Dual Rate Control for Security in Cyber-physical Systems

We consider malicious attacks on actuators and sensors of a feedback system which can be modeled as additive, possibly unbounded, disturbances at the digital (cyber) part of the feedback loop. We precisely characterize the role of the unstable poles and zeros of the system in the ability to detect stealthy attacks in the context of the sampled data implementation of the controller in feedback with the continuous (physical) plant. We show that, if there is a single sensor that is guaranteed to be secure and the plant is observable from that sensor, then there exist a class of multirate sampled data controllers that ensure that all attacks remain detectable. These dual rate controllers are sampling the output faster than the zero order hold rate that operates on the control input and as such, they can even provide better nominal performance than single rate, at the price of higher sampling of the continuous output

Time-and event-driven communication process for networked control systems: A survey

Copyright © 2014 Lei Zou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.In recent years, theoretical and practical research topics on networked control systems (NCSs) have gained an increasing interest from many researchers in a variety of disciplines owing to the extensive applications of NCSs in practice. In particular, an urgent need has arisen to understand the effects of communication processes on system performances. Sampling and protocol are two fundamental aspects of a communication process which have attracted a great deal of research attention. Most research focus has been on the analysis and control of dynamical behaviors under certain sampling procedures and communication protocols. In this paper, we aim to survey some recent advances on the analysis and synthesis issues of NCSs with different sampling procedures (time-and event-driven sampling) and protocols (static and dynamic protocols). First, these sampling procedures and protocols are introduced in detail according to their engineering backgrounds as well as dynamic natures. Then, the developments of the stabilization, control, and filtering problems are systematically reviewed and discussed in great detail. Finally, we conclude the paper by outlining future research challenges for analysis and synthesis problems of NCSs with different communication processes.This work was supported in part by the National Natural Science Foundation of China under Grants 61329301, 61374127, and 61374010, the Royal Society of the UK, and the Alexander von Humboldt Foundation of Germany

Recent advances on filtering and control for nonlinear stochastic complex systems with incomplete information: A survey

This Article is provided by the Brunel Open Access Publishing Fund - Copyright @ 2012 Hindawi PublishingSome recent advances on the filtering and control problems for nonlinear stochastic complex systems with incomplete information are surveyed. The incomplete information under consideration mainly includes missing measurements, randomly varying sensor delays, signal quantization, sensor saturations, and signal sampling. With such incomplete information, the developments on various filtering and control issues are reviewed in great detail. In particular, the addressed nonlinear stochastic complex systems are so comprehensive that they include conventional nonlinear stochastic systems, different kinds of complex networks, and a large class of sensor networks. The corresponding filtering and control technologies for such nonlinear stochastic complex systems are then discussed. Subsequently, some latest results on the filtering and control problems for the complex systems with incomplete information are given. Finally, conclusions are drawn and several possible future research directions are pointed out.This work was supported in part by the National Natural Science Foundation of China under Grant nos. 61134009, 61104125, 61028008, 61174136, 60974030, and 61074129, the Qing Lan Project of Jiangsu Province of China, the Project sponsored by SRF for ROCS of SEM of China, the Engineering and Physical Sciences Research Council EPSRC of the UK under Grant GR/S27658/01, the Royal Society of the UK, and the Alexander von Humboldt Foundation of Germany

Dynamic output-feedback passivity control for fuzzy systems under variable sampling

This paper concerns the problem of dynamic output-feedback control for a class of nonlinear systems with nonuniform uncertain sampling via Takagi-Sugeno (T-S) fuzzy control approach. The sampling is not required to be periodic, and the state variables are not required to be measurable. A new type fuzzy dynamic output-feedback sampled-data controller is constructed, and a novel time-dependent Lyapunov-Krasovskii functional is chosen for fuzzy systems under variable sampling. By using Lyapunov stability theory, a sufficient condition for very-strict passive analysis of fuzzy systems with nonuniformuncertain sampling is derived. Based on this condition, a novel fuzzy dynamic output-feedback controller is designed such that the closed-loop system is very-strictly passive. The existence condition of the controller can be solved by convex optimization approach. Finally, a numerical example is provided to demonstrate the effectiveness of the proposed method

Networked Control System Design and Parameter Estimation

Networked control systems (NCSs) are a kind of distributed control systems in which the data between control components are exchanged via communication networks. Because of the attractive advantages of NCSs such as reduced system wiring, low weight, and ease of system diagnosis and maintenance, the research on NCSs has received much attention in recent years. The first part (Chapter 2 - Chapter 4) of the thesis is devoted to designing new controllers for NCSs by incorporating the network-induced delays. The thesis also conducts research on filtering of multirate systems and identification of Hammerstein systems in the second part (Chapter 5 - Chapter 6). Network-induced delays exist in both sensor-to-controller (S-C) and controller-to-actuator (C-A) links. A novel two-mode-dependent control scheme is proposed, in which the to-be-designed controller depends on both S-C and C-A delays. The resulting closed-loop system is a special jump linear system. Then, the conditions for stochastic stability are obtained in terms of a set of linear matrix inequalities (LMIs) with nonconvex constraints, which can be efficiently solved by a sequential LMI optimization algorithm. Further, the control synthesis problem for the NCSs is considered. The definitions of H₂ and H∞ norms for the special system are first proposed. Also, the plant uncertainties are considered in the design. Finally, the robust mixed H₂/H∞ control problem is solved under the framework of LMIs. To compensate for both S-C and C-A delays modeled by Markov chains, the generalized predictive control method is modified to choose certain predicted future control signal as the current control effort on the actuator node, whenever the control signal is delayed. Further, stability criteria in terms of LMIs are provided to check the system stability. The proposed method is also tested on an experimental hydraulic position control system. Multirate systems exist in many practical applications where different sampling rates co-exist in the same system. The l₂-l∞ filtering problem for multirate systems is considered in the thesis. By using the lifting technique, the system is first transformed to a linear time-invariant one, and then the filter design is formulated as an optimization problem which can be solved by using LMI techniques. Hammerstein model consists of a static nonlinear block followed in series by a linear dynamic system, which can find many applications in different areas. New switching sequences to handle the two-segment nonlinearities are proposed in this thesis. This leads to less parameters to be estimated and thus reduces the computational cost. Further, a stochastic gradient algorithm based on the idea of replacing the unmeasurable terms with their estimates is developed to identify the Hammerstein model with two-segment nonlinearities. Finally, several open problems are listed as the future research directions

H∞ design of general multirate sampled-data control systems

Direct digital design of general multirate sampled-data systems is considered. To tackle causality constraints, a new and natural framework is proposed using nest operators and nest algebras. Based on this framework explicit solutions to the H∞ and H2 multirate control problems are developed in the frequency domain

Sampled-data Networked Control Systems: A Lyapunov-Krasovskii Approach

The main goal of this thesis is to develop computationally efficient methods for stability analysis and controller synthesis of sampled-data networked control systems. In sampled-data networked control systems, the sensory information and feedback signals are exchanged among different components of the system (sensors, actuators, and controllers) through a communication network. Stabilization of sampled-data networked control systems is a challenging problem since the introduction of multirate sample and holds, time-delays, and packet losses into the system degrades its performance and can lead to instability. A diverse range of systems with linear, piecewise affine (PWA), and nonlinear vector fields are studied in this thesis. PWA systems are a class of state-based switched systems with affine vector field in each mode. Stabilization of PWA networked control systems are even more challenging since they simultaneously involve switches due to the hybrid vector fields (state-based switching) and switches due to the sample and hold devices in the network (event-based switching). The objectives of this thesis are: (a) to design controllers that guarantee exponential stability of the system for a desired sampling period; (b) to design observers that guarantee exponential convergence of the estimation error to the origin for a desired sampling period; and (c) given a controller, to find the maximum allowable network-induced delay that guarantees exponential stability of the sampled-data networked control system. Lyapunov-Krasovskii based approaches are used to propose sufficient stability and stabilization conditions for sampled-data networked control systems. Convex relaxation techniques are employed to cast the proposed stability analysis and controller synthesis criteria in terms of linear matrix inequalities that can be solved efficiently

Modelado, análisis y diseño de sistemas de control con muestreo no convencional

[EN] The Thesis deals with the study of the non-conventional sampled-data systems, covering the following phases: · Modelling: On the one hand, a series of recurrence laws based on the internal representation appears. These laws are applicable to three different scenes: continuous process, continuous process preceded of ZOH and discrete process. On the other hand, a new external representation based modelling is introduced, which adopts an approximation (a kind of average for the discrete variable z) when the irregular sampling (within metaperiod) case is considered like a deviation of the regular one. Finally, by means of simulation it is easy to observe the considered approximation is excellent, by comparing the system response for the external representation modelling with the internal one (always exact). · Analysis: Both the time domain and the frequency one are considered, working with P-I-D controllers and from the point of view of the internal representation. The sampling pattern is shown to be considered as a valid design parameter. Given the complexity to study the subject in a classic simulation environment, an interactive application in SysQuake is developed. · Design: Starting from the rate decomposition of the controller with nonconventional structure and from the external representation, three kinds of different classic controllers are studied: PID, cancellation (minimum and finite time) and pole placement. The objective is to adapt them in order to work correctly when the system receives the control actions according to a non-conventional scheme. This fact usually happens when delays appear in the controller-system communication. In this case, it is observed a clear improvement over the system response when this kind of controllers versus the classical bi-rate ones is used. Finally, by means of the Tornado-VxWorks real-time operating system, a real-time platform is implemented to verify the simulation results.[ES] La Tesis se centra en el estudio de los sistemas muestreados de forma no convencional, cubriendo las fases siguientes: · Modelado: Por un lado, se presenta una serie de leyes de recurrencia basadas en la representación interna, las cuales son deducidas a partir del método de las sustituciones sucesivas sobre las ecuaciones de estado y salida del sistema. Dichas leyes son aplicables a tres escenarios distintos: proceso continuo, proceso continuo antecedido de ZOH y proceso discreto. Por otro lado, se propone un método novedoso de modelado de este tipo de sistemas basado en la representación externa, el cual adopta una aproximación (variable discreta z media) al considerar el caso de muestreo irregular dentro del metaperiodo como una desviación del regular. Finalmente, mediante simulación se comprueba que la aproximación considerada es excelente, comparando la respuesta que para un sistema ofrece el modelado en representación externa con la que se obtiene del modelado en representación interna (siempre más exacto). · Análisis: Se ha considerado tanto el dominio temporal como el frecuencial, trabajando con reguladores tipo P-I-D y siempre desde el punto de vista de la representación interna. Se pone de manifiesto que el patrón de muestreo puede ser considerado como un parámetro más de diseño. Dada la complejidad de estudiar el tema en un entorno de simulación clásico, se ha desarrollado una aplicación en SysQuake que ayuda a llevar a cabo la tarea de análisis de manera más sencilla, al permitir la interactividad. · Diseño: Basándose en la descomposición en frecuencias del controlador con estructura no convencional y en la representación externa, se estudia tres tipos de controladores clásicos distintos: PID, reguladores de cancelación (tiempo mínimo y finito) y de asignación de polos. Se trata de adaptarlos para lograr un funcionamiento apropiado cuando el sistema sobre el que actúen reciba dichas acciones siguiendo un esquema no convencional. Este hecho suele suceder al aparecer retardos en la comunicación controlador-planta. Se pone de manifiesto la mejora que supone sobre las prestaciones del sistema la utilización de este tipo de controladores frente a la utilización de controladores bifrecuencia clásicos, ya que éstos generan sus acciones de control sin tener en cuenta la existencia de posibles retardos. Mediante el uso de Tornado-VxWorks se ha implementado una plataforma de tiempo real que permite verificar los resultados de la[CA] La Tesi se centra en l'estudi dels sistemes mostrejats de forma no convencional, cobrint les fases següents: · Modelatge: D'una banda, es presenta una sèrie de lleis de recurrència basades en la representació interna, les quals són deduïdes a partir del mètode de les substitucions successives sobre les equacions d'estat i eixida del sistema. Dites lleis són aplicables a tres escenaris distints: procés continu, procés continu antecedit de ZOH i procés discret. D'altra banda, es proposa un mètode nou de modelatge d'este tipus de sistemes basat en la representació externa, el qual adopta una aproximació (variable discreta z mitjana) al considerar el cas de mostreig irregular dins del metaperiode com una desviació del regular. Finalment, per mitjà de simulació es comprova que l'aproximació considerada és excel·lent, comparant la resposta que per a un sistema oferix el modelatge en representació externa amb la que s'obté del modelatge en representació interna (sempre més exacte). · Anàlisi: S'ha considerat tant el domini temporal com el freqüencial, treballant amb reguladors tipus P-I-D i sempre des del punt de vista de la representació interna. Es posa de manifest que el patró de mostreig pot ser considerat com a un paràmetre més de disseny. Donada la complexitat d'estudiar el tema en un entorn de simulació clàssic, s'ha desenrotllat una aplicació en SysQuake que ajuda a dur a terme la tasca d'anàlisi de manera més senzilla, al permetre la interactivitat. · Disseny: Basant-se en la descomposició en freqüències del controlador amb estructura no convencional i en la representació externa, s'estudia tres tipus de controladors clàssics distints: PID, reguladors de cancel·lació (temps mínim i finit) i d'assignació de pols. Es tracta d'adaptar-los per a aconseguir un funcionament apropiat quan el sistema sobre el qual actuen reba dites accions seguint un esquema no convencional. Este fet sol succeir a l'aparéixer retards en la comunicació controlador-planta. Es posa de manifest la millora que suposa sobre les prestacions del sistema la utilització d'este tipus de controladors enfront de la utilització de controladors bifreqüència clàssics, ja que estos generen les seues accions de control sense tindre en compte l'existència de possibles retards. Per mitjà de l'ús de Tornado-VxWorks s'ha implementat una plataforma de temps real que peCuenca Lacruz, ÁM. (2004). Modelado, análisis y diseño de sistemas de control con muestreo no convencional [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/5510