Sharif Digital Repository

In this study, control and protection of low voltage microgrids have been analyzed. Studied microgrid takes the advantages of inverter based distributed generation. One of the most important issues in inverter based microgrids is the control of its distributed generation. Since, the low voltage networks have single phase loads and many imbalances, these imbalances can affect the control of IBDGs (inverter based distributed generations). This issue in the islanded mode of operation is the most important, because in this mode of operation, IBDGs control the network voltage and frequency.
In this thesis, the different control strategies of IBDGs are reviewed, and then modified to address... 

A hash function maps efficiently every finite length string to a fixed one. The output represents the entire content of the input, similar to digital fingerprint of input text. In order to be able to use a hash function in cryptography, it should be hard to find two distinct inputs with the same output (collision), because finding a collision in hash functions leads to malicious attacks on many security systems.We say a hash function is provably collision resistant, if finding a collision can be reduced to the known hard problems. Family of MD hash functions are one of the most famous applied hash functions which are considerably used in industrial applications. However, they are not... 

Pattern recognition in data streams using bounded memory and bounded time is a difficult task. There are many techniques for recognizing patterns but when we talk about data streams these algorithms became useless since there are no enough memory to store all data. In data stream model the entire data is not available at any time and we don’t have enough time processing each data.
In this thesis we consider current methods for recognizing patterns from a data streams. The goal pattern in this study was the minimum total area of k convex polygons encloses all data  

One of the methods to detect an unknown environment, is using the visual information about the environment. In this problem, we have an unknown two-dimensional environment consists of a set of edges and vertices that the geometric positions of vertices is not specified and only the combinatorial structure of the environment is available. In addition, certain information such as visibility of two vertices in the environment is known. The goal is to recognize the geometric shape of the environment with this information.This problem has many applications in Robotics particularly in dynamic envi- ronment where objects are moving.
In this thesis, we examine this problem in different conditions... 

We design a new algorithm for clustering data streams in any fixed di- mension, that use the framework of core-set to summarize data, in order to reduce the complexity of computation. Clustering is to separate data into distinct sets called clusters, which objects in the same cluster has the most similarity and objects in the different clusters has the least similarity.This problem has many application in the areas such as: machine learning,image processing, financial and stock transactions. Data stream has recently emerged as an important concept because in many applications, data is inherently streaming over a network or the data base is extremely large and sequential access is way faster... 

Due to recent advancements and wide usage of location detection devices, huge amount of data are collected by GPS and cellular technologies which exhibits moving objects trajectories. Using this information, it is possible to track a set of objects over a long period of time, as happens for instance in studying animal migration. Always, in these situations it is undesirable or even impossible (due to process and storage limits) to store the complete stream of positioning data. Then, simplifying a trajectory as a data reduction technique is the option for resolving such problems. Moreover, there is an increasing interest in queries capturing ”aggregate” behavior of trajectories as groups like... 

Visibility graph is a graph that comprises information about visibility of a set of objects in 2 or 3-dimensional space. By constructing this graph with respect to conditions of environment one can answer questions like minimum Euclidean distance. Inverse of this procedure is required too; i.e. knowing geometrical structure of objects and information about their visibility we want to guess their location in space and, as the saying goes, reconstruct the environment. For example some methods to reconstruct specific polygons knowing which vertices each vertex sees is proposed. In this dissertation we try to reconstruct a 2-dimensional environment that only has two x-monotone paths (chains)  

A triangulated irregular network (TIN) is a data structure that is usually used for representing and storing a geographic surface. TIN is a set of points and some segments between them that are distributed in space such that projection of segments on the plane is a triangulation for projection of points. Visibility graph of points is a graph in which there is a vertex correspond to each point of TIN and there is an edge between two vertexes if corresponding points can see each other. TIN’s visibility graph has many applications; for example, finding some points in a geographic surface that they can see all of the surface together, they are good places for radars.in first section of this... 

The visibility graph of a simple polygon represents visibility relations between its vertices. Since each vertex in a polygon is visible from the two vertices adjacent to it on the boundary of the polygon, this boundary is analogous to a Hamiltonian cycle in the visibility graph of the polygon. Therefore, visibility graphs are Hamiltonian. Finding this Hamiltonian cycle can be of great help when solving visibility graph recognition problems, in which one should decide whether a given graph is a visibility graph of a simple polygon; and reconstruction problems, which include constructing the polygon corresponding to a given visibility graph. These problems have been solved for the special... 

The geometric deformation of the polygon naturally changes its Visibility Graph. Also, any change in the Visibility Graph, which includes adding or removing the edge, requires a change in the position of the polygon vertices. Of course, some changes in the graph may cause no polygon corresponding to such a graph.In this thesis, we want to study the necessary and sufficient conditions to change the Visibility Graph of a polygon, so that the new Visibility Graph corresponds to the graph obtained by adding the edge to the initial graph. We consider this problem in the case of spiral, funnel, and quasi-triangular input polygons  

Computing shortest paths and minimum spanning trees are basic and well-known problems in computer science and graph theory; also they have been considered as basic subroutines for many practical algorithms. There exists optimal algorithms for solving these problems when information about the basic graph is certain and specified. But, in real applications when the graph is obtained by using measurement tools which have limited computing precision, practically we are facing a graph in which locations of vertices or weights of edges are estimated.In such cases determining upper bounds or lower bounds for solutions of the shortest path and the minimum spanning tree are estimations of the... 

In this study, the behavior of a new innovative bracing system under cyclic loading and according to AISC code is investigated. An Ideal bracing system, in addition to having sufficient stiffness, should have a suitable ductility. Also, in order to prevent inelastic behavior of other structural members, braces, in addition to possessing high architectural adaptivity, must be able to contain the damage within itself and behave like a fuse. Three specimens with different sections, IPE100, HEA100 and 2UNP100, were designed and tested. The results of these tests represent an elastic- (approximately) perfectly plastic behavior of the brace if an appropriate section is used. Moreover, this bracing... 

Nowadays the evaluation startups has been difficult and this problem increases the likelihood of misunderstanding price of company's stock. Companies need precise evaluation in different steps of their development. during fund-raising, To make the investors’ share determine, the value of companies should be calculated because the investors’ share equals the ratio of new investment to the value of company after investment. Besides, during the merge or possession of a company, to determine the value of the deal or the share of the dealers, we need to evaluate value of the company. to evaluate the Intrinsic value of company, we need to know potential revenue streams, financial statements, the... 

Finding optimize tours on a given sequence of objects has applications in robitic. A tour on a given sequence of objects is a path that touchs or cuts each of them, in order. In STOC′03 it is shown that finding such a shortest path for a sequence of convex polygons is polynomial solvable and it is NP-hard for non-convex polygons with intersections. The complexity of the problem for disjoint polygons is asked as the importest open peoblem. In 2008 an approximation algorithm is presented for this problem. We show that the problem is NP-hard in each Lp norm, even if each polygon consists of two unit line segments. Also, in 2003 the problem, with obstacles has been proposed as a future work. An...