• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 1072
  • 463
  • 266
  • 142
  • 81
  • 58
  • 49
  • 41
  • 41
  • 37
  • 32
  • 22
  • 20
  • 14
  • 14
  • Tagged with
  • 2777
  • 358
  • 293
  • 266
  • 263
  • 257
  • 209
  • 191
  • 161
  • 154
  • 153
  • 134
  • 128
  • 127
  • 122
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
821

A Study of the Grinding Process for the Optical-Fiber Endface with Double-Variable Curvatures

Chen, Jun-Hong 02 September 2010 (has links)
Mechanical grinding process is the most popular way to fabricate the fiber micro lenses, although there are some other methods, such as chemical etching, laser machining and focused ion beam micro-cutting. Mechanical grinding has its uniqueness in grinding Conical-Wedge-Shaped Fiber Endface, fiber endface with polygon-cone-shape, and fiber endface with double-variable curvatures. The double-variable curvatures fiber endface polisher, designed and manufactured by Mechanism Design Lab of NSYSU, is employed in this study. The normal force of the fiber endface is derived firstly and then the experimental parameters and data are substituted into the material removal rate (M.R.R.) formula to obtain M.R.R. and the Preston¡¦s constant K. The process parameters of the feed rate and polishing time on the fabrication of the fiber endface are analyzed. The polisher is calibrated and adjusted to improve the precision of the optical-fiber endface. A fiber endface with double-variable curvature is successfully fabricated in a single grinding process by properly controlling the fiber rotation angle, inclining angle, and the distant between the endface and the grinding film simultaneously. The grinding process developed in this study can be applied for fabricating optical fiber lenses in fiber optics communication as well as different types of micro probes, and micro spectroscopefors in other applications.
822

Policy interia hypothesis or unobserved variable hypothesis in Taiwan¡¦s Interest-rate rule?

Shiu, Kai-hung 29 June 2011 (has links)
This paper adopts an modle with endogenous variable to investigates policy inertia hypothesis or unobserved variable hypothesis in Taiwan¡¦s interest-rate rule 1981- 2010. Empirical result suggests that both policy inertia and omitted variable hypothesis are important in monetary policy in Taiwan,which is consistent with that of Gerlach- Kristen (2004).
823

Design of Shunt Semi-Active Power factor Correction Circuits

Chen, Bing-Hao 14 February 2012 (has links)
This study aims to design a Shunt Semi-Active Power Factor Correction Circuits , which can be applied to high power circuit by low switching frequency. The designed circuit can avoid power loss working with high switching frequency when using the method of active power factor correction .The experimental configuration based on DSP is applied to a compressor of air conditioner with varied load. The simulation check the developed circuit using Ispice . Both of the experimental and simulation results have guaranteed the derived configuration reach the expected goals.
824

Theoretical and Experimental Studies of Material Flow during the Friction Stir Welding Process

Cheng, Yu-Hsiang 16 February 2012 (has links)
In order to simulate the histories of temperature distributions and plastic flow of the dwell phase during a friction stir welding process, the Newton-Raphson method is used to solve the simultaneous equations of energy and momentum in the cylindrical-coordinate system. Comparing the simulation with the results of experiment, results show that the contact condition between the tool and the workpiece is at pure sliding without plastic flow at the beginning of the dwell phase until the temperature rises to about 300¢XC at the depth of 1.5 mm. In this period, the heat generation comes from the sliding friction between two surfaces. After the plastic flow occurs, the heat generation rises rapidly, and then decreases to a saturated value so that the temperature rise also achieves a constant value. Thermal expansion of the workpiece will increase the plunge force, so that the heat generation and the temperature raise increase. At the steady state condition, with increasing sticking proportion, the heat generation and the temperature quickly achieve a saturated value. For the steady-state condition, results show that the speed of plastic flow and shear strain rate increase with increasing rotational speed. The control of the contact state variable can effectively describe the heat generation and the distribution of plastic flow in different contact conditions. Comparing the simulation with the results of experiment, the contact condition can be identified.
825

A dB-Linear Programmable Variable Gain Amplifier and A Voltage Peak Detector with Digital Calibration for FPW-based Allergy Antibody Sensing System

Hsiao, Wei-Chih 10 July 2012 (has links)
This thesis proposes a dB-linear programmable variable gain amplifier (VGA) and a voltage peak detector with digital calibration for FPW-based antibody sensing system. In the first topic, a dB-linear programmable variable gain amplifier is proposed. By using two source followers as the input terminals, input signals with very low DC offset could be received. The linear local-feedback transconductors are employed to be trans-condurctor-stage and load-stage. Besides, a reconfiguration method is used to reduce the layout area and improve the linearity of the gain to attain gain error less than 0.86 dB measured on silicon. In the second topic, a voltage peak detector with digital calibration is proposed. The voltage peak of the input sine-wave signal is sampled and held by using an integra-tor, a digital-to-analog converter, and a voltage comparator to generate a square-wave signal. Besides, the voltage error caused by the propagation delay could be calibrated by the proposed digital calibration method. The frequency of input signal is up to 20 MHz and the voltage error is justified to be less than 0.81 % by simulations.
826

Convergece Analysis of the Gradient-Projection Method

Chow, Chung-Huo 09 July 2012 (has links)
We consider the constrained convex minimization problem: min_x∈C f(x) we will present gradient projection method which generates a sequence x^k according to the formula x^(k+1) = P_c(x^k − £\_k∇f(x^k)), k= 0, 1, ¡P ¡P ¡P , our ideal is rewritten the formula as a xed point algorithm: x^(k+1) = T_(£\k)x^k, k = 0, 1, ¡P ¡P ¡P is used to solve the minimization problem. In this paper, we present the gradient projection method(GPM) and different choices of the stepsize to discuss the convergence of gradient projection method which converge to a solution of the concerned problem.
827

The Sign-up Game, Sophisticated Learning and Learning Variable Demand

Watugala, Megha Weerakooon 15 May 2009 (has links)
This dissertation makes contributions in topics related to mechanism design and learn-ing in game theoretic environments through three essays. The rst essay deals withthe question of mechanism design in the principal-agent model. The main contribu-tion of this essay is in extending the work by Piketty (1993). It prescribes a mechanismin incomplete informational settings where the principal is able to implement rst-best contracts while extracting the entire surplus. Importantly, the mechanism issuch that the desired outcome can be uniquely obtained when agents play the actionthat survives iterative elimination of dominated strategies. Furthermore, given themechanism, the desired outcome is shown to be a truth-revealing Nash equilibriumwhich is also Pareto-ecient. It is shown that the proposed mechanism also has thefeature that none of the agents prefer any of the other possible Nash Equilibria tothe status quo. It thus gives insights into possible mechanisms in nite agent settingsthat could improve upon the traditional second-best results.In the second essay, a model of sophisticated learning is developed where itassumes that a fraction of the population is sophisticated while the rest are adaptive learners. Sophisticated learners in the model try to maximize their cumulative payoin the entire length of the repeated game and are aware of the way adaptive learnerslearn. Sophisticated learning contrasts other models of learning which typically tendto maximize the payo for the next period by extrapolating the history of play.The sophisticated learning model is estimated on data of experiments on repeatedcoordination games where it provides evidence of such learning behavior.The third essay deals with the optimal pricing policy for a rm in an oligopolythat is uncertain about the demand it faces. The demand facing the oligopoly, whichcan be learned through their pricing policy, changes over time in a Markovian fashion.It also deduces the conditions in which learning (experimentation) is not achievableand outlines the dierent learning policies that are possible in other settings. Themodel combines the monopoly learning literature with that of the literature on pric-ing behavior of rms over business cycles. The model has interesting insights onthe pricing behavior over business cycles. It predicts that prices jump as the beliefof a possible future boom rises over a certain threshold. The model also predictscompetition to be quite vigorous following a boom while rms are predicted not toexperiment with their (pricing) policies for many periods following a bust.
828

Analysis and Simulation of Mechanical Trains Driven by Variable Frequency Drive Systems

Han, Xu 2010 December 1900 (has links)
Induction motors and Variable Frequency Drives (VFDs) are widely used in industry to drive machinery trains. However, some mechanical trains driven by VFD-motor systems have encountered torsional vibration problems. This vibration can induce large stresses on shafts and couplings, and reduce the lifetime of these mechanical parts. Long before the designed lifetime, the mechanical train may encounter failure. This thesis focuses on VFDs with voltage source rectifiers for squirrel-cage induction motors of open-loop Volts/Hertz and closed-loop Field Oriented Control (FOC). First, the torsional vibration problems induced by VFDs are introduced. Then, the mathematical model for a squirrel-cage induction motor is given. Two common control methods used in VFD are discussed - open-loop Volts/Hertz and closed-loop FOC. SimPowerSystems and SimMechanics are used as the modeling software for electrical systems and mechanical systems respectively. Based on the models and software, two interface methods are provided for modeling the coupled system. A simple system is tested to verify the interface methods. The study of open-loop Volts/Hertz control method is performed. The closed-form of electromagnetic torque sideband frequency due to Pulse Width Modulation is given. A torsional resonance case is illustrated. The effects of non-ideal power switches are studied, which shows little in uence on the system response but which uses little energy consumption. A study of a non-ideal DC bus indicates that a DC bus voltage ripple can also induce a big torsional vibration. Next, the study of the closed-loop FOC control method is presented. Simulation for a complete VFD machinery train is performed. With the recti er and DC bus dynamic braking, the system shows a better performance than the ideal-DC bus case. Lastly, a parametric study of the FOC controller is performed. The effects of primary parameters are discussed. The results indicate that some control parameters (i.e. speed ramps, proportional gain in speed PI controller) are also responsible for the mechanical torsional vibration.
829

A recursive formula for computing Taylor polynomial of quantile

Kuo, Chiu-huang 28 June 2004 (has links)
This paper presents a simple recursive formula to compute the Taylor polynomial of quantile for a continuous random variable. It is very easy to implement the formula in standard symbolic programming system, for example Mathematica (Wolfram, 2003). Applications of the formula to standard normal distribution and to the generation of random variables for continuous distribution with bounded support are illustrated.
830

Analysis and Experimental Investigation on Energy Conservation of VRV Systems in Hot Humid Climates

Chuang, Yi-hung 08 July 2004 (has links)
Being located in subtropical area, the weather in Taiwan is hot and humid which imposing huge cooling load on buildings. Conventionally, central air-conditioning plants were designed using refrigerant compressors to make chilled water, and then pumped through the zone pumps to meet the cooling load, providing air-conditioning by Fan Coil Unit (FCU) or Air-Handling Units (AHU) by ductwork. To meet the varying cooling demand, two important systems were developed for energy savings, namely, the Variable Water Volume (VWV) system, and the Variable Air Volume (VAV) system, which has been widely adapted in Taiwan area. The working principle is mainly devoted to adjusting the volume of the chilled water and/or air volume delivered through inverter-driven pimps and fans to achieve energy saving. On the other hand, recently in Japan, an important energy-saving air-conditioning system has been developed which directly varying the refrigerant flow rate to meet the varying cooling demand by inverter-driven compressors, named VRV system. Comparative to the conventional air-conditioning system, the heat exchange mechanism of the VRV system has been effectively enhanced by direct exchange of the refrigerant and the cool air, which is in effect a combination of the VWV and VAV system. It provided huge energy saving potential for the application on buildings with moderate cooling loads, such as 100 USRT or so. It is the goal of this research project, to evaluate the performance of the VRV system in Taiwan¡¦s hot and humid climate, by performing full-scale experimental investigation so that energy savings effect can be validated quantitatively. Since VRV system is fairly new in Taiwan, the validation of the system performance under local weather condition is of particular importance. It is anticipated that through the changing of the operation conditions, such as different outdoor conditions, various partial load conditions, and different scheduling of the VRV system, the power consumption of the VRV vs. conventional system can be compared precisely and quantitatively. These experimental data will, in turn, provides valuable reference to the establishment of the building energy consumption index in Taiwan, which outwits the direct adoption of the foreign data such as from Japan, in achieving a much reliable database.

Page generated in 0.0585 seconds