Browsing by Author "Dr. M.K. Ramasubramanian, Committee Member"

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Actuation and Control Strategies for Miniature Robotic Surgical Systems
(2002-08-12) Stevens, Jason Michael; Dr. Edward Grant, Committee Member; Dr. Gregory Buckner, Committee Chair; Dr. M.K. Ramasubramanian, Committee Member
Over the past 20 years, tremendous advancements have been made in the fields of minimally invasive surgery (MIS) and minimally invasive robotic assisted (MIRA) surgery. Benefits from MIS include reduced pain and trauma, reduced risks of post-operative complications, shorter recovery times, and more aesthetically pleasing results. MIRA approaches have extended the capabilities of MIS by introducing three-dimensional vision, eliminating tremors, and enabling the precise articulation of smaller instruments. These advancements come with their own drawbacks, however. Robotic systems used in MIRA procedures are large, costly, and do not offer the miniaturized articulation necessary to facilitate tremendous advancements in MIS. This research tests the hypothesis that miniature actuation can overcome some of the limitations of current robotic systems by demonstrating accurate, repeatable control of a small end-effector. A 10X model of a two link surgical manipulator is developed, using antagonistic shape memory alloy (SMA) wires as actuators, to simulate motions of a surgical end-effector. Artificial neural networks (ANNs) are used in conjunction with real-time visual feedback to "learn" the inverse system dynamics and control the manipulator endpoint trajectory. Experimental results are presented for indirect, on-line learning and control. Manipulator tip trajectories are shown to be accurate and repeatable to within 0.5 mm. These results confirm that SMAs can be effective actuators for miniature surgical robotic systems, and that intelligent control can be used to accurately control the trajectory of these systems.
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.
Experimental Characterization and Modeling of Electro-Mechanically Coupled Ferroelectric Actuators
(2008-11-13) York, Alexander; Dr. Stefan Seelecke, Committee Chair; Dr. Gregory Buckner, Committee Member; Dr. M.K. Ramasubramanian, Committee Member; Dr. Ralph Smith, Committee Member
Piezoelectric actuators used in nano-positioning devices exhibit highly non-linear behavior and strong hysteresis. The rate-dependence of piezoelectric materials resulting from the kinetics of domain switching is an important factor that needs to be included in realistic modeling attempts. This thesis provides a systematic study of the rate-dependent hysteresis behavior of a commercially available PZT stack actuator. Experiments covering full as well as minor loops are conducted at different electro-mechanically coupled loading conditions with polarization and strain recorded. In addition, the creep behavior at different constant levels of the electric field is observed. These experiments provide evidence of kinetics being characterized by strongly varying relaxation times that can be associated with different switching mechanisms. Finally, an electro-mechanically coupled free energy model for polycrystalline ferroelectrics is presented that is based on the theory of thermal activation. It is capable of predicting the hysteretic behavior along with the frequency-dependence present in these materials. The electro-mechanically coupled model also predicts the behavior of spring coupled actuators under various pre-stress levels. The model will be coupled with a SDOF model of a commercial nano-positioning stage (Nano-OP30, Mad City Labs) and is the basis for future control applications.
Intelligent Event Detection in Aircraft Engines
(2008-05-19) Noel, Nana K.; Dr. Gregory Buckner, Committee Chair; Dr. M.K. Ramasubramanian, Committee Member; Dr. Paul Ro, Committee Member