ESTEGHLAL
Year:2006 | Volume:25 | Issue:1|Papers Count:18

Control of a class of uncertain nonlinear systems, which estimates unavailable state variables, is considered. A new approach for robust tracking control problem of satellite for large rotational maneuvers is presented in this paper. The features of this approach include a strong algorithm to estimate attitude, based on discrete extended Kalman filter combined with a continuous extended Kalman filter and attitude nonlinear model, and a robust controller based on sliding-mode with perturbation estimation. Estimation accuracy in this method is five times higher than other recent approaches based on Kalman filter. We have used sliding-mode controller in this paper. Not only the controller and the corresponding observer but also their composition must be robust. To make this controller robust against the uncertainty of parameters, the robust Kalman filter is used. Based on interval algebra, an upper bound and a lower bound are estimated for state variables of the system and considering these bounds in indicating the sliding conditions, stability of the controller in combination with the observer will be satisfied simultaneously. The simulation results show the capability of this method in spite of different uncertainty levels (up to %50).

Introducing distributed generation into a power system can lead to numerous benefits including technical, economic, environmental, etc. To attain these benefits, distributed generators with proper rating should be installed at suitable locations. Given the similar effects of distributed generators and capacitor banks on operation indices of a distribution system, simultaneous assignment of best locations and sizes to both will not only lead to greatest benefits from distributed generators but also to lower reactive power capacity requirements. In this paper, a new combined planning problem involving distributed generation and Volt/VAr control means planning is formulated and solved in which the quantity of distributed generators and reactive power sources are simultaneously assigned to buses in a distribution system. Also tap positions of voltage regulators are computed such that with a given distributed generation under peak load conditions, power losses and the reactive power capacity required are minimized. Like many other problems in power network planning, the problem formulated here is a nonlinear combinatorial one. Hence, we employ the tabu search algorithm to solve the optimization problem. The results from applying the algorithm to distribution networks with 6, 19, and 33 buses are presented and compared with those obtained from employing the second order method.

A novel version of Ant Colony Optimization (ACO) algorithms for solving continuous space problems is presented in this paper. The basic structure and concepts of the originally reported ACO are preserved and adaptation of the algorithm to the case of continuous space is implemented within the general framework. The stigmergic communication is simulated through considering certain direction vectors which are memorized. These vectors are normalized gradient vectors that are calculated using the values of the evaluation function and the corresponding values of object variables.The proposed Gradient-based Continuous Ant Colony Optimization (GCACO) method is applied to several benchmark problems and the results are compared and contrasted with other population-based algorithms such as Evolutionary Strategies (ES), Evolutionary Programming (EP), and Genetic Algorithms (GA). The results obtained from GCACO compare satisfactorily with those of other algorithms and in some cases are superior in terms of accuracy and computational demand.

This paper presents a method based on neural networks to detect broken rotor bars and end rings in squirrel cage induction motors. In the first part, detection methods are reviewed and traditional methods of fault detection as well as dynamic model of induction motors are introduced using the winding function method. In this method, all stator and rotor bars are considered independently in order to check the performance of the motor for any faults in the parts. Then the frequency spectrum of current signals is derived using the Fourier transform and analyzed under various conditions. In the second part of the paper, an analytical discussion of the theoretical principles is presented to arrive at a simple algorithm for fault detection based on neural networks. The neural network has been trained using the information from a 1.1 KW induction motor. Finally, the system is tested with different values of load torque and is found capable of working on-line to detect all normal and ill-performing conditions.

To assess the performance criteria of the reinforced-concrete, five-storey residential buildings common in Iran, an experimental study in the structural laboratory of the University of Tehran has been conducted. The test program includes cyclic and monotonic load tests of six beams that represent three-to-five storey buildings with rigid frame structures. Using definitions given in FEMA-356 and ATC-40, stages of immediate occupancy, life safety, and collapse prevention have been identified on the drift- force curves of all specimens. Based on the test results, values of the plastic rotation, ductility, strain in concrete cover and in longitudinal bar, crack width, damage index, and length of plastic region at different levels have been determined. It was found that the recommended values of plastic rotation and ductility for reinforced concrete beams by FEMA-356 are conservative. The length of plastic hinge region in the stage of immediate occupancy is about half the plastic hinge length in the stage of life safety and it increases by 20% from life safety to collapse prevention.

Bridges are potentially one of the most seismically vulnerable structures in the highway system during earthquake events. It is known that the seismic performance of transportation systems plays a key role in the post-earthquake emergency management. Hence, it is necessary to evaluate both physical and functional aspects of bridge structures. The physical aspects of the seismic performance of bridges are evaluated by seismic fragility functions or damage probability matrices of transportation facilities. The fragility curves represent the probability of structural damage due to various levels of ground shaking. The fragility curve describes a relationship between a ground motion and a level of damage.In this paper, the fragility curves (F.C) are developed. The vulnerability of a railway prestreed concrete bridge is assessed using fragility curves derived from dynamic nonlinear finite element analysis. A software package is developed in MATLAB to study the results obtained. Modeling of the bridge using 3D nonlinear models and modeling of abutments, bearings, effect of falling of girder on its bearings, and nonlinear interaction of soil-structure are some of the advantages of this research compared to previous ones.Reliability curves developed in this study are unique in their own kind. The proposed method as well as the results is presented in the form of vulnerability and structural reliability relations based on two damage functions.

Energy absorber systems like metallic dampers for controlling the structural vibrations due to earthquake have witnessed considerable development in the past few decades. Also there are some studies on the energy absorption of thin-walled tubes due to impact load. Thin-walled tubes have a large deformation capacity and are suitable energy absorbers in the structure during an earthquake provided that a suitable inelastic buckling mode obtains. This paper deals with the study of energy dissipation in accordion thin-walled tubes and their behavior due to axial cyclic loads. For this purpose, experimental and analytical studies have been performed. Experimental studies were conducted on specimens available in the market by dynamic tension and compression actuator. Analytical studies are based on finite element methods and nonlinear inelastic dynamic analysis. These studies are focused on the effects of mechanical and geometrical parameters of these tubes like shape, thickness, diameter, and length and material type of tube on the amount of energy dissipation and axial stiffness. The results show that accordion thin-walled tubes exhibit satisfactory energy absorption behavior and that proper selection of the parameters yields the optimum design of this metallic damper.

In this paper, an analytical model based on Modified Slab Method is presented for rolling of clad sheet or double-layers in which the two layers are bounded prior to rolling. This model considers the general case of asymmetrical rolling due to unequal surface speed, different contact friction, roll diameters, flow stress, and thickness ratios of the two layers. Using this model, rolling parameters such as pressure distribution along the arc of contact of the rolls and the clad sheet, rolling force, and torque with respect to reduction in thickness can be easily calculated. The analytical rolling force and torque computed by the proposed model were compared with the analytical results of other researchers and were shown to be in good agreement. The proposed model is very suitable for online control application due to its completeness and its capability of predicting the rolling parameters.

There is a demand for reduced power consumption in the vapor compression refrigeration cycle. Coefficient of performance of window-air conditioners considerably decreases and power consumption increases under very hot conditions.These problems have encouragecl studies aimed at improving the performance of window-air-conditioners by enhancing the heat transfer rate in the condenser. In this article, a new design for application of evaporative cooling in the condenser of window-air conditioners is introduced and experimentally investigated. In this design, two pads equipped with a water injection system are located on both sides of the air-conditioner to cool down the air flow passing over the condenser. The experimental results showed that thermodynamic characteristics of the system considerably improved while power consumption decreased by about 15% and the coefficient of performance increased by about 55%.

In this study, two CCCT diagrams are drawn to be compared with a CCT diagram. The CCCT diagrams represent continuous cooling transformations in stress assisted state. The increased Md and Bd temperatures of CCCT diagrams were also compared with those of the CCT diagrams and the cause was investigated from both thermodynamic and metallurgical viewpoints. Thermodynamic examinations revealed that stress causes the mechanical driving force to increase but the total free energy of transformation to decrease; hence, Md and Bd will rise. Metallurgical investigations showed that if deformation temperatures are selected in a manner to increase the structural strength of the original austenite grains prior to deformation, the shear force required for martensite and bainite transformations will arduously obtain; hence, Md and Bd will fall. However, if recrystallization or full recovery occurs during or after deformation, interior grain structure softens and the shear force required for martensite and bainite transformations will readily obtain; hence, Md and Bd will rise. It was also found that the nose in CCCT curves are shifted to the left as compared to that of CCT curves. This indicates that deformation of steel enhances bainite formation more readily than that of the martensite phase.

The effective parameters that influence in situ cast ferroTic composites were investigated. Centrifugal casting of specimens was carried out using ceramic & metallic molds. OM, SEM and XRD techniques were used to examine the existence of flows in the specimens. Results show that the control of chemical composition, processing, cooling rate and heat treatment has a promising effect on the quality of specimens. Also remelting process leads to the homogeneity of matrix by uniform distribution of secondary phase.

Aluminum matrix composites reinforced with Al2O3 and SiC particles (5 Vol %) were produced using the hot powder extrusion method. Extrusion temperature and extrusion reduction in area were chosen in the range of 500 to 600°C and 90 to 95%, respectively. The physical and mechanical properties of the extruded composites such as density, tensile strength, elongation and microhardness were evaluated and discussed as a function of extrusion parameters. The microstructure and fracture surface of the products were examined using SEM. The results showed that the composites were fully densified and reinforcement particles were distributed uniformly in the matrix. Presence of Al2O3 and SiC particles increased both strength and microhardness, but decreased the ductility of the composites. Experimental results for hot extrusion of the compacted powder billets also showed that the extrusion pressure was dependent on the ram speed or deformation strain rate.

The formation of microporosity in modified Al-Si alloys has been reviewed in the present study. A major concern in modification is the increased tendency to form microporosity in the macro-shrinkage free Al-Si alloy castings. It has also been demonstrated that at low hydrogen contents (0.1cc/ 100g, Al), where only shrinkage porosity should occur, the effect of Sr-modification on porosity content is not considerable, indicating that the increase in porosity is due to an increase in gas porosity. Modification treatment, however, does not add hydrogen to the melt, nor does it increase the rate of regassing of the liquid, revealing that it can not enhance pore formation by increasing the melt hydrogen content. Modification treatment raises the freezing range (4-10 oC), but this increased freezing range exerts only a very small effect on microporosity formation, which cannot, by itself, explain the increased tendency to microporosity formation observed in modified alloys. The presence of modifiers slightly decreases the surface tension of the melt (5%), although this decrease in surface tension is not sufficiently high to considerably enhance pore formation in modified alloys. Many researchers have reported that modification treatment might favour the formation of porosity due to its effect on oxide use in the heterogeneous pore formation although the systematic observation of pores has shown that SrO does not take part in pore formation in Sr-modified alloys. Strontium and other modifiers which increase pore formation (Na and Ca) in Al-Si alloys have a high chemical affinity to form complex intermetallic compounds with Si and Al. Systematic observation of pores have shown that Sr-rich intermetallics take part in pore formation. Thus, Sr-modification may increase the porosity content through the formation of Sr-rich compounds during solidification.

Anatase-to-rutile phase transformation was studied in milled and unmilled samples. Ball milling was carried out in two types of ball mills, planetary and tumbler, with a ball-to-powder ratio of 40:1 over 2-48 hours. First, the unmilled samples were heated in the furnace at various temperatures for different periods of time. The results revealed that the anatase-to-rutile transformation completed at 980 oC after 48 hours. The rate of transformation in milled samples was greatly higher than that of unmilled ones. Activation energy in unmilled samples was about 440 kj/mol. The rate of transformation in the planetary ball mill was higher than that in tumbler mill. In the former, transformation almost finished after 16 hours of milling while in the lattar, it did not finish even after 48 hours. XRD results revealed that the transformation proceeds through an intermediate srilankite phase in all milled samples. However, srilankite was not observed in the unmilled samples.

A method is presented for the stress analysis of flight vehicles under different flight conditions including gust and control surface deflection (or maneuver) using the governing equations of rigid-body motions and elastic deformations. The Lagrangian approach is used to derive the governing equations of motions. For this purpose, the basic equations of motions are derived in terms of potential energy, kinetic energy and generalized forces, which are, in turn, computed in terms of rigid-body motion variables, elastic mode shapes and cla(x) distribution for aerodynamic forces. By replacing them into the relations obtained, the governing equations for aeroelastic behavior of the vehicle are derived. The system of aeroelastic equations of motions is solved in time domain using numerical methods. The stress distribution is determined using the relation between modal variables and strain at each point. Finally, the prepared code is verified through comparison of the results obtained from the proposed method for the stability of a rocket and the same results reported by other studies. Also additional information such as maximum stress in the body is presented for various flight conditions.

It is empirically established that, due to a number of factors involved, a classical (linear) analysis of buckling pressure is impossible. Nonlinear theories of buckling are, therefore, required that involve effective factors such as imperfections and welding effects. In this study, models are developed which are as close to allowable standard deviations as possible. In the next stage, their buckling behavior is investigated both experimentally and numerically using finite element packages ADINA, ANSYS, COSMOS, and MARC based on specific capabilities of each. Results show that reasonable estimates of real buckling pressure will become possible when material and geometrical nonlinearities and initial imperfections are introduced into the analytical system. Finally, in the light of the results obtained, a submarine pressure hull is analyzed.