Year:2004 | Volume:38 | Issue:3 (85)|Papers Count:9

A novel concept for the diagnosis of a pure target gas (TG) in air is presented. The method employs a resistive gas sensor with a mm-thick highly porous electro ceramic gas-sensitive body. The diagnosis is based on the fact that the diffusion time of the TG through the porous body would depend on its molecular properties. The transient response of the device was analyzed based on a diffusion-reaction equation. The solutions of the equation were defined by three parameters, which were shown to be of diagnostic merits. Such a device was fabricated by partial sintering of a ZnO body formed by low-pressure pressing. The transient responses of the device to methanol, ethanol, poropanol and butanol were recorded. The analytical solutions were fitted to the experimental data, and the diagnostic parameters were resulted as the fitting parameters. Comparison of the parameters obtained with those available from the calibration experiments afforded TG identification.

Wevelet are able to approximate functions with different precisions, which makes them appropriate for modeling nonlinear systems. There are number of algorithms such as algorithms based on neural nets (wave net) and L2 learning algorithm, reported in the literatures for determination of wavelet parameters. In this paper several algorithms based on learning automata are reported. The proposed methods use the searching ability of learning automata to find the wavelet parameters. These methods unlike L2 learning algorithms, which requires high computations (inverting a large matrix) is very fast, and unlike the algorithms based on neural nets have a dynamic structure, that is, the number of base functions need not be known in advance and are determined during the parameters estimation process. The methods are tested on several problems and good results are obtained.

Radiation from a slotted planar waveguide array is investigated in this work. The effects of mutual coupling and wall thickness are taken into account in this analysis. The problem is formulated using integral equations and equivalence principle and is then solved using method of moments (MoM). To do this, we have used entire domain sinusoidal basis functions along with Galerkins method. For these basis functions, few terms are sufficient for convergence of the response. The moment method requires the admittance matrices in which various dyadic Greens functions are involved. The singularity in the integral of one of the admittance matrices corresponding to the half space is removed. Finally, the analysis results of the linear and planar arrays are compared to the existing measurement data.

In this paper, the magnetic coupling effects of phases windings in a three-phase 6/4 switched reluctance motor (SRM) is studied. Since the inductance models have no capability of flux-linkage models in dealing with the nonlinear magnetically/special behavior of SRM, then a nonlinear analytical model of flux-linkage of SRM in two-phase excitation regime is developed in which the magnetic coupling effects of phases windings are considered. Flux-linkage information used in development of model are obtained from two-dimensional finite element (FE) analysis. This information show that the magnetic coupling effects of phases windings are important specially for high values of phases currents. Among the advantages of the extended flux-linkage model is its similarity with the performance of a feed forward neural network that guarantee in advance the high ability of model in good agreement with the actual flux-linkage characteristics of motor. Simulation results confirm this claim.

Determination of transformer leakage reactance using magnetic cores has long been an area of interest to engineers involved in the design of power and distribution transformers. This is required for predicting the performance of transformers before actual assembly of the transformers. In this paper a closed form solution technique applicable to the leakage reactance calculations for transformers is presented. An emphasis is on the development of a simple method to calculate the leakage reactance of the distribution transformers and smaller transformers. Energy technique procedure for computing the leakage reactance's in distribution transformers is presented. This method is very efficient compared with the use of flux element and image technique and is also remarkably accurate. Examples of calculated leakage inductances and the short circuit impedance are given for illustration. For validation, the results are compared with the results obtained using test. This paper presents a novel technique for calculation of the leakage inductance in different parts of the transformer using the electromagnetic stored energy.

In cellular networks it is crucial to be able to use the available radio spectrum as efficiently as possible while providing a certain level of Quality of Service (QoS) for the users. Emergence of miscellaneous services has dramatically increased the complexity of this problem by creating a heterogeneous offered traffic environment. In this paper an efficient resource allocation scheme has been proposed for cellular networks with multimedia offered traffic, which combines classical intera-cell resource borrowing concept with a novel inter-cell resource-sharing scheme (unilateral cross-borrowing of video resources by audio call requests) between diverse classes of traffic. By assuming the heterogeneous offered traffic to be a combination of audio and video traffic types, through extensive simulations it will be shown that HCBA-UCB is capable of significantly improving audio telegraphic performance of the system while preserving fairness in service provision, i.e. without imposing additional expenses upon video QoS performance.

ATM technology is a multiplexing and switching technology for transmission of high-speed data packets. In contrast with IP, ATM supports quality of service in a prestablished connection and guarantees delivery of packets. Therefore, ATM has been considered as backbone for IP and future networks.Several techniques, including Next Hop Resolution Protocol (NHRP), supporting IP over ATM have been proposed and developed so far. In this paper, NHRP structure is designed and performance parameters, including Delay, Loss, Efficiency are evaluated using simulation.

Increasing applications of electric motors and the environmental problems associated with the production of electric energy in Power plants, together with the rapid increase in energy cost necessitate the development of new methods for designing and constructing of energy efficient motor drives. Optimal control of motors efficiency by minimization of electrical losses is an effective means for realizing this purpose. In this paper, after presenting a dynamic model of 3-phase induction motors including copper and iron losses, the motor electrical loss is analyzed as a function of motor variables, and the conditions of its minimization are described. A new control system for finding the minimum loss point and as a result the on-line optimization of motor efficiency at variable speed and load is then proposed. In this system, a motor control variable changes continuously in search for the minimum loss point. This new method, in comparison with existing on-line loss minimization methods of induction motors, leads to a more energy saving and a smoother motor operation. The comparison of simulation results of a vector controlled induction motor under the proposed control system and a conventional control system confirms the superiority of the proposed system.

Demands for low voltage power supply in mixed-signal designs are growing due to portable equipment capable of operating with minimum number of battery cells to reduce volume and weight with longer operating periods and also voltage limitations resulted from smaller feature sizes of modern IC technologies. Also with the recent developments in both wired and wireless communications, there is a need to design analog-to-digital converters (ADCs) at MHz speeds with high linearity.Sigma-delta modulators and pipelined ADCs are the two main candidates for high-resolution and high-speed applications. Pipelined converters need some type of calibration or error correction techniques to achieve accuracies beyond 12-bits, resulting in increased complexity and power dissipation. Some sigma-delta ADCs have been reported in the MHz ranges with resolutions about 14-bits and beyond. In this paper a novel sigma-delta modulator architecture suitable for very low-voltage and high-resolution in the MHz ranges is proposed. The main specifications of the proposed architecture are the reduced analog circuit requirements, large out-of-band gain in the noise transfer function (NTF) without any stability concerns to achieve high signal-to-noise ratio (SNR) with a low over sampling ratio (OSR), and unity-gain signal transfer function (STF) to reduce the harmonic distortions resulted from the analog circuit imperfections.The implemented prototype of the proposed modulator architecture with HSPICE gives SNDR and dynamic range of 90-dB and 92.5-dB, respectively, including the circuit noise with a single power supply voltage of 1.2-V. The circuit requirements for implementation of the modulator were simulated in a 0.13um CMOS technology. A new class A/AB operational Tran conductance amplifier with cascade compensation was proposed to implement the integrators, IIR filter, and gain-stage. The signal bandwidth and sampling frequency are 1.25 MHz and 40 MHz, respectively. The total power dissipation was about 40-mW.