Scientia Iranica
Year:2002 | Volume:9 | Issue:4 (ELECTRICAL ENGINEERING)|Papers Count:17

In this paper, a phase-code radar signal is considered in its most general form, in which the used Code has an arbitrary distribution. The suboptimal Electronic Support Measure (ESM) detector, for which the code is not known, is found for this kind of signal and the performance of the detector is evaluated. The deflection measure of this detector shows that the Low Probability of Detection (LPD) feature of the signal is effectively related to a defined quantity called certainty parameter. This new parameter depends on the first and the second order statistic of the code. Using this parameter, it is shown that a zero mean code is the best code to make LPD waveform. Then, the zero mean codes are considered and the code distribution effect is investigated. It is proven that distributions which have smaller Kurtosis show better LPD features in practical cases. It is, therefore. concluded that zero mean binary code is the best code.

In this paper, three-dimensional Finite Element Method in Time - Domain (FEMTD) has been used to solve the current distribution in an acid-copper bath. The effects of geometric figureson the electrode current distributions and deposited profiles have been studied. A powerful adaptive mesh generator has been carefully designed to meet the requirements on three-dimensional geometries. Almost all practical configurations, even discontinuities and very sharp features, are permitted. A mesh-deformation algorithm is used to follow the change of profiles in the time-domain. For this purpose, instead of becoming involved with complicated mathematical formulations, the morphing techniques of computer graphics have been utilized. The results are rendered to visualize the evolution of profiles, which has considerably simplified the interpretation of the program's output. Several examples have been analyzed numerically.

This paper has been prepared, based on reports regarding extraordinary induced ac voltage on buried gas pipelines in residential and industrial areas of the capital city of Tehran, especially in densely populated central areas. Also, some reports are considered on the existence of ac induced voltage in buried gas pipelines under the overhead transmission lines, for example, one case located 80 Km to the west of Tehran, in an agricultural region of the town of Abyek. In this paper, the results of an extensive theoretical, experimental and practical investigation and implementation of methods carried out during the last two years are introduced. Furthermore, this paper includes evaluation of the existence of ac induced voltage in gas pipelines, their effect on cathodic protection systems and the pipelines coatings. A method will be introduced for determination of electromagnetic induced ac voltage in gas pipelines under overhead transmission  lines. Finally, a method of ac induced voltage mitigation for application on buried gas pipelines under overhead transmission lines is introduced and verified by application in the above mentioned region, which resulted in a complete cancellation of induced ac voltage in the pipeline. A computer program is developed based on these methods.

From the energy conversion point of view. structure of all electrical machines can be simplified to an equivalent two-phase machine. At normal operation, this equivalent machine has a balanced structure which is the basis for vector control equations in this mode. Similarly, at a phase-cut off condition, this equivalent model has an unbalanced structure, which can be used to obtain vector control equations under fault conditions. In this paper, first a dual three-phase induction machine has been modeled with two phases of stator cut off, then vector control equations have been obtained from the d-q model of the machine. On the basis of these equations, a new "Rotor Field Oriented (R.F.O.) control" approach is presented, which indicates how the conventional RF.O. control can be adapted for fault conditions. Finally, a computer simulation has been presented to compare the operation of a conventional R.F.O. control with an improved R.F.O. control, while the two phases of the machine are cut off. Results show considerable improvement in drive performance, especially in reduction of torque oscillation.

An important subject in load pick up during power system restoration is the selection of suitable load steps, especially in the initial phase of restoration. Operators are often concerned with the size the load that can be safely picked up and the effectiveness of the generation reserve. load pick up in small increments tends to prolong the restoration period. With large increment, there is always the risk of falling into a frequency decline and to a recurrence of he system outage. In this paper, a novel method for dynamic calculation of load steps is presented. The proposed method considers' the dynamics of system components, as ell as their limitations and uses a suitable form for frequency behavior. It could be us d to assist the operator in the on-line selection of suitable load steps and/or to compute the minimal frequency for a given load step. Simulation tests, carried out on a 39-bus new England test system, verify the effectiveness of the proposed method.

In this paper, a new faul tolerant feed forward neural network architecture is first introduced, using fault tolerant neurl nodes in its output layer, which is the most critical layer in the network. Then, the conventional Back error Propagation (BP) algorithm is modified to apply to this architecture with he least learning degradation for fault tolerant nodes. The presented neural network, jointed with a systematic real convolutional code, contributes in an Algorithm Based Fault Tolerant (ABFT) scheme to protect a nonlinear data process block. The neural network is trained to pro uce an all zero syndrome sequence in the absence of any faults. The convolutional code guarantees that faults, representing errors in the processed data, will result in notable nonzero values in syndrome sequence. A majority logic decoder can easily detect and correct single faults by 0 serving the syndrome sequence. Simulation results for random s-a-0 faults, demonstrating error detection and correction behavior, are also presented.

In this paper, a systematic method is presented for modeling a multi-machine power system with multiple UPFC's installed and the dynamic interaction between different control functions of the UPFCs is investigated. Study undertaken on New England power systems demonstrates that the existence of dynamic interaction may lead to the instability of the whole system.

In this paper, the performance of EAM (Estimation After Modeling) and EBM (Estimation Before Modeling) techniques for aerodynamic parameter identification are compared. The results clea rly show that EAM has a superior performance over EBM. but both techniques fail when there is considerable modeling uncertainty. Given this shortcoming, a two-step procedure is used that is a combination of EAM an EBM for estimating the aerodynamic parameters. Simulation results show that this new technique outperforms EAM when modeling uncertainties are considered. This technique will be applied to the problem of identifying the aerodynamic parameters of an anti-tank missile and its performance is compared with both EAM and EBM approaches.

In this paper, the Transmission Line Matrix (TLM) method is used for the electromagnetic characterization of the Shielding Effectiveness (SE) of rectangular enclosures with apertures of arbitrary shape. Applying suitable Hanning windows in the time domain drastically reduces the number of time steps required to model the coupling behavior of resonant enclosures. Comparison of the numerical results With available measured data shows that the method is an efficient and accurate technique to evaluate the shielding  effectiveness of metallic shields in the time domain and over a wide frequency range. The proposed method can be used as an intermediate design tool for the evaluation of the effects of various aperture shapes on the effectiveness of shield assemblies.

The overall performance of a linear model predictive controller depends on proper adjustment of several design parameters. Most of these parameters have interdependent effects, which make their trial and error based tuning procedure very difficult. A systematic approach to overcome this problem is to reduce the number of adjustable parameters. This reduction is usually performed on the basis of sensitivity analysis, stability considerations or other objectives and constraints. The most reliable parameter, which can be independently tuned for performance improvement, is the control move suppression coefficient, l. In this paper, some tuning rules for adjusting this parameter, on the basis of specific performance criteria, are introduced. These rules are obtained from numerical analysis of the controller performance and are, therefore, applicable regardless of the existence of an approximated first-order model. The capabilities of the rules are demonstrated using simulations of regular and adaptive Dynamic Matrix Controllers (DMCs).

In this paper, set-membership identification is used to derive a simple algorithm which is a sign version of the normalized least mean square algorithm. Convergence analysis is carried out.With some simulation examples. the performance of the algorithm, in the cases of slow and fast variations of a parameter is compared with the modified Dasgupta-Huang optimal bounding ellipsoid algorithm. These examples show the performance of the proposed-algorithm.

A novel strategy based on Capon method is proposed for multiple signals extraction with an antenna array in TDMA/FDMA mobile cellular systems. In this method, called SF-Capon. each Intercell Cochannel (lC) user is alternatively asked to decrease its transmitted power for a few bits. This enables the estimation of a so-called Signal-Free (SF) correlation matrix, which is then used in connection with Capon method to compute an improved beamforming weight vector, with reduced sensitivity to DOA error and angular spread. The superiority of the proposed SF-Capon strategy over standard Capon and data independent least-squares beamformingis demonstrated via simulation in a fast Rayleigh fading channel.

Voltage security is an important concern in power networks as a result of system heavy loading. In this paper, a new continuation method is presented. which has three outstanding features for voltage security assessment. In this continuation method, adaptive step size is applied. For adaptation, a criterion, based on rate of voltage drop, is used. This method can monitor the voltage security status of power systems under various topologies, load levels and generation patterns. Different load and generation scenarios can be analyzed by this method. Also, the proposed continuation method uses local analysis for evaluation of power system voltage stability after contingency or pre ntive action. The proposed method has been examined on the test system of the New Zealand and Iran's power network. Obtained results confirm the validity of the developed approach. Also, the most vulnerable buses of these test systems have been sorted by a new ranking function. A discussion about this ranking function and its results has been presented.

In this paper, an efficient analytical method is described for evaluation of the element integrals, which occur in the variational axisymmetric finite element method with first order triangular elements, This method permits exact evaluation of such integrals without any need for numerical integration schemes. The key feature of this technique is that it permits finding the value of these integrals through solution of a simple linear algebraic system of equations.

In this paper, Joint Score Function (JSF) and Marginal Score Function (MSF) are first defined. It is then pointed out that their difference (SFO) can be treated as the stochastic gradient of mutual information and. hence, can be used in minimizing the mutual information with gradientbased methods. An estimator for SFO is then presented. based on nonlinear regression by means of spline smoothing. It is shown that SFO can be used to obtain a new non parametric algorithm for source separation in Post Non-Linear(PNL) mixtures. The method is very general and can be extended to convolutive mixtures. which is currently being studied.

In this paper, an intelligent CAD tool for the design of analog circuits is presented. First, the system uses fuzzy rule based reasoning to select alternative architectures for the circuits at each level of hierarchy and, then, deduces the device size using case-based reasoning, whose objective is to determine the plausible building blocks subspecifications on the basis of past design experience. In cases where some of the specifications have not been met, a qualitative reasoning-based approach is used to make final corrections. The above approach is demonstrated through the example of a CMOS Op-Amp.

In this paper, the robust stability analysis and stability bound improvement of perturbed parameter (e) in singularly perturbed systems are considered, using linear fractional transformations and structured singular values (m) approach. In this direction, by introducing the parametric and dynamic uncertainty in the singularly perturbed systems, the mentioned system is rewritten as a standard p,-interconnection framework by using linear fractional transformations. Also, a set of new stability conditions for the system is derived in the frequency domain. The exact solution of g-bound is characterized. It is shown that the g-bound obtained through this approach is larger compared to that of [1], in which only parametric uncertainty is considered. Simulation results show the efficiency of the approach.