Slobodan Ðukanovic, Mauro D’Arco, Leopoldo Angrisani
Instantaneous Frequency Estimation Of Multicomponent Signals Characterized By Interfering Linear Trajectories

The paper deals with signals that include nonstationary components. For these signals, the estimation of the instantaneous frequency of each component is especially difficult in time intervals where its trajectory interferes with another one, for example by crossing it. The attention is mainly paid to linear trajectories, seeing that any trajectory can be generally approximated as linear in the nearby of the crossing point. The authors propose a method based on the use of a two dimensional subspace of the chirplet transform, known as bowtie subspace, that allows accurate estimates of the instantaneous frequency even in the presence of crossing trajectories. The use of a restricted subspace allows a reduced computational burden and less time consumption with respect to several approaches based on advanced multiparametric transforms, typically exploited when multicomponent nonstationary signals are analyzed. The performance granted by the proposed method is assessed through a number of tests performed on simulated, emulated, and real signals.

Valeriy Martynyuk, Denis Makaryshkin, Juliy Boyko
Frequency Domain Analysis For Electrochemical Supercapacitors

The paper deals with the problem of creating a non-linear model of the electrochemical supercapacitors and measuring their electrical parameters in wide frequency range. Formalization of the model as a non-linear equivalent circuit enables to explain non-linear behavior of electrochemical supercapacitors such as dependence of their capacity and electrical series resistance (ESR) on the different voltage levels and frequencies. In order to determine the non-linear supercapacitor equivalent circuit parameters we carry out the impedance measurements and then perform fitting of obtained measurement results by means of impedance function, which describes the electrochemical supercapacitor non-linear properties with greater accuracy.

Boris Šnajder, Darko Vasic, Vedran Bilas
A Digital Signal Processing Technique for Pulsed Electromagnetic Inspection of Steel Tubes

A pulsed electromagnetic (PEM) technique is an effective method of nondestructive evaluation of steel tube inner diameter and wall thickness. Using time-spatial separation of direct-zone and remote-zone signals, it is possible to determine wall thickness and inner diameter with one receiving coil measuring voltage peak and zero-crossing time simultaneously. However, for fixed coil geometry the zero-crossing time can be recognized only for a limited range of wall thickness. In order to increase wall thickness range and shorten the probe, additional processing of the pulsed-eddy current signal must be applied. In this paper, we present a technique based on integration of the pick-up voltage and measurement of threshold-crossing time. The technique can be easily implemented in the existing PEM tool. We have verified the technique using numerically and experimentally obtained results for a range of tubes specified with API 5CT standard.

Matei Mihaela Florentina, Mihai Petruta, Claudia Popescu, Mihai Octavian Popescu
Testing and shaping for the performance of a system, used for solar energy production

The solar energy, like a permanent source of energy, is the optimal solution, and also a very important solution, for our investigations, because this source could solve a part of the problems related with inaccesibility and the high costs. A long series of projects are developing now, taking into consideration the importance of this problem. At University Politehnica of Bucharest, like a result of the PV Enlargement project exists a photovoltaic station. The actual system is analysed and a lot of conclusions are made, concerning the efficiency (as a result of these investigations we obtain an nergy excess of 73%) and the improving variances.

C. A. Nieto de Castro
THERMAL CONDUCTIVITY OF MOLTEN MATERIALS. IS EXPERIMENT NECESSARY?

The knowledge on the thermal conductivity of molten materials (salts, metals, semiconductors, polymers) is very scarce, both from the experimental and theoretical points of view. Knowing of the difficulty in obtaining accurate experimental data for most liquids, the task is uncouthly more difficult when the measurements are to be performed at medium and high temperatures with materials that are corrosive, easily reacting and good heat transfer media. Convective and radiative heat transfer effects affect in particular thermal conductivity measurements at high temperatures. These facts also make difficult theoretical calculations using molecular/ionic theories and drastic approximations, both from the phenomenological side, to the force field between particles, restrict the validity of the results obtained. Computer simulations are a possible alternative to overcome these problems and its development in recent years is noteworthy, induced by the improvements in theory, algorithms and computer hardware. Their applications to the study of molten materials have been very limited and with results of questionable validity. However, it was recently possible to apply equilibrium molecular dynamics simulations to the calculation molten salts thermal conductivity with a reasonable success. In this lecture a short review of the field will be presented, with especially emphasis in the actual situation, challenges faced and foreseen solutions, including microgravity experiments.