Sudarshan R. Nelatury

Dynamic characterization of small lead-acid cells

Abstract Three sizes of small valve regulated (VRLA) commercially available lead-acid cells were investigated and characterized for their dynamic properties by ac impedance spectroscopy and other electrochemical techniques. All cells were of the limited electrolyte type and no additional electrolyte was introduced during the studies. The data indicates a very significant increase in cell impedance at lower states of charge, as expected. In charging studies close to the fully charged state, some unexpected impedance data were observed. Complex impedance plots indicate a passive film formation, probably associated with the recombination surface film. The investigations included cells in various states of charge as well as cycling history including positional orientation studies. Equivalent circuits were derived from ac impedance spectroscopy and the parameters studied as a function of the cell’s state-of-charge. Furthermore, the voltage response of the cells was theoretically generated from the ac impedance spectroscopy using Fourier transform analysis and found to be similar to the measured cell responses.

Increasing the speed of convergence of the constant modulus algorithm for blind channel equalization

The constant modulus algorithm (CMA) is an excellent technique for blind channel equalization. A signed error version of CMA (SE-CMA) and dithered signed error version (DSE-CMA) have been proposed which afford overall computational efficiency. We propose three different error functions for faster convergence. This would be essential for communication systems, which cannot afford a high startup delay or for systems, where the channels impulse response is rapidly fluctuating. One of these algorithms relies on the idea of a variable step size, which increases the rate of convergence.

Propagation of surface waves at the planar interface of a columnar thin film and an isotropic substrate

Surface-wave propagation at the planar interface of a columnar thin film (CTF) -- a biaxial dielectric medium -- and an isotropic dielectric substrate may occur over a narrow range of propagation directions, range being dependent on (i) the bulk matgerial that is evaporated to deposit the CTF, (ii) the vapor incidence angle used for the deposition, and (iii) the refractive index of the substrate material.

A maximum power point tracking algorithm for photovoltaic applications

The voltage and current characteristic of a photovoltaic (PV) cell is highly nonlinear and operating a PV cell for maximum power transfer has been a challenge for a long time. Several techniques have been proposed to estimate and track the maximum power point (MPP) in order to improve the overall efficiency of a PV panel. A strategic use of the mean value theorem permits obtaining an analytical expression for a point that lies in a close neighborhood of the true MPP. But hitherto, an exact solution in closed form for the MPP is not published. This problem can be formulated analytically as a constrained optimization, which can be solved using the Lagrange method. This method results in a system of simultaneous nonlinear equations. Solving them directly is quite difficult. However, we can employ a recursive algorithm to yield a reasonably good solution. In graphical terms, suppose the voltage current characteristic and the constant power contours are plotted on the same voltage current plane, the point of tangency between the device characteristic and the constant power contours is the sought for MPP. It is subject to change with the incident irradiation and temperature and hence the algorithm that attempts to maintain the MPP should be adaptive in nature and is supposed to have fast convergence and the least misadjustment. There are two parts in its implementation. First, one needs to estimate the MPP. The second task is to have a DC-DC converter to match the given load to the MPP thus obtained. Availability of power electronics circuits made it possible to design efficient converters. In this paper although we do not show the results from a real circuit, we use MATLAB to obtain the MPP and a buck-boost converter to match the load. Under varying conditions of load resistance and irradiance we demonstrate MPP tracking in case of a commercially available solar panel MSX-60. The power electronics circuit is simulated by PSIM software.

Surface Electromagnetic Wave at a Tilted Uniaxial Bicrystalline Interface

The propagation of a surface electromagnetic wave at the interface of identical uniaxial dielectric half-spaces with a relative twist in their orientation about an axis perpendicular to the interface is formulated as a boundary value problem, the direction of propagation being along the bisector of the relative twist angle and both optic axes being tilted to the interface. Only positive uniaxiality supports surface-wave propagation. When the optic axes in both half-spaces are tilted relative to the interface, the range of the relative twist angle which supports surface waves diminishes and eventually disappears at some maximum value of the tilt angle which depends on the degree of uniaxiality. At optical frequencies and with relatively large degree of uniaxiality, the surface waves are localized within 10s to 100s of nm of the interface.

Surface waves with simple exponential transverse decay at a biaxial bicrystalline interface

The dispersion equation for surface waves--with simple transverse exponential decay at the interface of identical biaxial crystals with a relative twist about the axis normal to the interface and propagating along a bisector of the angle between the crystallographic configurations on either side of the interface--has several solutions of which only one is physical. The selected type of surface wave is possible only for a restricted range of the twist angle, which depends on the ratio of the maximum and the minimum of the principal refractive indexes and the angle between the optic ray axes.

ELECTRICAL CONTROL OF SURFACE-WAVE PROPAGATION AT THE PLANAR INTERFACE OF A LINEAR ELECTRO-OPTIC MATERIAL AND AN ISOTROPIC DIELECTRIC MATERIAL

Abstract Surface waves can propagate on the planar interface of a linear electro-optic (EO) material and an isotropic dielectric material for restricted ranges of the orientation angles of the EO material and the refractive index of the isotropic material. These ranges can be controlled by the application of a DC electric field, and depend on both the magnitude and the direction of the DC field. Thus, surface-wave propagation can be electrically controlled by exploiting the Pockels effect.

Uniqueness of torque-speed characteristics of an induction motor

This letter states and proves a uniqueness theorem for the torque-speed characteristics (TSC) of an induction motor. Given three arbitrary points at asynchronous speed on the TSC curve, it develops a method to interpolate the entire curve and indicate the pull-out torque point. The letter gives explicit formulas for approximately estimating the equivalent circuit parameters of the machine.

A new error function for fast phase recovery of QAM signals in CMA blind equalizers

A modification to the well-known constant modulus algorithm (CMA) for blind channel equalization is proposed. The conventional CMA has rotational ambiguity and hence cannot achieve carrier phase recovery. It happens because the cost function is made up of the absolute value of the equalizer output and phase information is not used. In this paper, we slightly modify the cost function and provide a different error function that restores the phase of the carrier. Simulations studies are shown for the case of 16-QAM signal constellation.

Additional correction to the impulse invariance method for the design of IIR digital filters

Almost four decades after the impulse invariance (i.i.) method has been in vogue, a correct way of its application has been emphasized independently, but at the same time, by Jackson and Mecklenbrauker. When there is a discontinuity at t=0 in the analog impulse response, the correct way requires that one should take half of its value. In spite of this correction, due to under-sampling there persists aliasing error in the frequency domain. In this paper the effect of aliasing is viewed as a 3 dB frequency warping and a pre-warping step prior to its application is proposed to reduce it, thereby making the technique appropriate from the viewpoint of frequency domain also. Simulation examples comparing this approach with the traditional method reveal significant improvement in the frequency response. Further, according to Jackson and Mecklenbrauker and also as written in many a standard textbook available today, i.i. technique cannot be used when the degrees of the numerator and denominator polynomials of the parent analog transfer function are equal. An example under this category is the Pre-emphasis circuit or a simple high pass filter. We shall show how the technique can still be used even for a Pre-emphasis circuit. Antoniou proposed (an intelligently) modified impulse invariance technique for certain types of filters by which, aliasing error is nearly eliminated. These cases are neither admitted by the traditional i.i. method nor by that of Jackson and Mecklenbrauker at all. Antonious modified method results in a digital filter of order almost twice as great with an additional task of stabilization. The method in this paper retains the stability and order but gives satisfactory response. In the standard textbooks, the i.i. technique is explained with the help of a first-order low-pass filter and for higher orders, partial fraction expansion is used. For the situation of repeated poles, application of i.i. method calls for extreme care and special handling. This paper alternatively employs the use of state variables to make i.i. technique more elegant.