Sharif Digital Repository

A new robust AQM strategy for dynamically varying TCP/AQM networks is proposed and its performance is investigated through computer simulations in MATLAB and ns-2 environments. The developed AQM is designed based on coefficient diagram method (CDM), which is a new indirect pole placement method that considers the speed, stability and robustness of the closed loop system in terms of time domain specifications. Simulation results indicate that the new method (CDM-AQM) performs very well for network variations both in topology and traffic. Besides, a new adaptive controller based on CDM as an AQM method is introduced. In the developed adaptive AQM (ACDM), the output feedback pole placement is... 

Packet-level observations show that the TCP/RED congestion control systems exhibit complex non-periodic oscillations which vary with the network/RED parameter variations. In this paper, it is investigated whether such complex behaviors are due to nonlinear deterministic chaotic dynamics or do they originate from nonlinear stochastic dynamics. To do this, various methods of linear and nonlinear time series analyses have been applied to the packet-level data gathered from a typical network simulated in ns-2. The results of the analysis for a wide range of variations in averaging weight of RED (as the most important bifurcation factor in TCP/RED networks) show that such behaviors are not due to... 

Predictive functional control (PFC) as a new active queue management (AQM) method in dynamic TCP networks supporting explicit congestion notification (ECN) is proposed. The ability of the controller in handling system delay along with its simplicity and low computational load makes PFC a privileged AQM method in the high speed networks. Besides, considering the disturbance term (which represents model/process mismatches, external disturbances, and existing noise) in the control formulation adds some level of robustness into the PFC-AQM controller. This is an important and desired property in the control of dynamically-varying computer networks. In this paper, the controller is designed based... 

In this paper, a new effective and computationally reduced method for congestion control in high speed dynamic computer networks is introduced. The controller is designed using the well-known predictive functional control (PFC) scheme and an ARMarkov model representation that considers the system delay explicitly. Use of the multi-step-ahead predictive ARMarkov model structure within the PFC results in a simple algebraic control law that does not require recursive model output computation in the so-called prediction horizon performed in the other Model Predictive Controllers (MPC). This combination not only reduces the required computational load, but the accumulative error due to the model... 

In this paper, a mathematical model and control theoretical framework for designing AQM controllers in networks supporting TCP Vegas is introduced. We have emphasized on a modified TCP Vegas algorithm that can respond to congestion signals through explicit congestion notification (ECN). The overall nonlinear delayed differential equations of the dynamics model of closed loop system have been derived based on TCP Vegas model. The model is then linearized to derive a transfer function representation between the packet marking probability and the bottleneck router queue length as the input and output of the modified TCP Vegas/AQM system. The model properties have been then examined especially... 

The performance of a new robust AQM strategy for dynamically varying TCP/AQM networks is proposed. Network variations include in both network topology and traffic variations. The developed AQM is designed based on Coefficient Diagram Method (CDM), which is a new indirect pole placement method that considers the speed, stability and robustness of the closed loop system in terms of time domain specifications. Simulation results show different features of the proposed method. ©2005 IEEE  

Ultrasonic motors (USM's) possess heavy nonlinear and load dependent characteristics such as dead-zone and saturation reverse effects, which vary with driving conditions. In this paper, behavior of an ultrasonic motor is modeled using Hammerstein model structure and experimental measurements. Also, model predictive controllers are designed to obtain precise USM position control. Simulation results indicate improved performance of the motor for both set point tracking and disturbance rejection. © 2005 ISA - The Instrumentation, Systems, and Automation Society  

Ultrasonic motors (USM) possess heavy nonlinear, and load dependent characteristics such as dead-zone and saturation reverse effects, which vary with driving conditions. These properties have made the position/velocity control of USM a difficult and challenging task. In this paper, a generalized model predictive (GPC) controller for precise USM position control is suggested. Simulation results indicate improved performance of the motor for both set point tracking and disturbance rejection. Since the motor and the controller both are of type one, the applied saturation would cause in wind up phenomenon. This drawback is removed by implementing the Quadratic GPC controller. © 2004 IEEE  

Ultrasonic motors (USM) possess heavy nonlinear and load dependent characteristics of ultrasonic motors (USM) such as dead-zone and saturation reverse effects, which vary with driving conditions. In this paper, behaviour of an ultrasonic motor is modelled using Hammerstein model structure and experimental measurements. Simulation results show the good matching between the developed model and experimental measurements. © 2004 IEEE  

In this paper, a new method is introduced for identification of transformer winding fault through transfer function analysis. For this analysis, vector fitting and probabilistic neural network are used. The results of transfer functions estimation through vector fitting are employed for training of neural network, and consequently, probabilistic neural network is used for classification of faults. The required data for fault type identification are obtained by measurements on two groups of transformers (one is a classic 20 kV transformer, and the other is a model transformer) under intact condition and under different fault conditions (axial displacement, radial deformation, disc space... 

As it is found in the related published literatures, the transfer function (TF) evaluation method is the most feasible method for detection of winding mechanical faults in transformers. Therefore, investigation of an accurate method for evaluation of the TFs is very important. This paper presents three new indices to compare the transformer TFs and consequently to detect the winding mechanical faults. These indices are based on estimated rational functions. To develop the method, the necessary measurements are carried out on a 1.3 MVA transformer winding, under intact condition, as well as different fault conditions (axial displacement of winding). The obtained results demonstrate the high... 

This study presents an intelligent fault classification method for identification of transformer winding fault through transfer function (TF) analysis. For this analysis support vector machine (SVM) is used. The required data for training and testing of SVM are obtained by measurement on two groups of transformers (one is a classic 20 kV transformer and the other is a model transformer) under intact condition and under different fault conditions (axial displacement, radial deformation, disc space variation and short circuit of winding). Two different features extracted from the measured TFs are then used as the inputs to SVM classifier for fault classification. The accuracy of proposed... 

Transfer function (TF) is an acknowledged method for power transformer mechanical faults detection. However the past published works mostly discovered how to specify the faults levels and paid less attention to detection of the type of faults using comparison of TFs. whereas, it seems important for most of the applications to specify the type of fault without opening the unit. This paper presents a new method based on vector fitting (VF) to compare the TFs and specify the type, level and location of the fault. For development of the method, and its verification the required measurements are carried out on four model transformers; under intact condition, and under different fault conditions... 

In this paper, a new bidding strategy for pay-as-bid market suppliers is introduced. This method is based on a systematic analysis of interactions of market with the suppliers via several market indices as well as forecasting important indices by artificial neural networks. Besides, the proposed method considers the practical limitations in the system and deals with incomplete information handling, closely. Next, a strategic bidding approach is proposed for optimal bidding by the suppliers. In these investigations, the paper focus is on the experimental situation of Iran electricity market as a pay-as-bid market and a sample generating company with several generating units from this market... 

Calculating leakage inductances between windings of multi-winding Traction transformers is one of the most important schemes in design procedure. In this paper, analytical and finite element methods (FEM) have been introduced for electromagnetic modeling of traction transformers with four pancake split windings. Using these models, the leakage inductances between the split windings are calculated and the models have been validated with the help of experimental data gathered from a typical 4000kVA cast-resin dry-type transformer. Some reformations are employed to improve the FEM results and effects of these reformations on accuracy of the model are studied. Employing the validated models, the... 

In this work, a new model of transformer winding is developed. The components in the model are determined by the geometric and electric data of the winding. Under different degrees of axial displacement and radial deformation in the winding, circuit parameters in the model will be changed and thus the character of the circuit will be influenced. After acquiring the model parameters in the intact and faulted cases, transfer function coefficients derived in model from nodal analysis. Then using introduce the new index based on coefficients we can specify the type and extent of penetration of the fault in the winding. Results presented demonstrate the potential of this method  

In this work, a new model of transformer winding is developed. The components in the model are determined by the geometric and electric data of the winding (detailed model) and using experimental data based on genetic algorithm. Under different degrees of axial displacement and radial deformation in the winding, the circuit parameters of the model will change and thus the equivalent circuit characteristics will be influenced. After acquiring the model parameters in the intact and faulted cases, transfer function coefficients are derived in model using nodal analysis. Subsequently, introducing a new index based on these coefficients, the type and extent of penetration of the fault in the... 

Calculating leakage inductances between windings of multi-winding traction transformers is one of the most important schemes in design procedure. In this paper, a new analytical method and finite element models have been introduced for electromagnetic modeling of traction transformers with four split windings. Using these methods, the leakage inductances between the split windings are calculated and the methods have been validated with the help of experimental data gathered from a typical 4000kVA cast-resin dry-type transformer. Some reformations are employed to improve the finite element results and effects of these reformations on accuracy of the models are studied. Employing the validated...