Sharif Digital Repository

One important aspect of privacy, is confidentiality. A common solution to preserve the confidentiality in network communications is ”Virtual Pri- vate Network”. VPNs todays are expected to be more secure and support higher throughput for their new applications, such as Inter-Cloud VPN and Virtual Private Cloud. block cipher is an important security com- ponent employed in most VPNs.On the other hand, most block ciphers have mathematical weaknesses in their structures, so the ttacker can use these weaknesses to break them faster than brute-force attacks. This thesis proposes a new method named ”Chaos-based Selective Key (CSK) Cryptosystem”, for increasing the security of block ciphers in a much... 

Streamline Curvature method is one of the most commonly used methods by industry for prediction of axial flow compressors. This method analyze axial compressor by solving radial equations in the meridional plane to locate streamlines and also by using empirical models to predict total pressure loss. Despite growth of CFD models, Streamline Curvature method has been considered as most practical method for compressor preliminary design and performance assessment because of it’s high speed and good precision. In the current work a C++ code (SLC) has been developed using streamline curvature for axial flow compressor performance analysis. This code receives compressor geometry, properties of... 

Water transportation through carbon nanotubes is key for designing nanodevices. The directed transport of water molecules through a rotating charged carbon nanotube (CNT) is investigated by molecular dynamics simulations. It is found that the net flux of continuous unidirectional water flow depended sensitively on the charge distribution, charge density and rotation of the CNT. we find that for a constant charge density, the water flux increases with the increase of the charge difference. Besides, we find that the water flux shows a nonlinear dependence on the angular velocity of the rotation. The rotation of the CNT with low angular velocities, can not generate a continuous water flux. The... 

In seismic regions, the construction of structures with appropriate seismic performance and re-centering capability that can reduce residual displacement and damage, are necessary. As an alternative, prestressed columns were developed in recent years which can limit the permanent deformation and structural damage effectively. After severe damage and residual displacement of bridge piers and building’s columns, extensive research was carried out on prestressed columns. Most researches have been conducted on the prestressing of bridge piers and precast columns in self-centering system but prestressed columns in common moment resisting frame and comparing their advantages and disadvantages with... 

Nowadays, power electronics converters with pulse width modulation (PWM) are widely used in different power circuits. Operation of semi-conductive switches during on and off conditions causes switching dissipation and consequently decreases the efficiency of the PWMs. Recently, by increasing the required power for power electronics applications, quasi-resonant converters attract attentions. Quasi-resonant converters, by adding resonant circuits to the PWM converters causes the soft switching operation (switching with zero current or voltage) and decreases the energy loss. In conventional control methods of these converters, the nonlinear average circuit model is linearized around the... 

DC-DC switching convertors are power electronic circuits that have various applications nowadays. Quasi-resonant convertors are a kind of these convertors that are of great interest due to their simple structure and soft switching. The goal of controlling these convertors is to achieve constant output voltage while varying the input voltage and the load by choosing a proper switching frequency. Classic controllers show drawbacks in the case of high input voltage and load tolerance, and this nonlinear behavior of the systems results in some practical challenges. In this thesis, nonlinear predictive controller for quasi-resonant Buck convertors is designed. Having designed the linear... 

Hydraulic steering technology has been used in vehicles for decades. Recently, a trend has been seen towards the use of electronic steer-by-wire systems that provide greater design flexibility by customizing the connection between the steering wheel and the steering mechanism. Steer-by-Wire systems (SbW) offer the potential to enhance steering functionality by enabling features such as automatic lane keeping, park assist, variable steer ratio, and advanced vehicle dynamics control. The lack of a steering intermediate shaft significantly enhances vehicle architectural flexibility. The task of a SbW system is two-fold: turning the steered wheels by tracking the hand wheel (HW) rotation and... 

Direct Torque Control (DTC) is one of the vector methods to control Permanent Magnet Synchronous Machines (PMSM). In order to achieve an appropriate control in vector control methods, an accurate estimation of flux is indispensable. Flux estimation should be fulfilled in a wide speed range without considerable dependence on motor parameters. Prevalent flux estimation methods are based on integration of stator voltage which is not feasible in low speed, or performed based on assumption of decoupled equations on d and q axis which is not precise for applications with fast dynamic. Different flux estimation methods are investigated in this thesis with the aim of improving the flux estimation in... 

Due to the growing oil price and environmental considerations, tendency to use local and environmentally friendly energy sources has increased.Bidirectional transmission can be one of the key features of the smart grid,so the use of car battery connected to the grid as local energy sources makes sense. This can provide network stability, especially during peak load times. Vehicles connected to grid (V2G) is an electric vehicle or a plug-in hybrid electric vehicles can delivered the electricity to the grid. These systems require either two separate power electronic converters can be used for transmission in either directions or a two-way transfer charger. The aim of this project is to design... 

In recent years, multiport bidirectional converters have been a center of attention in renewable energy systems. This research focuses on three-port bidirectional converters. First, series resonant bidirectional converter is precisely analyzed. This analysis includes sinusoidal, exact state space and small signal analysis through phasor concept. This converter can work in higher switching frequencies and higher power ratings in comparison with common DAB bidirectional converter due to its inductor impedance cancellation. To increase power rating a three-phase version is proposed which has reduced output filter capacitance. Then, three-port bidirectional resonant converter is proposed and... 

In this project the novel zero-voltage-switching topology for power factor correction bridgeless boost rectifier is proposed. By employing an improved ZVS pulse-width-modulation (PWM) switch cell, ZVS of all the main switches is achieved without additional current or voltage stress of main switches. Also for the auxiliary switch the zero-current-switching is provided. In all modes of opretion of this converter, the power path is provided by maximum two semiconductor so the conduction loss of converter is low as in conventional converter. With this features the efficiency of proposed converter is very high  

Accurate and fast position controlling is an important issue in today’s industrial needs. Drives used for position control require fast dynamics on speed control. Cascade linear controllers have sluggish response due to bandwidth limitations on speed and current loop. These structures limit the dynamics above all in high power applications where the switching frequency is low. In this thesis, deadbeat direct speed control is proposed, which overcomes limitations by cascade loops resulting in high-speed control dynamics. This approach uses a model of the plant to generate the control signals. According to measured speed and currents, the controller specifies the best voltage vector in order... 

The subject of control and stabilization of dc-dc series resonant converter (SRC) is still a challenge in power electronics. The conventional controller design and stability analysis for this converter are based on the model which is derived using the sinusoidal approximation and averaging followed by linearization about an operating point. This model is not applicable to a SRC that operates below resonance especially in discontinuous conduction mode (DCM) because the sinusoidal approximation is no longer acceptable. However, a SRC may be purposely designed to operate in DCM. Therefore, it is essential to investigate the stability analysis and controller design using a more sophisticated...