Journal of Advanced Materials in Engineering (Esteghlal)

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Dense Silicon nitride was investigated to determine the effect of its microstructural parameters and densification on thermo-mechanical properties and thermal stress resistance to fracture initiation during a hot or cold mechanical and thermal shock testing. The different materials and microstructures were obtained by changing the parameters such as the type of the powder, additive, forming process and sintering condition. Maximum crack growth and thermal shock resistance of dense Si3N4 are achieved after complete conversion of the aàB transformation, and after the change in grain morphology towards elongated grain and the relative crystallization of the second phases have been obtained. The characteristics are obtained by a high a phase content of the starting powder, high Y2O3, and sintering condition of higher temperature (2000ْC), longer soaking times (1h) and load application at the beginning of the thermal cycle. Keywords: Silicon nitride, Thermo- mechanical properties, Thermal shock resistance, Crack propagation resistance

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Designing a robust tracking control for a non-linear MIMO system with uncertainty is one of the most complicated control problems. In this paper, sliding mode changed to non-linear controllable canonical form by input-output linearization. This, sliding surfaces can be defined in a way that we can de-couple equations and indicate the sliding conditions of multi-variable controller system. The uncertain parameters will be estimated properly and the input equation improved to apply the restricted input condition. The control law will be improved in a way that in addition to increasing the tracking accuracy inside the boundary layer, the speed of convergence will increase outside of the boundary layer. In order to satisfy the balance of the filter, the thickness of the adaptive boundary layer is used. Thus, a robust tracking control is designed which can trace the angle of attitude of satellite for maneuvers with a very large angle (180 deg.) on a piece-wise smooth path. Finally, the simulation results are compared with Elmali & Olgac’s methods and it is shown that despite decreasing control signals, the tracking accuracy increases by several ten times. Keywords: Attitude, Control, Sliding-mode control, Non-linear System, Input-Output Linearzation

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Algorithmic skeleton has received attention as an efficient method of parallel programming in recent years. Using the method, the programmer can implement parallel programs easily. In this study, a set of efficient algorithmic skeletons is introduced for use in implementing parallel genetic algorithm (PGA).A performance modelis derived for each skeleton that makes the comparison of skeletons possible in order to select the best one for the application. The performance of the selected skeleton can be increased by specifying the virtual topology required by the appliation.This is a novel approach with no precedent. Nesting of skeletons used hereis another novelty of the study which has been employed only in few previous studies.

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In this paper, based on feedback linearization control method and using a special PI (propotational integrator) regulator (IP) in combination with a feed-forward controller, a three-phase induction servo-drive is speed controlled. First, an observer is employed to estimate the rotor d and q axis flux components. Then, two input-output state variables are introduced to control the dynamics of torque and the magnitude of the rotor flux independently. In addition, based on the model refrence adaptive system (MRAS) and the recursive least square (RLC) error techniques, the rotor time constant and the mechnical parameters (J, R) are simultaneously estimated. Finally, the efficiency of the proposed method is confirmed against results from computer simulation. Keywords: Adaptive speed ontrol, Inducation servo-drive, Feedback linearization, IP controller, Model reference, Adaptive system, Recursive least square.

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The objective of this study is to design a robust direct model reference adaptive controller (DMRAC) for a nonlinear cardiovascular model over a range of plant parameters representing a variety of physical conditions. The direct adaptive controllers used in thisd study require the plant to be almost strictly positive real (ASPR) that is, for a plant to be controlled there must exist a feedback gain such that the resulting closed loop system is strictly positive real. We designed a new compensator so that the system composed of the cardiovascular plant and the compensator satisfy the ASPR condition. Numerous studies in the past have considered a small range of gain variations of the cardiovascular system. In most cases, the controller was designed based on variations in either time delay or plant gains. Many of these workers treated the cardiovascular system as a single-input single output (SISo) plant in which the control output was Mean Arterial Pressure (MAO). We treated the cardiovascular system as a multi-input multi-output (MIMO) plant in which both the MAP and Cardiac Output (CO) are simultaneously controlled. In this study, a new linear model is presented that provides a better approximation thanthe one the original linear model does. By doing so and utilizing the DMRAC algorithm, we could satisfy the stability conditions for the nonlinear model while satisfactory responses obtained under every possible condition for the cardiovascular nonlinear model. Keywords: Adaptive control, Cardiovascular system, Blood pressure, Cardiac output

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The objective of this study is to design a robust direct model reference adaptive controller (DMRAC) for a nonlinear cardiovascular model over a range of plant parameters representing a variety of physical conditions. The direct adaptive controllers used in thisd study require the plant to be almost strictly positive real (ASPR) that is, for a plant to be controlled there must exist a feedback gain such that the resulting closed loop system is strictly positive real. We designed a new compensator so that the system composed of the cardiovascular plant and the compensator satisfy the ASPR condition. Numerous studies in the past have considered a small range of gain variations of the cardiovascular system. In most cases, the controller was designed based on variations in either time delay or plant gains. Many of these workers treated the cardiovascular system as a single-input single output (SISo) plant in which the control output was Mean Arterial Pressure (MAO). We treated the cardiovascular system as a multi-input multi-output (MIMO) plant in which both the MAP and Cardiac Output (CO) are simultaneously controlled. In this study, a new linear model is presented that provides a better approximation thanthe one the original linear model does. By doing so and utilizing the DMRAC algorithm, we could satisfy the stability conditions for the nonlinear model while satisfactory responses obtained under every possible condition for the cardiovascular nonlinear model.

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In the current engineering practices, buildings are commonly designed for an effective lifetime of 50 years. This lifetime can increase the cost of buildings with short lifetimes and can reduce the safety level of buildings with large lifetimes. In this paper, a “service-life factor” has been defined. Applying this factor into the nominal values of live, earthquake, wind, and snow loads, the effect of service-lifetime is taken into consideration. The study shows that the magnitude of seismic load can be reduced by 60% and those of other periodic loads by 30% for temporary buildings. An increase of 50% in the periodic loads was also observed for service-lifetime of 150– 200 years. These effects indicate a meaningful improvement in economy and safety of buildings if the service-life factor is considered

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This paper presents the results of a recent study about the following aspects relevant to tunneling in soft grounds:e) The domain of deformations due to tunneling in soft ground can be specified within a boundary of a parabolic shape. This boundary is defined by a parabolic formula as a function of a central angle which depends on the soil type i.e., either cohesive or cohesionless. This parabolic shape can also be verified by a finite element computation.f) A finite element program has been applied to investingate the deformation characteristics around and above circular tunnels and to find the settlement ratio as a function of known variables such as, depth ratio, modulus of elasticity, and the thickness of soil layer beneath the tunnel. The finite element computations were carried out by assuming a given distribution of displacements around the tunnel perimeter, for which reason the method may be called “compulsory displacements”. It was found that although all the variables mentioned affect both the settlement ratio and the type of soil deformations, changing the values of modulus of elasticity affects only on the amount of deformation components, but not the settlement ratio.g) The results of finite element computations for the settlement ratio have been compared to other analytical curves and empirical data from some available case studies from which excellent agreements were found. also the contours of Equal deformation components from the finite element program and from the simple formulae proposed by the author were found to be quite similar and in acceptable agreement.h) Because the results obtained from the proposed formulae for the distribution of settlement at the ground surface are in excellent agreement with the relationships recently proposed by Loganathan & Poulos and the empirical data available, it is concluded that the simple analysis proposed here and the finite element computations based on the elasticity assumption can both be used to predict the deformation pattern around excavations in soft ground.

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In this paper, dynamic and stability analysis of a flexible cam-follower system is investigated. Equation of motion is derived considering flexibility of the follower and camshaft. Viscous and Coulomb frictions are considered in the rocker arm pivot. The normalized equation of motion of the system is a 2nd- order differential equation with periodic coefficients. Floquet theory is employed to study parametric stability of the system. Stability diagrams are presented and the effects of varying cam profiles and motion events on the stability of the system are compared. Results show that viscous and Coulomb frictions stabilize the motion of the system

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This paper presents a numerical solution for a changing combustor geometry. The effects of the geometric change on the main parameters of the chamber are considered. For this purpose the original geometry and the new one are simulated numerically by a 3-D CFD code and the results are compared. Finally, comments are presented regarding this change. A model is used for turbulence modeling and an eddy dissipation model for reaction. Effect of thermal radiation is considered through solving an extra transport equation. The DO model is used to obtain radiation intensity.

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In this paper, the application of evaporative cooling for refrigeration cycle to reduce power consumption in hot climates is emphasized. Experimental and analytical investigations were performed in order to specify the effect of evaporative cooling condenser instead of the commonly used air cooling condenser in window-air-conditioners. Evaporative condenser can reject more heat, thereby preventing the reduction of cooling capacity and increasing power consumption of window-air-conditioners during very hot seasons. Two designs were developed for evaporative condensers. In the direct injection design, water is injected on the condenser coil directly while in the media pad design, water is injected on the media pad installed before the condenser. Thermodynamic properties of the systems after modification were measured and compared with the ordinary situation. Analysis of the results show that using these methods, the coefficient of performance increases by about 25% and power consumption decreases by about 13%. It is also anticipated that further modifications in these designs may yield better results

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For implementation of the free surface boundary condition, a new subroutine has been introduced to an existing steady 3-D body fitted code. This code was previously written for steady flow simulation in closed ducts. The algorithm used in this subroutine reduces the instability problem according to the free surface wave generation. For code validation, it was applied to two different open channels. The results obtained for these test cases were compared with existing data. The comparison of the two sets of data was promising and proved that with this subroutine it is possible to predict the free surface position very well. The results of this routine were also compared with the symmetry boundary condition at the free surface for two mixing flows and it showed that, if the changes of the free surface elevation were small, the symmetry boundary condition could be considered as an accurate enough method

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The purpose of this work was to production of ceramic thin films by using of Sol-Gel process. For this purpose deposition of SiO2 on substrates of soda-lime glasses has been carried out. Coating treatments on prepared specimen were conducted in a Sol solution by means of dipping at various times. After drying and performing appropriate heat treatment on each sample, the thickness of coated layer was measured by means of roughness method. Some of the specimens were also exposed to heat and chemical environment to evaluate the coating resistance in such media. SEM examination and EDAX and XRD analysis of coating layers was also conducted on some samples. The results indicated that by Sol-Gel method, it is easily possible to achieve thin layers in the scale of one hundredth micron meter. Any change of the thickness layer on the surface is negligible and the quality of the coating is excellent. Also, experiments indicated that deposited coatings by Sol-Gel process, are stable and give enough durability in various environments.

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Titanium is a highly reactive metal so that a thin layer of oxide forms on its surface whenever exposed to the air or other environments containing oxygen. This layer increases the corrosion resistance of titanium. The oxide film is electrochemically formed through anodizing. In this study, anodizing of titanium was performed in phosphate-base solutions such as H3Po4, NaH2Po4, and Na2Hpo4 at 9.75Ma/cm2 and 35ºC under galvanostatic conditions. The Potential-Time curves in the above solutions show that the anodic films formed on titanium are compact and their thickness depends on the solution type and concentration. The SEM and XRD techniques show that these layers are amorphous. In this paper, the effect of electrolyte concentration, composition and resistivity on breakdown voltage have been discussed in terms of Ikonopisov electron avalanche breakdown model. This model shows that the major factor contributing to the decrease in breakdown voltage is the increased electrolyte concentration leading to increased primary electronic current.

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In this study, colorimetric calibration of scanner has been done via perceptron neural network with three or four layers by back propagation algorithm for colored polyester fabrics. The results obtained for random training samples are not satisfactory but application of selective training samples for L*a*b* or RGB leads to good results, with better results obtained for the L*a*b* method. On the other hand, the color differences between calculation XYZ and real XYZ for unknown samples, are not only in agreement with the results of polynomials and regression methods, but are also better than the results obtained in previous studies where neural networkhad been used for colorimetric calibration of scanner.

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Many important problems in Artificial Intelligence can be defined as Constraint Satisfaction Problems (CSP). These types of problems are defined by a limited set of variables, each having a limited domain and a number of Constraints on the values of those variables (these problems are also called Consistent Labeling Problems (CLP), in which “Labeling" means assigning a value to a variable.) Solution to these problems is a set of unique values for variables such that all the problem constraints are satisfied. Several search algorithms have been proposed for solving these problems, some of which reduce the need for backtracking by doing some sort of looking to future, and produce more efficient solutions. These are the so-called Forward Checking (FC), Partially Lookahead (PL), and Fully Lookahead (FL) algorithms. They are different in terms of the amount of looking to the future, number of backtracks that are performed, and the quality of the solution that they find. In this paper, we propose a new search algorithm we call Modified Fully Lookahead (MFL) which is Shown to be more efficient than the original Fully Lookahead algorithm

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Pulse Width Modulation (PWM) techniques are commonly used to control the output voltage and current of DC to AC converters. Space Vector Modulation (SVM), of all PWM methods, has attracted attention because of its simplicity and desired properties in digital control of Three-Phase inverters. The main drawback of this PWM technique is its complex and time-consuming computations in real-time implementation. The time-consuming calculation as well as software and hardware complexities of the network grow dramatically as the number of inverter levels increases. Therefore, it is necessary to develop an exact, fast, and general computation SVM algorithm for multi-level converters. This paper introduces such an algorithm. Specifically, the SVM computation algorithm based on a vector classification technique, introduced for 2-level inverters in 1996, is developed and generalized to be applicable in determining the switching sequences and calculating the switching instants in m-level inverters. The proposed technique reduces hardware and software complexities, decreases the computation time, and increases the accuracy of the positioning of the switching instants when compared with the conventional implementation of the SVM in multi-level converters

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In this paper, a modified segmentation algorithm for printed Farsi words is presented. This algorithm is based on a previous work by Azmi that uses the conditional labeling of the upper contour to find the segmentation points. The main objective is to improve the segmentation results for low quality prints. To achieve this, various modifications on local baseline detection, contour labeling and segmentation rules have been applied. In an experiment, the correct segmentation rate was 97%. Based on the results obtained, a detailed error analysis is presented which should be useful for furthur research on this topic.