Obtención de imágenes de la distribución de impedancia eléctrica del subsuelo. Aplicación a la detección de objetos locales

El objetivo de este trabajo es la obtención de imágenes de la distribución de impedancia eléctrica en el subsuelo a partir de medidas realizadas desde la superficie. El estudio se centra en la localización de objetos locales a una profundidad de pocos metros y cuyas dimensiones sean incluso del orden de decenas de centímetros (tuberías, restos arqueológicos, etc.). El trabajo se ha dividido en tres partes principales: problema directo, instrumentación y problema inverso. En el problema directo se estudia el potencial originado en la superficie (anomalías) debido a la presencia de objetos esféricos y cilíndricos inmersos en un suelo homogéneo. Esto permite estimar la influencia de varios parámetros geométricos y físicos del objeto a detectar, como el tamaño, la profundidad y la resistividad. La detectabilidad de los objetos dependerá en gran parte de la anomalía producida y del nivel de ruido presente en las medidas. Se proponen varias configuraciones multielectródicas basadas en las configuraciones electródicas clásicas y se comparan las ventajas e inconvenientes de cada una. Se proponen también nuevas expresiones para determinar la profundidad y el radio de objetos esféricos y cilíndricos cuando los electrodos de inyección están cerca del objeto. Se han implementado varios sistemas de medida automáticos, dos para las medidas realizadas en el laboratorio y uno para las medidas de campo, permitiendo cualquier combinación de electrodos inyectores y detectores sobre una agrupación de 16 electrodos y 8 electrodos respectivamente. La señal inyectada es alterna y la detección es síncrona para evitar los errores provocados por el potencial de electrodo y por la interferencia a 50 Hz y las corrientes telúricas. La frecuencia máxima utilizable viene limitada por la profundidad de penetración y por el acoplamiento electromagnético entre el inyector y el detector. Uno de los sistemas implementados permite obtener la parte real e imaginaria de la impedancia medida. Se proponen nuevas técnicas de medida para evitar los errores originados por los acoplamientos capacitivos e inductivos. Para evitar los problemas de la elevada tensión de modo común la masa del detector es diferente a la del generador. En el problema inverso se han comparado diferentes algoritmos para la obtención de imágenes "reales" 2D y 3D de la distribución de la resistividad eléctrica del subsuelo y se han determinado los más adecuados para la detección de objetos locales. Esta comparación se realiza utilizando datos sintéticos (provenientes de las soluciones analíticas del problema directo). Se ha obtenido que el método de Marquardt-Levenberg es superior a los demás para la detección de objetos locales. Además, el algoritmo utilizado es de un solo paso, lo que reduce su complejidad y el tiempo de cálculo. La detección de objetos locales se valida experimentalmente con medidas de laboratorio sobre un modelo analógico compuesto por una cubeta de plástico (40 cm ´ 35 cm ´ 20 cm) llena de agua en la que se introducen objetos esféricos y cilíndricos. Los objetos se localizan correctamente hasta una profundidad igual a cuatro veces su radio. Las medidas de campo realizadas en un terreno agrícola situado en Santa Eulalia de Ronçana (Barcelona), han permitido detectar una tubería de plástico de 8 cm de radio, 1,06 m de largo, previamente enterrada a una profundidad de 24 cm. Tanto en las medidas de laboratorio como en las medidas de campo se ha de utilizar una medida de referencia para eliminar los efectos no debidos al objeto que se quiere detectar (inexactitud en la posición de electrodos, presencia de capas y paredes laterales, etc.).The aim of this work is to obtain 2-D and 3-D subsurface electrical impedance images to locate small local objects (tens of centimeter). An analytical solution for a spherical object embedded in a homogeneous medium yields synthetic data. A 16-electrode surface linear array yields 104 independent measurements. Shifting the electrode array parallel to it-self permits us to obtain 3-D images. Because the sensitivity matrix is ill-conditioned, so we have to use regularization techniques. We use a single-step Marquardt-Levenberg method where a regularization parameter is calculated automatically by the L-curve method. Experimental measurements involving spherical and cylindrical objects immersed in a water tank validate the proposed algorithm. We have implemented an automated electrical impedance-measuring system in order to speed the measurement and interpretation processes. The PROGEO system permits to choose any electrode pair for injection and detection from a 16-electrode array and obtains the real and imaginary parts of the soil impedance. We use dipole-dipole and Schlumberger-based electrode arrays. Images deteriorate for deeper targets. Taking a reference measurement before immersing the objects into the water tank reduces errors in the reconstructed image because of the uncertainty in electrode positioning and the finite dimensions of the tank. A field survey carried out on a farm field has permitted us to detect a 1 m plastic tube 8 cm in radius, buried at a 24 cm depth. A custom-built detector reduces the effect of capacitive and inductive coupling between the injecting and detecting wires by synchronously sampling square waveforms. The reconstruction algorithm is fast because it is non-iterative and takes only a few seconds to image the subsurface on a portable personal compute

Improving the efficiency of PV low-power processing circuits by selecting an optimal inductor current of the DC/DC converter

In the context of autonomous sensors powered by small-size photovoltaic (PV) panels, this work analyses how the efficiency of DC/DC-converter-based power processing circuits can be improved by an appropriate selection of the inductor current that transfers the energy from the PV panel to a storage unit. Each component of power losses (fixed, conduction and switching losses) involved in the DC/DC converter specifically depends on the average inductor current so that there is an optimal value of this current that causes minimal losses and, hence, maximum efficiency. Such an idea has been tested experimentally using two commercial DC/DC converters whose average inductor current is adjustable. Experimental results show that the efficiency can be improved up to 12% by selecting an optimal value of that current, which is around 300-350 mA for such DC/DC converters.Peer ReviewedPostprint (published version

Electrònica en les telecomunicacions: pràctiques de laboratori

2016/201

Optimal inductor current in boost DC/DC converters operating in burst mode under light-load conditions

This letter analyzes how the efficiency of boost dc/dc converters operating in burst mode under light-load conditions can be improved by an appropriate selection of the inductor current that transfers energy from the input to the output. A theoretical analysis evaluates the main power losses (fixed, conduction, and switching losses) involved in such converters, and how do they depend on the inductor current. This analysis shows that there is an optimal value of this current that causes minimum losses and, hence, maximum efficiency. These theoretical predictions are then compared with experimental data resulting from a commercial boost dc/dc converter (TPS61252), whose average inductor current is adjustable. Experimental results show that the use of the optimal inductor current, which was around 340 mA for an output voltage of 5 V, provides an efficiency increase of 7%.Peer ReviewedPostprint (author's final draft

Noniterative algorithms for electrical resistivity imaging applied to subsurface local anomalies

In this paper, we compare five noniterative (one-step) algorithms for two-dimensional electrical resistivity imaging applied to the location of subsurface local anomalies. Here, we analyze the performance of two backprojection algorithms and three algorithms based on a least-squares criterion. These five algorithms can also be adapted for process and medical tomography. Algorithm performance is first assessed from synthetic data derived from an analytical solution. We show that least-squares-based algorithms outperform backprojection algorithms in all situations considered. One of the least-squares algorithms was further validated with experimental measurements involving spherical objects immersed into a water tank. Data were obtained using a 16-electrode linear array and a computer-controlled data-acquisition system. A reference measurement before immersing the objects into the water tank reduced errors in the reconstructed image attributable to the uncertain electrode position and the finite dimensions of the tank. Images deteriorated for deeper objects, but neglecting measurements with the smallest signal-to-noise ratio improved the results.Peer Reviewe

Optimal Inductor Current in Boost DC/DC Converters Regulating the Input Voltage Applied to Low-Power Photovoltaic Modules

In energy-harvesting applications, inductor-based switching dc/dc converters are usually employed to regulate the operating voltage of the energy transducer and to transfer the harvested energy to a storage unit. In such a context, this paper analyses the optimal inductor current of the converter that leads to maximum power efficiency. This is evaluated assuming a low-power photovoltaic (PV) module connected to a boost dc/dc converter operating in burst mode so as to reduce the switching losses. The theoretical analysis and the experimental results reported herein prove that this optimal inductor current does not depend on the power generated by the PV module provided that the control circuit is powered from the output, but it does on the output voltage level of the storage unit. Experimental tests with a commercial boost dc/dc converter show that the use of this optimal inductor current provides up to 10% increase in efficiency.Postprint (author's final draft

Buck converter for low-power PV modules: A Comparative Study

Autonomous sensors that harvest energy from the environment usually employ a dc/dc converter to regulate the operating voltage of the energy transducer around its maximum power point (MPP). In this context, this work evaluates the efficiency of a buck converter when regulating the operating point of two low-power photovoltaic (PV) modules subjected to different irradiance levels. The buck converter operates in burst mode (BM) and is able to transfer the energy from the PV module to a storage unit through an optimal value of the inductor current. Experimental results show that an irradiance increase can cause either an increase or a decrease of the converter efficiency. This is because the higher the irradiance, the higher both the MPP voltage and current of the PV module, which involve opposite effects in terms of the converter efficiency.Peer ReviewedPostprint (published version

Experimental study on the power consumption of timers embedded into microcontrollers

An experimental study on the current consumption of timers embedded into microcontrollers is presented in this work. The study is carried out in two commercial microcontrollers (MSP430FR5969 and ATtiny2313) and the experimental results are co mpared with the scarce data provided in their datasheets. The sensitivity (expressed in ¿A/MHz) reported in the datasheet seems to be only applicable if the frequency divider of the timer equals one. Otherwise, such a sensitivity is lower but there is a significant offset component, leading to a higher power consumption at the same operating frequency. The knowledge extracted from this work is expected to provide guidelines to better use embedded timers in low-power sensor applicationsPostprint (updated version

Demodulating AM square signals via a digital timer for sensor applications

This paper evaluates theoretically and experimentally the performance of a timer-based demodulator applied to low-frequency amplitude-modulated (AM) square signals coming from sensor circuits. The demodulator extracts the amplitude of the AM square signal by measuring the period of a reference triangular signal that is altered by the AM signal itself, as already suggested in a previous paper but for AM sinusoidal signals.Postprint (published version

Power gain from energy harvesting sources at high MPPT sampling rates

Energy harvesting (EH) sources require the tracking of their maximum power point (MPP) to ensure that maximum energy is captured. This tracking process, performed by an MPP tracker (MPPT), is performed by periodically measuring the EH transducer’s output at a given sampling rate. The harvested power as a function of the sampling parameters has been analyzed in a few works, but the power gain achieved with respect to the case of a much slower sampling rate than the EH source’s frequency has not been assessed so far. In this work, simple expressions are obtained that predict this gain assuming a Thévenin equivalent for the EH transducer. It is shown that the power gain depends on the relationship between the square of AC to DC open circuit voltage of the EH transducer. On the other hand, it is proven that harvested power increases, using a suitable constant signal for the MPP voltage instead of tracking the MPP at a low sampling rate. Experimental results confirmed the theoretical predictions. First, a function generator with a series resistor of 1 k¿ was used, emulating a generic Thévenin equivalent EH. Three waveform types were used (sinus, square, and triangular) with a DC voltage of 2.5 V and AC rms voltage of 0.83 V. A commercial MPPT with a fixed sampling rate of 3 Hz was used and the frequency of the waveforms was changed from 50 mHz to 50 Hz, thus effectively emulating different sampling rates. Experimental power gains of 11.1%, 20.7%, and 7.43% were, respectively, achieved for the sinus, square, and triangular waves, mainly agreeing with the theoretical predicted ones. Then, experimental tests were carried out with a wave energy converter (WEC) embedded into a drifter and attached to a linear shaker, with a sinus excitation frequency of 2 Hz and peak-to-peak amplitude of 0.4 g, in order to emulate the drifter’s movement under a sea environment. The WEC provided a sinus-like waveform. In this case, another commercial MPPT with a sampling period of 16 s was used for generating a slow sampling rate, whereas a custom MPPT with a sampling rate of 60 Hz was used for generating a high sampling rate. A power gain around 20% was achieved in this case, also agreeing with the predicted gain.This work was supported by the European Innovation Council under the EU Horizon 287 Europe program ‐ Grant agreement No 101071179, project SUSTAIN (Smart Building Sensitive to 288 Daily Sentiment)”. “The second author was supported by the European Union – NextGenerationEU 289 and the Ministerio de Universidades – Plan de Recuperación, Transformación y Resiliencia under a 290 Margarita Salas post‐doctoral research fellowship (ref. 2022UPC‐MSC‐94068).Peer ReviewedPostprint (author's final draft