| 1. |
- Altaf, Faisal, et al.
(författare)
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Wireless event-triggered controller for a 3D tower crane lab process
- 2011
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Ingår i: 2011 19th Mediterranean Conference on Control and Automation, MED 2011. - 9781457701252 - 9781457701245 ; , s. 994-1001
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Konferensbidrag (refereegranskat)abstract
- This paper studies the design and real-time implementation of an event-triggered controller for a nonlinear 3D tower crane where the communication between the controller and the actuators is performed over a low-power wireless network. A flexible Event-Generation Circuit (EGC) is proposed in order to implement event-driven controllers for Networked Control Systems. Furthermore, a detailed experimental analysis on the performance of the event-triggered controller and the influence of packet losses on the transmitted actuation messages are presented. The results show that the event-triggered controllers in networked control systems are able to maintain the same level of performance as compared to periodic controllers, while increasing the sensors/actuators lifetime by reducing network bandwidth utilization.
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| 2. |
- Teixeira, Andre, et al.
(författare)
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Networked Control Systems under Cyber Attacks with Applications to Power Networks
- 2010
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Ingår i: 2010 AMERICAN CONTROL CONFERENCE. - : IEEE. - 9781424474271 ; , s. 3690-3696
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Konferensbidrag (refereegranskat)abstract
- Networked control systems under certain cyber attacks are analyzed. The communication network of these control systems make them vulnerable to attacks from malicious outsiders. Our work deals with two types of attacks: attacks on the network nodes and attacks on the communication between the nodes. We propose a distributed scheme to detect and isolate the attacks using observers. Furthermore, we discuss how to reduce the number of observer nodes while maintaining the coverage of the entire network. The results are applied to two classes of networked control systems: a network running the consensus protocol and a power network defined by the linearized swing equation. Sufficient conditions for the existence of the proposed attack detection scheme are provided for the first class of systems. For the second class, we provide a necessary condition for the existence of the proposed detection scheme.
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| 3. |
- Ramesh, Chithrupa, et al.
(författare)
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Design of State-Based Schedulers for a Network of Control Loops
- 2013
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Ingår i: IEEE Transactions on Automatic Control. - 0018-9286 .- 1558-2523. ; 58:8, s. 1962-1975
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Tidskriftsartikel (refereegranskat)abstract
- For a closed-loop system with a contention-based multiple access network on its sensor link, the medium access controller (MAC) may discard some packets when the traffic on the link is high. We use a local state-based scheduler to select a few critical data packets to send to the MAC. In this paper, we analyze the impact of such a scheduler on the closed-loop system in the presence of traffic, and show that there is a dual effect with state-based scheduling. In general, this makes the optimal scheduler and controller hard to find. However, by removing past controls from the scheduling criterion, we find that certainty equivalence holds. This condition is related to the classical result of Bar-Shalom and Tse, and it leads to the design of an innovations-based scheduler with a certainty equivalent controller. However, this controller is not an equivalent design for the optimal controller, in the sense of Witsenhausen. The computation of the estimate can be simplified by introducing a symmetry constraint on the scheduler. Based on these findings, we propose a dual predictor architecture for the closed-loop system, which ensures separation between scheduler, observer and controller. We present an example of this architecture, which illustrates a network-aware event-triggering mechanism.
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| 4. |
- Sandberg, Henrik
(författare)
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An Extension to Balanced Truncation With Application to Structured Model Reduction
- 2010
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Ingår i: IEEE Transactions on Automatic Control. - : IEEE. - 0018-9286 .- 1558-2523. ; 55:4, s. 1038-1043
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Tidskriftsartikel (refereegranskat)abstract
- Balanced truncation is a standard method for model reduction, possessing many desirable properties such as preservation of model stability and a priori error bounds. Balanced truncation is conducted using controllability and observability Gramians. Generalized Gramians can be found by solving a set of linear matrix inequalities. In this paper, we show that these linear matrix inequalities can be extended so that the number of decision variables are at least doubled, leading to the concept of extended Gramians. Herein it is shown that the desirable properties of balanced truncation also hold with extended Gramians. The extended Gramians are especially useful for improving error bounds and for models possessing additional structural constraints, as is demonstrated by means of examples herein.
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| 5. |
- Sou, Kin Cheong, et al.
(författare)
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On the Exact Solution to a Smart Grid Cyber-Security Analysis Problem
- 2013
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Ingår i: IEEE Transactions on Smart Grid. - : IEEE Press. - 1949-3053 .- 1949-3061. ; 4:2, s. 856-865
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Tidskriftsartikel (refereegranskat)abstract
- This paper considers a smart grid cyber-security problem analyzing the vulnerabilities of electric power networks to false data attacks. The analysis problem is related to a constrained cardinality minimization problem. The main result shows that an l1 relaxation technique provides an exact optimal solution to this cardinality minimization problem. The proposed result is based on a polyhedral combinatorics argument. It is different from well-known results based on mutual coherence and restricted isometry property. The results are illustrated on benchmarks including the IEEE 118-bus, IEEE 300-bus and the Polish 2383-bus and 2736-bus systems.
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| 6. |
- Teixeira, André, et al.
(författare)
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Cyber-Secure and Resilient Architectures for Industrial Control Systems
- 2015
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Ingår i: Smart Grid Security: Innovative Solutions for a Modernized Grid. - : Elsevier. - 9780128023549 ; , s. 149-183
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- In this chapter, we survey cyber security solutions for control and monitoring systems that are used to manage the Smart Grid. We start with a short review of the history and use of Industrial Control Systems (ICSs) and Supervisory Control and Data Acquisition (SCADA) systems, and how cyber security in control systems has recently become a major concern, in the wake of the Stuxnet and other recently discovered malware. We follow up with surveying information technology and control-centric security tools that can be used to improve the resilience of industrial control systems. Feedback control loops are core components in the Smart Grid, as they enable the maximal utilization of the physical infrastructure and its resources. As the number of control loops in the Smart Grid increases, the cyber security challenges faced by ICSs become increasingly important within the Smart Grid's context. To highlight such novel challenges, we give an overview of the envisioned control loops in future Smart Grids, and discuss the potential impact of cyber threats targeting critical Smart Grid functionalities. As a case study, false-data injection attacks on power transmission networks are considered. The level of resilience to such attacks is assessed through a control-centric risk assessment methodology, which is also used for allocating the deployment of more modern and secure equipment. The chapter ends with a discussion of future research challenges in the area.
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| 7. |
- Teixeira, André, et al.
(författare)
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Quantifying Cyber-Security for Networked Control Systems
- 2013
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Ingår i: Control of Cyber-Physical Systems. - Switzerland : Springer International Publishing. - 0170-8643. - 9783319011585 ; , s. 123-142
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Bokkapitel (refereegranskat)abstract
- In this paper we consider a typical architecture for a networked control system under false-data injection attacks. Under a previously proposed adversary modeling framework, various formulations for quantifying cyber-security of control systems are proposed and formulated as constrained optimization problems. These formulations capture trade-offs in terms of attack impact on the control performance, attack detectability, and adversarial resources. The formulations are then discussed and related to system theoretic concepts, followed by numerical examples illustrating the various trade-offs for a quadruple-tank process.
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| 8. |
- Teixeira, André, et al.
(författare)
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Security of smart distribution grids : Data integrity attacks on integrated volt/VAR control and countermeasures
- 2014
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Ingår i: American Control Conference (ACC), 2014. - : IEEE conference proceedings. - 9781479932726 ; , s. 4372-4378, s. 4372-4378
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Konferensbidrag (refereegranskat)abstract
- We examine the feasibility of an attack on themeasurements that will be used by integrated volt-var control(VVC) in future smart power distribution systems. The analysisis performed under a variety of assumptions of adversarycapability regarding knowledge of details of the VVC algorithmused, system topology, access to actual measurements, andability to corrupt measurements. The adversary also faces anoptimization problem, which is to maximize adverse impactwhile remaining stealthy. This is achieved by first identifyingsets of measurements that can be jointly but stealthily corrupted.Then, the maximal impact of such data corruptionis computed for the case where the operator is unaware ofthe attack and directly applies the configuration from theintegrated VVC. Furthermore, since the attacker is constrainedto remaining stealthy, we consider a game-theoretic frameworkwhere the operator chooses settings to maximize observabilityand constrain the adversary action space.
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| 9. |
- Teixeira, André, et al.
(författare)
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Voltage control for interconnected microgrids under adversarial actions
- 2015
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Ingår i: IEEE International Conference on Emerging Technologies and Factory Automation, ETFA. - : IEEE conference proceedings. - 9781467379298 ; , s. 1-8
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Konferensbidrag (refereegranskat)abstract
- In this paper, we study the impact of adversarial actions on voltage control schemes in interconnected microgrids. Each microgrid is abstracted as a power inverter that can be controlled to regulate its voltage magnitude and phase-angle independently. Moreover, each power inverter is modeled as a single integrator, whose input is given by a voltage droop-control policy that is computed based on voltage magnitude and reactive power injection measurements. Under mild assumptions, we then establish important properties of the nominal linearized closed-loop system, such as stability, positivity, and diagonal dominance. These properties play an important role when characterizing the potential impact of different attack scenarios. In particular, we discuss two attack scenarios where the adversary corrupts measurement data and reference signals received by the voltage droop controllers. The potential impact of instances of each scenario is analyzed using control-theoretic tools, which may be used to develop methodologies for identifying high-risk attack scenarios, as is illustrated by numerical examples.
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| 10. |
- Ziemann, Ingvar
(författare)
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Statistical Learning, Dynamics and Control : Fast Rates and Fundamental Limits for Square Loss
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Learning algorithms play an ever increasing role in modern engineering solutions. However, despite many recent successes, their performance in the context of dynamical and control systems is not exactly well-understood. This becomes especially pertinent as we move toward deploying these algorithms in safety-critical systems such as our power grids or smart mobility systems. These shortcomings motivate the present thesis. Broadly, by blending tools from control theory, information theory and statistical learning theory, this thesis seeks to further our understanding of when learning in the context of (controlled) dynamical systems is easy or hard. That is, we seek to quantify those properties which are key, and those which are not, for learning to succeed in this context.In the first part of the thesis, we study learning with square loss (regression) in a realizable time-series framework with martingale difference noise. Our main result is a fast rate excess risk boundwhich shows that whenever a\emph{trajectory hypercontractivity} condition holds, the risk of the least-squares estimator on dependent data matches the \iid\ rate order-wise after a burn-in time.In comparison, many existing results in learning from dependent data have rates where the effective sample size is deflated by a factor of the mixing-time of the underlying process, even after the burn-in time. However, recent work on linear dynamical systems and generalized linear models has shown that this is deflation is not always necessary. The main contribution of the first part of the thesis is to extend this observation to a much wider class of examples. Furthermore, our results allow the covariate process to exhibit long range correlations which are substantially weaker than geometric ergodicity. We present several examples for when this occurs: bounded function classes forwhich the $L^2$ and $L^{2+\e}$ norms are equivalent, ergodic finite state Markov chains, various parametric models including linear dynamical systems and generalized linear models, and a broad family of infinite dimensional $\ell^2(\mathbb{N})$-ellipsoids. In the second part of the thesis, we study fundamental limits to learning-based solutions of the linear quadratic Gaussian (LQG) problem. The majority of this part is devoted to the online (or adaptive) control problem, in which a learner (or control engineer) seeks to control an unknown linear system subject to a quadratic (square) loss function with minimal regret (cumulative suboptimality). %To solve this problem, the learner must solve a trade-off between exploration and exploitation. Put differently,The learner faces the dual objectives of controlling the system to the best of their present knowledge while simultaneously ensuring that enough information about the system is gathered. We prove fundamental limits - regret lower bounds - to this problem by relating the regret of any policy to an associated Fisher information and applying the Van Trees' Inequality (Bayesian Cramér-Rao). We instantiate our bounds for state-feedback and partially observed systems. We show that ill-conditioned systems - in terms of their controllability and observability structure - lead to larger regret. In other words, classical control-theoretic limitations carry over to the learning-based setting. We further show that systems as simple as integrators lead to \emph{exponential} in the problem dimension regret lower bounds. Finally, we also study the variance of policy gradient methods. We derive results similar in spirit to those for the online LQG problem: ill-conditioned systems, as per poor controllability or observability, yield noisier gradients.
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