Showing papers by "Tsinghua University published in 1998"

Analysis and design of fuzzy controller and fuzzy observer

Abstract: This paper addresses the analysis and design of a fuzzy controller and a fuzzy observer on the basis of the Takagi-Sugeno (T-S) fuzzy model. The main contribution of the paper is the development of the separation property; that is, the fuzzy controller and the fuzzy observer can be independently designed. A numerical simulation and an experiment on an inverted pendulum system are described to illustrate the performance of the fuzzy controller and the fuzzy observer.

Antimicrobial effects of metal ions (Ag+, Cu2+, Zn2+) in hydroxyapatite.

Abstract: The antimicrobial ceramics (AC) based on hydroxyapatite (HA) were made in a wet chemical process with additions of AgNO3, Cu(NO3)2. 3H2O and Zn(NO3)2. 6H2O. The ACs were composed of metal-ion substituted hydroxyapatite and nitrate-apatite, which was identified by X-ray diffraction. The viable count and turbidity measurement was adopted to observe the antimicrobial effects of the various ACs. The aerobic Escherichia coli was used in the study. An obvious antimicrobial effect against E. coli was observed in Ag+ AC. In contrast to Ag+ AC, it was difficult to ascertain any bactericidal effect in the case of Cu2+ and Zn2+ AC. The bactericidal effect of Ag+ was observed using a dialysis tube experiment. This suggests that Ag+ dissolved out and reacted with E. coli, thus inhibiting its growth.

Processing and properties of carbon nanotubes-nano-SiC ceramic

Abstract: Carbon nanotubes–nano-SiC ceramic has been fabricated by the hot-press method. The preparation steps involved the use of dispersing nano-SiC powders and carbon nanotubes in butylalcohol using an ultrasonic shaker. The reasonable relative density of about 95% has been achieved by hot-pressing at 2273 K (at 25 MPa in Ar for 1 h). The three-point bending strength and fracture toughness of the composite has about 10% increment over monolithic SiC ceramic which was fabricated under the same process. The reasons for the increment are the strengthening and toughening role of carbon nanotubes occuring in the matrix.

Tissue response to nano-hydroxyapatite/collagen composite implants in marrow cavity.

Abstract: The tissue response to a nano-hydroxyapatite/collagen composite implanted in a marrow cavity was investigated by histology and scanning electron microscopy. A Knoop microhardness test was performed to compare the mechanical behavior of the composite and bone. The ultrastructural features of the composite, especially the carbonate-substituted hydroxyapatite with low crystallinity and nanometer size, made it a bone-resembling material. It was bioactive, as well as biodegradable. At the interface of the implant and marrow tissue, solution-mediated dissolution and giant cell mediated resorption led to the degradation of the composite. Interfacial bone formation by osteoblasts was also evident. The process of implant degradation and bone substitution was reminiscent of bone remodeling. The composite can be incorporated into bone metabolism instead of being a permanent implant. For lack of the hierarchical organization similar to that of bone, the composite exhibited an isotropic mechanical behavior. However, the resistance of the composite to localized pressure could reach the lower limit of that of the femur compacta.

Preparation of nano-crystalline diamonds using pulsed laser induced reactive quenching

Abstract: Nano-crystalline diamond is prepared with a unique method in which a graphite target is etched by a high-power pulsed laser in water. Transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) indicate nano-crystalline diamond is obtained, having a hexagonal lattice or cubic lattice.

Design of a hybrid fuzzy logic proportional plus conventional integral-derivative controller

Abstract: Presents approaches to the design of a hybrid fuzzy logic proportional plus conventional integral-derivative (fuzzy P+ID) controller in an incremental form. This controller is constructed by using an incremental fuzzy logic controller in place of the proportional term in a conventional PID controller, By using the bounded-input/bounded-output "small gain theorem", the sufficient condition for stability of this controller is derived. Based on the condition, we modify the Ziegler and Nichols' approach to design the fuzzy P+ID controller. In this case, the stability of a system remains unchanged after the PID controller is replaced by the fuzzy P+ID controller without modifying the original controller parameters. When a plant can be described by any modeling method, the fuzzy P+ID controller can be determined by an optimization technique. Finally, this controller is used to control a nonlinear system. Numerical simulation results demonstrate the effectiveness of the fuzzy P+ID controller in comparison with the conventional PID controller, especially when the controlled object operates under uncertainty or in the presence of a disturbance.

Fault diagnostics and fault tolerant control

Abstract: A novel simultaneous fault detection and diagnostics (FDD) and fault tolerant control (FTC) strategy for nonlinear stochastic systems in closed loops based on a continuous stirred tank reactor (CSTR) is presented. The purpose of control is to track the reactant concentration setpoint. Instead of output feedback we propose here to use proportional-integral-derivative (PID) state feedback, which is shown essential to achieve FTC against sensor faults. A new concept of "equivalent bias" is proposed to model the sensor faults. Both the states and the equivalent bias are on-line estimated by a pseudo separate-bias estimation algorithm. The estimated equivalent bias is then evaluated via a modified Bayes' classification based algorithm to detect and diagnose the sensor faults. Many kinds of sensor faults are tested by Monte Carlo simulations, which demonstrate that the proposed strategy has definite fault tolerant ability against sensor faults, moreover the sensor faults can be on-line detected, isolated, and estimated simultaneously.

Stackelberg-Nash equilibrium for multilevel programming with multiple followers using genetic algorithms

Abstract: Multilevel programming offers a means of studying decentralized noncooperative decision systems. Unfortunately, multilevel programming is lacking efficient algorithms due to its computational difficulties such as nonconvexity and NP-hardness. This paper will design a genetic algorithm for solving Stackelberg-Nash equilibrium of nonlinear multilevel programming with multiple followers in which there might be information exchange among the followers. As a byproduct, we obtain a means for solving classical minimax problems. Finally, some numerical examples are provided to illustrate the effectiveness of the proposed genetic algorithm.

Switch-toughening of ferroelectrics subjected to electric fields

Abstract: Electric fields can influence the fracture toughness of ferroelectrics. For example, poled ferroelectrics exhibit fracture toughness anisotropy: the material is tougher for a crack parallel to the poling direction but less tough for a crack perpendicular to it. When an electric field is applied to a poled sample, a positive field reduces its fracture toughness but a negative field enhances it. Previous investigations attribute these phenomena to polarization switching. This paper proposes a model of stress-assisted 90 polarization switching to quantify the toughening process. Small scale switching and uniform electric fields are assumed. An analytical solution is presented for a mono-domain ferroelectric crystal undergoing a confined polarization switch. This solution and the domain orientation pattern enable us to estimate the fracture resistance against the steady state crack growth in ferroelectrics by a Reuss-type multiple-domain assembly. A dimensionless group of material parameters and an electric field function emerge, and form the key ingredients of switch-toughening. The model is used to delineate several observations, including: poling-induced anisotropy of the fracture toughness, asymmetric variation of the fracture toughness under positive and negative electric fields of a poled specimen; upside-down butterfly loop for the fracture toughness response under cyclic electric loading.

Preparation of bioactive microporous titanium surface by a new two-step chemical treatment.

Abstract: Microporous oxide layers allowing fast deposition of calcium phosphate layers (CPLs) were formed on commercially pure titanium (c.p.Ti) after the application of a newly developed two-step chemical treatment. The micropores were of submicrometre size. The two-step treatment was carried out by etching c.p.Ti samples with HCl and H2SO4 first and then treating them in boiling 0.2 N NaOH solution at 140 °C for 5 h. Conformal CPLs, about 20 μm thick, were deposited on the two-step treated c.p.Ti surface by means of a two-day immersion in an in vitro supersaturated calcification solution. The CPL was characterized to be mainly composed of two sublayers, i.e. an outside loose octacalcium phosphate crystal sublayer and an inside dense carbonated apatite sublayer. A scratching test indicated that the apatite sublayer was strongly bonded to the c.p.Ti substrate. Moreover, it was observed that the untreated or single-step treated c.p.Ti surfaces are not only morphologically different from one another but significantly different from the two-step treated one, in that no precipitation was observed on them up to 14 d immersion in the same calcification solution. It is indicated that the two-step chemical treatment is a simple and easily controllable method to prepare bioactive titanium surfaces and subsequently to induce the rapid precipitation of conformal and adherent CPL from in vitro supersaturated calcification solutions. © 1998 Chapman & Hall

Preparation of calcium phosphate coatings on titanium implant materials by simple chemistry

Abstract: A two-step chemical treatment has been developed in our group to prepare commercially pure titanium (cpTi) surfaces that will allow calcium phosphate (Ca-P) precipitation during immersion in a supersaturated calcification solution (SCS) with ion concentrations of [Ca2+] = 3.10 mM and [HPO42−] = 1.86 mM. It was observed that a precalcification (Pre-Ca) procedure prior to immersion could significantly accelerate the Ca-P deposition process. In this work, the bioactivity of chemically treated cpTi and Ti6Al4V was further verified by applying commercially available Hanks' balanced salt solution (HBSS), an SCS with very low ion concentrations of [Ca2+] = 1.26 mM and [HPO42−] = 0.779 mM, as the immersion solution. It was found that a uniform and very dense apatite coating containing magnesium impurities was formed if the Pre-Ca procedure was performed before immersion, as compared with the loose Ca-P layer obtained from the abovementioned high concentration of SCS. The formation of a microporous titanium dioxide thin surface layer on cpTi or Ti6Al4V by the two-step chemical treatment could be the main reason for the induction of apatite nucleation and growth from HBSS. Variations of pH values, Ca and P concentrations, and immersion time in HBSS were investigated to reveal the detailed process of Ca-P deposition. The described treatments provide a simple chemical method to prepare Ca-P coatings on both cpTi and Ti6Al4V. © 1998 John Wiley & Sons, Inc. J Biomed Mater Res, 41, 227–236, 1998.

Micronozzle/diffuser flow and its application in micro valveless pumps

Abstract: The nozzle/diffuser flow with different Reynolds number ranges and conical angles is analyzed first. It is found that the flow coefficients of nozzle/diffuser ξn and ξd vary with angle with different trends for large (> 105) and small (<50) Reynolds number ranges. The flow model of valveless pumps with nozzle/diffusers is proposed, and the relationship between nozzle/diffuser flow coefficients and the performance of the valveless pumps is investigated. In experiments, silicon nozzle/diffuser elements with conical angles of 5°, 7.5° and 10° and a silicon micro valveless pump wafer are fabricated. The experimental ξd, ξn, and ξn/ξd decrease with increasing nozzle/diffuser angles in the Re range ( 2000) of the experiment. The pumping direction of the fabricated valveless pump agrees with the nozzle/diffuser flow experiments and analysis. The pump works successfully with the output flow rate of 28 μl/min under the input power of 50 mW and 500 Hz.

A direct method for analysis of dynamic soil-structure interaction based on interface idea

Abstract: This paper presents a direct finite element method for analysis of dynamic soil-structure interaction based on the large general structure analysis software. The method can simulate not only the absorption of infinite soil to the scattering wave but also the elasticity recovery capacity of the far field media on the boundary. A new input method of wave motion dealing with dynamic soil-structure interaction is also proposed which can be used to exactly simulate seismic wave input with any angle. The accuracy of the methods presented in this paper is demonstrated by the numerical examples.

Lattice-Boltzmann models for high speed flows

Abstract: We formulate a lattice Boltzmann model to solve supersonic flows. The particle velocities are determined by the mean velocity and internal energy. The adaptive nature of particle velocities permits the mean flow to have a high Mach number. The introduction of a particle potential energy causes the model to be suitable for a perfect gas with an arbitrary specific heat ratio. The macroscopic conservation equations are derived by the Chapman-Enskog method. The simulations were carried out on the hexagonal lattice. However, the extension to both two- and three-dimensional square lattices is straightforward. As preliminary tests, we present the Sod shock-tube simulation and the two-dimensional shock reflection simulation.

Study of a slit cut on a microstrip antenna and its applications

Abstract: The slit cut on a microstrip antenna is studied in this paper, and a multiport series inductance network is established for analysis. Based on this study, two slit applications are proposed: to adjust the resonant frequency, and to make a small microstrip antenna. Experiments agree well with the theoretical analysis. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 18: 297–300, 1998.

Chinese Word Segmentation without Using Lexicon and Hand-crafted Training Data

Abstract: Chinese word segmentation is the first step in any Chinese NLP system. This paper presents a new algorithm for segmenting Chinese texts without making use of any lexicon and hand-crafted linguistic resource. The statistical data required by the algorithm, that is, mutual information and the difference of t-score between characters, is derived automatically from raw Chinese corpora. The preliminary experiment shows that the segmentation accuracy of our algorithm is acceptable. We hope the gaining of this approach will be beneficial to improving the performance (especially in ability to cope with unknown words and ability to adapt to various domains) of the existing segmenters, though the algorithm itself can also be utilized as a stand-alone segmenter in some NLP applications.

General matrix representations for B-splines

Abstract: The concept of basis matrix of B-splines is presented. A general matrix representation, which results in an explicitly recursive matrix formula, for nonuniform B-spline curves of an arbitrary degree is also presented by means of Toeplitz matrix. New recursive matrix representations for uniform B-spline curves and Bezier curves of an arbitrary degree are obtained as special cases of that for nonuniform B-spline curves. The recursive formula for basis matrix can be substituted for de Boor-Cox's one for B-splines, and it has better time complexity than de Boor-Cox's formula when used for conversion and computation of B-spline curves and surfaces between different CAD systems. Finally, some applications of the matrix representations are presented.

Fatigue crack growth in ferroelectrics driven by cyclic electric loading

Abstract: Fatigue crack growth has been observed recently in ferroelectrics under cyclic electric loading. Does the crack grow by electric breakdown, or by the stress field near the crack tip? The present paper provides a mechanistic explanation for the electric-field-induced fatigue crack growth. The non-uniform electric field near an insulated crack tip might cause domain switching which in turn produces a concentrated stress field characterized by a stress intensity factor. For ferroelectrics poled along a direction perpendicular to the crack, we are able to show quantitatively that: (1) the stress intensity factor under a negative electric field is nine times as large as the stress intensity factor under a positive electric field; (2) the crack starts to grow if the stress intensity factor is higher than the fracture toughness of the material, but the stress intensity factor decreases as the crack extends and eventually results in crack arrest; (3) by reversing the electric field, the stress intensity factor is increased and crack growth resumes; and (4) this model can predict the extent of fatigue crack growth. In contrast to the conventional perception of (mechanical) fatigue, the fatigue crack growth in ferroelectrics under cyclic electric loading is a step by step cleavage process caused by a domain switching sequence that generates a cyclic driving stress field near the crack tip.