Browsing by Author "Dr. Gregory D. Buckner, Committee Chair"

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Defect Identification in GRID-LOCK(R) Joints
(2006-12-21) Pandurangan, Pradeep; Dr. Kara J. Peters, Committee Member; Dr. Fuh-Gwo Yuan, Committee Member; Dr. Mohammed N. Noori, Committee Member; Dr. Gregory D. Buckner, Committee Chair
Bonded metallic GRID-LOCK® structures are being adopted for a variety of aerospace applications due to their structural efficiency and damage tolerance. The development of non-destructive evaluation (NDE) methods is necessary to identify bond defects that can lead to failures in these structures. However, this task is complicated by the lack of interior access and complex geometry of GRID-LOCK® components. In this dissertation, the feasibility of various NDE techniques for detecting the existence, location, and extent of bond defects in GRID-LOCK® joints is investigated. Experiments are conducted on customized test structures to compare the effectiveness of optical NDE, ultrasonic C-scans and vibration-based damage detection. Finite element analysis (FEA) is used to interpret experimental results and highlight the advantages of candidate methods. The qualitative effectiveness of optical NDE is further investigated using full-field surface slope measurements (shearography). Because accurate characterization of structural defects is critical to flight safety, a quantitative non-destructive evaluation (QNDE) method using artificial neural networks (ANNs) is developed. This method involves the use of radial basis function networks (RBFNs) trained and validated using FEA simulation data. The effectiveness of this QNDE approach is demonstrated using experimental data from a custom-built optical scanning system.
Design and Simulation of an Active Load Balancing System for High-Speed, Magnetically Supported Rotors
(2008-04-07) Robb, James Lawrence IV; Dr. M.K. Ramasubramanian, Committee Member; Dr. Paul I. Ro, Committee Member; Dr. Kari Tammi, Committee Member; Dr. Gregory D. Buckner, Committee Chair
Active magnetic bearings (AMBs) are being increasingly employed in the development of oil-free turbo machinery. One disadvantage of AMB systems, particularly AMB thrust bearings, is their limited dynamic load capacity relative to fluid film bearings. For centrifugal compressors, the most significant transient axial loads are associated with compressor surge, dictating that some of the AMB's load capacity be preserved to handle dynamic loads in this operating region. For other regions of operation, however, the AMB's load capacity may not be fully utilized, compromising compressor efficiency. One common solution to this problem involves the use of static balance pistons to keep thrust loads sufficiently small. Static balance pistons, however, employ seals that leak process gas flow and reduce machine performance. For these reasons, an active thrust load management system is sought. The active thrust balancing design proposed in this thesis seeks to improve the performance of AMB-supported turbo machines by maximizing load capacity and minimizing leakage across the machine's operating space. This design specifically targets high pressure ratio, single-overhung compressor systems that use magnetic thrust bearings. Detailed modeling and simulations are utilized to illustrate the limitations of magnetic thrust bearings and to discuss the pertinent design issues and benefits of regulating thrust loads. The modeling process addresses realistic dynamic effects such as amplifier saturation, magnetic flux saturation, and eddy currents. Simulation results are used to design an active thrust balancing system, and axial force and leakage flow characteristics of this active device are compared to a stationary design. The proposed active design is shown to offer average leakage reductions of 9.0% to 26.4% relative to static balancing devices. Finally, an observer-based controller is designed, and a gain-scheduling methodology is proposed to cover the compressor's full operating map.
Development of a Non-contacting Capacitive Displacement Sensor for Integrated Chatter Prediction on High Speed Machining Centers
(2003-06-09) Blue, Michael Dewayne; Dr. Kara J. Peters, Committee Member; Dr. Griff Bilbro, Committee Member; Dr. Gregory D. Buckner, Committee Chair
Chatter is an unstable forced vibration of the cutting tool during machining processes that can limit the productivity of high-speed machining. Chatter increases tool and machine wear rates, reduces tolerances, degrades surface finish, and can result in tool breakage. For these reasons, its avoidance is critical. Researchers at N.C. State University are focusing on the development of technologies to detect and avoid this dynamic instability. The primary objective of this project is to develop accurate, high bandwidth, low cost capacitive displacement sensors for use with a prototype electromechanically actuated chatter prediction device. These application-specific sensors are required to have at least 1.0 µm resolution, 1.0 kHz bandwidth, and linearity over the desired measurement ranges. These sensors must output analog DC voltages proportional to the vertical and horizontal displacements of a specifically engineered machine tool. A simulation-based design process is utilized to optimize the mechanical and electrical aspects of these capacitive probes, the electrical circuits, and the mechanical mounting hardware. Performance and stability are simulated using MATLAB® and SPICE® modeling software and are validated experimentally using static and dynamic test rigs.
A Dynamically Pressurized Heart Model to Facilitate the Development of Surgical Tools and Techniques for Mitral Valve Repair.
(2008-03-26) Richards, Andrew Latimer; Dr. Gregory D. Buckner, Committee Chair; Dr. Richard L. Goldberg, Committee Member; Dr. Denis R. Cormier , Committee Member
BACKGROUND: The development of a novel surgical tool or technique used in mitral valve repair can be hampered by the cost, complexity, and time associated with performing animal trials. We sought to develop a dynamically pressurized model which detects and quantifies mitral regurgitation in intact porcine hearts in order to preliminarily evaluate the effectiveness of mitral valve repair methods without the need for animal trials. METHODS: A computer controlled pulse duplication system was designed to accept freshly explanted porcine hearts and replicate a wide range of physiological conditions. To test the capabilities of this system in measuring mitral regurgitation, the cardiac output of four hearts was measured under two different peak left atrial pressures (120 and 150 mmHg) before and after induced mitral valve failures. Measurements were compared with clinically standard echocardiographic images. RESULTS: For all trials, cardiac output decreased as peak left atrial pressure was increased. After induction of mitral valve insufficiencies, cardiac output decreased, with a peak regurgitant fraction of 27%. These findings correlated well with the results from echocardiography. CONCLUSIONS: The resulting system is able to consistently and reliably detect and quantify mitral regurgitation and serves as an effective tool for the design of mitral valve repair techniques. The system is advantageous in its low experimental cost and time associated with each trial, while still allowing for surgical evaluations in an intact heart.
Force Feedback Control of Tool Deflection in Miniature Ball End Milling.
(2003-06-11) Hood, David Wayne; Dr. Thomas A. Dow, Committee Member; Dr. Ronald O. Scattergood, Committee Member; Dr. Gregory D. Buckner, Committee Chair
Previous research at North Carolina State University focused on open-loop compensation of machining errors associated with tool deflection in miniature ball end milling. These methods utilized tool force models to predict deflections an precompensate tool paths off-line to achieve dimensional tolerance and accuracy in finished parts. Accuracy depended on the tool force model, its cutting parameters, and workpiece alignment and dimensional accuracy. Real-time force feedback has the potential to further improve the accuracy of profiles created during machining. This paper demonstrates that force feedback can be used to predict tool deflection and compensate for deflection during the milling operation, reducing susceptibility to uncertainties in model parameters and workpiece alignment. Two specific force feedback approaches are presented here: cutting depth prediction (based on a non-dynamic cutting force model) and tool deflection prediction (using a non-dynamic model of tool stiffness). Real-time control algorithms incorporating both methods were implemented and evaluated on a high-speed air bearing spindle. A non-dynamic tool force model developed previously at the Precision Engineering Center used measured forces to predict depth of cut. A separate tool stiffness model was developed to predict tool deflections (axial and radial) based on measured forces. Experiments involving machined grooves in hard steel workpieces, including simple slotting cuts and three-dimensional finishing operations, were conducted at various tool tilt angles to evaluate the effectiveness of force feedback control. Results indicate that profile errors can be reduced up to 80% compared to non-compensated cases. These results confirm that real-time force feedback control can significantly improve the dimensional tolerance and accuracy of injection molds created using miniature ball end mills.
On the Design, Modeling, and Control of a Hybrid Pump System for Dynamic Pressurization of Explanted Mammalian Hearts
(2008-02-28) Dugan, Sean Patrick-Michael; Dr. Gregory D. Buckner, Committee Chair; Dr. Larry Silverberg, Committee Member; Dr. Denis R. Cormier, Committee Member; Dr. Andre P. Mazzoleni, Committee Member
A hybrid electromechanical pump system is proposed to mimic left ventricular blood pressure in a living mammalian heart. The system consists of a gear pump and voice coil actuated diaphragm (VCAD) pump connected in parallel. By combining a high-capacity, low-bandwidth gear pump with a low-capacity, high-bandwidth VCAD pump, the advantages of both can be realized, resulting in an economical high-bandwidth pumping system that may be used with animal hearts of arbitrary size. Mathematical models are developed to describe the system dynamics and develop a digital controller. Experimental results are also presented.