Showing papers by "Tsinghua University published in 1989"

An Average Reynolds Equation for Partial Film Lubrication With a Contact Factor

Abstract: Introduction d'un facteur de contact pour simplifier les calculs dans le cas de l'equation de Reynolds moyennee en lubrification avec film partiel

Nonlinear stabilizing control of multimachine systems

Abstract: It is of great importance to improve stability as well as dynamic performances of power systems for both small and large disturbance. Such issues have received a great deal of attention and many contributions have been made to this objective. However, some of the design methods developed previously are based on second-order synchronous generator model, so excitation systems or/and governor systems could hardly be considered by them, the others based on linear models, which are set up by approximate linearizing at an equilibrium point of the system. Since, in fact, a power system is a nonlinear dynamic one, the controller designed by using the approximate linearized model may cause untolerable errors, while the state point of the system is changed away from the equilibrium point at which the linearization is realized. A new approach to nonlinear decoupled optimal control based on the differential geometric control theory for interconnected power systems is proposed. And it is successfully applied to nonlinear steam valving control of multimachine systems. For an m-machine system the optimal steam valving control law of the ith-machine can be expressed as ui= TS; ii(K1-M& 1+ K2 cw-+ K3; | wdt + TsiPei+ (Pmj-Pmjo) (18) where Mi inertia coefficient (in sec.) Tsi, time constant of servomotor and steam (in sec.) wi, speed (in per unit) Di damping coefficient Pei electrical power (in per unit) Pmi mechanical power (in per unit) and K1j, K2j, and K3j are optimal feedback gain coefficients.

A multiscanning approach based on morphological filtering

Abstract: It is argued that the mathematical morphology method seems to be more reasonable and powerful in studying certain multiscaling vision problems than the approach that uses derivatives of Gaussian-shaped filters of different sizes. To show the validity of this method, the authors concentrated on an application that involves forming scale-space image of a 2-D shape using morphological opening filtering. A proof is given to show that morphological opening filtering has a property of not introducing additional zero-crossings as one moves to a coarser scale. This is a different result from the conclusion by A.L. Yuille and T.A. Poggio (ibid., vol.PAMI-8, Jan. 1986) that the Gaussian filter is the only filter with this property. In addition, opening filtering is computationaly simpler than the Gaussian filter. >

Low-Temperature Sintering of Lead-Based Piezoelectric Ceramics

Abstract: The low-temperature sintering of lead-based piezoelectric ceramics has been studied. The sintering temperature of lead zirconate titanate (PZT) ceramics could be reduced from ∼ 1250° to ∼960°C by the addition of a small amount of the lower-melting frit, B2O3–Bi2O3—CdO. It exhibited the following dielectric and piezoelectric properties: Kp= 0.52 to 0.58, Qm= 1000, eT33/e0= 800 to 1000, tan δ= 50 × 10−4, ρ= 7.56 to 7.64 g/cm3. Ceramics with the aid of suitable dopants (CdO, SiO2, and excess PbO) in the Pb-(Ni1/3Nb2/3)O3—PZT family could be sintered at 860° to 900°C. For these materials, Kp= 0.56 to 0.61, Qm= 1000, eT33/e0= 1500 to 2000, tan δ≤ 50 × 10−4, ρ= 7.80 to 8.03 g/cm3. The microstructure, sintering mechanism, and the effects of various impure additions have been analyzed by means of scanning electron microscopy, scanning transmission electron microscopy, electron probe microanalysis, and X-ray photoelectron spectroscopy.

Exact solutions for hydrogenic donor states in a spherically rectangular quantum well.

Abstract: Using different series forms in different regions of the radial equation, we have obtained exact solutions of donor states in a spherically rectangular quantum well by a numerical method. The calculated results show that there are stronger confinement and larger binding energy for a hydrogenic donor in the well of GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As than in quantum-well wires and two-dimensional quantum-well structures.

Round Robin computer simulation of ejection probability in sputtering

Abstract: We have studied the ejection of a copper atom through a planar copper surface as a function of recoil velocity and depth of origin. Results were obtained from six molecular dynamics codes, four binary collision lattice simulation codes, and eight Monte Carlo codes. Most results were found with a Born-Mayer interaction potential between the atoms with Gibson 2 parameters and a planar surface barrier, but variations on this standard were allowed for, as well as differences in the adopted cutoff radius for the interaction potential, electronic stopping, and target temperature. Large differences were found between the predictions of the various codes, but the cause of these differences could be determined in most cases. A fairly clear picture emerges from all three types of codes for the depth range and the angular range for ejection at energies relevant to sputter ejection, although a quantitative discussion would have to include an analysis of replacement collision events which has been left out here.

In vivo measurement of cortical bone lead using polarized x rays.

Abstract: An x‐ray fluorescence system which utilizes polarized radiation to measure lead in vivo in human subjects is described. The minimum detection limit is approximately 6.4 ppm wet weight lead in the cortex of the tibia with 4 mm of overlying soft tissue. This appears to be adequate for assessing lead stores in lead‐toxic preschool children. The measurement requires 16.5 min and is associated with an effective equivalent whole body dose to the subject of 2.5 μSv. The system, its calibration and its validation are described herein.

Electromagnetic scattering by a dielectric body with arbitrary inhomogeneity and anisotropy

Abstract: The scattering problem for a dielectric body is formulated in terms of the electric field integral equation where the scatterer is of general shape, inhomogeneity, and anisotropy. On applying the pulse-function expansion and the point-matching technique, the integral equation is solved using an efficient procedure involving the conjugate-gradient method and the fast Fourier transform (FFT). The solution procedure runs parallel to that of the two-dimensional case previously presented by the author (see ibid., vol.AP-35, p.1418-25, Dec. 1987). Most of the work presented involves generalizing two-dimensional Green's function and operations into corresponding three-dimensional ones. >

Design of feedback control laws for switching regulators based on the bilinear large signal model

Abstract: A method for the design of a linear feedback control law for switching regulators based on the bilinear large-signal model is presented. The resulting system guarantees a satisfactory stability region in the state space, and meets the requirements of large damping on transient responses with small feedback control energy. Large-signal transient responses are simulated and compared with the results obtained by R.W. Erickson et al. (1985). The proposed method is well suited for converter topologies with more than two state variables. >

Effect of Nonstoichiometry on Fracture Toughness and Hardness of Yttrium Oxide Ceramics

Abstract: This work deals with the changes in mechanical properties of yttrium oxide ceramics induced by nonstoichiometry. The maximum fracture toughness, Klc, is observed at the stoichiometric composition. For an oxygendeficient ceramic, a decrease of Klc is observed from 3.5 to 2.3 MPa.m1/2. On the other hand, the Vickers hardness seems to be less dependent on stoichiometry. These results are discussed in the frame of the evolution of the Y-O bond with the stoichiometry. They set in particular the problem of the role of electrostatic energy stored in a brittle material containing charged defects in the energy balance controlling crack propagation.

Decentralized output feedback stabilization of a class of nonlinear interconnected systems

Abstract: The problem of decentralized output feedback stabilization of a class of nonlinear interconnected systems is considered. On the basis of the linear-quadratic design, a sufficient condition for the existence of the decentralized output feedback control of the problem is presented. An estimation of the associated stability domain is also given. An application to the decentralized control of a large-scale telescope is presented to illustrate the use of the results. >

Drag on a metallic or nonmetallic particle exposed to a rarefied plasma flow

Abstract: Drag force on a metallic or nonmetallic spherical particle exposed to a plasma flow is studied for the extreme case of a free-molecule regime. Analytical expressions are derived for the drag components due to, respectively, atoms, ions, and electrons and for the total drag on the whole sphere due to all the gas species. It has been shown that the drag is proportional to the square of the particle radius or the drag coefficient is independent of the particle radius. At low gas temperatures with a negligible degree of ionization, the drag is caused mainly by atoms and could be predicted by using the well-known drag expression given in ordinary-temperature rarefied gas dynamics. On the other hand, the drag is caused mainly by ions at high plasma temperatures with a great degree of ionization. The contribution of electrons to the total drag is always negligible. Ignoring gas ionization at high plasma temperatures would overestimate the particle drag. There is a little difference between metallic and nonmetallic spheres in their total drag forces, with a slightly higher value for a metallic sphere at high plasma temperatures, but usually such a small difference could be neglected in engineering calculations. The drag increases rapidly with increasing gas pressure or oncoming speed ratio. For a two-temperature plasma, the drag increases at low electron temperatures but decreases at high electron temperatures with the increase in the electron/heavy-particle temperature ratio.

Tensile and fatigue properties of Fe-Mn-Al-C alloys

Abstract: The tensile properties and fatigue behaviour of three solution-treated Fe-29 Mn-9 Al-C (wt%) alloys having various carbon contents leading to different volume fractions of austenite and ferrite phases were investigated. The carbon contents were 1.06%, 0.60% and 0.26%, respectively and the corresponding volume fractions of austenite were 100%, 90% and 45%, respectively. The alloy having 1.06% carbon possessed the best tensile properties but its fatigue behaviour was only comparable to the other two alloys with lower carbon contents. The alloy having 0.60% carbon possessed the lowest yield strength, but its fatigue life was slightly better than other two alloys. The alloy having 0.26% C possessed lowest elongation and medium strength, and its fatigue life was comparable to the other two alloys. Their tensile properties and fatigue behaviour were explained in terms of crack initiation, crack propagation, grain size, constituent arrangement and constituent fraction.

A Numerical Analysis for the Transient EHL Process of a Cam-Tappet Pair in I. C. Engine

Abstract: Analyse numerique du processus de lubrification elastohydrodynamique en regime transitoire dans le cas du couple came-poussoir d'un moteur a combustion interne

Some Aspects of Determining the Flow Factors

Abstract: The uncertainty in calculated results of the flow factors greatly limits the application of the average flow model [1] which otherwise is an approach with broad prospects for studying the lubrication behavior of rough surfaces. The effects of boundary conditions, grid systems, and surface statistics on the flow factors are discussed in the paper, and the reason why the calculated results for the flow factors obtained by different researchers widely differ from each other is revealed. The research shows that ∂p /∂y over the micro-bearing has an effect on the calculated value of flow factors; the influence of the sideflow arises from the simulation approach itself, in which a micro-bearing is used for determining flow factors; the grid systems and the procedure to generate random surfaces may change the statistics of the surfaces used in calculation and affect the results thereby.

From quasicrystals to ordinary crystals

Abstract: A series of electron diffraction patterns of T2-AI6CuLi3 with various degrees of distortion have been obtained and interpreted It is shown that a certain quenched linear phason strain can introduce imperfections into icosahedral quasicrystals such that the structure deviates from its idealized form and changes towards a corresponding bcc lattice A possible approach to constructing an icosahedral quasicrystal structural model from the structure of the bcc phase is pointed out

Extended robust observation approach for failure isolation

Abstract: A new failure isolation scheme is proposed using an extended robust observation approach. A systematic procedure is developed for designing fault detection observers. This new failure isolation scheme can isolate a much larger class of failure modes than previous failure isolation schemes in the relevant literature, so that failure diagnosing precision may be greatly improved.

Deterministic approach to full-wave analysis of discontinuities in MICs using the method of lines

Abstract: Arbitrarily shaped one-port and in-line two-port discontinuities in any quasiplanar configuration can be analyzed by this one-step method. Using the hybrid homogeneous conditions, this approach simplifies the calculation, with Psi /sup e/ and Psi /sup h/ being discretized only in and near the discontinuity regions. Illustrative examples of S-parameter calculation are given. Computed results are compared with measured data and with the published results of other authors. >

Fabrication of holographic gratings using a moving point source

Abstract: In the manufacture of holographic gratings using coherent light there is an associated problem of optical noise, the conventional remedies for which involve various restrictions. In this work a moving point source is adopted and analysis indicates that if the path is a circle several of these restrictions are removed. The scheme is implemented and the results are highly satisfactory with high-efficiency gratings being produced at frequencies up to 2400 lines/mm, in sizes up to 65 mm square. Significant operational advantages also accrue in relation to alignment procedures and light utilization.