The admittance method is an important tool for the indirect determination of cardiac hemodynamics in animal research and has clinical potential as a hemodynamic monitor for implantable cardioverter-defibrillators (ICDs). Animal studies use a conductance catheter to determine left-ventricular (LV) volume, but ICDs must make use of existing pacing leads to perform an admittance measurement. This work aims to further the use of the admittance method by extending its use to multi-segment conductance catheters, determining parasitic circuit element's effects on electric property measurements, using biventricular pacing leads to determine cardiac hemodynamics, and analysis of spatial sensitivity using finite element models in various configurations. Experimental results show the admittance method can be used to determine LV stroke volume with biventricular pacing leads. Modeling results show removal of the muscle component focuses the measurement's spatial sensitivity towards the left-ventricular blood pool. / text

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Ce travail a pour objectif la détermination des propriétés fonctionnelles des matériaux piézoélectriques : les coefficients de couplage, les constantes élastiques, piézoélectriques et diélectriques, de même que les pertes électriques et mécaniques. De nos jours, les techniques conventionnelles d’identification de ces paramètres utilisent plusieurs échantillons. Dernièrement le laboratoire a développé (Delaunay et al.) une méthode ultrasonore de caractérisation de matériaux piézoélectriques permettant de déterminer ces propriétés à partir d’un seul échantillon. Cette méthode, basée sur la spectroscopie de résonance ultrasonore, examine les modes de vibrations d’un cube piézoélectrique et extrait des résonances mécaniques mesurées par interférométrie Laser les propriétés électromécaniques de l’échantillon. Dans ce travail, cette méthode a été modifiée afin d’obtenir les propriétés électromécaniques des matériaux à partir d’une seule mesure d’impédance électrique. Dans un premier temps, le problème direct est résolu ; les fréquences propres et les modes propres d’un cube sont modélisés par une méthode variationnelle ; les champs de déplacement et l’admittance électrique sont calculés en fonction de la fréquence. La géométrie étant fixée, forme cubique, l’admittance dépend seulement des propriétés du matériau et des conditions de métallisation de l’échantillon. La méthode est validée à travers la caractérisation d’un cube de PMN-34,5PT dont les propriétés sont connues. Les mesures électriques de l’impédance de l’élément sont comparées au spectre d’admittance prédit par la théorie. Les vitesses de vibration du matériau sont également mesurées et comparées aux résultats donnés par les modèles existants. La résolution du problème inverse, permet de déterminer les propriétés d’un matériau inconnu, à travers la convergence de courbe d’admittance théorique vers celle expérimentale. Les propriétés du PZ-21 sont extraites grâce à cette procédure. Une discussion sur ces valeurs et une comparaison avec celles de la littérature permet de valider les résultats obtenus. / This work deals with the determination of electromechanical properties of piezoelectric materials: coupling coefficients, elastic, dielectric and piezoelectric constants, electrical and mechanical losses. Until now, several samples are needed in conventional techniques to perform the complete identification of the material properties. Recently, Delaunay et al. proposed an ultrasonic protocol allowing the determination of these characteristics from only one sample. This method, referred to as Resonant Ultrasound Spectroscopy, is based on the comparison of the mode shapes and frequencies of modeled vibration modes of piezoelectric parallelepipeds with experimental data measured by Laser interferometry. It is here modified to obtain the electromechanical properties from electrical impedance measurements only. The direct problem is first solved: the resonance modes of a two face metalized piezoelectric cube are modeled and both mechanical displacements and electrical impedance are calculated as functions of the frequency. The method is first applied for validation on a PMN-34.5PT material with known properties. Electrical impedance and mechanical velocity measurements are performed and their agreement to the theoretical predictions is discussed. In order to determine the properties of unknown materials, the inverse problem is solved by fitting the theoretical impedance curves to experimental ones. This procedure is then applied to the identification of the properties of PZ-21. The results are discussed and compared to data from the literature.

Ce travail de recherche s'intéresse à l'utilisation d'actionneur piézoélectrique à onde progressive dans le cadre d'un contexte de retour d'effort comme un choix alternatif par rapport à l'actionneur électromagnétique utilisé très souvent dans le domaine haptique. Ainsi, à partir d'une étude sur les caractéristiques dynamiques et mécaniques du moteur USR30, on a proposé un modèle global en GIC qui prend en compte les phénomènes non linéaires intrinsèques dans le moteur. En inversant ce modèle de GIC, on arrive à valider deux types de commande à retour d'effort, enfin, en présentant les résultats expérimentaux à base de la plateforme de 1ddl de digitracker, la réalisation d'un environnement virtuel qui comprend un ressort et un mur virtuels devient possible. / This research work is interested in the use of travelling wave ultrasonic motor within the framework of a context of force feedback as an alternative choice with regard to the electromagnetic motor which is often used in the haptic domain. So, from a study on the dynamic and mechanical characteristics of the motor USR30, we proposed a global model in GIC who takes into account the intrinsic not linear phenomena in the motor. By inverting this model of GIC, we manage to validate two types of control in force feedback; finally, by presenting the experimental results based of the platform of 1ddl of digitracker, the realization of a virtual environment which includes a virtual spring and a virtual wall becomes possible.

Electrical and Computer Engineering / A high performance, wireless bio-impedance measurement system has been designed for the purpose of monitoring essential electrical properties of the heart during cardiac ablation. The system is broken into three parts: a spring-loaded device to house a tetrapolar surface probe and sensors, a wireless bio-impedance measurement system, and a desktop base station for graphical data display and acquisition. The system is specifically designed for a tetrapolar-electrode configuration where the two outer electrodes served as a current source operating at 20 kHz with an amplitude of 100 µArms and the two inner electrodes served as voltage sensing electrodes. In addition, the system also has a dedicated channel for current sense. The system is designed to be modular and reconfigurable for different measurement needs. Epochs of both discrete voltage and current samples generated by the voltage-controlled current source are processed using a digital signal processing algorithms to generate admittance measurements. In addition to the admittance’s magnitude and phase, pressure, electrocardiogram (EKG), and temperature (two channels) data are also acquired. The measurements are then wirelessly transmitted from the bio-impedance measurement system to a base station where data are processed and viewed graphically. The final system updates the admittance, pressure, EKG, and two temperature channels at 320 Hz, consumes less than 3 W, and has percent of measurement errors of 7 % and 2 % for capacitive and resistive measurements in the range of 100 pF to 10000 pF and 300 Ω to 1600 Ω, respectively. Instrument design, calibration, verification, and modeling are at the heart of this thesis. In the future, the instrument will be deployed for various bio-impedance measurements that require a high degree of linearity, precision, and a wide input range. / text

Traps are ubiquitously present in semiconductors. Their presence results in ineffective charge transport and thus limited the device performance. For organic semiconductors, traps can present intrinsically via structural disorder or extrinsically during synthesis or device fabrication. A thorough understanding of traps is important to optimize the device performance and material design. This thesis employs two trap measurement techniques, photothermal deflection spectroscopy (PDS) and thermal admittance spectroscopy (TAS), to investigate the trap density in the materials. The subgap optical absorptions of several high performance bulk-heterojunction (BHJ) systems for organic solar cells have been studied by PDS. The charge transfer (CT) states are, in particular, looked into detail. CT states are intermediate bound electron-hole pairs at the donor/acceptor (D/A) interface of an organic solar cell. The dynamics and energetics of CT states are crucial to free charge generation and recombination processes. With the help of PDS and external quantum efficiency (EQE) measurements, the CT states the delocalized CT states (hot) from the localized CT states (cold) are observed and differentiated directly. It is discovered that the localized CT states are more pronounced when the acceptor concentration reaches its percolation limit. As the acceptor concentration reaches its optimized composition, the intensity of these CT states is significantly reduced due to the reduced recombination. Using the CT energies measured from PDS, the open-circuit voltage losses from the BHJs are determined. Besides PDS, thermal admittance spectroscopy (TAS) is employed as an alternative method to measure the trap densities. TAS measures the frequency dependent capacitance response of a semiconductor under a small ac signal excitation. This technique is useful to measure the trap depth and trap density of a semiconductor. The defect profiles in two classes of materials are investigated, they are perovskite compounds and an organic hole transporter with an intentional dopant. The trap density are determined by TAS is compared with that obtained by PDS.

Slot dipole antennas fed by coplanar waveguide (CPW) on substrates consisting of a single dielectric layer exhibit various attractive qualities, including significantly wider impedance bandwidth than comparable microstrip patch antennas. For applications that call for unidirectional radiation, such as antennas on airframes, a conducting back plane is needed. A CPW on a conductor-backed single-dielectric-layer substrate will always experience power leakage into the TEM parallel-plate mode. On the other hand, it is possible to design CPW lines on conductor-backed two-layer substrates that are free from leakage into the substrate. However, once the CPW is used as feed line to a slot dipole, power leakage into the TM0 substrate mode caused by the transition between the CPW and the radiating slot, and by the radiating slot itself, may still severely compromise radiation efficiency. This study has two main contributions to offer. First, a paucity of work on CPW-fed slot antennas on conductor-backed two-layer substrates is alleviated by providing a fuller characterization of single-slot behaviour on two-layer parallel-plate substrates than is currently available, and by systematically investigating a practically feasible minimum antenna configuration, namely broadside twin slots, that is not debilitated by the problem of substrate mode leakage. Results obtained with the moment-method-based electromagnetic simulator IE3D that emphasize the trade-off between radiation efficiency and impedance bandwidth are presented; they can be used for design purposes. For instance, with respect to single slots on a substrate with an electrically thin top dielectric layer and an air bottom layer, it is shown that radiation efficiency increases and bandwidth decreases as height of the bottom substrate layer increases. For broadside twin slots, it is demonstrated that spacing close to half a wavelength of the two-layer parallel-plate TM0 mode apart can yield a large improvement in radiation efficiency over that of a single slot (a reduction in bandwidth however occurs). The second main contribution is the development of an approach for finding the mutual admittance Y12 between CPW-fed slots on conductor-backed two-layer substrates that can be more readily incorporated in an iterative array design procedure than a moment-method-based technique, yet is of comparable accuracy; it is built on a standard reciprocity-based expression. As an initial step, the mutual admittance between CPW-fed slots on a conductor-backed two-layer substrate with an air bottom layer is characterized using IE3D. This involves presenting curves for Y12 between twin slots against slot separation d along standard paths for slot half-lengths in the vicinities of the first and second resonant half-lengths of the corresponding isolated slots (such data might be used towards a first-order array design), and a study of the effect of back plane distance (i.e., bottom layer height) on mutual coupling. The bulk of the thesis however is devoted to the above reciprocity-expression approach. Simplifying assumptions are outlined that make it possible to determine Y12 against d by performing a once-only moment-method analysis of each slot in isolation, and then calculating external and internal reaction integrals at each value of d. This is significantly more economical than carrying out a full moment-method analysis of the whole twin-slot structure at every instance of d. Evaluation of the internal reaction integral requires the appropriate component of the spatial-domain Green’s function for the substrate, which is derived in a form containing Sommerfeld-type integrals; treatment of singularities is discussed. The reciprocity-expression approach is verified by comparing Y12 against d curves for twin slots and non-identical slot pairs on a variety of conductor-backed two-layer substrates to IE3D simulations. A procedure that involves judicious selection of reference planes is introduced by which agreement between the methods for the special case of twin slots with the same half-length as the corresponding isolated second-resonant slot can be even further improved. A measurement is provided that validate theoretical calculations. / Thesis (PhD)--University of Pretoria, 2008. / Electrical, Electronic and Computer Engineering / unrestricted

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In recent high-speed digital circuits, the simultaneous switching noise (SSN) or ground bounce noise (GBN) is induced due to the transient currents flowing between power and ground planes during the state transitions of the logic gates. In order to¡@analyze the effect of GBN on power delivery systems effectively and accurately, the impedance of power/ground is an important index to evaluate power delivery systems. In the operating frequency bandwidth, the power impedance must be less than the target impedance. The typical way to suppress the SSN is adding decoupling capacitors to create a low impedance path between power and ground planes. By using the admittance matrix method, we can evaluate the effect of decoupling capacitors mounted on PCB fast and accurately reducing the time needed from the empirical or try-and-error design cycle. In order to reduce the cost of decoupling capacitors, the genetic algorithm is employed to optimize the placement of decoupling capacitors to suppress the GBN. The decoupling capacitor are not effective in the GHz frequency range due to their inherent lead inductance. The electromagnetic bandgap(EBG) structure can produce a stopband to prevent the noise from disperseing at higher frequency. Combining decoupling capacitors with EBG structure to find the optimum placement for suppression of the SSN by using the genetic algorithm.

The design, fabrication, and in-vivo testing of an implantable device to measure the pressure-volume (PV) relationship in the hearts of conscious, untethered rats is presented. Volume is measured using a tetrapolar catheter positioned in the left-ventricle which emits a 20kHz current field across the LV blood pool and parallel heart tissue and measures the resulting voltage. The admittance method is used to instantaneously remove the contribution of the parallel heart muscle and Wei’s non-linear blood conductance-to-volume equation is used to calculate volume. Pressure is measured with a strain gauge sensor at the tip of the catheter. The implant was designed to be small, light, and low-power. An average implant occupies 5 cm3, weighs 8g, and on a single charge collects data for 2 months taking 43 samples per day. Collected data is transmitted wirelessly via RF to a base station where it is recorded. The functionality of the implant and measurement system was verified in six rat experiments. In all experiments, ambulatory PV loops were measured on implantation day. Viable pressure data was recorded for 11 days in one rat; in another rat viable admittance data was collected for 10 days. Changing catheter position and non-constant blood resistivity are considered as sources of error in the volume measurement. Pressure drift due to changing atmospheric pressure is considered as a source of error in the pressure measurement. Lastly, alternative uses for the implant and directions for future improvement are considered. / text