Electrical and Electronic Engineering - Theses
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ItemMulti-observer approach for estimation and control under adversarial attacks( 2019)Traditional control systems composed of interconnected controllers, sensors, and actuators use point-to-point communication architectures. This is no longer suitable when new requirements -- such as modularity, decentralisation of control, integrated diagnostics, quick and easy maintenance, and low cost -- are necessary. To meet these requirements, Networked Control Systems (NCSs) have emerged as a technology that combines control, communication, and computation, and offers the necessary flexibility to meet new demands in distributed and large scale systems. However, these new architectures, especially wireless NCSs, are more susceptible to adversarial attacks. For instance, one of the most well-known examples of attacks on NCSs is the StuxNet virus that targeted Siemens' supervisory control and data acquisition systems which are used in many industrial processes. Another very recent incident is the attack on the Ukraine power grid system, where an adversarial attack caused a power outage affecting more than 80,000 people for almost 3 hours. These incidents (and many other not mentioned here) show that there is an acute need for strategic defence mechanisms to identify and deal with adversarial attacks on NCSs. In this thesis, based on sensor and actuator redundancy, we develop a ``multi-observer based estimation framework'' to address the problem of state estimation for discrete-time nonlinear systems with general dynamics under sensor and actuator false data injection attacks. Although there exist results in the literature addressing similar problems, in general, they are only applicable to some specific classes of nonlinear systems. To the best of the author's knowledge, a unifying estimation framework that works for general nonlinear systems in the presence of attacks has not been proposed. The estimation scheme provided here can be applied to a large class of nonlinear systems as long as a bank of observers with certain stability properties exist. Once an estimate of the system states is obtained from the multi-observer estimator, we provide detection and isolation algorithms for attack detection and for identifying attacked sensors and actuators. For nonlinear systems in the presence of sensor attacks, process disturbance and measurement noise, we detect and isolate attacked sensors by designing multiple observers and comparing their estimates. For noise-free nonlinear systems under sensor and actuator attacks, we isolate attacked sensors and actuators by reconstructing the attack signals. Furthermore, for LTI systems, we provide a simple yet effective control method to stabilize the system despite of sensor and actuator attacks by switching off the isolated actuators and closing the system loop with the proposed estimator and a switching output feedback controller. Finally, we use a class of nonlinear systems with positive-slope nonlinearities under sensor attacks and measurement noise as a detailed case study where we provide a deeper discussion about the tools that we propose. In particular, we give sufficient conditions under which our tools are guaranteed to work; we also give sufficient conditions under which such methods cannot work. These results have been published in our previous conference papers.
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ItemModeling, stabilizing control and trajectory tracking of a spherical inverted pendulum( 2007)This dissertation addresses modelling, stabilization and trajectory tracking of a spherical inverted pendulum. The spherical inverted pendulum is a cylindrical beam attached to a horizontal plane via a universal joint. The universal joint is free to move in a plane, under the influence of a planar force the control signal. Because of the gravity, the downward and upward positions are respectively the stable equilibrium and the unstable equilibrium of the uncontrolled system. The model of the pendulum resembles several other systems found in robotics, defence and aerospace engineering and, hence, controller design for this system provides useful insight into control of several other engineering systems. For instance, the spherical inverted pendulum serves as an abstraction for a vector thrust body hovering at a constant altitude. The control objective for stabilization is to use the planar force to drive the spherical inverted pendulum in such a way that the upright position is asymptotically stable with a large domain of attraction (e.g., “global’, semi-global”). Moreover, the pendulum’s universal joint has to be returned to a given point on the plane and remain there. We refer to this control objective as non-local stabilization in contrast to the local stabilization with (small) bounded domain of attraction. The control objective for trajectory tracking is to use the planar force to drive the spherical inverted pendulum in such a manner that the universal joint of the pendulum tracks some smooth desired output profile asymptotically while keeping the pendulum pointing upwards. The control problem is challenging because the system in nonlinear, unstable (about the upper equilibrium), underactuated and MIMO. Our investigation concentrates on nonlinear control. We derive three nonlinear models in three sets of generalized coordinates for the pendulum by applying the Euler-Lagrange’s equations. This generalizes several simplified models found in the literature. For the purpose of comparison, we first present several benchmark linear controllers and identify their shortcomings (e.g., small bounded domain of attraction and large tracking errors due to limited bandwidth). Several nonlinear control strategies are proposed to deal with the full nonlinear model to overcome those shortcomings. First, and exact output tracking controller is given based on the nonlinear stable inversion tool for nonlinear non-minimum-phase systems. By specifying some desired smooth output profiles, the proposed controller asymptotically tracks these desired output trajectories while keeping the pendulum upwards. Second, a high-low gain stabilizing controller is formulated for the pendulum by incorporating the forwarding design methodology with several other design approaches, to yield a “global” domain of attraction (the whole upper hemisphere). Next, inspired by nested saturating design tools and some low gain design tools, a decentralized multi-time scale linear stabilizing control strategy is proposed for a class of nonlinear interconnected chains of integrators by taking advantage of singular perturbation ideas. This strategy is then applied to the spherical inverted pendulum. The controller yields a “semi-global” domain of attraction. For the purpose of comparison, we also compute a nonlinear control law for the pendulum using a design idea based on the method of controlled Lagrangians found in the literature. Finally and most importantly, we compare through extensive simulations some important aspects of the performance of the various controllers.
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ItemModelling and control design of river systems( 2011)Farming consumes a large amount of water usage and it is reported that large portion of this water is wasted through inefficient water distribution from river to farms. More efficient water distribution and preservation of environmental demands can be achieved through better control and decision support systems. In order to design better control and decision support systems, a river model is required. This model needs to be able to capture the relevant river dynamics and easy to be used for control design. Traditionally, the Saint Venant equations have been used to model river systems. These equations are nonlinear hyperbolic partial differential equation (PDE) and are solved numerically using Preissmann scheme. The simulated Saint Venant equations are compared against operational data from the Broken River, and it is found that the Saint Venant equations are accurate in representing the river systems. Through further study, it is found that a single segmentation, i.e. treating the river as one long stretch with uniform geometry is sufficiently accurate for representation of the river for the purpose of control design. For the representation of meandering river, the Saint Venant equations are as accurate a two-dimensional flow model. The nonlinearities in the Saint Venant equations are also investigated. From the nonlinearity test, it is found that the Saint Venant equations are approximately linear within an operating region. The Saint Venant equations are difficult to use for control design. An alternative model is therefore sought. Based on the operational data from the Broken River, simple time delay model (TDM) and integrator delay model (IDM) are proposed and estimated using system identification procedures. These models are found to be accurate in capturing the relevant dynamics of the river system. Furthermore, they are easy to use for control design. It is found that the time delay varies with the flow and hence controllers must be robust to variations in the time delay. A comparison between both TDM and IDM and the Saint Venant equations shows that they are as accurate as the Saint Venant equations within the operating range. The TDM and IDM are desirable as they are easier to be used for control design and decision support system. The TDM and IDM are used to design Model Predictive Control (MPC) to control the river system. The choice of using MPC is motivated by the fact that MPC handles constraints very well. Despite that, tuning the weights in the MPC cost function is not trivial. The methods of reverse engineering are used to obtain these weights. Building on the results of existing method of reverse engineering used in the literatures, two additional methods are developed. In addition, the design of MPC from scratch is also considered. A realistic year long simulations using both MPCs on the Broken River is carried out. The MPCs are compared with the current manual operation and a decentralised control configuration. The results show that with MPCs, significant water savings, improvement of water delivery service to the irrigators and the environmental demands satisfaction are achieved.