Browsing Dept. of Mechanical Engineering - Ph.D. / Sc.D. by Issue Date
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Item Open AccessDesign and fabrication of micro end mills for the machining of difficult-to-cut materials(Bilkent University, 2016-08) Oliaei, Samad Nadimi Bavil; Karpat, YiğitMicromilling is a cost-e ective method of fabricating miniaturized components with complex, three-dimensional features made from di cult-to-cut materials. Microcutting tools are exposed to harsh conditions during machining of such materials, which leads to short tool life and thus a ects the economics of the process. The aim of this thesis is to develop a systematic approach to the design and fabrication of high-precision micro-cutting tools. Machining characteristics of three di erent di cult-to-cut materials–stainless steel, titanium alloy, and silicon–have been investigated using experimental techniques. The results reveal the importance of interaction between tool micro geometry and work material mechanical properties. This observation leads to the development of tailored micro-end mills which are designed and fabricated based on the requirements of the specific machining task. This study also examines in detail built-up edge, an important but usually overlooked issue in micromachining of ductile materials, which a ects the process forces, tool wear, and tool deflections. The protective e ect of built-up edge has been exploited by creating micro-dimples on the tool surface using electrical discharge machining. Its positive influence on tool performance has been demonstrated. As for the micromachining of silicon, the flow of cut material around the cutting edge is paramount in tool design. A novel tool design for machining of silicon has been proposed and its e ectiveness has been validated through experiments. It has been shown that the selection of proper process parameters together with tailored tool design may increase the productivity of micromachining and improve surface quality and dimensional accuracy of micro-scale parts. Item Open AccessAdaptive control of cyberphysical human systems(Bilkent University, 2021-08) Tohidi, Seyed Shahabaldin; Yıldız, YıldırayThis dissertation focuses on the control of cyberphysical human systems in the presence of actuators’ redundancy and constraints. A novel adaptive control tech-nique is proposed to allocate control signals among redundant actuators in the presence of uncertainty and actuator saturation. The proposed method does not require any uncertainty identiﬁcation or persistency of excitation assumption. The stability of the proposed method is guaranteed using Lyapunov stability analysis. In addition, a modiﬁed projection operator that can be implemented to the adaptive control allocation is proposed. This operator enables the allo-cator to handle both magnitude and rate limits of actuators. A novel sliding mode controller with time-varying sliding surface is designed to complement the adaptive allocator and guarantee stability and reference tracking in the presence of uncertainty and actuator saturation. This controller is robust to both adap-tive control allocation error and external disturbance. Furthermore, an adaptive human model is proposed to mimic the human control response in the presence of uncertainty. The proposed structure is based on the model reference adaptive control, and the adaptive laws are obtained using the Lyapunov-Krasovskii stabil-ity criteria. To validate this model, an experimental setup is employed to collect data and a statistical analysis is conducted to measure the predictive power of the pilot model. Finally, the stability limits of a human-in-the-loop closed loop control system, where the plant to be controlled has redundant actuators with uncertain dynamics, are demonstrated. Various human models with and without time delays are investigated. Simulation results are provided to demonstrate the eﬀectiveness of the proposed methods in each chapter. Item EmbargoNanomechanical and microwave resonance sensing for characterization of individual virions and nanoparticles in atmospheric conditions(Bilkent University, 2023-09) Alkhaled, Mohammed; Hanay, Mehmet SelimThis dissertation focuses on Nanoelectromechanical-based Mass Spectrometry (NEMS-MS), an innovative technique for characterizing nanoparticles and biomolecules weighing above the working limit of commercial mass spectrometry tools. It suggests performing NEMS-MS under atmospheric conditions and enhancing its capabilities with a built-in focusing lens. Amid the COVID-19 pandemic, the study addresses urgent virus detection needs, proposing a label-free method using NEMS-MS for individual virus detection and characterization. Notably, the study achieves mass spectrometry measurement of the SARS-CoV-2 virus using a NEMS-MS system operating entirely under atmospheric pressure. As the first to pioneer NEMS-MS in air, the study examines challenges tied to this, particularly how NEMS response in dissipative environments, known as Mode Shape Attenuation. Mathematical models and experiments dissect factors contributing to this attenuation, resulting in improved mass spectra and contributing toward the utilization of NEMS-MS for real-world application. Taking innovation a step further, the study introduces a microwave-based sensor for inferring electrical properties of nanoparticles. This sensor works in the electro-magnetic domain, determining properties like dielectric constant and expanding the sensing possibilities. Overall, this dissertation propels NEMS-based sensing and characterization by combining mass spectrometry, microwave sensing, and atmospheric pressure operation. Addressing challenges and introducing innovative solutions, it advances NEMS-MS technology and offers a cost-effective tool for characterizing nanoparticles and biomolecules across various applications.