 Electrical and Electronic Engineering  Research Publications
Electrical and Electronic Engineering  Research Publications
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ItemNo Preview AvailableStable NearOptimal Control of Nonlinear Switched DiscreteTime Systems: An Optimistic PlanningBased ApproachGranzotto, M ; Postoyan, R ; Busoniu, L ; Nesic, D ; Daafouz, J (IEEEINST ELECTRICAL ELECTRONICS ENGINEERS INC, 20220501)Originating in the artificial intelligence literature, optimistic planning(OP) is an algorithm that generates nearoptimal control inputs for generic nonlinear discretetime systems whose input set is finite. This technique is therefore relevant for the nearoptimal control of nonlinear switched systems for which the switching signal is the control, and no continuous input is present. However, OP exhibits several limitations, which prevent its desired application in a standard control engineering context, as it requires for instance that the stage cost takes values in [0,1], an unnatural prerequisite, and that the cost function be discounted. In this paper, we modify OP to overcome these limitations, and we call the new algorithm OPmin. New nearoptimality and performance guarantees for OPmin are derived, which have major advantages compared to those originally given for OP. We also prove that a system whose inputs are generated by OPmin in a recedinghorizon fashion exhibits stability properties. As a result, OPmin provides a new tool for the nearoptimal, stable control of nonlinear switched discretetime systems for generic cost functions.

ItemFinitehorizon discounted optimal control: stability and performanceGranzotto, M ; Postoyan, R ; Busoniu, L ; Nesic, D ; Daafouz, J (Institute of Electrical and Electronics Engineers (IEEE), 2021)Motivated by (approximate) dynamic programming and model predictive control problems, we analyse the stability of deterministic nonlinear discretetime systems whose inputs minimize a discounted finitehorizon cost. We assume that the system satisfies stabilizability and detectability properties with respect to the stage cost. Then, a Lyapunov function for the closedloop system is constructed and a uniform semiglobal stability property is ensured, where the adjustable parameters are both the discount factor and the horizon length, which corresponds to the number of iterations for dynamic programming algorithms like value iteration. Stronger stability properties such as global exponential stability are also provided by strengthening the initial assumptions. We give bounds on the discount factor and the horizon length under which stability holds. In addition, we provide new relationships between the optimal value functions of the discounted, undiscounted, infinitehorizon and finitehorizon costs respectively, which appear to be very different from those available in the literature.

ItemAsynchronous Distributed Optimization via Dual Decomposition and Block Coordinate Subgradient MethodsLin, Y ; Shames, I ; Nesic, D (IEEEINST ELECTRICAL ELECTRONICS ENGINEERS INC, 20210901)In this article, we study the problem of minimizing the sum of potentially nondifferentiable convex cost functions with partially overlapping dependences in an asynchronous manner, where communication in the network is not coordinated. We study the behavior of an asynchronous algorithm based on dual decomposition and block coordinate subgradient methods under assumptions weaker than those used in the literature. At the same time, we allow different agents to use local stepsizes with no global coordination. Sufficient conditions are provided for almost sure convergence to the solution of the optimization problem. Under additional assumptions, we establish a sublinear convergence rate that, in turn, can be strengthened to the linear convergence rate if the problem is strongly convex and has Lipschitz gradients. We also extend available results in the literature by allowing multiple and potentially overlapping blocks to be updated at the same time with nonuniform and potentially timevarying probabilities assigned to different blocks. A numerical example is provided to illustrate the effectiveness of the algorithm.

ItemDetermination of the electrical impedance of neural tissue from its microscopic cellular constituentsMonfared, O ; Tahayori, B ; Freestone, D ; Nesic, D ; Grayden, DB ; Meffin, H (IOP Publishing, 20200201)The electrical properties of neural tissue are important in a range of different applications in biomedical engineering and basic science. These properties are characterized by the electrical admittivity of the tissue, which is the inverse of the specific tissue impedance. Objective. Here we derived analytical expressions for the admittivity of various models of neural tissue from the underlying electrical and morphological properties of the constituent cells. Approach. Three models are considered: parallel bundles of fibers, fibers contained in stacked laminae and fibers crossing each other randomly in all threedimensional directions. Main results. An important and novel aspect that emerges from considering the underlying cellular composition of the tissue is that the resulting admittivity has both spatial and temporal frequency dependence, a property not shared with conventional conductivitybased descriptions. The frequency dependence of the admittivity results in nontrivial spatiotemporal filtering of electrical signals in the tissue models. These effects are illustrated by considering the example of pulsatile stimulation with a point source electrode. It is shown how changing temporal parameters of a current pulse, such as pulse duration, alters the spatial profile of the extracellular potential. In a second example, it is shown how the degree of electrical anisotropy can change as a function of the distance from the electrode, despite the underlying structurally homogeneity of the tissue. These effects are discussed in terms of different current pathways through the intra and extracellular spaces, and how these relate to near and farfield limits for the admittivity (which reduce to descriptions in terms of a simple conductivity). Significance. The results highlight the complexity of the electrical properties of neural tissue and provide mathematical methods to model this complexity.

ItemOn the Latency, Rate and Reliability Tradeoff in Wireless Networked Control Systems for IIoTLiu, W ; Nair, G ; Li, Y ; Nesic, D ; Vucetic, B ; Poor, HV (Institute of Electrical and Electronics Engineers (IEEE), 2020)Wireless networked control systems (WNCSs) provide a key enabling technique for Industry Internet of Things (IIoT). However, in the literature of WNCSs, most of the research focuses on the control perspective, and has considered oversimplified models of wireless communications which do not capture the key parameters of a practical wireless communication system, such as latency, data rate and reliability. In this paper, we focus on a WNCS, where a controller transmits quantized and encoded control codewords to a remote actuator through a wireless channel, and adopt a detailed model of the wireless communication system, which jointly considers the interrelated communication parameters. We derive the stability region of the WNCS. If and only if the tuple of the communication parameters lies in the region, the average cost function, i.e., a performance metric of the WNCS, is bounded. We further obtain a necessary and sufficient condition under which the stability region is nbounded, where n is the control codeword blocklength. We also analyze the average cost function of the WNCS. Such analysis is nontrivial because the finitebit controlsignal quantizer introduces a nonlinear and discontinuous quantization function which makes the performance analysis very difficult. We derive tight upper and lower bounds on the average cost function in terms of latency, data rate and reliability. Our analytical results provide important insights into the design of the optimal parameters to minimize the average cost within the stability region.

ItemAdaptive Scan for Atomic Force Microscopy Based on Online Optimization: Theory and ExperimentWang, K ; Ruppert, MG ; Manzie, C ; Nesic, D ; Yong, YK (Institute of Electrical and Electronics Engineers, 20200501)A major challenge in atomic force microscopy is to reduce the scan duration while retaining the image quality. Conventionally, the scan rate is restricted to a sufficiently small value in order to ensure a desirable image quality as well as a safe tipsample contact force. This usually results in a conservative scan rate for samples that have a large variation in aspect ratio and/or for scan patterns that have a varying linear velocity. In this paper, an adaptive scan scheme is proposed to alleviate this problem. A scan linebased performance metric balancing both imaging speed and accuracy is proposed, and the scan rate is adapted such that the metric is optimized online in the presence of aspect ratio and/or linear velocity variations. The online optimization is achieved using an extremumseeking approach, and a semiglobal practical asymptotic stability result is shown for the overall system. Finally, the proposed scheme is demonstrated via both simulation and experiment.

ItemSecurity metrics and synthesis of secure control systemsMurguia, C ; Shames, I ; Ruths, J ; Nešić, D (Elsevier Inc., 20200501)The term stealthy has come to encompass a variety of techniques that attackers can employ to avoid being detected. In this manuscript, for a class of perturbed linear timeinvariant systems, we propose two security metrics to quantify the potential impact that stealthy attacks could have on the system dynamics by tampering with sensor measurements. We provide analysis tools to quantify these metrics for given system dynamics, control, and system monitor. Then, we provide synthesis tools (in terms of semidefinite programs) to redesign controllers and monitors such that the impact of stealthy attacks is minimized and the required attackfree system performance is guaranteed.

ItemPractical exponential stability and closeness of solutions for singularly perturbed systems via averagingDeghat, M ; Ahmadizadeh, S ; Nesic, D ; Manzie, C (PERGAMONELSEVIER SCIENCE LTD, 20210125)This paper studies the behavior of singularly perturbed nonlinear differential equations with boundarylayer solutions that do not necessarily converge to an equilibrium. Using the average for the derivative of the slow state variables and assuming the boundarylayer solutions converge exponentially fast to a bounded set, which is possibly parameterized by the slow variable, results on the closeness of solutions of the singularly perturbed system to the solutions of the reduced average and boundarylayer systems over a finite time interval are presented. The closeness of solution error is shown to be of order O(ε) where ε is the perturbation parameter. Moreover, under the additional assumption of exponential stability of the reduced average system, practical exponential stability of the solutions of the singularly perturbed system is provided.

ItemScan Rate Adaptation for AFM Imaging Based on Performance Metric OptimizationWang, K ; Ruppert, MG ; Manzie, C ; Nesic, D ; Yong, YK (Institute of Electrical and Electronics Engineers (IEEE), 202002)Constantforce contactmode atomic force microscopy (AFM) relies on a feedback control system to regulate the tip–sample interaction during imaging. Due to limitations in actuators and control, the bandwidth of the regulation system is typically small. Therefore, the scan rate is usually limited in order to guarantee a desirable image quality for a constantrate scan. By adapting the scan rate online, further performance improvement is possible, and the conditions to this improvement have been explored qualitatively in a previous study for a wide class of possible scan patterns. In this article, a quantitative assessment of the previously proposed adaptive scan scheme is investigated through experiments that explore the impact of various degrees of freedom in the algorithm. Further modifications to the existing scheme are proposed and shown to improve the closedloop performance. The flexibility of the proposed approach is further demonstrated by applying the algorithm to tappingmode AFM.

ItemUniting ObserversAstolfi, D ; Postoyan, R ; Nesic, D (Institute of Electrical and Electronics Engineers, 20200701)We propose a framework for designing observers possessing global convergence properties and desired asymptotic behaviors for the state estimation of nonlinear systems. The proposed scheme consists in combining two given continuoustime observers: One, denoted as global, ensures (approximate) convergence of the estimation error for any initial condition ranging in some prescribed set, while the other, denoted as local, guarantees a desired local behavior. We make assumptions on the properties of these two observers, and not on their structures, and then explain how to unite them as a single scheme using hybrid techniques. Two case studies are provided to demonstrate the applicability of the framework. Finally, a numerical example is presented.