Sharif Digital Repository

In line with the expansion of the Internet, the issue of information security has gained special importance. Many methods have been provided in order to establish hidden communications. In the meanwhile, steganography is one of a relatively new method. A method is introduced in this paper for exchanging information by steganography on HTML pages. The main idea in this method is to hide coded data in the ID attribute of the HTML document tags. Since numerous HTML pages are on the web, this method can be widely used. The suggested method is implemented by using the Java language. © 2006 Springer  

This paper presents a new controller for power system stabilizer (PSS) that, unlike the conventional linear controllers, consists of both a linear and a nonlinear part. The linear part is properly designed to improve the small-signal stability margin and the damping of the local electromechanical modes. The nonlinear part is then designed to improve transient stability margin of the system without violating the features offered by the linear controller. Both parts are designed based on a perspective which attributes the system oscillations to a Hopf bifurcation (HB) phenomenon. More specifically, the design of the nonlinear part is done through the definition of a nonlinear index that... 

Multi agent system is a system consist of multiple interacting agents. These systems tend to select the best solution for their problems. They can be used in different tasks which are hard for an individual or even a complex system to do. One of the most common algorithms which are used in multi agent systems is flocking. Here we introduce a theorem that multi agent systems could flock in environments with fixed obstacles without any collision between agents and obstacles. We use lyapunov theory and prior algorithms on flocking to extract a theorem which under those conditions in the theorem, collision never occurs between agents and obstacles. Results show that the theorem insures collision... 

In this paper, the power quality of a single-phase photovoltaic grid connected inverter when uses active islanding detection methods is investigated. Two main groups of these methods are analyzed in this paper. The main focus is on the output power quality of PV sources for these methods. Also the variation of output current THD, while the output power changes, is explored. The variation of output current THD is measured when the parameters of active islanding detection method are changed, and the limitation of these parameters for complying power quality standards in the nominal output power is detected. ©2009 IEEE  

In this paper, sliding-mode control (SMC) is applied to a z-source converter to control output dc voltage of impedance network. The converter is assumed working in continuous conduction mode (CCM). First a comprehensive review of the literature for z-source inverter (ZSI) and use of sliding mode control in switched converters are performed. The procedure of SMC design used in ZSI is presented. The simulations for the application of SMC in z-source inverter are made and the results are illustrated. The results verify the effectiveness and robustness of the controller. © 2008 IEEE  

The paper presents the vector model of the Brushless Doubly-Fed Machine (BDFM) in the rotor flux oriented reference frame. The rotor flux oriented reference frame is well known in the standard AC machines analysis and control. Similar benefits can be sought by employing this method for the BDFM The vector model is implemented in MATLAB/SIVIULINK to simulate the BDFM dynamic performance under different operating conditions. The predictions from the vector model are compared to those from the coupled circuit model in simulation. The results are shown for the cascade mode of operation. © 2008 IEEE  

This paper presents a performability estimation approach which measures the ability of a network to deliver its services in the presence of component failures. The measurement is based on two important network service parameters, the endto- end delay and the packet loss. To evaluate the approach, different network topologies are studied using the OPNET Modeler. The results of simulations for four random topologies and two routing protocols, the RIP and the OSPF are presented and compared. The OSPF protocol with different delay timer values is also evaluated. © 2007 Springer  

A method for real-time extraction and measurement of individual harmonic components of a power signal with time-varying characteristics is presented. The proposed method, which is based on the concept of adaptive notch filter (ANF), adaptively decomposes the measured power signal into its constituting components. The structural simplicity of the algorithm renders it desirable from the standpoint of practical, analog and hardware implementations. Simulation studies confirm the desirable transient and steady-state performances of the proposed method. © 2006 IEEE  

This paper is a summary and in-depth evaluation of some popular models for power electronic converters. Proper examples are given to support the evaluation with a complete comparison between the actual and simulated results. The paper presents the effort behind the design and analysis of DC-DC Converters and discuses their features. Some advantages and disadvantages of the studied models and the complicities of the nonlinear structure of the switching function of the converters are explained here. The paper offers some useful criteria for future selection and development on different DC-DC Converters. © 2006 IEEE  

The subject of modeling and stability analysis of dc-dc resonant converters is still a challenge. The conventional large signal nonlinear model of the resonant converter is derived using the sinusoidal approximation and averaging followed by linearization about an operating point. Models obtained with such method involve considerable approximation, and produce results that are limited for higher performance designs. Therefore, it is essential to investigate the stability of the resonant converters using a more sophisticated model. Because of semiconductors switching, dc-dc resonant converters are intrinsically hybrid systems consist of discrete input and continuous states. The complexity of... 

In this paper, we used echo state networks - a class of recurrent neural networks - for prediction of drought based on remote sensing data. To this end, the drought index was obtained for a number of stations in different clime zones of Iran. For each station, we also extracted the corresponding vegetation indices based on satellite imagery. Our model takes the satellite-based features as input and outputs the severity of drought. One of the major challenges of echo state networks is optimization of the reservoir parameters. Here we used a method based on Kronecker product in order to substantially reduce the parameter space to be optimized. We then used various optimization techniques... 

Accurate power factor definitions are of great importance for industries due to the fact that they affect the billing strategy. They also considerably affect the generation and distribution losses of the power system. At the moment, there are some ambiguities in power definitions to be used for this purpose. In recent years, because of a rapid growth in industry and the use of variable loads such as electrical furnaces and variable speed motors, there is a need for fast compensators that are capable of tracking load changes. One of these facilities is TSC, a capacitor bank that is controlled by static switches (thyristors). In this paper, the performance of these compensators in power factor... 

In this paper the dynamics and control of the underactuated flexible needle will be discussed. To evaluate the dynamics of the needle, the study uses Saint Venant-Kirchhoff and finite element method. The model is validated using the experimental data provided in the literature. It is also shown that using iterative decomposition of the dynamics equation the unactuated degree of freedom of the needle tip can be feedback linearized. The effect of the control signal exerted on the tip is projected via iterative decomposition of the dynamics equation. The efficiency of the approach will be next explored through some examples  

This paper presents a novel routing protocol based on the Learning Automata method for large scale Wireless Sensor Networks (WSNs) codenamed DRLR (distributed reinforcement learning routing). In this method, each node is equipped with learning automata so that it can learn the best path to transmit data toward the sink. The approach proved to be efficient, reliable, and scalable. It also prevents routing hole by considering network density and average of energy levels available. The approach also increases network lifetime by balancing energy consumption. We compared our approach to two other methods (MMSPEED and EESPEED) and the simulation results show our algorithm to better meet... 

FPGAs are interesting choices for control of power electronics converters and electrical drives. In this paper, implementation of the control method of a reduced switch- count five-leg converter is carried out. Two PWM methods are studied. For verification of the implemented controller in a practical manner, without risking the damaging of the real system, "FPGA in the loop" experiments are performed. It is shown that using the proposed methodology, FPGA implementation and verification is fast and effective. The provided results show the high performance of the implemented controller on the FPGA, therefore the feasibility and suitability of the FPGA for this application is approved  

The paper presents a novel vector control structure for the Brushless Doubly-Fed Machine (BDFM) which is derived based on the machine synchronous operation. In fact, the synchronous operation of the BDFM provides an efficient approach for determining the required reference angle in the machine vector control structure. The utilization of such reference angle makes the vector control structure presented in this paper different from and, in fact, more effective than the existing rotor flux and stator flux orientation schemes proposed for the machine. The results of implementing the vector control scheme in simulations confirm the effectiveness of the proposed approach for the BDFM control  

A new method to evaluate the steady state performance of a three-phase self excited induction generator based on conductance minimization is proposed. It can be simply used to take series compensation into account. Among the priorities of this method are the absence of convergence problem and flexibility. Simple methods to find the frequency and magnetizing reactance have been proposed. Simulation results show the efficiency of this method  

The steer by wire system for vehicles has attracted much attention in recent years. Steer by wire systems provide many benefits in terms of functionality and at the same time present significant challenges too. It is essential to investigate the stability of steer by wire systems using a more sophisticated model rather than common approximated models. Fractional order modeling offers a good modeling framework for complex dynamical systems involving flexible structures such as multi-mass drives. In this paper a nonlinear fractional order model for steer by wire systems is presented  

In this paper, optimal control strategy is applied to parallel three-phase inverters using SVPWM technique. The three-phase system is transformed to a synchronous-stationary frame, suitable cost functions are defined, and the switching schemes are determined for the inverters. The control strategy minimizes the circulating current between the inverters, therefore, proper load sharing scheme is achieved. Simulations are performed for steady state and load change operation, and the results which show the suitability of the controller are provided. ©2009 IEEE  

In most approaches to adaptive robust control design for manipulators, a constant upper bound is considered for unmodeled dynamics. However, experiments show that the nature of the control input affects the magnitude of the unmodeled dynamics. For example, in an almost rigid manipulator where the mechanical flexibilities are considered as unmodeled dynamics, high frequency components of the input torque which are resulted form non-smooth control laws, can themselves excite system flexibilities. In this paper, we consider different characterizations of the unmodeled dynamics to take into account the effect of control input signal on the unmodeled dynamics. Simulation results illustrate the...