Robust controllability assessment via semi-definite programming

This paper presents a new framework for systematic assessment of the controllability of uncertain linear time-invariant (LTI) systems. The objective is to evaluate controllability of an uncertain system with norm-bounded perturbation over the entire uncertain region. The method is based on a singular-value minimization problem, over the entire complex plane. To solve the problem, first, a necessary and sufficient condition is proposed to avert the difficulties of griding over the complex plane, and to verify the controllability of directed and undirected networks in a single step. Secondly, the results are utilized to formulate the problem as two Lyapunov-based linear matrix inequalities (LMIs) for undirected networks, and four LMIs, for directed ones. The proposed approach is subsequently extended to evaluate the maximum guaranteed distance to uncontrollability of a system through a quasi-convex optimization problem whose solution is guaranteed to be globally optimal. By duality, analogous results are established for robust observability of directed and undirected networks. The proposed framework is implemented efficiently using LMI tools, and provides a fast and reliable tool for the assessment of robust controllability (and by duality, robust observability). It is also extendable to robust control system design problems such as control node selection for uncertain systems.     

Performance improvement of robust controllers for polynomially uncertain systems

This paper is concerned with the high-performance robust control of discrete-time linear time-invariant (LTI) systems with semi-algebraic uncertainty regions. It is assumed that a robustly stabilizing static controller is given whose gain depends polynomially on the uncertain variables. The problem of tuning this parameter-dependent gain with respect to a prescribed quadratic cost function is formulated as a sum-of-squares (SOS) optimization. This method leads to a near-optimal controller whose performance is better than that of the initial controller. It is shown that the results derived in the present work encompass the ones obtained in a recent paper. The efficacy of the results is elucidated by an example.     

Structural properties and poles assignability of LTI singular systems under output feedback

This paper studies the structural properties of both finite poles and the infinite pole of linear time-invariant singular systems under output feedback. Three main problems are studied, namely, (1) the algebraic structures of both finite poles and the infinite pole; (2) the assignability of finite poles and the elimination of the infinite pole by output feedback; and (3) the controllability and observability of the system with minimal number of inputs and outputs. New generic solutions to these problems are presented in terms of some new concepts defined in this paper including the geometric multiplicity of the infinite pole, the finite and impulsive output feedback cycle indices of the system. Determination of these multiplicities and indices are discussed. An assignability equivalence is established between the variable finite poles and the poles of a controllable and observable non-singular system. The number of the independent infinite poles that can be reduced is given in terms of the system matrices. The minimal number of inputs and outputs that guarantee controllability and observability are shown to be the output feedback cycle indices.     

Robust and fault-tolerant supervisory control of discrete event systems with partial observation and model uncertainty

This paper proposes the notions of faults and failures in discrete event systems (DESs) with partial observation. They are associated with controllability and an observability property. The proposed notions are used to address the notion of tolerable fault event sequences which represents fault-tolerant behaviour of systems as a desired specification. A robust and fault-tolerant supervisor is a controller which is robust to model uncertainty and guarantees fault-tolerant behaviour of a system. In this paper we present necessary and sufficient conditions for the existence of a robust and fault-tolerant supervisor. The developed conditions capture the concepts of controllability and observability which are cores in the control of DESs with partial observation     

Distributed Control of Nonlinear Uncertain Systems: A Cyclic-small-gain Approach

This paper presents a cyclic-small-gain approach to distributed control of nonlinear multi-agent systems for output agreement. Through a novel nonlinear control law design, the output agreement problem is transformed into a stabilization problem, and the closed-loop multi-agent system is transformed into a large-scale system composed of input-to-state stability (ISS) subsystems which are interconnected with each other through redefined outputs. By forcing the redefined outputs to go to arbitrarily small neighborhoods of the origin, practical consensus is achieved for the agents in the sense that their outputs ultimately converge to each other within an arbitrarily small region. A recently developed cyclic-small-gain result is adopted to assign appropriately the ISS gains to the transformed interconnected system. Moreover, if the system is disturbancefree, then consensus can be guaranteed. Interestingly, the closedloop multi-agent system is also robust to bounded time-delays and disturbances in information exchange.      

Multivariate control theory in linear sequential machines

This paper presents a unified survey of some modern multivariate control theory aspects and techniques applied to linear sequential machines over a Galois field GF(p), Utilizing the concepts of controllability and observability for a Mealy-type machine the canonical decomposition problem, the state minimization problem, the identification problem, the transformation to canonical form problem, and the controllability/observability problem of combined machines are studied. Then the problem of designing a linear feedback controller for driving any state of a linear machine to the zero state after a minimum number of time steps A(clock periods), as well as the dual problem of designing a time-optimal state reconstructor for the same machine are solved. Finally, the problems of inverting a linear sequential machine and decoupling its inputs and outputs by using state feedforward and feedback are examined. Several examples illustrate the theoretical results.     

Observability of linear systems with commensurate delays and unknown inputs

This paper investigates the observability analysis for linear time systems whose outputs are affected by unknown inputs. Three different definitions of observability are proposed. By introducing the Smith form and comparing the invariant factors, a sufficient condition is deduced for each proposed observability definition. Three examples are given for the purpose of highlighting the effectiveness of the proposed approach.     

New results in stability of a class of hereditary linear systems

Some basic connections between bounded-input, bounded-output stability and exponential asymptotic stability are presented for a class of linear time-varying hereditary systems. For that class of systems, controllability and observability are closely related to that of an equivalent linear system. This eases the analytical treatment of such properties which are then used to derive the stability results. The requirement of boundedness of inputs and outputs in the BIBO definition is modified by requesting that their ‘energy content’ over a fixed-length interval should be bounded independently of the position of the interval. For the class of hereditary linear systems studied, it is found that EAS and BIBO stability are equivalent under uniform complete controllability and observability.     

On observability of sampled-data systems

In this paper we consider sampled-data systems derived from continuous, time-invariant systems whose inputs are piecewise constant. The problem dealt with is that of finding necessary and sufficient conditions for observability of the sampled-data system, given that the continuous-time system is observable. By comparison With some previously obtained results pertaining to the preservation of controllability under sampling, it is shown that the latter requires a stronger condition than that required for the preservation of observability.     

基于模型参考和内模原理的线性系统鲁棒控制设计

不确定性因素会使系统性能恶化,它包括系统不确定性参数和各种外部干扰等.本文针对这种不确定性线性系统,利用模型参考和内模原理,建立鲁棒控制系统结构,分析了其能控性条件,通过选择参考模型和内模实现系统能控,在此条件下,将鲁棒控制设计转换为控制系统的LQR问题进行研究,并运用最优控制理论,计算其反馈控制律.仿真结果表明该设计方法将"模型参考"与"内模原理"有机结合起来,提高了系统的鲁棒稳定性,有效抑制干扰,并实现系统性能的改善.     

Span-reachability and observability of bilinear hybrid systems

The paper presents an algebraic characterization of observability and span-reachability of bilinear hybrid systems without guards, i.e. hybrid systems whose continuous dynamics are determined by bilinear control systems, and whose discrete transitions are triggered externally. The proposed characterization provides necessary and sufficient conditions for observability and span-reachability in terms of rank conditions for generalized observability and controllability matrices. This characterization is useful for the system theory of bilinear hybrid systems, in particular, it is used in the realization theory of bilinear hybrid systems, (Petreczky, 2006).     

Robust analysis and synthesis for a class of uncertain nonlinear systems with multiple equilibria

This paper focuses on a class of uncertain nonlinear systems which are subject to norm-bounded parameter uncertainty in the forward path and a vector-valued periodic nonlinearity in the feedback path, and addresses robust analysis and synthesis problems for such systems. Sufficient conditions for global asymptotic stability are derived in terms of linear matrix inequalities (LMIs) and a technique for the estimation of the uncertainty bound is proposed by solving a generalized eigenvalue minimization problem. The problem of robust synthesis is concerned with designing a feedback controller such that the resulting closed-loop system is globally asymptotically stable for all admissible uncertainties. It is shown that a solution to the robust synthesis problem for the uncertain system can be obtained by solving a synthesis problem for an uncertainty free system. A concrete example is presented to demonstrate the applicability and validity of the proposed approach.     

Minimal controllability for systems with delays†

In this paper we consider the problem of minimal controllability for a general class of linear spectrally controllable systems. We show that the smallest number of controls that are necessary in order that spectral controllability may hold depends only upon the matrices of the uncontrolled system and that it is equal to the smallest number of outputs necessary for the dual problem of ‘spectral observability’; moreover, any spectrally controllable system can be made minimally controllable by acting upon the control matrix only, and any spectrally observable system can be made minimally observable by acting upon the output matrix only. In the sequel this result is explained in the cases of the delayed systems and of some kind of generalized systems.     

Necessary and sufficient conditions for quadratic controllability of a class of uncertain systems

We introduce a controllability notion for uncertain systems, namely, quadratic controllability. For a specific class of norm bounded uncertain systems, we show that this notion is equivalent to controllability of a nominal system and the satisfaction of a certain structural condition by the uncertainty. For systems in which the uncertain parameters affect all control inputs, this structural condition is simply the familiar matching condition commonly encountered in the literature on robust and adaptive control of uncertain systems.     

Band criteria and recursive tests of complete controllability and observability of linear algebraic-differentiable systems

New criteria are introduced of complex controllability and observability of controllable linear stationary algebraic-differentiable systems with many inputs and outputs, which reduce to the calculation of the rank of special band matrices. Recursive tests are set up, which permit reducing computational expenses in the verification for the fulfillment of the conditions of complete controllability and observability of algebraic-differentiable systems of a large dimension.     

Robust disturbance modeling for model predictive control with application to multivariable ill-conditioned processes

In this paper the disturbance model, used by MPC algorithms to achieve offset-free control, is optimally designed to enhance the robustness of single-model predictive controllers. The proposed methodology requires the off-line solution of a min-max optimization problem in which the disturbance model is chosen to guarantee the best closed-loop performance in the worst case of plant in a given uncertainty region. Application to a well-known ill-conditioned distillation column is presented to show that, for ill-conditioned processes, the use of a disturbance model that adds the correction term to the process inputs guarantees a robust performance, while the disturbance model that adds the correction term to the process outputs (used by industrial MPC algorithms) does not.     

Local observability and controllability of stirred tank reactors

This paper deals with the observability and controllability properties of the linearized tangent dynamic model of (bio)chemical processes in isothermal stirred tank reactors (STRs). It is shown that a necessary condition for the observability (controllability) of isothermal (bio)chemical processes is that the number of measured components (the number of control inputs) is larger than or equal to the number of process components minus the number of reactions involved in the process. This gives a simple test for detecting possible lack of observability and/or controllability of isothermal STRs.     

Exact pole assignment by output feedback Part 2

We continue the study in Fletcher and Magni (1987) concerning controllable and observable linear time-invariant multivariable control systems in which the number of inputs plus the number of outputs exceeds the number of states. In this paper we shall prove that exact assignment of distinct poles by means of a real output feedback is always possible if the set of poles to be assigned consists entirely of complex conjugate pairs of non-real numbers. We examine the problem of assigning these eigenvalues in complex conjugate pairs by means of the algorithm described in the first paper. We reduce the problem to the case in which just one of the observability and controllability indices of the system is larger than 2, and further detailed analysis then disposes of this case.     

Bisimilarity control of partially observed nondeterministic discrete event systems and a test algorithm

In this paper, the bisimilarity control of discrete event systems (DESs) under partial observations is investigated, where the plant and the specification are allowed to be nondeterministic. A notation of simulation-based controllability and a synchronization scheme for the supervised system are formalized based on the simulation relation between the specification and the plant. It is shown that the existence of bisimilarity supervisors is characterized by the notions of the simulation-based controllability and the language observability, which extends the traditional results of supervisory control from language equivalence to bisimulation equivalence. In addition, a polynomial algorithm to test the simulation-based controllability is developed by constructing a computing tree. This algorithm together with the test of language observability can be used to check the existence of bisimilarity supervisors.