256 research outputs found
3D documentation and preservation of historical monument using terrestrial laser scanning
No full textA STUDY ON GABA TRANSPORTERS IN THE TRIGEMINAL SENSORY NUCLEUS AND THEIR ROLE IN HYPERALGESIA
No full textPh.DDOCTOR OF PHILOSOPH
Strain-tuning of periodic optical devices : tunable gratings and photonic crystals
No full textThesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2003.Includes bibliographical references (p. [161]-173).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.The advancement of micro- and nano-scale optical devices has heralded micromirrors, semiconductor micro- and nano-lasers, and photonic crystals, among many. Broadly defined with the field of microphotonics and microelectromechanical systems, these innovations have targeted applications in integrated photonic chips and optical telecommunications. To further advance the state-of-the-art, dynamically tunable devices are required not only for demand-based reconfiguration of the optical response, but also for compensation to external disturbances and tight device fabrication tolerances. In this thesis, specific implementations of strain-tunability in two photonic devices will be discussed: the fundamental diffractive grating element, and a photonic band gap microcavity waveguide. For the first part, we demonstrate high-resolution analog tunability in microscale diffractive optics. The design concept consists of a diffractive grating defined onto a piezoelectric-driven deformable membrane, microfabricated through a combination of surface and bulk micromachining. The grating is strain-tuned through actuation of high-quality thin-film piezoelectric actuators. Device characterization shows grating period tunability on the order of a nanometer, limited by measurement uncertainty and noise. The results are in good agreement with analytical theory and numerical models, and present immediate implications in research and industry. For the second part, we generalize the piezoelectric strain-tunable membrane platform for strain-tuning of a silicon photonic band gap microcavity waveguide. Additional motivation for this strain-tuning approach in silicon photonic crystals lies in:(cont.) (a) the virtual absence of electro-optic effects in silicon, and (b) the ability to achieve tuning with low power requirements through piezoelectric actuation. Compared to current thermo-optics methods, piezoelectric actuation affords faster and more localized tuning in high-density integrated optics. The small-strain perturbation on the optical resonance is analyzed through perturbation theory on unperturbed full 3D finite-difference time-domain numerical models. Device fabrication involves X-ray nanolithography and multi-scale integration of micro- and nano-fabrication methods. Experimental characterization achieved dynamically-tunable resonances with 1.54 nm tunable range (at 1.55 Mum optical wavelengths), in good agreement with theory. This is the first demonstration of strain tunability in photonic crystals and contributes to the development of smart micro- and nano-scale photonics.by Chee Wei Wong.Sc.D
BIFURCATED DOMESTICITY : IDENTITY NEGOTIATIONS WITHIN THE DIASPORIC SOJOURN OF A BANGLADESHI MIGRANT WORKER IN SINGAPORE
No full textMaster'sMASTER OF ARCHITECTURE (M.ARCH
The rise of the information battlespace : realities from the Iraq wars
No full textThe Information Age has presented many new opportunities and challenges to warfighting. Advanced technologies, especially in the information domain, had transformed military capabilities and fighting doctrines. But at the same time, these capabilities have also complicated the notion of war victory.Master of Science (Strategic Studies
Sub-fm/Hz(1/2) displacement measurement on MgF2 whispering gallery mode microcavity
No full textWe present sub-fm/Hz(1/2) displacement measurement on the MgF2 whispering gallery mode microcavity. We analyze power spectral density of the displacement from 1 Hz to 100 kHz of the Fourier offset frequency. (C) 2020 The Author(s
End-to-end autonomous driving based on reinforcement learning
No full textIn this project, an RGB camera will be used as data input to explore an end-to-end method based on visual based reinforcement learning. The project will be carried out with the Unity game engine as the training environment, along with Unity’s ML-Agents package that provides out of the box deep Reinforcement Learning (RL) algorithms to interface with their environment. The results of training a simulated donkey car to drive in its own lane with an on-policy method, Proximal Policy Optimization (PPO), and an off-policy method, Soft Actor-Critic (SAC) will be compared. An ablation study, consisting of adding Generative Adversarial Imitation Learning (GAIL), semantic segmentation and stacked visual inputs, will be performed. Additionally, RL based obstacle avoidance will be explored. The results, based on stability of control and ability to stay in lane, indicate that the best performing method is PPO. Code is available at: https://github.com/MrOCW/Autonomous-Driving-RL-UnityBachelor of Engineering (Mechanical Engineering
A discovery process for initializing ad hoc underwater acoustic networks
No full textSeaweb is an underwater acoustic wide-area network connecting autonomous, distributed nodes. Prior iterations of Seaweb relied on operator intervention to initialize and manually configure the network routes. This thesis implements a network discovery process that enables a field of spontaneously deployed, ad hoc nodes to auto-configure for networking purposes. Network routing is initialized as nodes in the network are discovered, with routes chosen according to comparative evaluation of a cost function for all candidate routes. The implemented network discovery process is tested using computer simulation and sea trial data. The resultant network routes obtained upon completion of the ad hoc network discovery process are compared with those derived from Dijkstra's algorithm. It is concluded that the network discovery process always produces a shortest-path route from a master node to any other discovered nodes in the network. Sensitivity studies on the route cost evaluation function are performed, and an alternative network discovery scheme is discussed.Approved for public release; distribution is unlimited.Republic of Singapore Navy author.http://archive.org/details/adiscoveryproces10945377
Characterization of heat transfer process in selective laser melting
No full textThis report serves as a documentation of the work and research that the student has accomplished for his final year project on the Characterization of Heat Transfer Process in Selective Laser Melting. The report consists of a literature review on the theory of Additive Manufacturing and its various techniques. Six different machines across varying additive manufacturing concepts are brought together and examined in detail. One of these machines, M2 Cusing from Concept Laser is selected to produce specimens for this study. All specimens are made of AlSi10Mg alloy.
Experiments are conducted to investigate certain properties of the specimens and each experimental setup is clearly described. The specimen properties that are investigated in this study includes dimensional and geometrical accuracy, parallelism and flatness, surface roughness, density, porosity, microstructure and precipitate identification. The effects of several post-processing methods such as sandblasting and heat treatment are also investigated. Results are tabulated according to its different properties and are subsequently discussed. Relevant theories of heat transfer are used to explain some of the phenomena observed from the results. Relationships between the quality of certain properties and the machine’s processing parameters are developed.
This report also recommends improvements on certain aspects of the experiment as well as suggests specific areas for future studies.Bachelor of Engineering (Mechanical Engineering
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