Showing papers by "University of Macau published in 2001"

Laser surfacing of S31603 stainless steel with engineering ceramics for cavitation erosion resistance

Abstract: As a preliminary study on incorporating ceramic particles into stainless steels for improving cavitation erosion resistance, ceramic-reinforced coatings were fabricated on austenitic stainless steel UNS S31603 using a laser surfacing technique. Ceramic powders, including WC, Cr 3 C 2 , SiC, TiC, CrB 2 and Cr 2 O 3 , were used to produce surface metal matrix composites (MMCs) with high volume fractions of reinforcement. The powders were pasted on the surface of UNS S31603 first, followed by surface melting using a high power laser. The microstructures and structures of the surfaces were analyzed. The cavitation erosion characteristics of the surface-modified specimens in 3.5% NaC1 solution at 23°C were studied by means of a 20-kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 μm. The cavitation erosion resistance R e of all the laser-modified specimens was improved, except for the case of Cr 2 O 3 , with the improvement depending on the microstructure formed. The R e of surface MMCs reinforced with CrB 2 , WC and TiC increased to 9.4, 8.5 and 1.7 times that of monolithic S31603, respectively. For the specimens modified with Cr 3 C 2 and SiC, a eutectic mixture of austenite and carbides or silicide was formed and the R e was increased by a factor of 4.8 and 2, respectively.

Three-dimensional pulse-width modulation technique in three-level power inverters for three-phase four-wired system

Abstract: Shunt-connected trilevel power inverter in three-phase four-wired system as an active filter or individual current supply (peak-load supply) is studied by a novel technique: three-dimensional (3-D) voltage vectors pulse width modulation (PWM). In past decades, almost all the study for PWM is limited to the two-dimensional (2-D) domain, /spl alpha/ and /spl beta/ frames, in a three-phase three-wired system. However, in practical operation, there are many three-phase four-wired systems in distribution sites. The generalized study of 3-D two-level and three-level inverters is achieved in this paper so as to perform the basic theory of 3-D multilevel space vector switching PWM technique. The sign cubical hysteresis control strategy is proposed and studied with simulation results in 3-D aspect. The 3-D PWM technique in three-level inverters is accomplished.

Cavitation erosion-corrosion behaviour of laser surface alloyed AISI 1050 mild steel using NiCrSiB

Abstract: A Ni-based hardfacing alloy NiCrSiB (Ni–16.5%Cr–15.5%Fe–3.5%Si–3.8%B–1%C) was surface alloyed on AISI 1050 mild steel (Fe–0.2%Cr–0.4%Mn–0.5%C) specimens using a two-step process. NiCrSiB powder was preplaced on the substrate by flame spraying and then remelted by a 2 kW continuous wave Nd:YAG laser to achieve surface alloying. The cavitation erosion–corrosion characteristics of the surface alloyed specimens in 3.5% NaCl solution at 23°C were determined by means of a 20 kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 μm and a potentiostat. At a dilution ratio of 12% (corresponding to a laser scanning speed of 25 mm s −1 ), the overall cavitation erosion–corrosion resistance of the alloyed surface was 8.9 times that of the as-received AISI 1050 specimen. The corrosion resistance was also improved as reflected by a reduction in the current density of least one order of magnitude as compared with the as-received specimen at the same potential. The improvement in cavitation erosion resistance could be attributed to the superior mechanical properties of the NiCr-alloyed matrix formed and the presence of borides and boro-carbides which increased the hardness. The improvement in corrosion resistance was due to the increase in Cr and Ni content in the alloyed layer. The relative contributions of pure mechanical erosion, electrochemical corrosion, and synergism between erosion and corrosion to the overall cavitation erosion–corrosion were also determined.

Formal and use-case driven requirement analysis in UML

Abstract: We have recently proposed a formalization of the use of UML in requirement analysis. This paper applies that formalization to a library system as a case study. We intend to show how the approach supports a use case-driven, step-wised and incremental development in building models for requirement analysis. The actual process of building the models shows the importance and feasibility of the formalization itself.

Analysing the curriculum development process: three models

Abstract: This article attempts to analyse the curriculum development process by using three models, i.e. the modern model, the postmodern model and the model suggested by the actor-network theory. It is argued that no matter what context we are in, curriculum is the manifestation of the power distribution in society. It is critical to note that power is not a fixed entity, but a strong network formed by heterogeneous components. Therefore, the study of change in this network formation can greatly enhance the analysis of curriculum.

The conditional relation between beta and returns in the Hong Kong stock market

Abstract: Published results of empirical tests over the past two decades indicate that the risk-return relation in the Hong Kong stock market is negative. Such findings refute the positive risk-returnrelation stipulatedinthe traditional CAPM. However, traditional CAPM invokes expected or ex-ante returns while empirical tests have used ex-post returns as an imperfect proxy. Thus, in this paper, the risk-return relationship in the Hong Kong stock market is examined using the conditional method based on the work of Pettengill et al., which takes into consideration the dominating ex-post negative excess market returns found in the Hong Kong stock market. Under the conditional Pettengill et al. method, test results demonstrate a strong conditional positive and negative risk-return relationships in the Hong Kong stock market. The results show that the estimated risk premiums in both up and down markets are insignificantly different from the corresponding expected risk premiums. But the estimated risk premiums of the up a...

Convergence of the Multigrid Method of Ill-conditioned Block Toeplitz Systems

Abstract: We study the solutions of block Toeplitz systems Amnu = b by the multigrid method (MGM). Here the block Toeplitz matrices Amn are generated by a nonnegative function f (x,y) with zeros. Since the matrices Amn are ill-conditioned, the convergence factor of classical iterative methods will approach 1 as the size of the matrices becomes large. These classical methods, therefore, are not applicable for solving ill-conditioned systems. The MGM is then proposed in this paper. For a class of block Toeplitz matrices, we show that the convergence factor of the two-grid method (TGM) is uniformly bounded below 1 independent of mn and the full MGM has convergence factor depending only on the number of levels. The cost per iteration for the MGM is of O(mn log mn) operations. Numerical results are given to explain the convergence rate.

Laser-fabricated Fe–Ni–Co–Cr–B austenitic alloy on steels. Part I. Microstructures and cavitation erosion behaviour

Abstract: Laser surface alloying using NiCoCrB alloy (Ni–17.1% Co–19.6% Cr–14.5% Fe–3.5% B–1% C–0.9% Si) on mild steel AISI 1050 and austenitic stainless steel AISI 316L was attempted. Both steels alloyed with NiCoCrB contained austenite as the main phase, with carbides and borides as the minor phases. The maximum hardness of laser-alloyed 1050 and 316L increased to 545 and 410 Hv, respectively, with the cobalt content ranging from 4.5 to 12 wt.%. The cavitation erosion resistance R e in deionized water was determined by an ultrasonic vibration system. R e was found to increase with the Co content in the alloyed layer, reaching a value 4.4- and 12-fold that of the substrate for 1050 and 316L, respectively. The increase in R e was attributed to a decrease in the stacking-fault energy and enhancement of strain-induced martensitic transformability due to the presence of Co. Increase in hardness due to the presence of carbides and borides and a refined microstructure resulting from laser treatment also contributed to an enhanced resistance.

An integrative framework for knowledge extraction in collaborative virtual environments

Abstract: Collaborative virtual environments are becoming an intrinsic part of professional practices. In addition to providing collaboration support, they have the potential to collect vast amounts of data about collaborative activities. The aim of this research is to utilize this data effectively, extract meaningful insights out of it and feeding discovered knowledge back into the environment. The paper presents a framework for integrating knowledge discovery techniques with collaborative virtual environments, starting from early conceptual development. Discovered patterns are deposited in an organizational memory which makes these available within the virtual environment. Two examples of the application of the framework are included.

Laser-fabricated Fe–Ni–Co–Cr–B austenitic alloy on steels. Part II. Corrosion behaviour and corrosion–erosion synergism

Abstract: A cobalt-containing hardfacing alloy NiCoCrB (Ni–171% Co–196% Cr–145% Fe–35% B–1% C–09% Si) was surface-alloyed on mild steel AISI 1050 and stainless steel AISI 316L as presented in Part I, together with their microstructures and cavitation erosion behaviour in deionised water In Part II, the corrosion and cavitation erosion–corrosion behaviour in 35% NaCl at 23°C was studied with a potentiodynamic polarisation technique and ultrasonic vibration tester, respectively The corrosion resistance of mild steel 1050 was somewhat increased, owing to the presence of Cr in the alloyed layer, while that of stainless steel 316L decreased, due to the presence of borides and boro-carbides which weakened the passive film The cavitation erosion–corrosion resistance R ec in 35% NaCl was improved by a factor of 83 and 99 for 1050 and 316L, respectively Corrosion and erosion–corrosion synergism played a significant role in the overall cavitation erosion–corrosion resistance, leading to a drop in resistance of approximately 50–70% for laser-alloyed 1050 specimens and approximately 20% in the case of laser-alloyed 316L specimens Thus, it might be concluded that it is difficult to simultaneously achieve significant improvements in both erosion and corrosion resistance

Distribution of polycyclic aromatic hydrocarbons in aerosols and dustfall in Macao.

Abstract: Polycyclic aromatic hydrocarbons (PAHs), widespread environmental pollutants, have been measured in aerosols and dustfall in Macao. In this paper, we compare concentration distributions and determine the partitioning of PAHs in aerosols and dustfall for different areas, sampling times and sampling heights. The results demonstrate that the concentrations of ΣPAHs in aerosols and dustfall vary at the different sites and heights. The concentrations of ΣPAHs and most of the individual PAHs in aerosols at night were higher than those in the daytime when using the unit of μg/g and lower than those when using the unit of μg m-3. It is shown that the distribution of individual PAHs in aerosols differs significantly from that in dustfall.

Rotation-invariant texture classification using a two-stage wavelet packet feature approach

Abstract: A novel two-stage wavelet packet feature approach for classification of rotated textured images is discussed. In the first stage, a set of sorted and dominant wavelet packet features is extracted from a texture image and a Mahalanobis distance classifier is employed to output N best classes. In the second stage, another set of wavelet packet features is extracted from the polarised form of the sample texture image and the most dominant wavelet packet features are selected and passed to the radial basis function (RBF) classifier with the N best classes to output the final matched class. Experimental results, based on a large sample data set of twenty distinct natural textures selected from the Brodatz album with different orientations, show that the proposed method outperforms the similar wavelet methods and the other rotation invariant texture classification schemes, and an overall accuracy rate of 91.4% was achieved.

A novel algorithm for phasor calculation based on wavelet analysis [power system analysis]

Abstract: A novel discrete wavelet transform (DWT) approach to the computation of synchronized phasors is presented in this paper. Under steady state, the error in the phasor obtained by the proposed algorithm is very small, i.e. 0.001 p.u. in magnitude error and 0.05/spl deg/ in angle error. During transients and considering frequency deviation, the performance of the proposed algorithm is far higher than that in FFT.

Teaching HCI with scenario-based design: the constructivist's synthesis

Abstract: This paper describes the application of scenario-based design in the teaching of Human-Computer Interaction (HCI), in an undergraduate Software Engineering program. Specifically, we describe how the ideas of constructivism can be synthesized with the team-based efforts in managing software requirements. The paper serves as an experience report of an ongoing action research the author has been executing to revise the curriculum and pedagogy of a junior core course entitled Software Psychology. In particular, we depict some problem scenarios, helping the evolution of the course content, and developing our students as self-directed work teams of software professionals. The paper concludes with the author's lessons learned with this course enactment plus the necessary reflective evaluations therein.

Kinematics Control of Redundant Manipulators Using CMAC Neural Network

Abstract: The inverse kinematics problems of redunda nt manipulators have been investigated for many years. The conventional method of solving this problem analytically is by applying the Jacobian Pseudoinverse Algorithm. It is effective and able to resolve the redundancy for additional constraints. However, its demand for computational effort makes it not suitable for real-time control. Recently, neural networks have been widely used in robotic control because they are fast, fault-tolerant and able to learn. In this paper, we will present the application of CMAC (Cerebellar Model Articulation Controller) neural network for solving the inverse kinematics problems in real time. Simulations will be carried out for evaluating the performance of the CMAC neural network.

Short-term load forecasting techniques using ANN

Abstract: This paper presents some practical techniques of using artificial neural networks for the short-term load forecasting problem. The model described in this paper is a backpropagation based multilayer perceptron including the temperature factor. In order to expedite the training process, the quasi-Newton method is employed. An intelligent treatment to holiday factors in order to improve the forecasting accuracy is discussed. The average forecasting error of this system is 3.2%.

A Comparative Study of ANT-Based Optimization for Dynamic Routing

Abstract: An ANT is a mobile agent that is capable of solving various kinds of routing and congestion problems in computer networking by continuously modifying routing tables in respond to congestion. In a distributed problem solving paradigm, a society of ANTs (each contributing some information) collaborate to solve a larger problem. In recent years, Ant-based algorithms were used to solve classical routing problems such as: Travelling Salesman Problem, Vehicle Routing Problem, Quadratic Assignment Problem, connection-oriented/connectionless routing, sequential ordering, graph coloring and shortest common supersequence. By introducing the general idea of Ant-based algorithms with a focus on Ant Colony Optimization (ACO) and their mathematical models, this paper brings together a collection of ACO algorithms discussed their features, strength and weaknesses.

Visualizing structural and behavioural aspects of virtual collaboration

Abstract: Collaborative virtual environments are becoming increasingly commonplace, moving out of research labs and into professional practice. Among others, they support knowledge-intensive emergent work performed by distributed teams. In these settings where conventional awareness is absent, making sense of the virtual collaboration becomes challenging. This paper introduces information visualization that aims to reveal structural and behavioural aspects of virtual collaboration, referring respectively to relationships among elements of the collaboration, and the interactions that occur as part of it. Visualization tools have to be tailored to, and express notions from, the ontology of the virtual environment employed. An example is introduced where instances of collaboration that have taken place within the LiveNet system are visualized, in terms of LiveNet's ontology. Finally, the paper offers an outlook on how visualization tools can be assisted by software agents that learn from and actively support their human users.

Web-based asynchronous support for collaborative learning

Abstract: This paper investigates the construction of a WWW-enabled course support environment for learner-centered education. The system is designed to encourage students' responsibility, to make learning meaningful, and to support active knowledge construction in the specific curriculums of their study. This system, currently named REAL, carries the connotation of a Rich Environment for Active Learning, whose pedagogy comes from the constructivist's theory of learning. REAL facilitates the interaction among students and teachers by two notions. The first is maintaining a course-specific Web-site for students to look up course-related information. The second is providing a collaborative Web-based service with which students could initiate query requests through using specific inquiry Web-page, which acts as an interactive medium for teachers and students to exchange ideas, record actions, use e-mails, and upload/download files. Each interaction is captured asynchronously and maintained in a searchable archive for other students' reference. The REAL system, developed through the use of iterative prototyping, provides feedback for perceived learning. The paper also discusses lessons learned for teachers developing REAL, in the reflection of instructional methods, as well as some future development of the environment.

Two new approaches to analysis of inner-fault of squirrel-cage rotor for three-phase induction motors

Abstract: The paper details two new methods for analyzing inner-fault of squirrel-cage rotor for three-phase induction motors. The basic ideas of the first method we have developed in this paper are the "bi-directional revolving magnetic fields" concept and the superposition theorem. According to the procedures of the first method, we calculate the additional stator currents respectively at frequency f/sub 1/ and (1-2s)f/sub 1/, which are caused by breaking one rotor-bar. Then those stator currents are added to the original stator currents in the healthy case so as to form the total fault stator currents during the inner-fault of a squirrel-cage rotor. For the second method, based on the hybrid system of 1,2,0 axes and d,q, 0 axes, we find out the ratio between the amplitude of the stator current at frequency (1-2s)f/sub 1/ and that at frequency f/sub 1/. On the basis of these ideas, the mathematical models of these two new methods have been built. The simulation and experimental results have also been obtained and compared.

A new prototype of diagnosis system of inner-faults for three-phase induction motors developed by expert system

Abstract: During practical operation of three-phase induction motors in industry, the inner faults may occur in their rotor and stator windings. These kinds of faults seriously influence the operation of the motor. Moreover, due to insufficient research and development in the diagnosis system of inner-faults of an induction motor, new research on the prototype of this diagnosis system of inner-faults for three-phase induction motors, developed by the expert system are presented. This paper provides a comprehensive understanding of the fundamental signal processing techniques that underpin the diagnosis systems, and demonstrate how they are applied. As we have found in analyzing the inner-fault of a squirrel-cage rotor for three-phase induction motors by "bi-directional revolving magnetic fields" concept, the stator currents at frequencies f/sub 1/ and (1-2s)f/sub 1/ exists. Signal processing involves the measurement and analysis of the fluctuations about the selected variables in order to provide the detection of the motor malfunctions.

Corrosion resistance of laser-fabricated metal-matrix composite layer on stainless steel 316L

Abstract: Laser fabrication of ceramic-reinforced metal-matrix composite (MMC) layer on stainless steel 316L was achieved by a two-step process. Ceramic powders (WC, SiC, CrB2) and alloy-ceramic powders (Ni-WC, Co-WC, Ni-A12O3, Ni-Cr3C2) were preplaced on 316L specimens by pasting or flame spraying and then irradiated by a high power laser to form MMCs with low (up to 28%) and high (over 80%) volume fractions of ceramic particles. The microstructure and composition of the MMC layers were studied by optical microscopy, scanning electron microscopy, energy dispersive X-ray analysis and image analysis. The corrosion behavior of the laser-fabricated layers in 3.5% NaCl solution at 23°C was studied by cyclic potentiodynamic polarization technique. The pitting potential of specimens laser-modified with Co-WC and Ni-WC was shifted in the noble direction relative to 316L while there was no improvement in the corrosion resistance in the case of Ni-Al2O3 and Ni-Cr3C2. For specimens modified with ceramic powders, the corrosion resistance deteriorated. The results of the present study indicated that a high volume fraction of ceramic particles in an MMC layer was unfavorable for corrosion resistance, owing to the difficulty in the formation of intact passive films.Laser fabrication of ceramic-reinforced metal-matrix composite (MMC) layer on stainless steel 316L was achieved by a two-step process. Ceramic powders (WC, SiC, CrB2) and alloy-ceramic powders (Ni-WC, Co-WC, Ni-A12O3, Ni-Cr3C2) were preplaced on 316L specimens by pasting or flame spraying and then irradiated by a high power laser to form MMCs with low (up to 28%) and high (over 80%) volume fractions of ceramic particles. The microstructure and composition of the MMC layers were studied by optical microscopy, scanning electron microscopy, energy dispersive X-ray analysis and image analysis. The corrosion behavior of the laser-fabricated layers in 3.5% NaCl solution at 23°C was studied by cyclic potentiodynamic polarization technique. The pitting potential of specimens laser-modified with Co-WC and Ni-WC was shifted in the noble direction relative to 316L while there was no improvement in the corrosion resistance in the case of Ni-Al2O3 and Ni-Cr3C2. For specimens modified with ceramic powders, the corrosio...

An analytical linearization method for CMOS MMIC power amplifier using Multiple Gated Transistors

Abstract: A novel analytical linearization method for CMOS MMIC power amplifiers based on parallel connection of transistors (also called Multiple Gated Transistors) is proposed. By this method, the power amplifier's IMD/sub 3/ can be eliminated analytically by operating the transistors within the determined g/sub m/" linearized range. In order to demonstrate the usefulness of the proposed method, a 900 MHz Class AB power amplifier using 4 NMOS transistors with maximum aspect ratio (W/L)=1300/0.5 is designed based on a 0.35 /spl mu/m CMOS process. The simulation results demonstrate that -76 dB IMD/sub 3/ can be obtained with 13 dB power gain.

A unified algorithmic framework for variable-rate TDM switching assignments

Abstract: A unified algorithmic framework for time-optimal transmission scheduling is obtained for satellite-switched time-division multiple access systems with M uplink and N downlink beams. System capability may vary from the basic configuration of one carrier per beam to complex configurations of variable number of carriers for each beam and variable transmission rate for each beam. The computational complexity for time-optimal transmission scheduling remains strongly polynomial at O(M/sup 2/N/sup 2/) and the number of switching configurations is at most MN+1 for all system configurations considered.

leaf movement simulation

Abstract: This paper presents a method for the animation of leaf movement. Leaves are classified into two classes: moveable and non-moveable. For moveable leaves, their movements consist of two parts: leafstalk rotation and leaf surface rotation. For each rotation, a movement model was constructed and the movement was computer based on each model, respectively. The final leaf movements were obtained by superposition of these two rotations. The method’s principle is simple and easy to use.