MS, University of Cincinnati, 2020, Engineering and Applied Science: Mechanical Engineering

Bayesian optimization (BO) is a popular method for solving optimization problems involving expensive objective functions. Although BO has been applied across various fields, its use in structural optimization area is in its early stages. Origami folding structures provide a complex design space where the use of an efficient optimizer is critical. In this research work for the first time the ability of BO to solve origami-inspired design problems is demonstrated. A Gaussian process (GP) is used as the surrogate model that is trained to mimic the response of the expensive finite element (FE) objective function. The ability of this BO-FE framework to find optimal designs is verified by applying it to two well known origami design problems: chomper and twist chomper. The performance of the proposed approach is compared to traditional gradient-based optimization techniques and genetic algorithm methods in terms of ability to discover designs, computational efficiency and robustness. BO has many user-defined components and parameters, and intuitions for these for structural optimization are currently limited. In this work, the role of hyperparameter tuning and the sensitivity of Bayesian optimization to the quality and size of the initial training set is studied. Taking a holistic view of the computational expense, various heuristic approaches are proposed to reduce the overall cost of optimization. A methodology to include derivative information of the objective function in the formulation of the GP surrogate is described, and its advantages and disadvantages are discussed. Additionally, an anisotropic GP surrogate model with independent length scales for each design variable is studied. A procedure to reduce the overall dimension of the problem using information from anisotropic models is proposed. The results show that Bayesian optimization is an efficient and robust alternative to traditional methods. It allows for the discovery of optimal designs using f (open full item for complete abstract)

Committee: Kumar Vemaganti Ph.D. (Committee Chair); Philip Buskohl Ph.D. (Committee Member); Manish Kumar Ph.D. (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2014, Electrical and Computer Engineering

As technology progresses, mobile devices such as laptops, tablets, cell phones, and two-way radios have become smaller in size. Consequently antennas become electrically small to fit inside aggressive packaging requirements with rapidly changing real and imaginary impedances. As such, these antennas are very narrow in bandwidth with high-Q and input impedance which is very sensitive to environmental effects. The radiation efficiency of the device is drastically decreased as the antenna is detuned and signal quality is degraded. As the number of mobile devices we use increases, adaptive impedance tuners have and will become a bigger necessity, especially as more radios are integrated into a single device. This dissertation presents novel improvements to closed loop tuning topologies from a system level perspective addressing impedance tuners, sensing techniques, and how they apply to different antennas. The biggest design hindrance to impedance tuners are losses due to small signal resistance, and loss due to circuit resonances and radiation. A detailed explanation of these loss mechanisms is developed, providing designers with the knowledge to minimize the impact of said losses and improve system efficiency. By exploiting loss mechanisms, a novel small and low cost VHF impedance synthesizer is presented to characterize impedance tuners in load pull measurements. With full consideration of circuit loss mechanisms, a new directional coupler based tuning topology is presented. Traditional tuning topologies aim to minimize |S11| of the matching network. As demonstrated in this work, such a method has the potential to maximize losses in the circuit, especially in multi-stage tuners. Alternative directional coupler based topologies are presented which maximize the system transducer gain. Furthermore, a novel method of sensing a tuned state through the use of a near field probe that detects far field radiated power is introduced. A design guide is detailed with sev (open full item for complete abstract)

Committee: John Volakis Professor (Advisor); Chi-Chih Chen Professor (Committee Member); Chris Baker Professor (Committee Chair) Subjects: Electrical Engineering; Electromagnetics; Electromagnetism