Stabilizing Gain Selection of Networked Variable Gain Controller to Maximize Robustness Using Particle Swarm Optimization

Networked Control Systems (NCSs) are often associated with problems like random data losses which might lead to system instability. This paper proposes a method based on the use of variable controller gains to achieve maximum parametric robustness of the plant controlled over a network. Stability using variable controller gains under data loss conditions is analyzed using a suitable Linear Matrix Inequality (LMI) formulation. Also, a Particle Swarm Optimization (PSO) based technique is used to maximize parametric robustness of the plant.Comment: 6 pages, 6 figure

Custom optimization algorithms for efficient hardware implementation

The focus is on real-time optimal decision making with application in advanced control systems. These computationally intensive schemes, which involve the repeated solution of (convex) optimization problems within a sampling interval, require more efficient computational methods than currently available for extending their application to highly dynamical systems and setups with resource-constrained embedded computing platforms. A range of techniques are proposed to exploit synergies between digital hardware, numerical analysis and algorithm design. These techniques build on top of parameterisable hardware code generation tools that generate VHDL code describing custom computing architectures for interior-point methods and a range of first-order constrained optimization methods. Since memory limitations are often important in embedded implementations we develop a custom storage scheme for KKT matrices arising in interior-point methods for control, which reduces memory requirements significantly and prevents I/O bandwidth limitations from affecting the performance in our implementations. To take advantage of the trend towards parallel computing architectures and to exploit the special characteristics of our custom architectures we propose several high-level parallel optimal control schemes that can reduce computation time. A novel optimization formulation was devised for reducing the computational effort in solving certain problems independent of the computing platform used. In order to be able to solve optimization problems in fixed-point arithmetic, which is significantly more resource-efficient than floating-point, tailored linear algebra algorithms were developed for solving the linear systems that form the computational bottleneck in many optimization methods. These methods come with guarantees for reliable operation. We also provide finite-precision error analysis for fixed-point implementations of first-order methods that can be used to minimize the use of resources while meeting accuracy specifications. The suggested techniques are demonstrated on several practical examples, including a hardware-in-the-loop setup for optimization-based control of a large airliner.Open Acces

Investigations into a multiplexed fibre interferometer for on-line, nanoscale, surface metrology

Current trends in technology are leading to a need for ever smaller and more complex featured surfaces. The techniques for manufacturing these surfaces are varied but are tied together by one limitation; the lack of useable, on-line metrology instrumentation. Current metrology methods require the removal of a workpiece for characterisation which leads to machining down-time, more intensive labour and generally presents a bottle neck for throughput. In order to establish a new method for on-line metrology at the nanoscale investigation are made into the use of optical fibre interferometry to realise a compact probe that is robust to environmental disturbance. Wavelength tuning is combined with a dispersive element to provide a moveable optical stylus that sweeps the surface. The phase variation caused by the surface topography is then analysed using phase shifting interferometry. A second interferometer is wavelength multiplexed into the optical circuit in order to track the inherent instability of the optical fibre. This is then countered using a closed loop control to servo the path lengths mechanically which additionally counters external vibration on the measurand. The overall stability is found to be limited by polarisation state evolution however. A second method is then investigated and a rapid phase shifting technique is employed in conjunction with an electro-optic phase modulator to overcome the polarisation state evolution. Closed loop servo control is realised with no mechanical movement and a step height artefact is measured. The measurement result shows good correlation with a measurement taken with a commercial white light interferometer

Advanced digital resonant control techniques for grid-connected voltage source converters

It is a fact that the presence of power distributed generation sources in the electrical sector is growing exponentially worldwide. This can mainly be explained by the increase of renewable energy production, which involves principally grid-connected single- and three-phase Voltage Source Converters (VSCs). Regarding the regulations for medium and high power inverters, it is not enough to generate currents with low harmonic content and a unity-power-factor operation: grid-connected VSCs also have to work properly when the electrical grid presents non-ideal characteristics such as frequency variations, sags, swells or a high impedances, among other disturbances. In line with the above and to improve the grid currents quality, LCL filters are widely used at the input of VSCs for their attenuation capabilities at high frequencies. However, they present a big resonance that may produce robustness issues. In order to study this problem and to stablish a basis for the controllers design, the grid-connected VSC mathematical models have been obtained, analysed and discretised, including passive and active LCL resonance damping techniques. Adaptive Feedforward Cancellation (AFC) is the control technique addressed in this research work, leading to robust designs which are able to face all the grid disturbances aforementioned. AFC controllers, formed by resonators, have been considered in continuous- and discrete-time forms, and presenting infinite and finite gain, along with an anti-windup system for limitation purposes. The development of an AFC control design method (directly in discrete time), and the use of automatic code generation tools, have allowed a fast implementation of the resonant controllers into a Digital Signal Processor (DSP). The experimental results obtained from the VSC prototypes (also developed during this thesis), prove the robustness of this control technique.Es un hecho que la presencia de fuentes de generación de energía distribuida en el sector eléctrico está creciendo exponencialmente a nivel mundial. Esto se debe esencialmente al incremento de la producción de energías renovables, que están estrechamente relacionadas con convertidores estáticos de potencia monofásicos y trifásicos (VSC) conectados a la red eléctrica. Con respecto a la normativa en el ámbito de los inversores de media y alta potencia, ya no es solamente necesario que éstos generen unas corrientes con un bajo contenido en armónicos y un factor de potencia unitario: el funcionamiento de dichos equipos debe ser correcto frente a situaciones no ideales de la red como variaciones de frecuencia, huecos de tensión o redes débiles, entre otros. En consonancia con lo anterior y para mejorar la calidad de las corrientes de red, los filtros LCL son ampliamente utilizados a la entrada de los VSC por su gran atenuación a altas frecuencias. Sin embargo, la gran resonancia que presentan puede generar problemas de robustez. Para estudiar este fenómeno y crear una base para el diseño de los controladores, se han obtenido, analizado y discretizado los modelos matemáticos de los VSC conectados a la red, incluyendo técnicas de amortiguación pasivas y activas para el filtro LCL. La técnica de control abordada en este trabajo de investigación se denomina Adaptive Feedforward Cancellation (AFC), con la que se han conseguido diseños robustos y capaces de hacer frente a las perturbaciones en la red detalladas anteriormente. Los controladores AFC, formados por resonadores, han sido considerados en tiempo continuo y discreto, y con ganancias infinita y finita, junto con un sistema de limitación anti-windup. El desarrollo de un método de diseño de control AFC (directamente en tiempo discreto) y el uso de herramientas de generación automática de código, han permitido una rápida implementación de los controladores resonantes en un procesador digital de señales (DSP). Los resultados experimentales obtenidos con los prototipos de convertidores monofásico y trifásico (también desarrollados durante esta tesis), confirman la robustez de esta técnica de control.C’est un fait que la présence de sources de génération d’énergie distribuée dans le secteur électrique est en train de croître exponentiallement à niveau mondiale. Ceci est principalement à cause de l’augmentation de la production d’énergies renouvelables, qui est étroitement liée à des convertisseurs statiques de puissance monophasés et triphasés (VSC) connectés au réseau électrique. Par rapport aux régulations dans le domaine des onduleurs de moyenne et forte puissance, il n’est pas seulement nécessaire qu’ils génèrent des courants à bas contenu harmonique avec un facteur de puissance unitaire: le fonctionnement de ces systèmes doit être correct face à des situations non idéales du réseau comme variations de fréquence, creux de tension ou réseaux faibles, parmi d’autres. Dans cet esprit et pour améliorer la qualité des courants du réseau, les filtres LCL sont vastement utilisés à l’entrée des VSC pour leur capacité d’atténuation aux hautes fréquences. Néanmoins, la grande résonance qu’ils présentent peut générer des problèmes de robustesse. Pour l’étude de ce phénomène et créer une base pour la conception des contrôlleurs, les modèles mathématiques des VSC ont été obtenus, analysés et discretisés, y compris techniques d’amortissement passives et actives pour le filtre LCL. La technique de commande abordée dans ce travail de recherche s’appelle Adaptive Feedforward Cancellation (AFC), avec laquel il a été possible d’obtenir des contrôlleurs robustes et capables de faire face aux perturbations du réseau nommées précédemment. Les contrôlleurs AFC, constitués de résonateurs, ont été considerés en temps continu et discret, et avec des gains infinis et finis, ainsi qu’un système de limitation anti-windup. Le développement d’une méthode de conception de commande AFC (directement en temps discret) et l’utilisation d’outils de génération de code automatique, ont permis une implementation rapide des contrôlleurs résonantes dans un processeur de signal numérique (DSP). Les résultats expérimentaux obtenus avec les prototypes des convertisseurs monophasé et triphasé (aussi développés pendant cette thèse), réalerment la robustesse de cette technique de commande.És un fet que la presència de fonts de generació d’energia distribuïda al sector elèctric està creixent exponencialment a nivell mundial. Això es deu principalment a l’increment de la producció d’energies renovables, directament vinculades a convertidors estàtics de potència monofàsics i trifàsics (VSC) connectats a la xarxa elèctrica. Pel que fa a la normativa en l’àmbit dels inversors de mitjanes i altes potències, ja no és solament necessari que els convertidors generin corrents amb baix contingut harmònic amb un factor de potència unitari, sinó que el funcionament d’aquests equips deu ser correcte enfront de situacions no ideals de la xarxa com ara variacions de freqüència, forats de tensió o xarxes febles, entre altres. D’acord amb l’anterior, i per millorar la qualitat dels corrents de xarxa, els filtres LCL són àmpliament utilitzats a l’entrada dels VSC per la seva capacitat d’atenuació a altes freqüències. No obstant això, la gran ressonància que presenten pot generar problemes de robustesa. Per estudiar aquest fenomen i crear una base per al disseny dels controladors, s’han obtingut, analitzat i discretitzat els models matemàtics dels VSC connectats a la xarxa, incloent tècniques d’esmorteïment passives i actives per al filtre LCL. La tècnica de control abordada en aquest treball de recerca es denomina Adaptive Feedforward Cancellation (AFC), amb la qual s’han aconseguit dissenys robustos i capaços de fer front a les pertorbacions de la xarxa detallades anteriorment. Els controladors AFC, formats per ressonadors, han sigut considerats en temps continu i discret, i amb ganàncies infinita i finita, juntament amb un sistema de limitació anti-windup. El desenvolupament d’un mètode de disseny de control AFC (directament en temps discret) i l’ús d’eines de generació automàtica de codi, han permès una ràpida implementació dels controladors ressonants en un processador digital de senyals (DSP). Els resultats experimentals obtinguts amb els prototips de convertidors monofàsic i trifàsic (també desenvolupats durant aquesta tesi), confirmen la robustesa d’aquesta tècnica de contro

Control of fluid flows and other systems governed by partial differential-algebraic equations

The motion of fluids, such as air or water, is central to many engineering systems of significant economic and environmental importance. Examples range from air/fuel mixing in combustion engines to turbulence induced noise and fatigue on aircraft. Recent advances in novel sensor/actuator technologies have raised the intriguing prospect of actively sensing and manipulating the motion of the fluid within these systems, making them ripe for feedback control, provided a suitable control model exists. Unfortunately, the models for many of these systems are described by nonlinear, partial differential-algebraic equations for which few, if any, controller synthesis techniques exist. In stark contrast, the majority of established control theory assumes plant models of finite (and typically small) state dimension, expressed as a linear system of ordinary differential equations. Therefore, this thesis explores the problem of how to apply the mainstream tools of control theory to the class of systems described by partial differential-algebraic equations, that are either linear, or for which a linear approximation is valid. The problems of control system design for infinite-dimensional and algebraically constrained systems are treated separately in this thesis. With respect to the former, a new method is presented that enables the computation of a bound on the n-gap between a discretisation of a spatially distributed plant, and the plant itself, by exploiting the convergence rate of the v-gap metric between low-order models of successively finer spatial resolution. This bound informs the design, on loworder models, of H[infinity] loop-shaping controllers that are guaranteed to robustly stabilise the actual plant. An example is presented on a one-dimensional heat equation. Controller/estimator synthesis is then discussed for finite-dimensional systems containing algebraic, as well as differential equations. In the case of fluid flows, algebraic constraints typically arise from incompressibility and the application of boundary conditions. A numerical algorithm is presented, suitable for the semi-discrete linearised Navier-Stokes equations, that decouples the differential and algebraic parts of the system, enabling application of standard control theory without the need for velocity-vorticity type methods. This algorithm is demonstrated firstly on a simple electrical circuit, and secondly on the highly non-trivial problem of flow-field estimation in the transient growth region of a flat-plate boundary layer, using only wall shear measurements. These separate strands are woven together in the penultimate chapter, where a transient energy controller is designed for a channel-flow system, using wall mounted sensors and actuators

An Energy Efficient non-volatile FPGA Digital Processor for Brain Neuromodulation

PhD ThesisBrain stimulation technologies have the potential to provide considerable clinical benefits for people with a range of neurological disorders. Recent neuroscience studies have shown that considerable information of brain states is contained in the low frequency local field potential (If-LFP; below 5Hz) recordings with application in real-time closed-loop neurostimulation for treating neurological disorders. Given these signals can be sampled at low sampling rate and hence provide sparse data streams, there is an opportunity to design implantable neuroprosthesis with long battery lifecycles which enables enough processing power to implement long-term, real-time closed loop control algorithms. In this thesis, a closed-loop embedded digital processor has been created for use in rodent neuroscience experiments. The first contribution of this work is to develop a mathematical analytical design approach of feedback controller for suppressing high-amplitude epileptic activity in the neuron mass model to form a better understanding of how to perform a better closed-loop stimulation to control seizures. The second contribution and the third contribution are combined to present an exploratory energy-efficient digital processor architecture built with commercial off-the-shelf non-volatile FPGAs and microcontroller for sparse data processing of brain neuromodulation. A digital hardware design of an exemplar PID control algorithm has been implemented on this proposed digital architecture. A new power computing diagram of this time-driven approach significantly reduced the power consumption which suggests that a digital combined control system of non-volatile FPGAs and microcontroller outweighs a digital control system of microcontroller with microcontroller regarding computing time cost and energy consumption supposing one microcontroller is always required. Taken together, this digital energy-efficient processor architecture gives important insights and viewpoints for the further advancements of neuroprosthesis for brain neurostimulation to achieve lower power consumption for sparse sampling data rate