Output feedback control of flow separation over an aerofoil using plasma actuators

We address the problem of controlling the unsteady flow separation over an aerofoil, using plasma actuators. Despite the complexity of the dynamics of interest, we show how the problem of controlling flow separation can be formulated as a simple set-point tracking problem, so that a simple control strategy may be used. A robust output feedback control is designed, on the basis of a low-order, linear, dynamical model approximating the incompressible Navier-Stokes equations, obtained from the snapshots of 2D laminar finite element simulations at Re=1,000. Fast flow reattachment is achieved, along with both stabilisation and increase/reduction of the lift/drag, respectively. Accurate 2D finite element simulations of the full-order nonlinear equations illustrate the effectiveness of the proposed approach: good dynamic performances are obtained, as both the Reynolds number and the angle of attack are varied. The chosen output can be experimentally measured by appropriate sensors and, despite its simplicity, the proposed set-point tracking controller is sufficient to suppress the laminar separation bubble; moreover, its extension to 3D turbulent configurations is straightforward, thus illustrating the effectiveness of the designed control algorithm in more practical conditions, which are far from the design envelope

Uniform finite time stabilisation of non-smooth and variable structure systems with resets

This thesis studies uniform finite time stabilisation of uncertain variable structure and non-smooth systems with resets. Control of unilaterally constrained systems is a challenging area that requires an understanding of the underlying mechanics that give rise to reset or jumps while synthesizing stabilizing controllers. Discontinuous systems with resets are studied in various disciplines. Resets in states are hard nonlinearities. This thesis bridges non-smooth Lyapunov analysis, the quasi-homogeneity of differential inclusions and uniform finite time stability for a class of impact mechanical systems. Robust control synthesis based on second order sliding mode is undertaken in the presence of both impacts with finite accumulation time and persisting disturbances. Unlike existing work described in the literature, the Lyapunov analysis does not depend on the jumps in the state while also establishing proofs of uniform finite time stability. Orbital stabilization of fully actuated mechanical systems is established in the case of persisting impacts with an a priori guarantee of finite time convergence between t he periodic impacts. The distinguishing features of second order sliding mode controllers are their simplicity and robustness. Increasing research interest in the area has been complemented by recent advances in Lyapullov based frameworks which highlight the finite time Convergence property. This thesis computes the upper bound on the finite settling time of a second order sliding mode controller. Different to the latest advances in the area, a key contribution of this thesis is the theoretical proof of the fact that finite settling time of a second order sliding mode controller tends to zero when gains tend to infinity. This insight of the limiting behaviour forms the basis for solving the converse problem of finding an explicit a priori tuning formula for the gain parameters of the controller when and arbitrary finite settling time is given. These results play a central role ill the analysis of impact mechanical systems. Another key contribution of the thesis is that it extends the above results on variable structure systems with and without resets to non-smooth systems arising from continuous finite time controllers while proving uniform finite time stability. Finally, two applications are presented. The first application applies the above theoretical developments to the problem of orbital stabilization of a fully actuated seven link biped robot which is a nonlinear system with periodic impacts. The tuning of the controller gains leads to finite time convergence of the tracking errors between impacts while being robust to disturbances. The second application reports the outcome of an experiment with a continuous finite time controller

Experiments Relevant to the Development of Laser Interferometric Gravitational Wave Detectors

The development of gravitational wave detectors has been in progress for approximately twenty-five years. As yet there has been no clear evidence for the successful detection of such propagating fluctuations in the curvature of spacetime, but the prospects seem good that detectors of sufficient sensitivity to detect gravitational waves of astrophysical origin can be constructed in the near future. The most promising form of detector is the long baseline laser interferometer, and prototypes are being developed at a number of sites around the world. A 10 metre prototype is currently being developed in Glasgow. This thesis is an account of work based on the Glasgow prototype. After an elementary introduction to the theoretical foundations of gravitational waves, various sources of gravitational radiation, the nature of their emitted signal and their strengths are considered. Suitable detectors and their possible sensitivities are reviewed. Noise sources which could limit the sensitivity of laser interferometer detectors and the constraints which these place on the design of the detector are discussed. Since the test masses in an interferometer detector must be freely suspended as pendulums, yet their orientation must be accurately controlled to maintain correct alignment of the optical cavities forming the interferometer, an active orientation control system was developed and installed on the Glasgow prototype. This system provides a high degree of positional and angular stabilisation at low frequencies while leaving the test mass essentially free at high frequencies. Some of the potential limitations and noise sources are noted and their magnitudes calculated. A digital recording system was designed and used to record data from the prototype detector at Glasgow. The effects of the detector's response are analysed and techniques to recover the gravitational wave signal from the recorded data are described. The analysis of some data recorded with this system is then reported. The pulse statistics of the interferometer are analysed and the implications for searches for millisecond pulses of gravitational waves are discussed. The results of a search for periodic signals in the detector output are presented. Various sources of contamination which may be present in the detector output are identified, limitations of the recorded data are noted, and techniques which may be used to reduce the importance of these effects are described

Ultrasonic transducer calibration

When a material is placed under stress, small changes within the specimen release ultrasonic energy in the form of stress waves. The change may, for example, be a dislocation movement or the advancement of a crack tip. These ultrasonic pulses are termed Acoustic Emission and may be detected at the material surface by ultrasonic transducers. The detected pulse shape is related to the generating source, to the material geometry through which the pulse propagates and to the response of the ultrasonic transducer used to detect the waves. Work has been carried out to measure both the effect of wave propagation and to calibrate the response of ultrasonic transducers. Three types of ultrasonic wave may exist in a material with a non-zero shear modulus; these are longitudinal waves, shear waves and surface or Rayleigh waves. In a large number of specimen geometries, the surface wave has the largest amplitude. The response of a transducer to this wave is therefore very important. Most transducers respond to the out of plane motion of a material surface carrying ultrasonic waves. Therefore, to successfully calibrate a transducer, some absolute measurement of the out of plane motion due to surface waves must be made. An interferometer has been designed and constructed for this purpose. The calibration of ultrasonic transducers has enabled some development work to be carried oLt on high-fidelity piezoelectric transducers and on piezomagnetic transducers. It is not always possible to measure an ultrasonic pulse directly with a calibrated interferometric detector and therefore to enable a wider range of propagation problems to be investigated, various methods of ultrasonic pulse generation have been studied. These artificial sources of acoustic emission have included brittle fracture, laser impact and stimulation by piezoelectric transducers. This work has enabled theoretical calculations on pulse propagation to be verified

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

Operational modal analysis - Theory and aspects of application in civil engineering

In recent years the demand on dynamic analyses of existing structures in civil engineering has remarkably increased. These analyses are mainly based on numerical models. Accordingly, the generated results depend on the quality of the used models. Therefore it is very important that the models describe the considered systems such that the behaviour of the physical structure is realistically represented. As any model is based on assumptions, there is always a certain degree of uncertainty present in the results of a simulation based on the respective numerical model. To minimise these uncertainties in the prediction of the response of a structure to a certain loading, it has become common practice to update or calibrate the parameters of a numerical model based on observations of the structural behaviour of the respective existing system. The determination of the behaviour of an existing structure requires experimental investigations. If the numerical analyses concern the dynamic response of a structure it is sensible to direct the experimental investigations towards the identification of the dynamic structural behaviour which is determined by the modal parameters of the system. In consequence, several methods for the experimental identification of modal parameters have been developed since the 1980ies. Due to various technical restraints in civil engineering which limit the possibilities to excitate a structure with economically reasonable effort, several methods have been developed that allow a modal identification form tests with an ambient excitation. The approach of identifying modal parameters only from measurements of the structural response without precise knowledge of the excitation is known as output-only or operational modal analysis. Since operational modal analysis (OMA) can be considered as a link between numerical modelling and simulation on the one hand and the dynamic behaviour of an existing structure on the other hand, the respective algorithms connect both the concepts of structural dynamics and mathematical tools applied within the processing of experimental data. Accordingly, the related theoretical topics are revised after an introduction into the topic. Several OMA methods have been developed over the last decades. The most established algorithms are presented here and their application is illustrated by means of both a small numerical and an experimental example. Since experimentally obtained results always underly manifold influences, an appropriate postprocessing of the results is necessary for a respective quality assessment. This quality assessment does not only require respective indicators but should also include the quantification of uncertainties. One special feature in modal testing is that it is common to instrument the structure in different sensor setups to improve the spacial resolution of identified mode shapes. The modal information identified from tests in several setups needs to be merged a posteriori. Algorithms to cope with this problem are also presented. Due to the fact that the amount of data generated in modal tests can become very large, manual processing can become extremely expensive or even impossible, for example in the case of a long-term continuous structural monitoring. In these situations an automated analysis and postprocessing are essential. Descriptions of respective methodologies are therefore also included in this work. Every structural system in civil engineering is unique and so also every identification of modal parameters has its specific challenges. Some aspects that can be faced in practical applications of operational modal analysis are presented and discussed in a chapter that is dedicated specific problems that an analyst may have to overcome. Case studies of systems with very close modes, with limited accessibility as well as the application of different OMA methods are described and discussed. In this context the focus is put on several types of uncertainty that may occur in the multiple stages of an operational modal analysis. In literature only very specific uncertainties at certain stages of the analysis are addressed. Here, the topic of uncertainties has been considered in a broader sense and approaches for treating respective problems are suggested. Eventually, it is concluded that the methodologies of operatinal modal analysis and related technical solutions have been well-engineered already. However, as in any discipline that includes experiments, a certain degree of uncertainty always remains in the results. From these conclusions has been derived a demand for further research and development that should be directed towards the minimisation of these uncertainties and to a respective optimisation of the steps and corresponding parameters included in an operational modal analysis

On the control of paraplegic standing using functional electrical stimulation

This thesis is concerned with the restoration of upright standing after spinal cord injury (SCI) by the means of Functional Electrical Stimulation. In particular, the work presented in this thesis is concerned with unsupported standing, i.e. standing without any support by the arms for stabilisation. Firstly, the experimental apparatus and feedback control approach is described. Secondly, the experimental work is divided into three parts. The motivation, experimental setup and procedure as well as results and conclusions are given for each of them. The feasibility of the investigated approach was usually tested on a neurologically intact subject. The results were subsequently confirmed with a paraplegic subject. First the feasibility and fundamental limitations of unsupported standing were investigated. Assuming the subject as a single-link inverted pendulum, an improved fully dynamic control approach was employed in the first step, confirming existing results. Here, the voluntary influence by the central nervous system was minimised. However, it is naturally desirable to take advantage of the residual sensory-motor abilities of the paraplegic subject to ease the task of stabilising the body. Ankle stiffness control has been proposed in the literature to accomplish this task. Hitherto, ankle stiffness was provided by artificial actuators. In the second part we investigated the feasibility and limitations of ankle stiffness control by means of FES. The same single-link approach was employed as above. Ankle stiffness control by FES was used in the third part to enable paraplegic standing. Here, the subject was required to participate actively in the task of stable standing and, while doing so, behaving like a double-link inverted pendulum. It could be shown that FES-controlled ankle stiffness contributed crucially to the subject's ability to stand. The thesis concludes with propositions for future work

Repetitive Control Systems: Stability and Periodic Tracking beyond the Linear Case

Periodic output regulation studies the problem of steering the output of a dynamical system along a periodic reference. This is a fundamental control problem which has a great interest from a practical point of view, since most industrial activities oriented to production are based on tasks with a cyclic nature. Nevertheless this interest extends rapidly to a theoretical framework once the problem is formalized. Mathematical tools coming from different fields can be used to provide an insight to the output regulation problem in different ways. An important control technique that is classically used to achieve periodic out- put regulation si called Repetitive Control (RC) and this thesis focuses on (but is not limited to) the development and the analysis with novel tools of RC schemes. Periodic output regulation for nonlinear dynamical systems is a challenging topic. This thesis, besides of providing consistent and practically useful results in the linear case, introduces promising tools dealing with the nonlinear periodic output regulation problem, whose solution is presented for particular classes of systems. The contribution of this research is mainly theoretical and relies on the use of mathematical tools like infinite-dimensional port-Hamiltonian systems and autonomous discrete-time systems to study stability and tracking properties in RC schemes and periodic regulation in general. Differently from the classical continuous-time formulation of RC, internal model arguments are not directly used is this work to study asymptotic tracking. In this way the linear case can be reinterpreted under a new light and novel strategies to consistently attack the nonlinear case are presented. Furthermore an application-oriented chapter with experimental results is present which describes the possibility of implementing a discrete-time RC scheme involving trajectory generation and non-minimum phase systems

Bio-mimetic Spiking Neural Networks for unsupervised clustering of spatio-temporal data

Spiking neural networks aspire to mimic the brain more closely than traditional artificial neural networks. They are characterised by a spike-like activation function inspired by the shape of an action potential in biological neurons. Spiking networks remain a niche area of research, perform worse than the traditional artificial networks, and their real-world applications are limited. We hypothesised that neuroscience-inspired spiking neural networks with spike-timing-dependent plasticity demonstrate useful learning capabilities. Our objective was to identify features which play a vital role in information processing in the brain but are not commonly used in artificial networks, implement them in spiking networks without copying constraints that apply to living organisms, and to characterise their effect on data processing. The networks we created are not brain models; our approach can be labelled as artificial life. We performed a literature review and selected features such as local weight updates, neuronal sub-types, modularity, homeostasis and structural plasticity. We used the review as a guide for developing the consecutive iterations of the network, and eventually a whole evolutionary developmental system. We analysed the model’s performance on clustering of spatio-temporal data. Our results show that combining evolution and unsupervised learning leads to a faster convergence on the optimal solutions, better stability of fit solutions than each approach separately. The choice of fitness definition affects the network’s performance on fitness-related and unrelated tasks. We found that neuron type-specific weight homeostasis can be used to stabilise the networks, thus enabling longer training. We also demonstrated that networks with a rudimentary architecture can evolve developmental rules which improve their fitness. This interdisciplinary work provides contributions to three fields: it proposes novel artificial intelligence approaches, tests the possible role of the selected biological phenomena in information processing in the brain, and explores the evolution of learning in an artificial life system