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  • Conference Object
    Citation - WoS: 5
    Citation - Scopus: 6
    Fault-Tolerant Control of Discrete-Event Systems With Lower-Bound Specifications
    (Elsevier, 2015) Moor, Thomas; Schmidt, Klaus Werner
    Fault-tolerant control addresses the control of dynamical systems such that they remain functional after the occurrence of a fault. To allow the controller to compensate for a fault, the system must exhibit certain redundancies. Alternatively, one may relax performance requirements for the closed loop behaviour after the occurrence of a fault. To achieve fault tolerance for a hierarchical control architecture, a combination of both options appears to be advisable: on each individual level of the hierarchy, the controller may compensate the fault as far as possible, and then pass on responsibility to the next upper level. This approach, when further elaborated for discrete-event systems represented by formal languages, turns out to impose a hard lower-bound inclusion specification on the closed-loop behaviour. The present paper discusses the corresponding synthesis problem and presents a solution. (C) 2015, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.
  • Conference Object
    Citation - Scopus: 2
    Computation of Supervisors for Fault-Recovery and Repair for Discrete Event Systems
    (Univelt Inc., 2014) Sülek, A.N.; Schmidt, K.W.
    In this paper, we study the fault-recovery and repair of discrete event systems (DES). To this end, we first develop a new method for the fault-recovery of DES. In particular, we compute a fault-recovery supervisor that follows the specified nominal system behavior until a fault-occurrence, that continues its operation according to a degraded specification after a fault and that finally converges to a desired behavior after fault. We next show that our method is also applicable to system repair and we propose an iterative procedure that determines a supervisor for an arbitrary number of fault occurrences and system repairs. We demonstrate our method with a manufacturing system example. © IFAC.
  • Conference Object
    Citation - Scopus: 8
    Computation of Fault-Tolerant Supervisors for Discrete Event Systems
    (IFAC Secretariat, 2013) Sülek, A.N.; Schmidt, K.W.
    Fault-tolerance addresses the problem of operating a system even in case of faults. In this paper, we study fault-tolerance in the supervisory control framework for discrete event systems (DES). We consider DES, where certain events might no longer be possible in case a fault happens. In this setting, we first identify necessary and suficient conditions for the existence of a supervisor that realizes a given behavioral specification both in the non-faulty and in the faulty case. We further show that it is possible to determine a supremal fault-tolerant sublanguage in case the existence condition is violated. Finally, we propose an algorithm for the computation of this sublanguage and prove its correctness. Different from existing work, our fault-tolerant supervisor allows fault occurrences and system repairs at any time. The concepts and results developed in this paper are illustrated by a manufacturing system example. © 2013 IFAC.
  • Conference Object
    Citation - Scopus: 12
    Applied Supervisory Control for a Flexible Manufacturing System
    (IFAC Secretariat, 2010) Moor, T.; Schmidt, K.; Perk, S.
    This paper presents a case study in the design and implementation of a discrete event system (DES) of real-world complexity. Our DES plant is a flexible manufacturing system (FMS) laboratory model that consists of 29 interacting components and is controlled via 107 digital signals. Regarding controller design, we apply a hierarchical and decentralised synthesis method from earlier work in order to achieve nonblocking and safe closed-loop behaviour. Regarding implementation, we discuss how digital signals translate to discrete events from a practical point of view, including timing issues. The paper demonstrates how both, design and implementation, are supported by the open-source software tool libFAUDES.
  • Conference Object
    Citation - Scopus: 5
    Abstraction-Based Supervisory Control for Recon-Gurable Manufacturing Systems
    (IFAC Secretariat, 2013) Khalid, H.M.; Kirik, M.S.; Schmidt, K.W.
    Reconfiguration control for discrete event systems (DES) is concerned with the realization of different system configurations by modification of the supervisory control loop. In this paper, we study the reconfiguration supervisor design for reconfigurable manufacturing systems (RMS) that comprise multiple components. We construct a modular supervisor for each configuration and system component in order to realize each active configuration and to quickly change between configurations. Different from the existing literature that is focused on monolithic design, our method is abstraction-based, and, hence applicable to large-scale DES. © 2013 IFAC.
  • Article
    Citation - WoS: 8
    Citation - Scopus: 8
    Reconfigurability of Behavioural Specifications for Manufacturing Systems
    (Taylor & Francis Ltd, 2017) Schmidt, Klaus Werner
    Reconfigurable manufacturing systems (RMS) support flexibility in the product variety and the configuration of the manufacturing system itself in order to enable quick adjustments to new products and production requirements. As a consequence, an essential feature of RMS is their ability to rapidly modify the control strategy during run-time. In this paper, the particular problem of changing the specified operation of a RMS, whose logical behaviour is modelled as a finite state automaton, is addressed. The notion of reconfigurability of specifications (RoS) is introduced and it is shown that the stated reconfiguration problem can be formulated as a controlled language convergence problem. In addition, algorithms for the verification of RoS and the construction of a reconfiguration supervisor are proposed. The supervisor is realised in a modular way which facilitates the extension by new configurations. Finally, it is shown that a supremal nonblocking and controllable strict subautomaton of the plant automaton that fulfils RoS exists in case RoS is violated for the plant automaton itself and an algorithm for the computation of this strict subautomaton is presented. The developed concepts and results are illustrated by a manufacturing cell example.
  • Conference Object
    The Controllability Prefix for Supervisory Control Under Partial Observation With an Application To Fault-Tolerant Control
    (Elsevier, 2017) Moor, Thomas; Schmidt, Klaus Werner
    The controllability prefix is known as a useful concept for the discussion and solution of synthesis problems in supervisory control of cp-languages, i.e., formal languages of infinite-length words. There, the controllability prefix is defined as the set of all finite-length prefixes that can be controlled to satisfy prescribed liveness and safety properties. In this paper, we discuss a variation of the controllability prefix to address supervisory control under partial observation for regular *-languages, i.e., formal languages of finite-length words. We derive algebraic properties that are useful for a quantitative analysis on how an upper-bound language-inclusion specification affects achievable lower-bound specifications. Our study is motivated by the synthesis of fault-tolerant supervisory controllers, where the possible occurrence of a fault may restrict the achievable pre-fault behaviour so severe, that a relaxation of the upper-bound specification becomes a practical option. As our study shows, such a relaxation can be systematically constructed in terms of the controllability prefix. (C) 2017, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.
  • Article
    Citation - WoS: 9
    Citation - Scopus: 9
    Efficient Abstractions for the Supervisory Control of Modular Discrete Event Systems
    (Ieee-inst Electrical Electronics Engineers inc, 2012) Schmidt, Klaus Werner; Ribeiro Cury, Jose Eduardo; Cury, José Eduardo Ribeiro
    The topic of this technical note is the nonblocking and maximally permissive abstraction-based supervisory control for modular discrete event systems (DES). It is shown, that an efficient abstraction technique, that was developed for the nonconflict verification of modular DES, is also suitable for the nonblocking supervisory control. Moreover, it is proved that this abstraction technique can be extended by the condition of local control consistency, in order to achieve maximally permissive supervision. Different from existing approaches, the presented abstraction does not require to preserve the shared events among the system components in the respective abstraction alphabets, and hence leads to potentially smaller system abstractions. The obtained results are illustrated by a flexible manufacturing system example.
  • Article
    Citation - WoS: 17
    Citation - Scopus: 17
    A Framework for State Attraction of Discrete Event Systems Under Partial Observation
    (Elsevier Science inc, 2014) Schmidt, Klaus Werner; Breindl, Christian
    State attraction for discrete event systems (DES) addresses the problem of reaching a desired subset of the plant state space after a bounded number of event occurrences. The problem of state attraction arises for example in fault-tolerant supervisory control or in the control of reconfigurable manufacturing systems, and is also applicable to systems biological problems such as the control of gene regulatory networks. State attraction is investigated with the assumption of full event observation in the existing literature. This paper extends the concept of state attraction to the case of partial observation. The notion of weak attraction under partial observation (WAPO) is introduced and necessary and sufficient conditions for the existence of a supervisor under partial observation that achieves WAPO are derived. Furthermore, a solution algorithm is proposed that finds such supervisor whenever it exists. It is shown that such supervisor can always be realized as a subautomaton of the observer automaton of the DES plant. An application example from systems biology illustrates the obtained results. (C) 2014 Elsevier Inc. All rights reserved.
  • Article
    Citation - WoS: 9
    Citation - Scopus: 10
    Computation of Supervisors for Reconfigurable Machine Tools
    (Springer, 2015) Schmidt, Klaus Werner
    The rapid reconfiguration of manufacturing systems is an important issue in today's manufacturing technology in order to adjust the production to varying product demands and types. In this paper, we study the control of reconfigurable machine tools (RMTs) with the aim of fast reconfiguration and an easy controller implementation. We first formulate a particular reconfiguration problem for RMTs in a discrete event system setting, and then provide a necessary and sufficient condition for its solution. Moreover, we propose a polynomial-time algorithm for the construction of a reconfiguration supervisor as the composition of one modular supervisor for each separate RMT configuration. Each modular supervisor operates in three modes. In the first mode, it tracks the plant state if its corresponding configuration is inactive. In the second mode, it performs a configuration change if its corresponding configuration becomes active and in the third mode, it follows the specified behavior of its corresponding configuration if the configuration is active. An important property of the proposed reconfiguration supervisor is that it performs reconfigurations in a bounded number of event occurrences. In addition, the modular realization of our reconfiguration supervisor enables controller modifications such as adding or removing configurations during run-time. All results presented in the paper are illustrated by an RMT example.