Browsing by Author "Kim, Woon Kyung"

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    Design and Analysis of Switching Circuits for Energy Harvesting in Piezostrutures
    Kim, Woon Kyung (Virginia Tech, 2012-08-06)
    This study deals with a general method for the analysis of a semi-active control technique for a fast-shunt switching system. The benefit of the semi-active system is the reduction in power consumption, which is a significant disadvantage of a fully active system compared with a passive system. A semi-active system under consideration is a semi-actively shunted piezoelectric system, which converts the strain energy into electrical energy through strong electromechanical coupling achieved though the piezoelectric phenomenon. Our proposed semi-active approach combines a PZT-based energy harvesting with a fast switching system driven by a Pulse-Width Modulated (PWM) signal. The fast switching system enables continuous adaptation of vibration energy control/harvesting by varying the PWM duty cycle. This contrasts with a conventional capacitance switching system that can only change the capacitance at discrete values. The analysis of the current piezoelectric system combined with a fast-switching system poses a considerable challenge as it contains both continuous and discrete characteristics. The study proposes an enhanced averaging method for analyzing the piecewise linear system. The simulation of the averaged system is much faster than that of the time-varying system. Moreover, the analysis derives error bounds that characterize convergence in the time domain of the averaged system to the original system. The dissertation begins with the derivation of the equations governing the physics of a piezostructure combined with an electrical switching shunt network. The results of the averaging analysis and numerical simulation are presented in order to provide a basis for estimating the structural responses that range between open- and short-circuit conditions which constitutes two limiting conditions. An experimental study demonstrates that the capacitive shunt bimorph piezostructure coupled with a single switch can be adjusted continuously by varying the PWM duty cycle. And the behavior of such hybrid system can be well predicted by the averaging analysis.
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    Design Optimization Of Slotted Waveguide Antenna Stiffened Structures (Swass)
    Kim, Woon Kyung; Canfield, Robert A. (2014-04-15)
    The objective of the research is to investigate computational methods for design optimization of a Conformal Load-Bearing Antenna Structure (CLAS) concept. Research centers on investigating computational methods for design optimization of a slotted waveguide antenna stiffened structure (SWASS). The goal of this concept is to turn the skin of aircraft into a radio frequency (RF) antenna. SWASS is a multidisciplinary blending of RF slotted waveguide technology and stiffened composite structures technology. Waveguides provide channels for RF signal transmission, as well as structural stiffening. A SWASS skin or stiffener will have numerous slots that allow the RF energy to radiate to the atmosphere. Slot design for maximum RF performance with minimum structural performance degradation due to the slots will be the multidisciplinary, multiobjective design challenge. Initially, waveguides acting as hat stiffeners were considered in this research; then, waveguides that constituted the core of a sandwich panel were designed for loads in the aircraft skin. The concept design requires parameterization of slot shape, size, location, and spacing in conjunction with stiffener or core sizing and spacing, composite material selection, and laminate layout in order to simultaneously meet desired structural and RF performance.