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PROPERTIES OF THE WEIERSTRASS FUNCTION IN THE TIME AND FREQUENCY DOMAINS

CHAOS SOLITONS & FRACTALS 1995年第1期5卷 1-8页

作者： NICOLETTI, D Electrical and Computer Engineering Department Worcester Polytechnic Institute 100 Institute Road Worcester MA 01609 USA

The Weierstrass function is continuous everywhere and differentiable nowhere. It is used advantageously for modeling fractally coarse media. The frequency domain expression is analyzed using the Laplace transform of a truncated Weierstrass function. The poles of this function constitute a geometric sequence on the imaginary axis of the complex frequency plane. computer calculations show that the zeros change little as the order is increased. Theoretical development reveals the nature of the dependence of the zeros on order and the parameters of the function. The effect of the scale of observation on the function's properties is examined in both time and frequency domains.

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Determination of grain-size distribution from ultrasonic attenuation: Transformation and inversion

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JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA 1997年第2期101卷 686-689页

作者： Nicoletti, D Anderson, A Electrical and Computer Engineering Department Worcester Polytechnic Institute 100 Institute Road Worcester Massachusetts 01609

Although researchers often describe a particular medium with only a single grain size or an average grain size, distributions of grain sizes can have significant effects on the physical characteristics of the material. Ultrasonic attenuation due to energy scattered at the grain boundaries depends on the ultrasonic wavelength, the grain size, and certain material properties such as elastic constants. This paper examines the relationship between the wavelength dependency of ultrasonic attenuation and grain-size distributions. A transformation is developed which solves a general class of Fredholm equations of the first kind. The equation relating attenuation to grain-size distributions belongs to this class of equations. The transformation allows attenuation to be calculated by discrete convolution, and provides a method for estimating the grain-size distribution directly from attenuation measurements. (C) 1997 Acoustical Society of America.

关键词： wavelengths Particle size distribution size distribution Material properties Physical characteristics Ultrasonic attenuation Researchers

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ON ADAPTIVE REFINEMENT FOR BOUNDARY INTEGRAL METHODS IN ELECTROSTATICS

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IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION 1994年第6期1卷 963-974页

作者： SPASOJEVIC, M LEVIN, PL Computational Fields Laboratory Department of Electrical and Computer Engineering Worcester Polytechnic Institute Worcester MA USA

Numerical techniques based upon boundary integral methods like charge simulation or Galerkin boundary elements are still warmly received in the HV community because they are relatively easy to implement and use, and because they are accurate. However, the results can be sensitive to the quality of the discretization. In this work we introduce an efficient refinement scheme that automatically chooses the sections of conducting and dielectric boundaries where new nodes should be placed. The numerical results compare favorably to canonical solutions, and the method is shown to be effective on more complicated practical geometries.

关键词： Integral equations Electrostatics Geometry Computational modeling Costs Interpolation Laboratories computer simulation Moment methods Dielectrics

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THEORETICAL BASIS FOR A UNIFIED CONSERVATION LAW DESCRIPTION OF THE ELECTROMAGNETIC ACOUSTIC TRANSDUCTION PROCESS

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IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL 1992年第4期39卷 476-480页

作者： LUDWIG, R Department of Electrical and Computer Engineering Worcester Polytechnic Institute Worcester MA USA

A unified theoretical formulation describing the electro-acoustic transduction processes governing an electromagnetic acoustic transducer is presented. This new approach establishes separate electrical, mechanical and material subsystem descriptions based on the momentum conservation forms of Maxwell's field equations in the quasi-static limit and Cauchy's law of motion. Instead of accounting for the electromagnetic acoustic material interaction by direct linkage of mechanical stress and magnetic flux through constitutive relations, the coupling of the field tensors is achieved as part of an additional material subsystem. As a result, the complete set of governing equations and associated boundary conditions can be derived from first principles in a concise way that lends itself to a quantitative numerical implementation in the time domain. Aspects of the numerical formulation and the subsequent computer model predictions are discussed in a companion paper.

关键词： Magnetic materials Magnetic flux Tensile stress Magnetostriction Maxwell equations Electromagnetic fields Acoustic materials Acoustic transducers Electromagnetic coupling Surface acoustic waves

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SHALLOW LEVEL RECOMBINATION CURRENT DOMINANCE IN TRANSISTOR BETAS

IEEE ELECTRON DEVICE LETTERS

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IEEE ELECTRON DEVICE LETTERS 1993年第2期14卷 49-50页

作者： LANYON, HPD Department of Electrical and Computer Engineering Worcester Polytechnic Institute Worcester MA USA

A new recombination mechanism, via the high density of shallow donor and acceptors present in the base-emitter junction of integrated circuit transistors, is suggested. In contrast to the recombination via deep centers, this mechanism has an ideal voltage dependence, leading to a beta essentially independent of the base-emitter voltage. It appears to be the dominant component to the base current of integrated circuit transistors. The temperature dependence of the beta of the transistors arises from the ionization energy of the donors and acceptors. The rate-setting capture cross section by neutral acceptor and donors is 1.3(5) x 10(-15) cm2.

关键词： Voltage Spontaneous emission Charge carrier processes Electron traps Silicon Temperature dependence Energy capture Ionization Doping Diodes

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A finite-element time-domain method using prism elements for microwave cavities

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IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY 1995年第4期37卷 519-527页

作者： Sacks, ZS Lee, JF Department of Electrical and Computer Engineering Worcester Polytechnic Institute Worcester MA USA

A finite-element time-domain (FETD) method for solving Maxwell's equations was developed by combining prism-based edge elements with central differencing in the time domain. This method solves for the electric field as a vector function of space and time in finite cylindrical cavity geometries of arbitrary geometry. Two stability analyses of this method are performed using the growth factor technique and the Z-transform. Furthermore, the computational complexity of the present approach is also investigated. This method has been implemented and its validity tested by studying the resonant frequencies of various microwave cavities.

关键词： Finite element methods Time domain analysis Microwave theory and techniques Geometry Maxwell equations Difference equations Finite difference methods Magnetic fields Stability analysis Computational complexity

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ANALYTICAL EXPRESSIONS FOR THE TIME-DOMAIN DISPLACEMENT RESPONSE OF A PIEZOELECTRIC PLATE RESONATOR

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IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL 1995年第4期42卷 801-803页

作者： USTUNDAG, A LUDWIG, R LEVIN, P Computational Electromagnetics and Ultrasonics Group Department of Electrical and Computer Engineering Worcester Polytechnic Institute Worcester MA USA

An analytical one-dimensional series solution for the time-domain displacement response of a piezoelectric plate is derived. Rather than the usual charge driven electric boundary value formulation, this paper develops the solution for a voltage-driven step function excitation where one electrode is stress-free and the other is displacement fixed. The advantage of this rapidly converging series lies in the ability to test numerical methods such as piezoelectric finite element time domain (FETD) algorithms.

关键词： Time domain analysis Electrodes Delay Finite element methods Equations Displays Dispersion Frequency dependence Acoustic reflection Time series analysis

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A COMPARATIVE-STUDY OF NONPARAMETRIC SPECTRAL ESTIMATORS FOR APPLICATION IN MACHINE VIBRATION ANALYSIS

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MECHANICAL SYSTEMS AND SIGNAL PROCESSING 1992年第6期6卷 551-574页

作者： NAJAFI, B HAKIM, H Electrical and Computer Engineering Department Worcester Polytechnic Institute Worcester MA 01609 U.S.A.

The vibration of a machine carries important information about the health of its internal components; this information, if properly extracted, can be extremely useful in the detection of developing faults within the machine. One of the most effective methods to extract this information is the computation of the vibration power spectrum, but since the statistical properties of the vibration signal are generally unknown, the power spectrum has to be estimated from the available finite-length data. A number of methods have been proposed for vibration spectral estimation, each presenting advantages and disadvantages. In each specific application, it is often necessary to evaluate and choose the optimum spectral estimation method. This paper presents an analysis of four widely accepted non-parametric spectral estimation methods: Periodogram, Blackman-Tukey, Welch, and Nuttall-Carter. It compares them based on their mean, variance, resolution, and computational requirements. It is found that all methods, except the Periodogram, can achieve the same mean, variance, and resolution but differ in leakage suppression and computational complexity. The Welch method has the best leakage suppression and the Nuttall-Carter method has the lowest computational burden in most cases.

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INTEGRATION OF STEREO VISION AND OPTICAL-FLOW BY USING AN ENERGY-MINIMIZATION APPROACH

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JOURNAL OF THE OPTICAL SOCIETY OF AMERICA A-OPTICS IMAGE SCIENCE AND VISION 1989年第6期6卷 900-907页

作者： NASRABADI, NM CLIFFORD, SP LIU, Y []Computer Vision Research Group Department of Electrical Engineering Worcester Polytechnic Institute Worcester Massachusetts 01609

A cooperative method is proposed in which image intensity (brightness) and optical-flow information are integrated into a single stereo technique by modeling the input data as coupled Markov random fields (MRF’s). The Bayesian probabilistic estimation method and the MRF–Gibbs equivalence theory are used to integrate the optical flow and the gray-level intensity information to obtain an energy function that will explicitly represent the depth discontinuity and occlusion constraints on the solution. This energy function involves the similarity in intensity (or edge orientation) and the optical flow between corresponding sites of the left and right images as well as the smoothness constraint on the disparity solution. If a simple MRF is used to model the data, the energy function will yield a poor disparity by smoothing across object boundaries, particularly when occluding objects are present. We use optical-flow information to indicate object boundaries (depth discontinuities) and occluded regions, in order to improve the disparity solution in occluded regions. A stochastic relaxation algorithm (simulated annealing) is used to find a favorable disparity solution by minimization of the energy equation.

关键词： Brightness Medical imaging Motion estimation Optical flow Remote sensing Stereo matching

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Tolerance analysis based computer-aided setup planning

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Journal of the Chinese institute of Industrial Engineers 2003年第3期20卷 247-257页

作者： Zhang, Y. Hu, W. Rong, Y. Computer-aided Manufacturing Laboratory Department of Mechanical Engineering Worcester Polytechnic Institute Worcester MA 01609 United States Department of Mechanical Engineering United States Computer-aided Manufacturing Laboratory Worcester Polytechnic Institute United States

Setup Planning is to determine the number/sequence of setups in a production line and operation details in each setup. To bridge the gap between computer-aided process planning (CAPP) and computer-aided fixture design (CAFD), an automated setup planning and tolerance decomposition method is developed. Directed graph is extended to represent feature/dimension/tolerance relationships (FTG) and datum/machining feature relationships (DMG). According to different production schemes and manufacturing resource capabilities, setup planning principles and algorithms are explored to automatically extract DMG from FTG. Under the true positioning frame, tolerance decomposition models are concluded to partition a tolerance into interoperable machining errors, such as locating error, tool alignment error, and random process error. As one of the deterministic components of machining errors, locating error is primarily caused by positional variations of locating elements in fixture design and dimensional variations of locating features on workpieces. In this research, mathematical models are developed to analyze locating variations and their influences on machining features. The analysis results of locating error effects can be used in budgeting the locating precision and improving the setup plan. The technique has been implemented on a commercial CAD system with applications to automobile manufacturing industry.

关键词： computer aided design

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