model build time. Each component model, derived from physical laws, interacts with other models according to block connection. The presented engine simulation platform includes a semi-predictive spark ignition combustion model that correlates the burn rate to combustion chamber geometry, laminar flame speed, and turbulence. Combustion is represented by a spherical flame propagating from the spark plug. To accurately predict the burn rate, the quasi-dimensional model requires tuning. A method is proposed for fitting turbulence and burn rate parameters across an engine’s operating space. The method reduces optimization time by eliminating the intake and exhaust flow models when evaluating the fitness function. Using the proposed method, 12 combustion model parameters were optimized to match cylinder pressure. Optimization and validation results are given for a 2.0 L Mazda Skyactiv-G engine.
Development of neural network-based computer vision system for automated grading operation of a hydraulic excavator
(University of Alabama Libraries, 2018) Xu, Jiaqi; Yoon, Hwan-Sik; University of Alabama Tuscaloosa
Hydraulic excavators are widely used in the construction and mining industries. While conventional hydraulic excavators have been manually operated by a human operator, automated systems are being developed as an effective alternative to manual operations for common tasks that excavators routinely perform in typical work sites. An example is an automated ground grading system that can enhance the productivity of an excavator by assisting an operator to perform ground grading in a fast and accurate manner. For this purpose, a sliding mode controller is developed for automated grading of a hydraulic excavator in this research. First, an excavator manipulator model is developed in Simulink by using SimMechanics and SimHydraulics toolboxes. Then, the sliding mode controller is used to control the manipulator to trace a predefined trajectory for a grading task. The simulation results show that the sliding mode controller can control the grading operation with less tracking errors than a PI controller. As an alternative to conventional displacement sensors in an automated excavation system, a novel approach to estimate the position of a hydraulic manipulator using a neural network-based vision system is also studied in this research. A webcam is used to capture images of a moving manipulator, and the captured images are used to train neural networks. Then, the trained neural networks can be used to estimate the position of the excavator manipulator for a feedback control system. A case study was conducted to investigate the factors that affect the performance of the neural networks. A simulation study shows a stable grading performance when a PI controller is used to control the manipulator based on the estimated manipulator position.
Dynamic behavior and degradation of a fuel cell system
(University of Alabama Libraries, 2010) Jung, Minjae; Williams, Keith A.; University of Alabama Tuscaloosa
In this dissertation, the effect of dynamic operation of a Ballard Nexa 1.2kw fuel cell system is investigated. Three specific topics are considered: the first is an analysis of the dynamic behavior of the fuel cell system, the second is an evaluation and examination of fuel cell membrane degradation during dynamic operation and the last is numerical simulation to predict the transient response in the cell voltage. To enable the analysis of the fuel cell system's dynamic behavior, a simple method for analyzing the system's voltage response to a step change in load resistance is presented. A modified Randles model is used as the system model, where two resistors and two capacitors are implemented for the Warburg impedance. Using that model, the response is fitted with three exponential curves. Six independent equations corresponding to six parameters of the model can be solved using the fitted values, under a specific assumption for the initial state. The impedance is also simulated using the estimated parameters. Cyclic operation is thought to have a negative impact on fuel cell lifetime. The frequency effect of the cyclic operation on chemical degradation is investigated. After calculating each parameter value through exponential curve fitting, the dynamic behaviors of the three resistor-capacitor pairs are simulated using MATLAB Simulink®. In addition, fluoride release as the change of the frequency of cyclic operation is evaluated by measuring the concentration of fluoride ion in effluent of a fuel cell. The frequency effect on chemical degradation is explained by comparing the simulated result and the fluoride release result. Finally, a single-phase numerical model to predict the transients in voltage of a PEMFC is presented. A new approach is developed by classifying the current density by two groups; charging currents which are accumulated in the interfaces where the reaction occurs, and faradaic currents which are charge transfer currents. The successive change of the activation overpotential is calculated by using the charging currents and the element law for an ideal linear capacitor, and then the transient voltage response to a step load change is shown in results.
The effects of drive cycle accessory load and degree of hybridization on fuel economy and emissions for hybrid electric buses
(University of Alabama Libraries, 2010) Chen, Dennis; Midkiff, K. Clark; University of Alabama Tuscaloosa
Hybrid Electric Vehicles (HEVs) have gained much attention in recent years. This is mainly due to rising fuel prices and increasing environmental awareness. By implementing electricity as one of the power sources, a HEV can not only reduce fuel consumption but can also decrease tailpipe emissions. In this thesis, the software package Powertrain Systems Analysis Toolkit (PSAT) was chosen as the simulation tool to model several bus powertrain configurations - conventional, three different degrees of hybridization parallel hybrid electric (PHEB), and a series hybrid electric (SHEB) to predict fuel economy and emissions level. The simulations were run with a typical accessory load, 15 kW, for a 40-foot transit bus as well as for no accessory load. The effect of accessory load on fuel economy was identified. Four different drive cycles - Manhattan, UDDS, CBD, and WVU City cycles - that covered a wide range of driving conditions were chosen as the testing cycles for the simulations. For no accessory load, it was found that the PHEB1, which had the highest degree of hybridization, yielded the best fuel economy improvement on all four drive cycles. The highest fuel economy improvement without accessory load, 121.9%, was found for the Manhattan cycle. The maximum fuel economy improvement, 51.8%, for a 15 kW accessory load also occurred running the Manhattan cycle, and was achieved by the PHEB1 as well. The maximum fuel economy reduction with a 15 kW accessory load was 48.9%. The largest emissions reductions with a 15 kW of accessory load were achieved by the PHEB1 operated in the Manhattan cycle, with NOx and PM emissions reductions of 73.4% and 45.9% over the conventional bus, respectively. Based on the emissions analysis, a bus with better fuel economy tends to have lower emissions and a bus with lower gas mileage usually produces more emissions, although there were some exceptions in the inverse relationship between gas mileage and emissions level.
Elastic-plastic characterization of nanocrystalline Al-Mg alloy using nanoindentation
(University of Alabama Libraries, 2012) Harvey, Evan Matthews; Ladani, Leila J.; University of Alabama Tuscaloosa
A nanocrystalline Al-Mg alloy synthesized via cryomilling and consolidated by cold isostatic pressing with subsequent extrusion was subjected to nanoindentation testing. The data collected from these tests was subjected to different data analysis techniques to investigate the capabilities of such techniques in full, accurate elastic-plastic mechanical characterization. A commercially available, coarse-grained sample of this same Al-Mg alloy was also tested to investigate these models' capabilities of distinguishing between the two types of material. Finite element analysis was used as a verification mechanism for the property values extracted from the nanocrystalline material, and initial results showed signs of good accuracy of characterization. Additionally, extensive testing was performed in an effort to investigate how the levels of certain test parameters might affect the resulting values of calculated mechanical properties. The test parameters that were varied during experimentation were maximum applied load, loading/unloading rate, and length of hold period at maximum load. Tests were conducted on specimens cut longitudinal and transverse to the extrusion direction of the bulk sample to investigate any effects anisotropy might play in the characterization process. Results show the levels of the studied parameters can hold effect over the measured values of hardness (H), elastic modulus (E), yield stress (σ_y), and strain-hardening exponent (n). This implies that individual materials may require specific levels of testing parameters if accurate values of mechanical properties are to be measured. Extensive comparison of load-depth behavior indicated that longitudinal specimens exhibit greater material non-homogeneity than transverse specimens as expected trends showed greater variation in the former.
Electronic and Electromechanical Tester for physiological sensors
(University of Alabama Libraries, 2013) Grace, John Bryant; Haskew, Tim A.; University of Alabama Tuscaloosa
Historically, physiological sensors used for measuring respiratory, cardiovascular and electrodermal activity have been used with polygraph devices and sleep laboratories. Periodic testing of these sensors is imperative to maintain sound performance of the measurement device. This thesis presents an Electronic and Electromechanical Tester (EET) for physiological sensors used with polygraph instruments that can accurately and repeatedly reproduce both physiological signals originating from the human body as well as computer-generated signals. The tester is interfaced to a personal computer via USB and contains the following four time-synchronous channels: two electromechanical simulators for testing abdominal and thoracic respiratory sensors, an electromechanical simulator for testing a sphygmomanometer used to capture cardiovascular activity, and an electronic simulator for testing electrodermal sensors. All of the simulated physiological channels apply direct physical actuation to the corresponding sensors. Specifics of software architecture and hardware implementation are included along with validation examples and test results. System identification techniques are discussed and transfer function models are defined. Based on these transfer function models, a compensator is designed with a goal of improving validation test data. The EET demonstrated its ability to reproduce physiological signals with adequate accuracy and repeatability. Finally, future systematic improvements as well as additional application areas are explored.
Enery harvesting through wind excitation of a piezoelectric flag-like harvester
(University of Alabama Libraries, 2013) Truitt, Andrew; Mahmoodi, S. Nima; University of Alabama Tuscaloosa
This study seeks to propose a novel approach to wind-based piezoelectric energy harvesting. A brief literature review of energy harvesting followed by a discussion of piezoelectric system dynamics is offered. Biomedical applications for piezoelectric energy harvesting are then presented offering a segue into fluid based energy harvesting. Fluid based energy harvesting is a relatively young subfield within piezoelectric energy harvesting, but it is increasingly pursued due to the ubiquitous nature of the excitation source as well as the strong correlation with other types of excitation. Vortex-induced vibrations (VIV), as well as vibrations induced by bluff bodies, and the effect of their shape on potential gains have been investigated. The interactions of VIVs on a flag-like membrane form the foundation for the piezoelectric energy harvester in this study. Polyvinylidene fluoride (PVDF) piezoelectric energy harvesters are chosen due to their desirable flexibility. Modeling of flag-like systems is review followed by system modeling of a PVDF piezoelectric flag. Numerical and experimental results from the PVDF flag-like piezoelectric energy harvester are generated and compared. A maximum power output of 1.5 mW is achieved with the flag-like system which is competitive when compared to power output and energy density levels of other studies. The power output of this system provides concrete evidence for the effective use of not only this type of energy harvester system model but also for the use of PVDFs in wind-based applications.

Browsing by Author "Williams, Keith A."

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