Sharif Digital Repository

In this thesis, in addition to reviewing the previous work done in the study of ideal access structures, we present the author's recent results in this field. As a first result, we introduce a new technique for reducing the size of the secret space in ideal homomorphic secret sharing schemes. The concept of decomposition of secret sharing schemes is formally introduced for the first time. In this regard, we show that ideal homomorphic and abelian secret sharing schemes are decomposable. We also examine the inherent group-characterizability of secret sharing schemes and show that an ideal secret sharing scheme is not necessarily inherently group-characterizable. Some weaker definitions of the... 

Due to the increase of computer networks and their application, the need for secure communication has become a very important concern. We introduce Cmail, a secure messaging system that works without any central party and in addition protects privacy of users. This system is on top of a cryptocurrency system such as Bitcoin. Bitcoin is cryptocurrency that works on the Internet. The suply of bitcoins and verification of transactions is done by Bitcoin miners, which are some typical client of Bitcoin system, in a decentralizd maner. In Cmail the miners have additional task, they should transfer messages of users and if needed, they hold it for a limited time. In order to increase incentive of... 

In this project, TiO2 nanoparticles were prepared by the sol gel method. The band gap of TiO2 was modified with transition metal ions Pt, Pd, Ni. The kinetics and photocatalytic degradation of AB92 investigated. The effect of some parameters such as pH and amount of photocatalysis and calcination tempreture were examined. Also effect of transition metal increase on surface morphology and photocatalytic activity were investigated by XRD, BET, SEM, DRS and FT-IR Techniques. Dyes are an abundant class of colored organic compounds that represent and increasing environmental danger. Azo dyes are well-known carcinogenic organic substances. Advanced oxidation processes (AOPs) have been proposed as... 

In this study, an adaptive controller design for nonlinear fractional order systems in the strict-feedback form has been investigated. The system is subject to unknown dynamics, unmeasured state variables, quantized input and output, and input nonlinearity. A linear observer is used to solve the problem of unmeasured state variables. The fuzzy logic system is used to estimate the unknown functions, and instead of updating all the regressor weights, only the upper bound of their norms is updated, which significantly reduces the computational load. In the controller design, limitations due to the bandwidth of data transmission have been considered by applying quantizers in the input and output... 

Kinetic roughening has attracted a lot of attention over the last few decades not only because of its practical importance for the growth of solid films, but olso as an example for a dynamical mechanism
that drives a system into a spatially and temporally scale invariant state. Most of the models of kinetic roughening studies so far can be described by the KPZ equation. In this project growth processes which
cannot be described by KPZ will be studied. The first nonequilibrium growth models including deposition and surface diffusion were intruduced independently by Wolf and villain. The Schramm-Loewner evolution (SLE_, where _ is the diffusivity) proposed recently is a very successful... 

Realistic simulation of tool-tissue interactions can help developing more effective surgical training systems and simulators. In this study we used finite element and meshless modeling approaches to simulate the grasping procedure of a large intra-abdominal organ, i.e., kidney, during laparoscopic surgery. Results indicated that the accuracy of meshless method was comparable with that of the finite element, with the RMS errors in the range of 0.8 to 3.4 mm in different directions. The computational cost of the meshless method however, was much less than that of the finite element model. In order to verify the model, a silicon sample was used with the ship kidney geometry. For determination... 

Common induction motors are controlled with the stator voltage and so all the power passes throw the converter. In the Brushless Doubly-fed Induction Motor (BDFM) a converter with smaller size and lower cost can be used. Also its brushless operation and cage rotor are advantages in comparison with doubly-fed induction motors with rotor excitation. Direct Torque Control is a method to control induction motors. Its advantages include fast responses to flux and torque commands, not requiring to know machine parameters except winding resistors, and easy implementation. In this thesis application of this method to control the BDFM is studied. At the first, electromagnetic mechanism and coupling... 

High voltage dc-dc converters are widely used in industrial applications. One of the methods for generating the high voltage is series connection of isolated voltage sources which can be produced by a multiple output converter. One major problem of multiple output converters is their cross regulation. In this project, the operation of a full-bridge based dc-dc converter (FBBC) is investigated in both single output and multiple output cases and analytical expressions for its output voltage and cross regulation are derived. Then, the effect of different parameters of the converter on the cross regulation is studied. It has been shown that increasing the input voltage of the converter improves... 

Today, in different industries,as the use of high voltage DC power supplies is increasing, the need of output voltage precision also is increasing. One way to make High DC voltage is to use a multi-output DC-DC converter and makethe output series.Existing problems in measuring high voltages, results in this idea That in these converters, we measure and regulate only one of the outputs and estimate the others by circuit elements. In this estimatin, differences between the transformer parasitic elements are very effectiveon transformer various output voltages. In this thesis after a comprehensive study on the pulse transformer parasitic elements and choose a suitable model for it, the... 

High voltage pulse generators are widely used in the radar systems, laser, medical, food processing and etc. One of the pulse quality criteria that was analysed in this thesis is the pulse to pulse regulation. Pulse to pulse regulation is important in various fields. In the radar applications pulse to pulse instability caused undesired errors in the signal processing and limitations in the tracking of the targets. In Free Electron Lasers (FELs) non identical pulses cause RF phase shift in the each pulsed power supply and creates instabilities in the electron beam. Pulse quality is essential in medical application so that in the treatment of cancer an improved precision in delivering the... 

In industial applications such as linear accelerators, pulsed lasers, nuclear fusion, radar systems, medical applications and etc. that need great ratio of instantanuse to the average power, pulsed power supplies are used. These power supplies store energy in an energy storage element and discharge this energy to load in a short period of time. The most important characterisitics of them are output pulse quality, reliability and longevity of the resource, efficiency and cost. One of the devices that produce high instantaneous RF power is magnetron tube. In some applications such as radar systems, quality of the output RF is very important and the most important factor that affect the quality... 

The growth of high voltage dc-dc switching power supplies in various applications such as medical, industrial and laboratory research is undeniable. Design of these power supplies, respect to their expectation have some restrictions. Among the main characteristics, appropriate output voltage regulation, high efficiency, fast dynamic response and low EMI problems, are expected. Transformers constraints as one of the key components of high voltage dc-dc switching power supplies, plays essential role in the selection of converter topology and its control algorithm. In this thesis, first, with regard to issues of high frequency and high voltage transformers, provides a good approximation model... 

In various industries as a solution to energy production for a short period of time in order to have a high instantanuse power pulsed sources are widely used. This resource is a relatively long period of energy stored in the energy storage element and then this energy in a short period of time the exhaust. Among the most important applications of these resources can be used in radar systems, linear accelerators, pulsed lasers, nuclear fusion, medical applications, etc. Among the most important targets are the following sources can be output pulse quality, reliability and longevity of these resources, efficiency and cost them. From the perspective of the quality of the output pulse, rise time... 

High DC voltage is widely used in many applications such as X-Ray machines ,CRT , photocopy machines and etc. One idea to produce this high DC voltage is to use transformers, but it will be large in volume. We can increase frequency to reduce the volume of transformers but high voltage of primary and secondary winding will limited this volume reduction. Furthermore high turn ratio in transformers increases non idealities that cause voltage and current spike, and increase loss and noise. The main goal of this thesis is to reduce differential mode voltages on transformers to reduce the volume of power supply. Voltage multiplier will be used to reduce differential mode voltage. Voltage... 

Microwave tubes are widely used in the telecommunications transmitters. high voltage power supplies are used to feed them. According to the specifications that arises for microwave tubes, To take advantage of these tubes need to maintain and stabilize voltage in a certain amount to be felt. If the tube supply voltage changes, the desired output parameters such as RF output power quality will change. To stabilize the voltage is within acceptable limits provided in different ways. One common method is the use of switching regulators due to the high efficiency compared to linear regulators considered. But it also will bring problems. Some of these problems can be pointed to electromagnetic...