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Title: ROBUSTNESS ANALYSIS FOR TURBOMACHINERY STALL FLUTTER.
Creator: Forhad, Md, Xu, Yunjun, University of Central Florida
Abstract / Description: Flutter is an aeroelastic instability phenomenon that can result either in serious damage or complete destruction of a gas turbine blade structure due to high cycle fatigue. Although 90% of potential high cycle fatigue occurrences are uncovered during engine development, the remaining 10% stand for one third of the total engine development costs. Field experience has shown that during the last decades as much as 46% of fighter aircrafts were not mission-capable in certain periods due to high...
Show moreFlutter is an aeroelastic instability phenomenon that can result either in serious damage or complete destruction of a gas turbine blade structure due to high cycle fatigue. Although 90% of potential high cycle fatigue occurrences are uncovered during engine development, the remaining 10% stand for one third of the total engine development costs. Field experience has shown that during the last decades as much as 46% of fighter aircrafts were not mission-capable in certain periods due to high cycle fatigue related mishaps. To assure a reliable and safe operation, potential for blade flutter must be eliminated from the turbomachinery stages. However, even the most computationally intensive higher order models of today are not able to predict flutter accurately. Moreover, there are uncertainties in the operational environment, and gas turbine parts degrade over time due to fouling, erosion and corrosion resulting in parametric uncertainties. Therefore, it is essential to design engines that are robust with respect to the possible uncertainties. In this thesis, the robustness of an axial compressor blade design is studied with respect to parametric uncertainties through the Mu analysis. The nominal flutter model is adopted from . This model was derived by matching a two dimensional incompressible flow field across the flexible rotor and the rigid stator. The aerodynamic load on the blade is derived via the control volume analysis. For use in the Mu analysis, first the model originally described by a set of partial differential equations is reduced to ordinary differential equations by the Fourier series based collocation method. After that, the nominal model is obtained by linearizing the achieved non-linear ordinary differential equations. The uncertainties coming from the modeling assumptions and imperfectly known parameters and coefficients are all modeled as parametric uncertainties through the Monte Carlo simulation. As compared with other robustness analysis tools, such as Hinf, the Mu analysis is less conservative and can handle both structured and unstructured perturbations. Finally, Genetic Algorithm is used as an optimization tool to find ideal parameters that will ensure best performance in terms of damping out flutter. Simulation results show that the procedure described in this thesis can be effective in studying the flutter stability margin and can be used to guide the gas turbine blade design.
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Date Issued: 2011
Identifier: CFE0003999, ucf:48666
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0003999

Title: VISION BASED TRAJECTORY TRACKING OF SPACE DEBRIS IN CLOSE PROXIMITY VIA INTEGRATED ESTIMATION AND CONTROL.
Creator: Li, Ni, Xu, Yunjun, University of Central Florida
Abstract / Description: 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.
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Date Issued: 2011
Identifier: CFE0003731, ucf:48775
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0003731

Title: Process Optimization Towards the Development of an Automated CNC Monitoring System for a Simultaneous Turning and Boring Operation.
Creator: Hernandez, Manuel, Xu, Chengying, Chew, Phyekeng, Xu, Yunjun, University of Central Florida
Abstract / Description: Manufacturing operations generate revenue by adding value to material through machine work and the cost associated with part production hinders the maximum profit available. In order to remain competitive, companies invest in research to maximize profit and reduce waste of manufacturing operations. This results in cheaper products for the customer without sacrificing quality. The purpose of this research was to identify machine settings of an Okuma LC 40 Turning Center and optimize the cost...
Show moreManufacturing operations generate revenue by adding value to material through machine work and the cost associated with part production hinders the maximum profit available. In order to remain competitive, companies invest in research to maximize profit and reduce waste of manufacturing operations. This results in cheaper products for the customer without sacrificing quality. The purpose of this research was to identify machine settings of an Okuma LC 40 Turning Center and optimize the cost of machining in terms of tool cost and energy consumption while maintaining part quality at a productive cycle time. Studying the relationship between energy consumption, tool life, and cycle time with the speed and feed settings through statistical Analysis of Variance (ANOVA) method will allow the production plant to make profitable financial decisions concerning simultaneous turning operation of forged chrome-alloy steel. The project was divided into three phases; the first phase began with a literature survey of sensors used in current manufacturing research and the adaptation of our sensors to the Okuma LC 40 turning center. Then, phase II used design of experiments to identify spindle speed and feedrate settings that optimize multiple responses related to the turning process. The result was a saving in energy consumption (kWh) by 11.8%, a saving in cutting time by 13.2% for a total cost reduction from $1.15 per tool pass to $1.075 per tool pass. Furthermore, this work provides the foundation for phase III to develop an intelligent monitoring system to provide real-time information about the state of the machine and tool. For a monitoring system to be implemented in production, it should utilize cost effective sensors and be nonintrusive to the cutting operation.
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Date Issued: 2012
Identifier: CFE0004278, ucf:49510
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004278

Title: Pressure Image Based Attitude Controller for Small Unmanned Aerial Vehicles.
Creator: Thompson, Kenneth, Xu, Yunjun, Gou, Jihua, Lin, Kuo-Chi, University of Central Florida
Abstract / Description: As technology improves, small unmanned aerial vehicles (SUAV) have been identified for their utility in a variety of applications in which larger unmanned craft may be incapable of accomplishing mission objectives. These aircraft with their small size and long flight durations are ideal for hazardous inspection and long duration surveillance missions. One challenge preventing the widespread adoption of these systems is their instability to abrupt changes in the flow field around them due to...
Show moreAs technology improves, small unmanned aerial vehicles (SUAV) have been identified for their utility in a variety of applications in which larger unmanned craft may be incapable of accomplishing mission objectives. These aircraft with their small size and long flight durations are ideal for hazardous inspection and long duration surveillance missions. One challenge preventing the widespread adoption of these systems is their instability to abrupt changes in the flow field around them due to wind gusts or flow separation.Currently, traditional rigid body based sensors are implemented in their flight control systems, which are sufficient in higher inertia aircraft for accurate control.However, in low inertia SUAV applications during a flow event, often, the inertial sensors are incapable of detecting the event before catastrophic failure.A method of directly measuring the flow information around the SUAV in order to generate control commands will improve the stability of these systems by allowing these systems to directly react to flow events.In contrast, established inertial based control systems can only react to changes in vehicle dynamics caused by flow events.Such a method is developed utilizing a network of pressure and shear sensors embedded in the wing and used to create (")flow images(") which can be easily manipulated to generate control commands.A method of accurately calculating the aerodynamic moment acting on the aircraft based on the flow image is also developed for implementation of flow image-based control in real world systems.
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Date Issued: 2018
Identifier: CFE0007417, ucf:52722
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007417

Title: A Framework for Miniaturized Mechanical Characterization of Tensile, Creep, and Fatigue Properties of SLM Alloys.
Creator: Torres-Caceres, Jonathan, Orlovskaya, Nina, Xu, Yunjun, Das, Tuhin, University of Central Florida
Abstract / Description: With the heightened design complexity that may be achieved through additive manufacturing (AM) comes an equally complex set of distinct material characteristics. To properly characterize new materials for use in selective laser melting (SLM), extensive analysis is necessary. Traditional testing techniques, however, can be prohibitive in time and cost incurred. The small punch test (SPT) has been developed for such purposes, where material is scarce or costly. Although lacking standardization,...
Show moreWith the heightened design complexity that may be achieved through additive manufacturing (AM) comes an equally complex set of distinct material characteristics. To properly characterize new materials for use in selective laser melting (SLM), extensive analysis is necessary. Traditional testing techniques, however, can be prohibitive in time and cost incurred. The small punch test (SPT) has been developed for such purposes, where material is scarce or costly. Although lacking standardization, SPT has been successfully employed with various materials to assess material properties such as the yield and ultimate strength and verified by traditional testing results. With the accompaniment of numerical simulations for use in the inverse method and determining correlation factors, several methods exist for equating SPT results with traditional results. There are, however, areas of weakness with SPT which require development, and the solution of the inverse method can be demanding of time and resources. Additionally, the combination of SPT and SLM is relatively unexplored in literature, though studies have shown that SPT is sensitive to the types of structures and unique material characteristics present in SLM components. The present research therefore focuses on developing a framework for characterizing SLM materials via the small punch test. Several types of SLM materials in various orientations and processing states are small punch tested to evaluate the ability of the SPT to track the effects of these as they cause the materials to evolve. A novel cyclic test method is proposed to fill the gap in SPT fatigue testing. Results from these tests are evaluated via numerical modelling using the inverse method solved with the least squares method. Samples were also inspected using digital microscopy to connect fracture morphology to processing parameter variations. A framework is thus presented with which SPT may be utilized to more economically and expeditiously characterize SLM materials.
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Date Issued: 2018
Identifier: CFE0007109, ucf:51952
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007109

Title: Design and Motion Control of a Four Degree of Freedom Robotic Needle Guide for MRI-Guided Intervention.
Creator: Zhang, Shihao, Song, Sang-Eun, Xu, Yunjun, Bagci, Ulas, University of Central Florida
Abstract / Description: In the past several MRI compatible robotic needle guide devices for targeted prostate biopsy have been developed. The large and complex structure have been identified as the major limitations of those devices. Such limitations, in addition to complex steps for device to image registration have prevented widespread implementation of MRI-guided prostate biopsy despite the advantages of MRI compared to TRUS. We have designed a compact MRI-guided robotic intervention with the capability to have...
Show moreIn the past several MRI compatible robotic needle guide devices for targeted prostate biopsy have been developed. The large and complex structure have been identified as the major limitations of those devices. Such limitations, in addition to complex steps for device to image registration have prevented widespread implementation of MRI-guided prostate biopsy despite the advantages of MRI compared to TRUS. We have designed a compact MRI-guided robotic intervention with the capability to have angulated insertion to avoid damage to any anatomical feature along the needle path. The system consists of a novel mechanism driven Robotic Needle Guide (RNG). The RNG is a 4-DOF robotic needle manipulator mounted on a Gross Positioning Module (GPM), which is locked on the MRI table. The RNG consists of four parallel stacked disks with an engraved profile path. The rotary motion and positioning of the discs at an angle aids in guiding the biopsy needle. Once a clinician selects a target for needle insertion, the intervention provides possible insertion angles. Then, the most suitable angle is selected by the clinician based on the safest trajectory. The selected target and insertion angle are then computed as control parameters of RNG i.e. the discs are then rotated to the required angle. Insertion is followed by quick confirmation scans to ascertain needle position at all times.
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Date Issued: 2018
Identifier: CFE0007116, ucf:51942
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007116

Title: Printable Carbon Nanotube Based Multifunctional Nanocomposites for Strain Sensing and Self-heating.
Creator: Wang, Xin, Gou, Jihua, Challapalli, Suryanarayana, Xu, Yunjun, University of Central Florida
Abstract / Description: The unique properties of carbon nanotubes (CNTs) represent a potential for developing a piezo-resistive strain sensor and a resistive heating sheet with a smart structure. Conventional fabrication techniques of CNT based nanocomposites such as molding, casting or spray coating lack the ability to control the geometry and properties of fabricated composites. In order to meet the various requirements of strain sensing or self-heating applications, nanocomposites with complex geometry and...
Show moreThe unique properties of carbon nanotubes (CNTs) represent a potential for developing a piezo-resistive strain sensor and a resistive heating sheet with a smart structure. Conventional fabrication techniques of CNT based nanocomposites such as molding, casting or spray coating lack the ability to control the geometry and properties of fabricated composites. In order to meet the various requirements of strain sensing or self-heating applications, nanocomposites with complex geometry and controllable properties are in high demand. Digital printing technique is able to fabricate CNT films with precisely controlled geometry with the help of computer aided design, and their properties could also be controlled by adjusting the printing parameters. The objective of this study is to investigate the printing-structure-property relationship of CNT based multifunctional nanocomposites fabricated by digitally controlled spray deposition process for strain sensing and self-heating. A spray deposition modeling (SDM) printer that uses a 12-array inkjet nozzle attached to an x-y plotter was developed for the fabrication of CNT layers. Most of previously-reported CNT based nanocomposite strain sensors only have limited stretchability and sensitivity for measuring diverse human motions. Additionally, strain sensors fabricated by traditional techniques are only capable of measuring strain in a single direction, but for monitoring human motion with complicated strain condition, strain sensors that can measure strain from multi-direction are favorable. In this dissertation, highly stretchable (in excess of 45% strain) and sensitive (gauge factor of 35.75) strain sensors with tunable strain gauge factors were fabricated by incorporating CNT layers into polymer substrate using SDM printing technique. The cyclic loading-unloading test results revealed that the composite strain sensors exhibited excellent long-term durability. Due to the flexibility of the printing technique, rosette-typed sensors were fabricated to monitor complicated human motions. These superior sensing capabilities of the fabricated nanocomposites offer potential applications in wearable strain sensors. Resistive heating properties of CNT based nanocomposites were also investigated. The electrically resistive heating of these composites can be a desirable stimulus to activate the shape memory effect of polymer matrix. CNT based nanocomposites fabricated by traditional techniques showed a slow heating rate and same shape recovery ratio at different locations in nanocomposites. However, from the practical applications like smart skin or smart tooling perspective, programmable shape recovery ratio at specified locations are desirable. In this dissertation, the CNT based nanocomposites with a fast heating rate and controllable maximum surface temperature were fabricated using SDM technique. The study on the shape memory effect of nanocomposites showed that their shape recoverability was approximately 100% taking 30s under a low voltage of 40V. It is worth noting that through programming the number of printed CNT layers at different locations, the shape recovery rate could be controlled and localized actuation with the desired recovery ratio was achieved. The high efficiency of heating coupling with wide adjustability of surface temperature and shape recovery ratio at specified locations make the fabricated nanocomposites a promising candidate for electrical actuation applications.
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Date Issued: 2017
Identifier: CFE0006819, ucf:52892
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006819

Title: A Smart UAV Platform for Railroad Inspection.
Creator: Debevec, Ryan, Elgohary, Tarek, Xu, Yunjun, Lin, Kuo-Chi, University of Central Florida
Abstract / Description: Using quadcopters for analysis of an environment has been an intriguing subject of study recently. The purpose of this work is to develop a fully autonomous UAV platform for Railroad inspection The dynamics of the quadrotor is derived using Euler's and Newton's laws and then linearized around the hover position. A PID controller is designed to control the states of the quadrotor in a manner to effectively follow a vision-based path, using the down facing camera on a Parrot Mambo quadrotor....
Show moreUsing quadcopters for analysis of an environment has been an intriguing subject of study recently. The purpose of this work is to develop a fully autonomous UAV platform for Railroad inspection The dynamics of the quadrotor is derived using Euler's and Newton's laws and then linearized around the hover position. A PID controller is designed to control the states of the quadrotor in a manner to effectively follow a vision-based path, using the down facing camera on a Parrot Mambo quadrotor. Using computer vision the distance from the position of the quadrotor to the position of the center of the path was found. Using the yaw controller to minimize this distance was found to be an adequate method of vision-based path following, by keeping the area of interest in the field of view of the camera. The downfacing camera is also simultaneously observing the path to detect defects using machine learning. This technique was able to detect simulated defects on the path with around 90% accuracy.
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Date Issued: 2019
Identifier: CFE0007623, ucf:52555
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007623

Title: Vision-Based Testbeds for Control System Applicaitons.
Creator: Sivilli, Robert, Xu, Yunjun, Gou, Jihua, Cho, Hyoung, Pham, Khanh, University of Central Florida
Abstract / Description: In the field of control systems, testbeds are a pivotal step in the validation and improvement of new algorithms for different applications. They provide a safe, controlled environment typically having a significantly lower cost of failure than the final application. Vision systems provide nonintrusive methods of measurement that can be easily implemented for various setups and applications. This work presents methods for modeling, removing distortion, calibrating, and rectifying single and...
Show moreIn the field of control systems, testbeds are a pivotal step in the validation and improvement of new algorithms for different applications. They provide a safe, controlled environment typically having a significantly lower cost of failure than the final application. Vision systems provide nonintrusive methods of measurement that can be easily implemented for various setups and applications. This work presents methods for modeling, removing distortion, calibrating, and rectifying single and two camera systems, as well as, two very different applications of vision-based control system testbeds: deflection control of shape memory polymers and trajectory planning for mobile robots. First, a testbed for the modeling and control of shape memory polymers (SMP) is designed. Red-green-blue (RGB) thresholding is used to assist in the webcam-based, 3D reconstruction of points of interest. A PID based controller is designed and shown to work with SMP samples, while state space models were identified from step input responses. Models were used to develop a linear quadratic regulator that is shown to work in simulation. Also, a simple to use graphical interface is designed for fast and simple testing of a series of samples. Second, a robot testbed is designed to test new trajectory planning algorithms. A template-based predictive search algorithm is investigated to process the images obtained through a low-cost webcam vision system, which is used to monitor the testbed environment. Also a user-friendly graphical interface is developed such that the functionalities of the webcam, robots, and optimizations are automated. The testbeds are used to demonstrate a wavefront-enhanced, B-spline augmented virtual motion camouflage algorithm for single or multiple robots to navigate through an obstacle dense and changing environment, while considering inter-vehicle conflicts, obstacle avoidance, nonlinear dynamics, and different constraints. In addition, it is expected that this testbed can be used to test different vehicle motion planning and control algorithms.
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Date Issued: 2012
Identifier: CFE0004601, ucf:49187
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004601

Title: Optimal Attitude Control Management for a Cubesat.
Creator: Develle, Michael, Xu, Yunjun, Lin, Kuo-Chi, Chew, Phyekeng, University of Central Florida
Abstract / Description: CubeSats have become popular among universities, research organizations, and government agencies due to their low cost, small size, and light weight. Their standardized configurations further reduce the development time and ensure more frequent launch opportunities. Early cubesat missions focused on hardware validation and simple communication missions, with little requirement for pointing accuracy. Most of these used magnetic torque rods or coils for attitude stabilization. However, the...
Show moreCubeSats have become popular among universities, research organizations, and government agencies due to their low cost, small size, and light weight. Their standardized configurations further reduce the development time and ensure more frequent launch opportunities. Early cubesat missions focused on hardware validation and simple communication missions, with little requirement for pointing accuracy. Most of these used magnetic torque rods or coils for attitude stabilization. However, the intrinsic problems associated with magnetictorque systems, such as the lack of three-axis control and low pointing accuracy, make them unsuitable for more advanced missions such as detailed imaging and on-orbit inspection. Three-axis control in a cubesat can be achieved by combining magnetic torque coils with other devices such as thrusters, but the lifetime is limited by the fuel source onboard. To maximize the missionlifetime, a fast attitude control management algorithm that could optimally manage the usage of the magnetic and thruster torques is desirable. Therefore, a recently developed method, the B-Spline-augmented virtual motion camouflage, is presented in this defense to solve the problem. This approach provides results which are very close to those obtained through other popular nonlinear constrained optimal control methods with a significantly reduced computational time.Simulation results are presented to validate the capabilities of the method in this application.
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Date Issued: 2011
Identifier: CFE0004099, ucf:49102
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004099

Title: Study of High Efficiency Micro Thermoelectric Energy Harvesters.
Creator: Pedrosa, Steven, Chen, Quanfang, Cho, Hyoung, Xu, Yunjun, University of Central Florida
Abstract / Description: Thermal energy sources, including waste heat and thermal radiation from the sun, are important renewable energy resources. Thermal energy can be converted into electricity by thermoelectric phenomena; the thermoelectric phenomena can also be operated in reverse when provided an electric current, producing a temperature gradient across the device. Thermoelectric devices are scalable, renewable, and cost effective products that offer capabilities to harness waste heat or environmental heat...
Show moreThermal energy sources, including waste heat and thermal radiation from the sun, are important renewable energy resources. Thermal energy can be converted into electricity by thermoelectric phenomena; the thermoelectric phenomena can also be operated in reverse when provided an electric current, producing a temperature gradient across the device. Thermoelectric devices are scalable, renewable, and cost effective products that offer capabilities to harness waste heat or environmental heat sources, and convert the captured heat into usable electricity. The operating principle of a thermoelectric device requires that a temperature gradient be present across the device, which induces the flow of electrons from the hot side of the device to the cold side. Thermoelectric devices are currently hampered by the low conversion efficiencies and strict operating temperatures for certain materials. This study investigates the main factors affecting efficiencies of thermoelectric devices as energy harvesters and aims to optimize the devices for maximum efficiency and lower costs by using microfabrication processes and self-assembled materials for complete thermoelectric modules (TEMs). By first establishing operating conditions and a desired mode of operation, optimization equations have been established to determine device dimensions and performance parameters. Compact integration realized by microfabrication technologies that allow for multiple output voltages from a single chip was also investigated. Additionally, cost savings were found by reducing the number of fabrication processing steps and eliminating the need for precious metals during fabrication. The optimized design proposed in this study utilizes copper electrodes and requires fewer applications of photoresist than previous proposed designs. In fabrication of thin film based micro devices, the film quality and the composition of the film are essential elements for producing TEMs with desired efficiencies. Although Bi2Te3 has been investigated as thermoelectric material, this study determined that there was a possibility that both N-type and P-Type Bi2Te3 could be created from a single electrolyte solution by controlling the amount of Te present in the film. Films were produced with both AC and DC signals and varied composition of Te at.% of Bi2Te3 was achieved by controlling the average current density during electrochemical deposition. A linear relationship was established between the average current density and the resultant Te content. SEM and EDS were used to characterize the morphology and the composition of the thin films created. With the fabricated thermoelectric materials, analytical models could be developed using known material properties of thermoelectric films with a given Te content. The analytical results obtained by the developed optimization equations were comparable with the FEA models produced by using COMSOL, a multiphysics program with powerful solving algorithms that was used to evaluate designs. Further improvements to device performance can be achieved by designing a segmented thermoelectric device with multiple layers of thermoelectric material to allow the device to operate across a larger temperature gradient.
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Date Issued: 2011
Identifier: CFE0004486, ucf:49318
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004486

Title: Simulation of a Capacitive Micromachined Ultrasonic Transducer with a Parylene Membrane and Graphene Electrodes.
Creator: Sadat, David, Chen, Quanfang, Xu, Yunjun, Foroosh, Hassan, University of Central Florida
Abstract / Description: Medical ultrasound technology accounts for over half of all imaging tests performed worldwide. In comparison to other methods, ultrasonic imaging is more portable and lower cost, and is becoming more accessible to remote regions where traditionally no medical imaging can be done. However, conventional ultrasonic imaging systems still rely on expensive PZT-based ultrasound probes that limit broader applications. In addition, the resolution of PZT based transducers is low due to the limitation...
Show moreMedical ultrasound technology accounts for over half of all imaging tests performed worldwide. In comparison to other methods, ultrasonic imaging is more portable and lower cost, and is becoming more accessible to remote regions where traditionally no medical imaging can be done. However, conventional ultrasonic imaging systems still rely on expensive PZT-based ultrasound probes that limit broader applications. In addition, the resolution of PZT based transducers is low due to the limitation in hand-fabrication methods of the piezoelectric ceramics.Capacitive Micromachined Ultrasonic Transducers (CMUTs) appears as an alternative to the piezoelectric (PZT) ceramic based transducer for ultrasound medical imaging. CMUTs show better ultrasound transducer design for batch fabrication, higher axial resolution of images, lower fabrication costs of the elements, ease of fabricating large arrays of cells using MEMS fabrication, and the extremely important potential to monolithically integrate the 2D transducer arrays directly with IC circuits for real-time 3D imaging.Currently most efforts on CMUTs are silicon based. Problems with current silicon-based CMUT designs include low pressure transmission and high-temperature fabrication processes. The pressure output from the silicon based CMUTs cells during transmission are too low when compared to commercially available PZT transducers, resulting in relatively blurry ultrasound images. The fabrication of the silicon-based cells, although easier than PZT transducers, still suffers from inevitable high temperature process and require specialized and expensive equipment. Manufacturing at an elevated temperature hinders the capability of fabricating front end analog processing IC circuits, thus it is difficult to achieve true 3D/4D imaging. Therefore novel low temperature fabrication with a low cost nature is needed. A polymer (Parylene) based CMUTs transducer has been investigated recently at UCF and aims to overcome limitations posted from the silicon based counterparts. This thesis describes the numerical simulation work and proposed fabrication steps of the Parylene based CMUT. The issue of transducer cost and pressure transmission is addressed by proposing the use of low cost and low temperature Chemical Vapor Deposition (CVD) fabrication of Parylene-C as the structural membrane plus graphene for the membrane electrodes. This study focuses mainly on comparing traditional silicon-based CMUT designs against the Parylene-C/Graphene CMUT based transducer, by using MEMS modules in COMSOL. For a fair comparison, single CMUT cells are modeled and held at a constant diameter and the similar operational frequency at the structural center. The numerical CMUT model is characterized for: collapse voltage, membrane deflection profile, center frequency, peak output pressure transmission over the membrane surface, and the sensitivity to the change in electrode surface charge. This study took the unique approaches in defining sensitivity of the CMUT by calculating the membrane response and the change in the electrode surface charge due to an incoming pressure wave. Optimal design has been achieved based on the simulation results. In comparison to silicon based CMUTs, the Parylene/Graphene based CMUT transducer produces 55% more in volume displacement and more than 35% in pressure output. The thesis has also laid out the detailed fabrication processes of the Parylene/Graphene based CMUT transducers. Parylene/Graphene based ultrasonic transducers can find wide applications in both medical imaging and Non destructive evaluation (NDE).
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Date Issued: 2012
Identifier: CFE0004333, ucf:49458
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004333

Title: Numerical Simulation of Electrolyte-Supported Planar Button Solid Oxide Fuel Cell.
Creator: Aman, Amjad, Orlovskaya, Nina, Xu, Yunjun, Das, Tuhin, University of Central Florida
Abstract / Description: Solid Oxide Fuel Cells are fuel cells that operate at high temperatures usually in the range of 600oC to 1000oC and employ solid ceramics as the electrolyte. In Solid Oxide Fuel Cells oxygen ions (O2-) are the ionic charge carriers. Solid Oxide Fuel Cells are known for their higher electrical efficiency of about 50-60% [1] compared to other types of fuel cells and are considered very suitable in stationary power generation applications. It is very important to study the effects of different...
Show moreSolid Oxide Fuel Cells are fuel cells that operate at high temperatures usually in the range of 600oC to 1000oC and employ solid ceramics as the electrolyte. In Solid Oxide Fuel Cells oxygen ions (O2-) are the ionic charge carriers. Solid Oxide Fuel Cells are known for their higher electrical efficiency of about 50-60% [1] compared to other types of fuel cells and are considered very suitable in stationary power generation applications. It is very important to study the effects of different parameters on the performance of Solid Oxide Fuel Cells and for this purpose the experimental or numerical simulation method can be adopted as the research method of choice. Numerical simulation involves constructing a mathematical model of the Solid Oxide Fuel Cell and use of specifically designed software programs that allows the user to manipulate the model to evaluate the system performance under various configurations and in real time. A model is only usable when it is validated with experimental results. Once it is validated, numerical simulation can give accurate, consistent and efficient results. Modeling allows testing and development of new materials, fuels, geometries, operating conditions without disrupting the existing system configuration. In addition, it is possible to measure internal variables which are experimentally difficult or impossible to measure and study the effects of different operating parameters on power generated, efficiency, current density, maximum temperatures reached, stresses caused by temperature gradients and effects of thermal expansion for electrolytes, electrodes and interconnects.Since Solid Oxide Fuel Cell simulation involves a large number of parameters and complicated equations, mostly Partial Differential Equations, the situation calls for a sophisticated simulation technique and hence a Finite Element Method (FEM) multiphysics approach will be employed. This can provide three-dimensional localized information inside the fuel cell. For this thesis, COMSOL Multiphysics(&)#174; version 4.2a will be used for simulation purposes because it has a Batteries (&) Fuel Cells module, the ability to incorporate custom Partial Differential Equations and the ability to integrate with and utilize the capabilities of other tools like MATLAB(&)#174;, Pro/Engineer(&)#174;, SolidWorks(&)#174;. Fuel Cells can be modeled at the system or stack or cell or the electrode level. This thesis will study Solid Oxide Fuel Cell modeling at the cell level. Once the model can be validated against experimental data for the cell level, then modeling at higher levels can be accomplished in the future. Here the research focus is on Solid Oxide Fuel Cells that use hydrogen as the fuel. The study focuses on solid oxide fuel cells that use 3-layered, 4-layered and 6-layered electrolytes using pure YSZ or pure SCSZ or a combination of layers of YSZ and SCSZ. A major part of this research will be to compare SOFC performance of the different configurations of these electrolytes. The cathode and anode material used are (La0.6Sr0.4)0.95-0.99Co0.2Fe0.8O3 and Ni-YSZ respectively.
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Date Issued: 2012
Identifier: CFE0004349, ucf:49431
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004349

Title: Vibration Reduction of Mistuned Bladed Disks via Piezoelectric-Based Resonance Frequency Detuning.
Creator: Lopp, Garrett, Kauffman, Jeffrey L., Das, Tuhin, Xu, Yunjun, University of Central Florida
Abstract / Description: Recent trends in turbomachinery blade technology have led to increased use of monolithically constructed bladed disks (blisks). Although offering a wealth of performance benefits, this construction removes the blade-attachment interface present in the conventional design, thus unintentionally removing a source of friction-based damping needed to counteract large vibrations during resonance passages. This issue is further exacerbated in the presence of blade mistuning that arises from small...
Show moreRecent trends in turbomachinery blade technology have led to increased use of monolithically constructed bladed disks (blisks). Although offering a wealth of performance benefits, this construction removes the blade-attachment interface present in the conventional design, thus unintentionally removing a source of friction-based damping needed to counteract large vibrations during resonance passages. This issue is further exacerbated in the presence of blade mistuning that arises from small imperfections from otherwise identical blades and are unavoidable as they originate from manufacturing tolerances and operational wear over the lifespan of the engine. Mistuning is known to induce vibration localization with large vibration amplitudes that render blades susceptible to failure induced by high-cycle fatigue. The resonance frequency detuning (RFD) method reduces vibration associated with resonance crossings by selectively altering the blades' structural response. This method utilizes the variable stiffness properties of piezoelectric materials to switch between available stiffness states at some optimal time as the excitation frequency sweeps through a resonance. For a single-degree-of-freedom (SDOF) system, RFD performance is well defined. This research provides the framework to extend RFD to more realistic applications when the SDOF assumption breaks down, such as in cases of blade mistuning. Mistuning is inherently random; thus, a Monte Carlo analysis performed on a computationally cheap lumped-parameter model provides insight into RFD performance for various test parameters. Application of a genetic algorithm reduces the computational expense required to identify the optimal set of stiffness-state switches. This research also develops a low-order blisk model with blade-mounted piezoelectric patches as a tractable first step to apply RFD to more realistic systems. Application of a multi-objective optimization algorithm produces Pareto fronts that aid in the selection of the optimized patch parameters. Experimental tests utilizing the academic blisk with the optimized patches provides validation.
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Date Issued: 2018
Identifier: CFE0007488, ucf:52639
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007488

Title: Decentralized Consensus-based Control Allocation For Some Dynamical Systems.
Creator: Mark, August, Xu, Yunjun, Gou, Jihua, Lin, Kuo-Chi, University of Central Florida
Abstract / Description: In this dissertation, three separate studies, wherein techniques from graph theory and consensus control are used to address control allocation problems, are presented. In the first study, a decentralized allocator is presented for synthetic jet actuators and control surfaces onboard a small unmanned aerial vehicle to cooperatively generate desired aerodynamic moments. First order linear dynamics are assumed for both the synthetic jet actuators and control surfaces. A weighted consensus...
Show moreIn this dissertation, three separate studies, wherein techniques from graph theory and consensus control are used to address control allocation problems, are presented. In the first study, a decentralized allocator is presented for synthetic jet actuators and control surfaces onboard a small unmanned aerial vehicle to cooperatively generate desired aerodynamic moments. First order linear dynamics are assumed for both the synthetic jet actuators and control surfaces. A weighted consensus algorithm with limited feedback is used for the aerodynamic moment contribution allocator considering constraints. In the second study, the same allocation problem as in the first study is considered, but the actuator dynamics are now assumed to behave according to second order nonlinear dynamics. In the third study, a spray allocator is presented for an array of nozzles used to cool a large heated surface in order to address the local disagreement in surface temperature within sprayed sections. Within each study, the stability of each system is proven, and the performance of each allocator is demonstrated via simulations.
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Date Issued: 2019
Identifier: CFE0007496, ucf:52636
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007496

Title: Spray Deposition Modeling of Carbon Nano-Inks.
Creator: Sparkman, John, Gou, Jihua, Xu, Yunjun, Lin, Kuo-Chi, University of Central Florida
Abstract / Description: Carbon nanopaper (CNP) exhibits qualities that are desirable for a number of applications such as flame retardancy, lightning protection, and flexible printed circuit boards. CNP has become a desired engineering material in many important sectors of industries such as space, automotive, aviation, and military. However the production of consistent thicknesses and dispersion remains a challenge for practical use. Most of the standard methods of production do not allow for continuous...
Show moreCarbon nanopaper (CNP) exhibits qualities that are desirable for a number of applications such as flame retardancy, lightning protection, and flexible printed circuit boards. CNP has become a desired engineering material in many important sectors of industries such as space, automotive, aviation, and military. However the production of consistent thicknesses and dispersion remains a challenge for practical use. Most of the standard methods of production do not allow for continuous applications or digital fabrication of the CNP. In this work, CNP is produced two different ways that allows for continuous production and digital fabrication. The continuous CNP making technique uses vacuum infiltration along with air atomization and a continuous drive belt system to produce a continuous roll of the CNP. This system is able to produce an 11 (&)#181;m (&)#177; 2 (&)#181;m CNP at 6 inches per min with an electrical resistivity of 59 ? per square. The major advantage of this production process is the ability to mass manufacture the CNP. Spray deposition modeling (SDM) is a digital fabrication process that uses a 12 array bubble jet nozzle attached to a digital control x-y plotter combined with a heated substrate which induces evaporation. This process is able to produce paper with variable thicknesses in defined locations. The maximum thickness of the CNP produced is 10 (&)#181;m with a resistivity of 95.7 ? per square. A strong advantage of this CNP production method comes from the ability to digitally print images. The controllable thickness and selective location printing presents an effective alternative to costlier methods and provides a solution to many geometrical CNP issues.
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Date Issued: 2015
Identifier: CFE0006231, ucf:51073
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006231

Title: Characteristics of Hydrogel-Wetted Thin Films.
Creator: Owens, James, Putnam, Shawn, Chow, Louis, Xu, Yunjun, University of Central Florida
Abstract / Description: The meniscus region of a thin film is known to have high heat transfer properties due to high evaporation rates and activation of latent heat. The region known as the thin film meniscus (?_film(
Show moreThe meniscus region of a thin film is known to have high heat transfer properties due to high evaporation rates and activation of latent heat. The region known as the thin film meniscus (?_film(<)2 (&)#181;m ) can account for more than half of the total heat transfer of a droplet or film. This study focuses on the potential elongation and curvature amplification of the thin film meniscus region by the implementation of a layer of high hydrogen bonding (hydrogel) film on which the liquid meniscus is built. Forced wetting via liquid propagation though this hydrogel layer in the radial direction increases the surface area of the film. By analyzing the mass flux of liquid lost through evaporation and using both spectroscopic and optical methods to obtain the curvature of the film, relationships between hydrogel thickness and the resulting mass flux were made. Isothermal and steady state assumptions were used to relate hydrogel thickness layers to meniscus curvature, evaporative mass flux, and overall heat transfer coefficients. The experimental results demonstrate, that steady state conditions are achievable with small percentage change in film profile over time. These results are promising toward the addition of the hydrogel coatings and further advancements in heat piping and high heat flux cooling systems for micro electronic devices.
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Date Issued: 2017
Identifier: CFE0006634, ucf:51257
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006634

Title: Determination of Frequency-Based Switch Triggers for Optimal Vibration Reduction via Resonance Frequency Detuning.
Creator: Lopp, Garrett, Kauffman, Jeffrey, Das, Tuhin, Xu, Yunjun, University of Central Florida
Abstract / Description: Resonance frequency detuning (RFD) is a piezoelectric-based vibration reduction approach that applies to systems experiencing transient excitation through the system's resonance(-)for example, turbomachinery experiencing changes in rotation speed, such as on spool-up and spool-down. This technique relies on the inclusion of piezoelectric material and manipulation of its electrical boundary conditions, which control the stiffness of the piezoelectric material. Resonance frequency detuning...
Show moreResonance frequency detuning (RFD) is a piezoelectric-based vibration reduction approach that applies to systems experiencing transient excitation through the system's resonance(-)for example, turbomachinery experiencing changes in rotation speed, such as on spool-up and spool-down. This technique relies on the inclusion of piezoelectric material and manipulation of its electrical boundary conditions, which control the stiffness of the piezoelectric material. Resonance frequency detuning exploits this effect by intelligently switching between the open-circuit (high stiffness) and short-circuit (low stiffness) conditions as the excitation approaches resonance, subsequently shifting the natural frequency to avoid this resonance crossing and limit the response. The peak response dynamics are then determined by the system's sweep rate, modal damping ratio, electromechanical coupling coefficient, and, most importantly, the trigger (represented here in terms of excitation frequency) that initiates the stiffness state switch. This thesis identifies the optimal frequency-based switch trigger over a range of sweep rates, damping ratios, and electromechanical coupling coefficients. With perfect knowledge of the system, the optimal frequency-based switch trigger decreases approximately linearly with the square of the coupling coefficient. Furthermore, phase of vibration at the time of the switch has a very small effect; switching on peak strain energy is marginally optimal. In practice, perfect knowledge is unrealistic and an alternate switch trigger based on an easily measurable parameter is necessary. As such, this thesis also investigates potential methods using the open-circuit piezoelectric voltage response envelope and its derivatives. The optimal switch triggers collapse to a near linear trend when measured against the response envelope derivatives and, subsequently, an empirical control law is extracted. This control law agrees well with and produces a comparable response to that of the optimal control determined using perfect and complete knowledge of the system.
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Date Issued: 2015
Identifier: CFE0005829, ucf:50909
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005829

Title: General Vector Explicit - Impact Time and Angle Control Guidance.
Creator: Robinson, Loren, Qu, Zhihua, Behal, Aman, Xu, Yunjun, University of Central Florida
Abstract / Description: This thesis proposes and evaluates a new cooperative guidance law called General Vector Explicit -Impact Time and Angle Control Guidance (GENEX-ITACG). The motivation for GENEX-ITACGcame from an explicit trajectory shaping guidance law called General Vector Explicit Guidance(GENEX). GENEX simultaneously achieves design specifications on miss distance and terminalmissile approach angle while also providing a design parameter that adjusts the aggressiveness ofthis approach angle. Encouraged by...
Show moreThis thesis proposes and evaluates a new cooperative guidance law called General Vector Explicit -Impact Time and Angle Control Guidance (GENEX-ITACG). The motivation for GENEX-ITACGcame from an explicit trajectory shaping guidance law called General Vector Explicit Guidance(GENEX). GENEX simultaneously achieves design specifications on miss distance and terminalmissile approach angle while also providing a design parameter that adjusts the aggressiveness ofthis approach angle. Encouraged by the applicability of this user parameter, GENEX-ITACG is anextension that allows a salvo of missiles to cooperatively achieve the same objectives of GENEXagainst a stationary target through the incorporation of a cooperative trajectory shaping guidancelaw called Impact Time and Angle Control Guidance (ITACG).ITACG allows a salvo of missile to simultaneously hit a stationary target at a prescribed impactangle and impact time. This predetermined impact time is what allows each missile involvedin the salvo attack to simultaneously arrived at the target with unique approach angles, whichgreatly increases the probability of success against well defended targets. GENEX-ITACG furtherincreases this probability of kill by allowing each missile to approach the target with a uniqueapproach angle rate through the use of a user design parameter.The incorporation of ITACG into GENEX is accomplished through the use of linear optimal controlby casting the cost function of GENEX into the formulation of ITACG. The feasibility GENEXITACGis demonstrated across three scenarios that demonstrate the ITACG portion of the guidancelaw, the GENEX portion of the guidance law, and finally the entirety of the guidance law. Theresults indicate that GENEX-ITACG is able to successfully guide a salvo of missiles to simultaneouslyhit a stationary target at a predefined terminal impact angle and impact time, while alsoallowing the user to adjust the aggressiveness of approach.
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Date Issued: 2015
Identifier: CFE0005876, ucf:50868
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005876

Title: Coordinated Optimal Power Planning of Wind Turbines in an Offshore Wind Farm.
Creator: Vishwakarma, Puneet, Xu, Yunjun, Kapat, Jayanta, Kauffman, Jeffrey, Behal, Aman, University of Central Florida
Abstract / Description: Wind energy is on an upswing due to climate concerns and increasing energy demands on conventional sources. Wind energy is attractive and has the potential to dramatically reduce the dependency on non-renewable energy resources. With the increase in wind farms there is a need to improve the efficiency in power allocation and power generation among wind turbines. Wake interferences among wind turbines can lower the overall efficiency considerably, while offshore conditions pose increased...
Show moreWind energy is on an upswing due to climate concerns and increasing energy demands on conventional sources. Wind energy is attractive and has the potential to dramatically reduce the dependency on non-renewable energy resources. With the increase in wind farms there is a need to improve the efficiency in power allocation and power generation among wind turbines. Wake interferences among wind turbines can lower the overall efficiency considerably, while offshore conditions pose increased loading on wind turbines. In wind farms, wind turbines' wake affects each other depending on their positions and operation modes. Therefore it becomes essential to optimize the wind farm power production as a whole than to just focus on individual wind turbines. The work presented here develops a hierarchical power optimization algorithm for wind farms. The algorithm includes a cooperative level (or higher level) and an individual level (or lower level) for power coordination and planning in a wind farm. The higher level scheme formulates and solves a quadratic constrained programming problem to allocate power to wind turbines in the farm while considering the aerodynamic effect of the wake interaction among the turbines and the power generation capabilities of the wind turbines. In the lower level, optimization algorithm is based on a leader-follower structure driven by the local pursuit strategy. The local pursuit strategy connects the cooperative level power allocation and the individual level power generation in a leader-follower arrangement. The leader, could be a virtual entity and dictates the overall objective, while the followers are real wind turbines considering realistic constraints, such as tower deflection limits. A nonlinear wind turbine dynamics model is adopted for the low level study with loading and other constraints considered in the optimization. The stability of the algorithm in the low level is analyzed for the wind turbine angular velocity. Simulations are used to show the advantages of the method such as the ability to handle non-square input matrix, non-homogenous dynamics, and scalability in computational cost with rise in the number of wind turbines in the wind farm.
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Date Issued: 2015
Identifier: CFE0005899, ucf:50896
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005899

Robotics (3)	+ -
SOFC (3)	+ -
optimal control (3)	+ -
vibration reduction (3)	+ -
Computer Vision (2)	+ -

CubeSat (2)	+ -
Fuel Cells (2)	+ -
Grasping (2)	+ -
Optimal trajectory design (2)	+ -
Robust Control (2)	+ -
Shape Memory Polymer (2)	+ -
Solid Oxide Fuel Cells (2)	+ -
Testbed (2)	+ -
UAV (2)	+ -
additive manufacturing (2)	+ -
attitude control (2)	+ -
autogyro (2)	+ -
autorotation (2)	+ -
cooperative control (2)	+ -
missile (2)	+ -
robust control (2)	+ -
trajectory planning (2)	+ -
virtual motion camouflage (2)	+ -
wind energy (2)	+ -
3D printing (1)	+ -
A Smart UAV Platform for Railroad Inspection (1)	+ -
Adaptive Control (1)	+ -
Aerodynamics (1)	+ -
Agriculture (1)	+ -
Agriculture Ground Robot (1)	+ -

University of Central Florida (50)	+ -
Xu, Yunjun (50)	+ -
Lin, Kuo-Chi (20)	+ -
Das, Tuhin (11)	+ -
Gou, Jihua (10)	+ -

Behal, Aman (8)	+ -
Haralambous, Michael (6)	+ -
Kauffman, Jeffrey (5)	+ -
Orlovskaya, Nina (5)	+ -
Qu, Zhihua (4)	+ -
Boloni, Ladislau (3)	+ -
Cho, Hyoung (3)	+ -
Kauffman, Jeffrey L. (3)	+ -
Song, Sang-Eun (3)	+ -
Aman, Amjad (2)	+ -
Bradley, Eric (2)	+ -
Challapalli, Suryanarayana (2)	+ -
Chen, Quanfang (2)	+ -
Chew, Phyekeng (2)	+ -
Defterli, Sinem (2)	+ -
Elgohary, Tarek (2)	+ -
Kassab, Alain (2)	+ -
Li, Ni (2)	+ -
Lopp, Garrett (2)	+ -
Mark, August (2)	+ -
Moslehy, Faissal (2)	+ -
Pourmohammadi Fallah, Yaser (2)	+ -
Thompson, Kenneth (2)	+ -
Torres-Caceres, Jonathan (2)	+ -
Wang, Xin (2)	+ -

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