Comparison of V2IC control with voltage mode and current mode controls for high frequency (MHz) and very fast response applications

High switching frequencies (several MHz) allow the integration of low power DC/DC converters. Although, in theory, a high switching frequency would make possible to implement a conventional Voltage Mode control (VMC) or Peak Current Mode control (PCMC) with very high bandwidth, in practice, parasitic effects and robustness limits the applicability of these control techniques. This paper compares VMC and CMC techniques with the V2IC control. This control is based on two loops. The fast internal loop has information of the output capacitor current and the error voltage, providing fast dynamic response under load and voltage reference steps, while the slow external voltage loop provides accurate steady state regulation. This paper shows the fast dynamic response of the V2IC control under load and output voltage reference steps and its robustness operating with additional output capacitors added by the customer.

Accurate Analysis of Subharmonic Oscillations of V^2 and V^2Ic Controls Applied to Buck Converter

V2Ic control provides very fast dynamic performance to the Buck converter both under load steps and under voltage reference steps. However, the design of this control is complex since it is prone to subharmonic oscillations and several parameters affect the stability of the system. This paper derives and validates a very accurate modeling and stability analysis of a closed-loop V2Ic control using the Floquet theory. This allows the derivation of sensitivity analysis to design a robust converter. The proposed methodology is validated on a 5-MHz Buck converter. The work is also extended to V2 control using the same methodology, showing high accuracy and robustness. The paper also demonstrates, on the V2 control, that even a low bandwidth-linear controller can affect the stability of a ripple-based control.

Discrete element analysis for the assessment of the accuracy of load cell-based dynamic weighing systems in grape harvesters under different ground conditions

Dynamic weighing systems based on load cells are commonly used to estimate crop yields in the field. There is lack of data, however, regarding the accuracy of such weighing systems mounted on harvesting machinery, especially on that used to collect high value crops such as fruits and vegetables. Certainly, dynamic weighing systems mounted on the bins of grape harvesters are affected by the displacement of the load inside the bin when moving over terrain of changing topography. In this work, the load that would be registered in a grape harvester bin by a dynamic weighing system based on the use of a load cell was inferred by using the discrete element method (DEM). DEM is a numerical technique capable of accurately describing the behaviour of granular materials under dynamic situations and it has been proven to provide successful predictions in many different scenarios. In this work, different DEM models of a grape harvester bin were developed contemplating different influencing factors. Results obtained from these models were used to infer the output given by the load cell of a real bin. The mass detected by the load cell when the bin was inclined depended strongly on the distribution of the load within the bin, but was underestimated in all scenarios. The distribution of the load was found to be dependent on the inclination of the bin caused by the topography of the terrain, but also by the history of inclination (inclination rate, presence of static periods, etc.) since the effect of the inertia of the particles (i.e., representing the grapes) was not negligible. Some recommendations are given to try to improve the accuracy of crop load measurement in the field.

A procedure to track vibration modes under changing external factors: application to a pedestrian bridge

A low-cost vibration monitoring system has been developed and installed on an urban steel- plated stress-ribbon footbridge. The system continuously measures: the acceleration (using 18 triaxial MEMS accelerometers distributed along the structure), the ambient temperature and the wind velocity and direction. Automated output-only modal parameter estimation based on the Stochastic Subspace Identification (SSI) is carried out in order to extract the modal parameters, i.e., the natural frequencies, damping ratios and modal shapes. Thus, this paper analyzes the time evolution of the modal parameters over a whole-year data monitoring. Firstly, for similar environmental/operational factors, the uncertainties associated to the time window size used are studied and quantified. Secondly, a methodology to track the vibration modes has been established since several of them with closely-spaced natural frequencies are identified. Thirdly, the modal parameters have been correlated against external factors. It has been shown that this stress-ribbon structure is highly sensitive to temperature variation (frequency changes of more than 20%) with strongly seasonal and daily trends

Optical sensing of the fatigue damage state of CFRP under realistic aeronautical load sequences

We present an optical sensing methodology to estimate the fatigue damage state of structures made of carbon fiber reinforced polymer (CFRP), by measuring variations on the surface roughness. Variable amplitude loads (VAL), which represent realistic loads during aeronautical missions of fighter aircraft (FALSTAFF) have been applied to coupons until failure. Stiffness degradation and surface roughness variations have been measured during the life of the coupons obtaining a Pearson correlation of 0.75 between both variables. The data were compared with a previous study for Constant Amplitude Load (CAL) obtaining similar results. Conclusions suggest that the surface roughness measured in strategic zones is a useful technique for structural health monitoring of CFRP structures, and that it is independent of the type of load applied. Surface roughness can be measured in the field by optical techniques such as speckle, confocal perfilometers and interferometry, among others.

Agronomic performance of Graciano, Petit verdot and Tempranillo grapevine cultivars (Vitis vinífera L.) under two water regimes.

Fort the last decades Spanish vineyard has experience a deep transformation by allowing irrigation, changing the traditional bush into a possible full mechanized VSP and cultivar changing map was also took into account. Tempranillo has led the change to replace the white grapevines, such as in La Mancha; however results in these warm areas were not as expect. Under this conditions, low yield, low acidity and uncouple sacharimetric and phenol maturity usually is obtained. Within this frame an experimental trial was set up with the goal to know other cultivars that could better perform than Tempranillo in warm areas in a sustainable viticulture.

Capacitive load based on IGBTs for on-site characterization of PV arrays

This paper describes the practical design of a portable capacitive load based on insulated gate bipolar transistors (IGBTs), which is used to measure the I–V characteristics of PV arrays with short-circuit currents up to 80 A and open circuit voltages up to 800 V. Such measurement allows on-site characterization of PV arrays under real operating conditions and also provides information for the detection of potential array anomalies, such as broken cells or defective connections. The presented I–V load is easy to reproduce and low-cost, characteristics that are within the reach of small-scale organizations involved in PV electrification projects.

Novel Inductor‐less Conversion Strategy Based on Multiphase Transformer‐Coupled Converters: Analysis, Design and Applications

There are many the requirements that modern power converters should fulfill. Most of the applications where these converters are used, demand smaller converters with high efficiency, improved power density and a fast dynamic response. For instance, loads like microprocessors demand aggressive current steps with very high slew rates (100A/mus and higher); besides, during these load steps, the supply voltage of the microprocessor should be kept within tight limits in order to ensure its correct performance. The accomplishment of these requirements is not an easy task; complex solutions like advanced topologies - such as multiphase converters- as well as advanced control strategies are often needed. Besides, it is also necessary to operate the converter at high switching frequencies and to use capacitors with high capacitance and low ESR. Improving the dynamic response of power converters does not rely only on the control strategy but also the power topology should be suited to enable a fast dynamic response. Moreover, in later years, a fast dynamic response does not only mean accomplishing fast load steps but output voltage steps are gaining importance as well. At least, two applications that require fast voltage changes can be named: Low power microprocessors. In these devices, the voltage supply is changed according to the workload and the operating frequency of the microprocessor is changed at the same time. An important reduction in voltage dependent losses can be achieved with such changes. This technique is known as Dynamic Voltage Scaling (DVS). Another application where important energy savings can be achieved by means of changing the supply voltage are Radio Frequency Power Amplifiers. For example, RF architectures based on ‘Envelope Tracking’ and ‘Envelope Elimination and Restoration’ techniques can take advantage of voltage supply modulation and accomplish important energy savings in the power amplifier. However, in order to achieve these efficiency improvements, a power converter with high efficiency and high enough bandwidth (hundreds of kHz or even tens of MHz) is necessary in order to ensure an adequate supply voltage. The main objective of this Thesis is to improve the dynamic response of DC-DC converters from the point of view of the power topology. And the term dynamic response refers both to the load steps and the voltage steps; it is also interesting to modulate the output voltage of the converter with a specific bandwidth. In order to accomplish this, the question of what is it that limits the dynamic response of power converters should be answered. Analyzing this question leads to the conclusion that the dynamic response is limited by the power topology and specifically, by the filter inductance of the converter which is found in series between the input and the output of the converter. The series inductance is the one that determines the gain of the converter and provides the regulation capability. Although the energy stored in the filter inductance enables the regulation and the capability of filtering the output voltage, it imposes a limitation which is the concern of this Thesis. The series inductance stores energy and prevents the current from changing in a fast way, limiting the slew rate of the current through this inductor. Different solutions are proposed in the literature in order to reduce the limit imposed by the filter inductor. Many publications proposing new topologies and improvements to known topologies can be found in the literature. Also, complex control strategies are proposed with the objective of improving the dynamic response in power converters. In the proposed topologies, the energy stored in the series inductor is reduced; examples of these topologies are Multiphase converters, Buck converter operating at very high frequency or adding a low impedance path in parallel with the series inductance. Control techniques proposed in the literature, focus on adjusting the output voltage as fast as allowed by the power stage; examples of these control techniques are: hysteresis control, V 2 control, and minimum time control. In some of the proposed topologies, a reduction in the value of the series inductance is achieved and with this, the energy stored in this magnetic element is reduced; less stored energy means a faster dynamic response. However, in some cases (as in the high frequency Buck converter), the dynamic response is improved at the cost of worsening the efficiency. In this Thesis, a drastic solution is proposed: to completely eliminate the series inductance of the converter. This is a more radical solution when compared to those proposed in the literature. If the series inductance is eliminated, the regulation capability of the converter is limited which can make it difficult to use the topology in one-converter solutions; however, this topology is suitable for power architectures where the energy conversion is done by more than one converter. When the series inductor is eliminated from the converter, the current slew rate is no longer limited and it can be said that the dynamic response of the converter is independent from the switching frequency. This is the main advantage of eliminating the series inductor. The main objective, is to propose an energy conversion strategy that is done without series inductance. Without series inductance, no energy is stored between the input and the output of the converter and the dynamic response would be instantaneous if all the devices were ideal. If the energy transfer from the input to the output of the converter is done instantaneously when a load step occurs, conceptually it would not be necessary to store energy at the output of the converter (no output capacitor COUT would be needed) and if the input source is ideal, the input capacitor CIN would not be necessary. This last feature (no CIN with ideal VIN) is common to all power converters. However, when the concept is actually implemented, parasitic inductances such as leakage inductance of the transformer and the parasitic inductance of the PCB, cannot be avoided because they are inherent to the implementation of the converter. These parasitic elements do not affect significantly to the proposed concept. In this Thesis, it is proposed to operate the converter without series inductance in order to improve the dynamic response of the converter; however, on the other side, the continuous regulation capability of the converter is lost. It is said continuous because, as it will be explained throughout the Thesis, it is indeed possible to achieve discrete regulation; a converter without filter inductance and without energy stored in the magnetic element, is capable to achieve a limited number of output voltages. The changes between these output voltage levels are achieved in a fast way. The proposed energy conversion strategy is implemented by means of a multiphase converter where the coupling of the phases is done by discrete two-winding transformers instead of coupledinductors since transformers are, ideally, no energy storing elements. This idea is the main contribution of this Thesis. The feasibility of this energy conversion strategy is first analyzed and then verified by simulation and by the implementation of experimental prototypes. Once the strategy is proved valid, different options to implement the magnetic structure are analyzed. Three different discrete transformer arrangements are studied and implemented. A converter based on this energy conversion strategy would be designed with a different approach than the one used to design classic converters since an additional design degree of freedom is available. The switching frequency can be chosen according to the design specifications without penalizing the dynamic response or the efficiency. Low operating frequencies can be chosen in order to favor the efficiency; on the other hand, high operating frequencies (MHz) can be chosen in order to favor the size of the converter. For this reason, a particular design procedure is proposed for the ‘inductorless’ conversion strategy. Finally, applications where the features of the proposed conversion strategy (high efficiency with fast dynamic response) are advantageus, are proposed. For example, in two-stage power architectures where a high efficiency converter is needed as the first stage and there is a second stage that provides the fine regulation. Another example are RF power amplifiers where the voltage is modulated following an envelope reference in order to save power; in this application, a high efficiency converter, capable of achieving fast voltage steps is required. The main contributions of this Thesis are the following: The proposal of a conversion strategy that is done, ideally, without storing energy in the magnetic element. The validation and the implementation of the proposed energy conversion strategy. The study of different magnetic structures based on discrete transformers for the implementation of the proposed energy conversion strategy. To elaborate and validate a design procedure. To identify and validate applications for the proposed energy conversion strategy. It is important to remark that this work is done in collaboration with Intel. The particular features of the proposed conversion strategy enable the possibility of solving the problems related to microprocessor powering in a different way. For example, the high efficiency achieved with the proposed conversion strategy enables it as a good candidate to be used for power conditioning, as a first stage in a two-stage power architecture for powering microprocessors.

Coupled models for the dynamics of bridges under high-speed rail traffic

The dynamic effects of high-speed trains on viaducts are important issues for the design of the structures, as well as for determining safe running conditions of trains. In this work we start by reviewing the relevance of some basic moving load models for the dynamic action of vertical traffic loads. The study of lateral dynamics of running trains on bridges is of importance mainly for the safety of the traffic, and may be relevant for laterally compliant bridges. These studies require 3D coupled vehicle-bridge models and consideration of wheel to rail contact. We describe here a fully nonlinear coupled model, formulated in absolute coordinates and incorporated into a commercial finite element framework. An application example is presented for a vehicle subject to a strong wind gust traversing a bridge, showing the relevance of the nonlinear wheel-rail contact model as well as the interaction between bridge and vehicle.

Modelling Pinus pinea forest management to attain natural regeneration under present and future climatic scenarios

Natural regeneration-based silviculture has been increasingly regarded as a reliable option in sustainable forest management. However, successful natural regeneration is not always easy to achieve. Recently, new concerns have arisen because of changing future climate. To date, regeneration models have proved helpful in decision-making concerning natural regeneration. The implementation of such models into optimization routines is a promising approach in providing forest managers with accurate tools for forest planning. In the present study, we present a stochastic multistage regeneration model for Pinus pinea L. managed woodlands in Central Spain, where regeneration has been historically unsuccessful. The model is able to quantify recruitment under different silviculture alternatives and varying climatic scenarios, with further application to optimize management scheduling. The regeneration process in the species showed high between-year variation, with all subprocesses (seed production, dispersal, germination, predation, and seedling survival) having the potential to become bottlenecks. However, model simulations demonstrate that current intensive management is responsible for regeneration failure in the long term. Specifically, stand densities at rotation age are too low to guarantee adequate dispersal, the optimal density of seed-producing trees being around 150 stems·ha−1. In addition, rotation length needs to be extended up to 120 years to benefit from the higher seed production of older trees. Stochastic optimization confirms these results. Regeneration does not appear to worsen under climate change conditions; the species exhibiting resilience worthy of broader consideration in Mediterranean silviculture.

Assessment of troughput performance under NS2 in Mobile Ad Hoc Networks (MANETs)

Providing QoS in the context of Ad Hoc networks includes a very wide field of application from the perspective of every level of the architecture in the network.In order for simulation studies to be useful, it is very important that the simulation results match as closely as possible with the test bed results. In this Paper, we study the throughput performance (parameter QoS) in Mobile Ad Hoc Networks (MANETs) and compares emulated test bed results with simulation results from NS2 (Network Simulator). The performance of the Mobile Ad Hoc Networks is very sensitive to the number of users and the offered load. When the number of users/offered load is high then the collisions increase resulting in larger wastage of the medium and lowering overall throughput. The aim of this research is to compare the throughput of Mobile Ad Hoc Networks using three different scenarios: 97, 100 and 120 users (nodes) using simulator NS2. By analyzing the graphs in MANETs, it is concluded When the number of users o nodes is increased beyond the certain limit, throughput decreases.

Space-state robust control of a Buck converter with amorphous core coil and variable load

Pulse-width modulation is widely used to control electronic converters. One of the most frequently used topologies for high DC voltage/low DC voltage conversion is the Buck converter. These converters are described by a second order system with an LC filter between the switching subsystem and the load. The use of a coil with an amorphous magnetic material core rather than an air core permits the design of smaller converters. If high switching frequencies are used to obtain high quality voltage output, then the value of the auto inductance L is reduced over time. Robust controllers are thus needed if the accuracy of the converter response must be preserved under auto inductance and payload variations. This paper presents a robust controller for a Buck converter based on a state space feedback control system combined with an additional virtual space variable which minimizes the effects of the inductance and load variations when a switching frequency that is not too high is applied. The system exhibits a null steady-state average error response for the entire range of parameter variations. Simulation results and a comparison with a standard PID controller are also presented.

Finite element simulation of sandwich panels of laminated plaster and rockwool under mixed mode fracture

Sandwich panels of laminated gypsum and rock wool have shown large pathology of cracking due to excessive slabs deflection. Currently the most widespread use of this material is as vertical elements of division or partition, with no structural function, what justifies that there are no studies on the mechanism of fracture and mechanical properties related to it. Therefore, and in order to reduce the cracking problem, it is necessary to progress in the simulation and prediction of the behaviour under tensile and shear load of such panels, although in typical applications have no structural responsability.

Estudio de la permeabilidad y de la resistencia a tracción en hormigones precomprimidos

Entre los requisitos que deben cumplir las estructuras se debe garantizar que estas posean la durabilidad necesaria para permanecer en servicio a lo largo de todo el periodo de vida útil para el que han sido proyectadas. Para conseguir este objetivo las normativas han ido incorporando prescripciones para el diseño del hormigón, en base a distintas clases de exposición dependiendo del origen y magnitud de la agresividad exterior. En ambientes con una elevada agresividad, una de las comprobaciones que debe cumplir el hormigón es que tenga una permeabilidad inferior a los valores máximos fijados según la clase de exposición, y que en caso de considerar como ensayo de referencia el de penetración de agua, analiza el frente de penetración limitando las profundidades de penetración media y máxima. Adicionalmente a las condiciones de diseño según el tipo de ambiente, principalmente basadas en la dosificación del hormigón en términos de la relación agua/cemento y el mínimo contenido de cemento y el recubrimiento de las armaduras, durante la vida en servicio las estructuras pueden están solicitadas por distintas acciones imprevistas que pueden provocar cambios en la microestructura interna del hormigón que modifican su permeabilidad y resistencia, y por tanto pueden alterar la durabilidad inicialmente prevista. Es conocido el efecto de cansancio del hormigón cuando está solicitado por cargas de compresión mantenidas en el tiempo, provocando bajas en su resistencia debido al incremento de la microfisuración. Dada la relación entre la permeabilidad y la microfisuración del hormigón, es previsible el aumento de la permeabilidad en hormigones que han sido precomprimidos durante un periodo largo de tiempo. Los estudios de la permeabilidad en hormigones previamente comprimidos se han realizado analizando periodos de tiempo de compresión cortos que no permiten evaluar el efecto del cansancio sobre la permeabilidad. La presente tesis doctoral investiga la permeabilidad y resistencia a tracción en hormigones que previamente han sido comprimidos en carga mantenida durante distintos plazos de tiempo, al objeto de conocer su evolución en base al tiempo de precompresión. La investigación se apoya en el estudio de otras dos variables como son el tipo de hormigón de acuerdo a su dosificación según el tipo de ambiente considerando una agresividad baja, media o alta, y el grado de compresión aplicado respecto de su carga última de rotura. En los resultados del plan experimental desarrollado se ha obtenido que la permeabilidad presenta un incremento significante con el tiempo de precompresión, que dependiendo del valor inicial de la permeabilidad que tiene el hormigón puede provocar que hormigones que previamente satisfacen las limitaciones de permeabilidad pasen a incumplirlas, pudiendo afectar a su durabilidad. También se confirma la influencia del tiempo de precompresión sobre la resistencia a tracción obteniendo bajas de resistencia importantes en los casos pésimos ensayados, que deben ser tenidas en consideración en tanto afectan a la capacidad resistente del hormigón como a otros aspectos fundamentales como el anclaje de las armaduras en el hormigón armado y pretensado. One of the requirements that structures must meet is to guarantee their durability to remain in service throughout all the working life period for which they have been designed. To achieve this goal, building standards and codes have included specifications for the design of concrete structures, based on different exposure classes depending on the environmental conditions and their origin and magnitude. In severe aggressive environments, one of the specifications the concrete must meet is to have a permeability lower than the maximum values set for a certain exposure class. If this parameter is referenced to water penetration on specimens, then the average and maximum depths of front penetration are analyzed. In addition to the design conditions depending on the exposure class, which regulate the dosage of concrete in terms of the water/cement ratio, minimum samples that have been pre-compressed for a long period of time. Previous studies on permeability have been carried on pre-compressed concrete elements analyzing short periods of time. However, they have not studied the effects of compression forces on concrete in the long term. This Thesis investigates permeability and tensile strength of concrete samples that have been previously compressed under loads applied for different periods of time. The goal is to understand its evolution based on the time exposed to compression. The research variables also include the type of concrete according to the dosage used - depending on the environmental exposure it will have low, medium or high aggressiveness-, and the amount of compression applied in relation to its failure load. Results of the experimental tests showed that permeability increases significantly over the time of pre-compression. Depending on the initial value of permeability, this change could make the concrete not meet the original permeability restrictions and therefore affect its durability. These investigations also confirmed the influence of time of pre-compression in tensile strength, where some cases showed a significant decrease of resistance. These issues must be taken into consideration as they affect the bearing capacity of the material and other key features such as the anchoring of steel bars in reinforced and pre-stressed concrete. amount of cement content and the minimum concrete cover of the steel bars, during their working life structures may be subject to various unforeseen actions. As a result, the concrete’s internal microstructure might be affected, changing its permeability and resistance, and possibly altering the original specified durability. It is a known fact that when concrete is loaded in compression maintained over a long time, its resistance to compression forces is diminished due to the increase in micro-cracking. Considering the relationship between permeability and microcracking of concrete, an increase in permeability may be expected in concrete

Wind-induced loads over double cantilever bridges under construction

Within the last century the interest in wind-induced loads over civil engineering structures has become more and more important, the reason being that the development of new construction techniques and materials has allowed engineers and architects to design new structures far from the traditional concepts, and in many cases wind actions over these singular structures are not included in the existing codes of practice. In this paper the windinduced static loads over bridges constructed by the double cantilever method during erection stages are considered. The aerodynamic load over a double cantilever bridge under a yawing-angled wind produces a yawing (torsional) moment on the bridge deck, which can lead to undesirable rotation of the deck about the supporting pier. The effects of the wind yaw angle and the length of the deck are analysed. The wind action caused by the presence of sliding concrete forms at the ends of the deck is also studied.