No. 4, 2003


Vratislav Kafka, David Vokoun : Creep and relaxation modeled on the mesoscale, pp.325-338

The general mesomechanical model of the first author is applied to the description of creep and relaxation. Although the material properties and internal microstructures of different materials are different, the curves of creep and relaxation are usually very similar. Our model tries to grasp the common mesomechanical features that are responsible for the character of these curves. The method of determination of the model parameters is presented and exemplified. It is shown that with appropriate model parameters, the experimental creep curve of the material chosen for demonstration can be described very closely. The theoretical diagrams for different kinds of prescribed loading and constraint of deformation qualitatively agree with what is generally observed. It is hypothesized that the limited capacity of the material with regard to the comprised elastic energy described on the mesoscale could serve as the criterion of strength.

You Xiaojie, Li Yongdong, Viktor Valouch : Identification of reactive and harmonic current of nonlinear load with consideration of non-ideal mains voltage and its application, pp.339-351

This paper presents a novel method to detect the harmonic and reactive current of nonlinear load under the non-ideal supply voltages, i.e. unsymmetrical and distorted voltage conditions. The proposed method is based on the rotating coordinate transformation and projection of three-phase currents and voltages simultaneously. It can extract positive-sequence fundamental active component of the load current, and thereby determine the harmonic and reactive current of the loads. Furthermore, it can extract any single order of harmonic current. Used in the control of shunt active power filter, the proposed method achieved satisfactory simulation and experimental results.

Jaroslav Kucera, Karel Stransky : The dependence of carbon diffusion coefficients in austenitic ternary alloys on concentration of additive elements, pp.353-364

The diffusion interaction parameters dCC, dCi expressing the dependence of carbon diffusivities D(C) on carbon N(C) and alloying elements N(i) concentrations in austenitic ternary alloys Fe-C-M are presented. The parameters dCC, dCi differ from the parameters bCC, bCi expressing the D(C) dependence on N(C) and N(i) concentrations in polycomponent alloys and steels. The values dCC and dCi are calculated on the basis of literature data. The parameters bCC and bCi published in our previous papers are introduced for comparison sake. The results of analysis showing the dependence of D(C) diffusivity on its own concentration N(C) in gFe are added.

Jaroslava Zilkova, Jaroslav Timko, Peter Girovsky : An inverse neural model for controlling non-linear dynamic systems, pp.365-377

Recently, various control strategies using artificial neural networks (ANN) have been elaborated. To realize the strategies, neural networks have been employed as estimators or models of controlled systems. The paper deals with a quasi-inverse model which is intended to work as a controller of a non-linear dynamic system simulating an induction motor. The presented method of control takes advantage of approximating properties of multi-layer perceptron (MLP) networks. Obtained simulation results are a contribution to theoretical discussion of the problem.

Lesaw Topór-Kaminski, Marian Pasko : Digitally tuned universal filters with multiple input operational amplifiers, pp.379-394

The conception of multiple input voltage operational amplifier (MIOA) and its realization in discrete and integrate techniques is presented. Its application in systems of voltage and current amplifiers having digitally tuned gain is shown. Two versions of the realization of universal filters with digitally tuned parameters are described. It is done by the use of digital potentiometers or switched inputs of MIOA. Simulation of frequency characteristics of voltage amplifier and a universal filter was carried out in SPICE programme. The influence of tuning their parameters on the shape of their characteristics is examined.

Jaroslav Feda : Irregular creep in granular materials, pp.395-410

Regular creep of granular materials follows a semilogarithmic relation e vs logt (e-void ratio, t-time). There are some cases (weak grains, large load) where grain crushing occurs. In such a case, bilinear creep, diffusion collapse and garlandlike compression take place. These unusual forms of creep are experimentally documented with silicagel and fragmentary clay as model materials. Grain crushing is quantified and the stability of garlandlike compression underlined.

Jaroslav Novak, Vladimir Schejbal : The possibility of mathematical model utilization for real-time analyses of induction machine, pp.411-426

The design and utilization of measuring equipment for testing electromechanical systems, especially drive units with induction motors, are presented. The equipment function is based on computation of induction machine current model with the subsequent increasing of accuracy. The main attention is paid to indirect torque determination by real-time computation.

You Xiaojie, Li Yongdong : Three-phase PWM AC/DC converter using digital control with the consideration of unbalanced power network, pp.427-436

This paper presents a novel control approach of three-phase PWM AC/DC converter under the condition of unsymmetrical and non-sinusoidal AC source voltage. Two parts are included: one is the determination of input current reference and the other is calculation of duty cycles of switching devices. The determination of input current reference is based on the rotating coordinate transformation of three-phase source voltage. The resulting current reference is in phase with the fundamental positive-sequence component of the source voltage. The duty cycles of switching devices are calculated directly in order that the converter input current tracks its reference over one switching period. The simulation and experimental results validated the effectiveness of the proposed control approach.