274 research outputs found
A Method for the Design of Multirate Sampled-Data Digital Flight Control Systems of Piloted Aircraft
The initial flight-test operations of piloted aircraft, in which Digital Flight Control (DFC) systems were first employed, exposed handling qualities problems that were not predicted during the design stage. Subsequent studies attributed the cause of these problems to the techniques used in the design of the digital control systems. The particular feature which unites the reported difficulties is that, an infinite-resolution sampled-data model is assumed for the design process but the practical DFC implementation is realised as an amplitude-quantised sampled-data system
Resource-Constrained Acquisition Circuits for Next Generation Neural Interfaces
The development of neural interfaces allowing the acquisition of signals from the cortex of the brain has seen an increasing amount of interest both in academic research as well as in the commercial space due to their ability to aid people with various medical conditions, such as spinal cord injuries, as well as their potential to allow more seamless interactions between people and machines. While it has already been demonstrated that neural implants can allow tetraplegic patients to control robotic arms, thus to an extent returning some motoric function, the current state of the art often involves the use of heavy table-top instruments connected by wires passing through the patient’s skull, thus making the applications impractical and chronically infeasible.
Those limitations are leading to the development of the next generation of neural interfaces that will overcome those issues by being minimal in size and completely wireless, thus paving a way to the possibility of their chronic application. Their development however faces several challenges in numerous aspects of engineering due to constraints presented by their minimal size, amount of power available as well as the materials that can be utilised.
The aim of this work is to explore some of those challenges and investigate novel circuit techniques that would allow the implementation of acquisition analogue front-ends under the presented constraints. This is facilitated by first giving an overview of the problematic of recording electrodes and their electrical characterisation in terms of their impedance profile and added noise that can be used to guide the design of analogue front-ends.
Continuous time (CT) acquisition is then investigated as a promising signal digitisation technique alternative to more conventional methods in terms of its suitability. This is complemented by a description of practical implementations of a CT analogue-to-digital converter (ADC) including a novel technique of clockless stochastic chopping aimed at the suppression of flicker noise that commonly affects the acquisition of low-frequency signals. A compact design is presented, implementing a 450 nW, 5.5 bit ENOB CT ADC, occupying an area of 0.0288 mm2 in a 0.18 μm CMOS technology, making this the smallest presented design in literature to the best of our knowledge.
As completely wireless neural implants rely on power delivered through wireless links, their supply voltage is often subject to large high frequency variations as well voltage uncertainty making it necessary to design reference circuits and voltage regulators providing stable reference voltage and supply in the constrained space afforded to them. This results in numerous challenges that are explored and a design of a practical implementation of a reference circuit and voltage regulator is presented. Two designs in a 0.35 μm CMOS technology are presented, showing respectively a measured PSRR of ≈60 dB and ≈53 dB at DC and a worst-case PSRR of ≈42 dB and ≈33 dB with a less than 1% standard deviation in the output reference voltage of 1.2 V while consuming a power of ≈7 μW.
Finally, ΣΔ modulators are investigated for their suitability in neural signal acquisition chains, their properties explained and a practical implementation of a ΣΔ DC-coupled neural acquisition circuit presented. This implements a 10-kHz, 40 dB SNDR ΣΔ analogue front-end implemented in a 0.18 μm CMOS technology occupying a compact area of 0.044 μm2 per channel while consuming 31.1 μW per channel.Open Acces
Transient and stochastic dynamics in cellular processes
This Thesis studies different cellular and cell population processes driven by non-linear and stochastic dynamics.
The problems addressed here gravitate around the concepts of transient dynamics and relaxation from a perturbed to a steady state. In this regard, in all processes studied, stochastic fluctuations, either intrinsically present in or externally applied to these systems play an important and constructive role, by either driving the systems out of equilibrium, interfering with the underlying deterministic laws, or establishing suitable levels of heterogeneity.
The first part of the Thesis is committed the analysis of genetically regulated transient cellular processes. Here, we analyse, from a theoretical standpoint, three genetic circuits with pulsed excitable dynamics. We show that all circuits can work in two different excitable regimes, in contrast to what was previously speculated.
We also study how, in the presence of molecular noise, these excitable circuits can generate periodic polymodal pulses due to the combination of two noise induced phenomena: stabilisation of an unstable spiral point and coherence resonance.
We also studied an excitable genetic mechanism for the regulation of the transcriptional fluctuations observed in some pluripotency factors in Embryonic Stem cells. In the embryo, pluripotency is a transient cellular state and the exit of cells from it seems to be associated with transcriptional fluctuations.
In regard to pluripotency control, we also propose a novel mechanism based on the post-translational regulation of a small set of four pluripotency factors. We have validated the theoretical model, based on the formation of binary complexes among these factors, with quantitative experimental data at the single-cell level. The model suggests that the pluripotency state does not depend on the cellular levels of a single factor, but rather on the equilibrium of correlations between the different proteins. In addition, the model is able to anticipate the phenotype of several mutant cell types and suggests that the regulatory function of the protein interactions is to buffer the transcriptional activity of Oc4, a key pluripotency factor.
In the second part of the Thesis we studied the behaviour of a computational cell signalling network of the human fibroblast in the presence of external fluctuations and signals. The results obtained here indicate that the network responds in a nontrivial manner to background chatter, both intrinsically and in the presence of external periodic signals. We show that these responses are consequence of the rerouting of the signal to different network information-transmission paths that emerge as noise is modulated.
Finally, we also study the cell population dynamics during the formation of microbial biofilms, wrinkled pellicles of bacteria glued by an extracellular matrix that are one of the simplest cases of self-organised multicellular structures. In this Thesis we develop a spatiotemporal model of cellular growth and death that accounts for the experimentally observed patterns of massive bacterial death that precede wrinkle formation in biofilms. These localised patterns focus mechanical forces during biofilm expansion and trigger the formation of the characteristic ridges. In this sense, the proposed model suggests that the death patterns emerge from the mobility changes in bacteria due to the production of extracellular matrix and the spatially inhomogeneous cellular growth. An important prediction of the model is that matrix productions is crucial for the appearance of the patterns and, therefore for winkle formation. We have also experimentally validated validated this prediction with matrix deficient bacterial strains, which show neither death patterns nor wrinkles.En aquesta Tesi s’estudien diferents processos intracel·lulars i de poblacions cel·lulars regits per dinà mica estocà stica i no lineal. El problemes biològics tractats graviten al voltant el concepte de dinà mica transitòria i de relaxació d’un estat dinà mic pertorbat a l’estat estacionari. En aquest sentit, en tots els processos estudiats, les fluctuacions estocà stiques, presents intrÃnsecament o aplicades de forma externa, hi tenen un paper constructiu, ja sigui empenyent els sistemes fora de l’equilibri, interferint amb les lleis deterministes subjacents, o establint els nivells d’heterogeneïtat necessaris.
La primera part de la Tesi es dedica a l’estudi de processos cel·lulars transitoris regulats genèticament. En ella analitzem des d’un punt de vista teòric tres circuits genètics de control de polsos excitables i, contrà riament al que s’havia especulat anteriorment, establim que tots ells poden treballar en dos tipus de règim excitable. Analitzem també com, en presència de soroll molecular, aquests circuits excitables poden generar polsos periòdics i multimodals degut a la combinació de dos fenòmens induïts per soroll: l’estabilització estocà stica d’estats inestables i la ressonà ncia de coherència.
D’altra banda, estudiem com un mecanisme genètic excitable pot ser el responsable de regular a nivell transcripcional les fluctuacions que s’observen experimentalment en alguns factors de pluripotència en cèl·lules mare embrionà ries. En l’embrió, la pluripotència és un estat cel·lular transitori i la sortida de les cèl·lules d’aquest sembla que està associada a fluctuacions transcripcionals. En relació al control de la pluripotència, presentem també un nou mecanisme basat en la regulació post-traduccional d’un petit conjunt de 4 factors de pluripotència. El model teòric proposat, basat en la formació de complexos entre els diferents factors de pluripotència, l’hem validat mitjançant experiments quantitatius en cèl·lules individuals. El model postula que l’estat de pluripotència no depèn dels nivells cel·lulars d’un únic factor, sinó d’un equilibri de correlacions entre diverses proteïnes. A més, prediu el fenotip de cèl·lules mutants i suggereix que la funció reguladora de les interaccions entre les quatre proteïnes és la d’esmorteir l’activitat transcripcional d’Oct4, un dels principals factors de pluripotència.
En el segon apartat de la Tesi estudiem el comportament d’una xarxa computacional de senyalització cel·lular de fibroblast humà en presència de senyals externs fluctuants i cÃclics. Els resultats obtinguts mostren que la xarxa respon de forma no trivial a les fluctuacions ambientals, fins i tot en presència d’una senyal externa. Diferents nivells de soroll permeten modular la resposta de la xarxa, mitjançant la selecció de rutes alternatives de transmissió
de la informació.
Finalment, estudiem la dinà mica de poblacions cel·lulars durant la formació de biofilms, pel·lÃcules arrugades d’aglomerats de bacteris que conformen un dels exemples més simples d’estructures multicel·lulars autoorganitzades.
En aquesta Tesi presentem un model espai-temporal de creixement i mort cel·lular motivat per l’evidència experimental sobre l’aparició de patrons de mort massiva de bacteris previs a la formació de les arrugues dels biofilms. Aquests patrons localitzats concentren les forces mecà niques durant l’expansió del biofilm i inicien la formació de les arrugues caracterÃstiques. En aquest sentit, el model proposat explica com es formen els patrons de mort a partir dels canvis de mobilitat dels bacteris deguts a la producció de matriu extracel·lular combinats amb un creixement espacialment heterogeni. Una important predicció del model és que la producció de matriu és un procés clau per a l’aparició dels patrons i, per tant de les arrugues. En aquest aspecte, els nostres resultats experimentals en bacteris mutants que no produeixen components essencials de la matriu, confirmen les prediccions
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A high precision accelerometer-based sensor unit for the acquisition of ultra low distortion seismic signals
Over 800,000 people worldwide lost their lives to earthquakes in the last decade and on average 171 people die every day due to earthquake related damage to structures and buildings. Precisely understanding the effects ground motion has on manmade structures is crucial to making them earthquake resistant. This can only be achieved by the precise measurement, recording, and analysis of ground displacement trends during a seismic event.
Although there is a vast amount of recorded seismological data available, current technology and processing methods fail to represent accurate ground displacement over time as the considerable technological challenges have yet to be overcome.
Raw seismic data has so far been primarily acquired with instruments utilising geophone or accelerometer based sensors. These instruments produce prominent time domain displacement errors due to the various system and sensor inaccuracies, and due to non-linear response. Since accelerometers provide acceleration over time data: whilst geophones are velocimeters, and therefore provide velocity over time data; in order to derive true ground displacement over time, a double, or single numerical integration is required respectively. During this essential numerical integration processes of data from such sensors, even small in magnitude errors accumulate to yield rather large displacement trend offsets over a typical event recording period of 60 to 120 seconds. In addition, the numerical integration process itself poses considerable challenges due to the theoretically infinite number of samples and the accurate determination of initial conditions required for an exact mathematical result to be obtained. The latter, is currently performed by averaging an up to 60 second pre-event data trend stored on the instrument.
Most post-integration data from current instruments appears to contain low frequency drifts amongst other noise artefacts, and generally requires baseline correction algorithms in an attempt to correct for these effects. Such corrections, although helpful, only aid to minimise the perceived effects of an assumed and collective source of error, and hence are largely unable to tackle the individual error contribution of each element within the system. Since individual element contribution is of a dynamic nature, the validity of these algorithms is limited by the accuracy of the initial assumptions made about a specific set of data. Faced with such a multivariable and uncertain dynamic behaviour, where even mathematical system modelling is of inadequate long term accuracy, a solution that aims to directly minimise these errors at source, rather than attempt to correct them postacquisition, is of immense importance when it comes to the recording, analysis, and understanding of earthquakes.
This thesis describes the design, implementation, and evaluation of a High Precision Active Gyro Stabilised (HPAGS) sensor unit of exceptional performance for the provision of highly accurate ground displacement data. Experimental results demonstrated that the device described herein, was able to diminish the inherent non-linear and environment-dependant effects of current sensors, and thus was able to provide highly improved time domain displacement data
Atoms in microcavities : detection and spectroscopy
This thesis presents work undertaken with cold rubidium atoms interacting with an optical
microcavity. The optical microcavity used is unique in its design, being formed between an
optical fibre and silicon micromirror. This allows direct optical access to the cavity mode,
whilst the use of microfabrication techniques in the design means that elements of the system
are inherently scalable. In addition, the parameters of the system are such that a single atom
has a substantial impact on the cavity field.
In this system, two types of signal arise from the atoms' interaction with the cavity field;
a `reflection' signal and a `fluorescence' signal. A theoretical description for these signals is
presented, followed by experiments which characterise the signals under a variety of experimental
conditions. The thesis then explores two areas: the use of the microcavity signals for
atom detection and the investigation of how higher atom numbers and, as a result, a larger
cooperative interaction between the atoms and the cavity field, impacts the signals.
First, the use of these signals to detect an effective single atom and individual atoms whilst
falling and trapped is explored. The effectiveness of detection is parameterised in terms of
detection confidence and signal to noise ratio, detection fidelity and dynamic range.
In the second part of this thesis, the effect of higher atom numbers on the reflection and fluorescence signals is investigated. A method for increasing the atom number is presented,
alongside experiments investigating the impact on the measured signals. This is followed by
experiments which explore the dispersive nature of the atom-cavity interaction by measuring
the excitation spectrum of the system in reflection and fluorescence. In doing so, it is shown
that, for weak coupling, these two signals are manifestly different
Sigma delta modulation of a chaotic signal
Sigma delta modulation SDM has become a widespread method of analogue to digital conversion, however its operation has not been completely defined. The majority of the analysis carried out on the circuit has been from a linear standpoint, with non-linear analysis hinting at hidden complexities in the modulator's operation. The sigma delta modulator itself is a non-linear system consisting, as it
does, of a number of integrators and a one bit quantiser in a feedback loop. This configuration can be generalised as a non-linearity within a feedback path, which is a classic route to chaotic behaviour.
This initially raises the prospect that a sigma delta modulator may be capable of chaotic modes of operation with a non-chaotic input. In fact, the problem does not arise and we show why not.
To facilitate this investigation, a set of differential equations is formulated to represent SDM; these equations are subsequently utilised in a stability study of the sigma delta modulator.
Of more interest, and more uncertainty, is the effect sigma delta modulation may have on a chaotic signal. If SDM makes a chaotic signal more chaotic then this will have serious repercussions on the predictability of that signal. In the past, analysis of the circuit has tended to be based around a steady state input or a slowly moving non-chaotic input such as a low frequency sine wave. This has greatly eased the complexity of such analyses, but it does not address the problem at hand.
In this thesis we present the results of comparing the sigma delta modulation of a chaotic signal to a direct quantisation of the same signal. The tool we use to investigate this is the Lyapunov spectrum of the time series, measured using an algorithm developed at Edinburgh University. The Lyapunov exponents of a chaotic signal are presented before and after both SDM and direct quantisation, and it is shown that SDM does not increase the chaos of the signal. Indeed, it is shown that SDM has no more effect on the predictability of the signal, as measured by the Lyapunov spectrum, than direct quantisation.
As such, we conclude that sigma delta modulation provides a reliable method for analogue to digital conversion of chaotic signals.
It should be pointed out that, due to the incompleteness of rigorous analysis of SDM and the complex processes involved in applying such analysis to a chaotic signal, the results of this thesis are largely based upon experimentation and observation from a simulation of a sigma delta modulator
Internally Sensed Optical Phased Arrays
The performance of existing ground-based space debris laser
ranging systems can be improved by directing more light onto
space debris by coherently combining multiple lasers using an
optical phased array (OPA). If the power delivered to target is
sufficiently high then these systems may also provide the
capability to remotely manoeuvre space debris via photon
radiation pressure and/or ablation. By stabilising the relative
output phase of multiple lasers, OPAs form a coherent optical
wave-front in the far field. Since the phase of each laser can be
controlled independently, they also have the ability to
dynamically manipulate the distribution of optical power in the
far field, potentially enabling them to compensate for
atmospheric turbulence. This beam-forming functionality, combined
with their inherent scalability and high power handling
capabilities make OPAs a promising technology for future space
debris laser ranging and manoeuvring systems.
In this thesis, we describe the iterative development of a
high-power compatible internally sensed OPA, which---in contrast
to externally sensed OPAs that sense the output phase of each
laser externally using free-space optics---relies on the small
fraction of light that is reflected back into the fibre at the
output of the OPA to stabilise its relative output phase. This
allows internally sensed OPAs to be implemented entirely within
fibre without any dependence on free-space optics at the output,
offering potential advantages over externally sensed techniques
when operating in the presence of shock and vibration.
A proof-of-concept experiment demonstrated the viability of
internal sensing, but also highlighted a number of weaknesses
that would affect its utility, specifically in supporting high
optical powers greater than 100s of mW. An improved high-power
compatible internally sensed OPA was designed to overcome these
restrictions by isolating sensitive optical components from high
optical powers using asymmetric fibre couplers. This concept was
initially demonstrated experimentally using slave lasers offset
phase-locked to a single master laser, and then again using fibre
amplifiers in a master oscillator power amplifier configuration.
The experimental demonstration of the fibre amplifier compatible
OPA stabilised the relative output phase of three commercial 15 W
fibre amplifiers, demonstrating a root-mean-squared output phase
stability of , and the ability to steer the beam at
up to 10 kHz.
The internally sensed OPA presented here requires the
simultaneous measurement, and control of the phase of each
emitter in the OPA. This is accomplished using digitally enhanced
heterodyne interferometry and digitally implemented phasemeters,
both of which rely heavily on high-speed digital signal
processing resources provided by field-programmable gate-arrays
Heterogeneous and hybrid control with application in automotive systems
Control systems for automotive systems have acquired a new level of complexity. To fulfill the requirements of the controller specifications new technologies are needed. In many cases high performance and robust control cannot be provided by a simple conventional controller anymore. In this case hybrid combinations of local controllers, gain scheduled controllers and global stabilisation concepts are necessary. A considerable number of state-of-the-art automotive controllers (anti-lock brake system (ABS), electronic stabilising program (ESP)) already incorporate heterogeneous and hybrid control concepts as ad-hoc solutions. In this work a heterogeneous/hybrid control system is developed for a test vehicle in order to solve a clearly specified and relevant automotive control problem. The control system will be evaluated against a state-of-the-art conventional controller to clearly show the benefits and advantages arising from the novel approach. A multiple model-based observer/estimator for the estimation of parameters is developed to reset the parameter estimate in a conventional Lyapunov based nonlinear adaptive controller. The advantage of combining both approaches is that the performance of the controller with respect to disturbances can be improved considerably because a reduced controller gain will increase the robustness of the approach with respect to noise and unmodelled dynamics. Several alternative resetting criteria are developed based on a control Lyapunov function, such that resetting guarantees a decrease in the Lyapunov function. Since ABS systems have to operate on different possibly fast changing road surfaces the application of hybrid methodologies is apparent. Four different model based wheel slip controllers will be presented: two nonlinear approaches combined with parameter resetting, a simple linear controller that has been designed using the technique of simultaneously stabilising a set of linear plants as well as a sub-optimal linear quadratic (LQ)-controller. All wheel slip controllers operate as low level controllers in a modular structure that has been developed for the ABS problem. The controllers will be applied to a real Mercedes E-class passenger car. The vehicle is equipped with a brake-by-wire system and electromechanical brake actuators. Extensive real life tests show the benefits of the hybrid approaches in a fast changing environment
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