Publications (145)39.6 Total impact
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ABSTRACT: We discuss the role of supervisor synthesis in automated software code generation.•The proposed approach is systematic and based on process theory.•We implemented a modelbased systems and software engineering framework.•The framework has been applied to multiple industrial studies. 
Conference Paper: Turing meets Milner
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ABSTRACT: We enhance the notion of a computation of the classical theory of computing with the notion of interaction from concurrency theory. In this way, we enhance a Turing machine as a model of computation to a Reactive Turing Machine that is an abstract model of a computer as it is used nowadays, always interacting with the user and the world.  [Show abstract] [Hide abstract]
ABSTRACT: We conservatively extend an ACPstyle discretetime process theory with discrete stochastic delays. The semantics of the timed delays relies on time additivity and time determinism, which are properties that enable us to merge subsequent timed delays and to impose their synchronous expiration. Stochastic delays, however, interact with respect to a socalled race condition that determines the set of delays that expire first, which is guided by an (implicit) probabilistic choice. The race condition precludes the property of time additivity as the merger of stochastic delays alters this probabilistic behavior. To this end, we resolve the race condition using conditionallydistributed unit delays. We give a sound and groundcomplete axiomatization of the process theory comprising the standard set of ACPstyle operators. In this generalized setting, the alternative composition is no longer associative, so we have to resort to special normal forms that explicitly resolve the underlying race condition. Our treatment succeeds in the initial challenge to conservatively extend standard time with stochastic time. However, the ‘dissection’ of the stochastic delays to conditionallydistributed unit delays comes at a price, as we can no longer relate the resolved race condition to the original stochastic delays. We seek a solution in the field of probabilistic refinements that enable the interchange of probabilistic and nondeterministic choices.  [Show abstract] [Hide abstract]
ABSTRACT: A supervisory controller controls and coordinates the behavior of different components of a complex machine by observing their discrete behaviour. Supervisory control theory studies automated synthesis of controller models, known as supervisors, based on formal models of the machine components and a formalization of the requirements. Subsequently, code generation can be used to implement this supervisor in software, on a PLC, or embedded microprocessor. In this article, we take a closer look at the control loop that couples the supervisory controller and the machine. We model both eventbased and statebased observations using process algebra and bisimulationbased semantics. The main application area of supervisory control that we consider is coordination, referred to as supervisory coordination, and we give an academic and an industrial example, discussing the processtheoretic concepts employed.  [Show abstract] [Hide abstract]
ABSTRACT: We revisit the central notion of controllability in supervisory control theory from processtheoretic perspective. To this end, we investigate partial bisimulation preorder, a be havioral preorder that is coarser than bisimulation equivalence and finer than simulation preorder. It is parameterized by a subset of the set of actions that need to be bisimulated, whereas the actions outside this set need only to be simulated. This preorder proves a viable means to define controllability in a nondeterministic setting as a refinement relation on processes. The new approach provides for a generalized characterization of controllability of nondeterministic discreteevent systems. We characterize the existence of a deterministic supervisor and compare our approach to existing ones in the literature. It helped identify the coarsest minimization procedure for nondeterministic plants that respects controllability. At the end, we define the notion of a maximally permissive supervisor, nonblocking property, and partial observability in our setting. I. INTRODUCTION  [Show abstract] [Hide abstract]
ABSTRACT: A widely accepted method to specify (possibly infinite) behaviour is to define it as the solution, in some process algebra, of a recursive specification, i.e., a system of recursive equations over the fundamental operations of the process algebra. The method only works if the recursive specification has a unique solution in the process algebra; it is wellknown that guardedness is a sufficient requirement on a recursive specification to guarantee a unique solution in any of the standard process algebras.In this paper we investigate to what extent guardedness is also a necessary requirement to ensure unique solutions. We prove a theorem to the effect that all unguarded recursive specifications over BPA have infinitely many solutions in the standard models for BPA. In contrast, we observe that there exist recursive specifications over PA, necessarily involving parallel composition, that have a unique solution, or finitely many solutions in the standard models for PA. 
Article: Reactive Turing Machines
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ABSTRACT: We propose reactive Turing machines (RTMs), extending classical Turing machines with a processtheoretical notion of interaction, and use it to define a notion of executable transition system. We show that every computable transition system with a bounded branching degree is simulated modulo divergencepreserving branching bisimilarity by an RTM, and that every effective transition system is simulated modulo the variant of branching bisimilarity that does not require divergence preservation. We conclude from these results that the parallel composition of (communicating) RTMs can be simulated by a single RTM. We prove that there exist universal RTMs modulo branching bisimilarity, but these essentially employ divergence to be able to simulate an RTM of arbitrary branching degree. We also prove that modulo divergencepreserving branching bisimilarity there are RTMs that are universal up to their own branching degree. Finally, we establish a correspondence between executability and finite definability in a simple process calculus. 
Conference Paper: Computations and Interaction
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ABSTRACT: We enhance the notion of a computation of the classical theory of computing with the notion of interaction. In this way, we enhance a Turing machine as a model of computation to a Reactive Turing Machine that is an abstract model of a computer as it is used nowadays, always interacting with the user and the world.  [Show abstract] [Hide abstract]
ABSTRACT: Due to increasing system complexity, timetomarket and development costs reduction, there are higher demands on engineering processes. Modelbased engineering processes can play a role here because they support system development by enabling the use of various modelbased analysis techniques and tools. As a result, they are able to cope with complexity and have the potential to reduce timetomarket and development costs. Moreover, supervisory control synthesis can be integrated in this setting, which can contribute to the development of control systems. This paper gives an overview of recently developed supervisor synthesis techniques and tools. To evaluate the applicability of these techniques and to show how they can be integrated in an engineering process, a few industrial cases are discussed. The supervisors synthesized for these cases have successfully been implemented and integrated in the existing resourcecontrol platform. 
Conference Paper: Expressiveness modulo Bisimilarity of Regular Expressions with Parallel Composition (Extended Abstract)
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ABSTRACT: The languages accepted by finite automata are precisely the languages denoted by regular expressions. In contrast, finite automata may exhibit behaviours that cannot be described by regular expressions up to bisimilarity. In this paper, we consider extensions of the theory of regular expressions with various forms of parallel composition and study the effect on expressiveness. First we prove that adding pure interleaving to the theory of regular expressions strictly increases its expressiveness up to bisimilarity. Then, we prove that replacing the operation for pure interleaving by ACPstyle parallel composition gives a further increase in expressiveness. Finally, we prove that the theory of regular expressions with ACPstyle parallel composition and encapsulation is expressive enough to express all finite automata up to bisimilarity. Our results extend the expressiveness results obtained by Bergstra, Bethke and Ponse for process algebras with (the binary variant of) Kleene's star operation. 
Conference Paper: A ProcessTheoretic Look at Automata
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ABSTRACT: Automata theory presents roughly three types of automata: finite automata, pushdown automata and Turing machines. The automata are treated as language acceptors, and the expressiveness of the automata models are considered modulo language equivalence. This notion of equivalence is arguably too coarse to satisfactorily deal with a notion of interaction that is fundamental to contemporary computing. In this paper we therefore reconsider the automaton models from automata theory modulo branching bisimilarity, a wellknown behavioral equivalence from process theory that has proved to be able to satisfactorily deal with interaction. We investigate to what extent some standard results from automata theory are still valid if branching bisimilarity is adopted as the preferred equivalence. 
Article: Design of asynchronous supervisors
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ABSTRACT: One of the main drawbacks while implementing the interaction between a plant and a supervisor, synthesised by the supervisory control theory of \citeauthor{RW:1987}, is the inexact synchronisation. \citeauthor{balemiphdt} was the first to consider this problem, and the solutions given in his PhD thesis were in the domain of automata theory. Our goal is to address the issue of inexact synchronisation in a process algebra setting, because we get concepts like modularity and abstraction for free, which are useful to further analyze the synthesised system. In this paper, we propose four methods to check a closed loop system in an asynchronous setting such that it is branching bisimilar to the modified (asynchronous) closed loop system. We modify a given closed loop system by introducing buffers either in the plant models, the supervisor models, or the output channels of both supervisor and plant models, or in the input channels of both supervisor and plant models. A notion of desynchronisable closed loop system is introduced, which is a class of synchronous closed loop systems such that they are branching bisimilar to their corresponding asynchronous versions. Finally we study different case studies in an asynchronous setting and then try to summarise the observations (or conditions) which will be helpful in order to formulate a theory of desynchronisable closed loop systems. 
Conference Paper: A Basic Parallel Process as a Parallel Pushdown Automaton
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ABSTRACT: We investigate the set of basic parallel processes, recursively defined by action prefix, interleaving, 0 and 1. Different from literature, we use the constants 0 and 1 standing for unsuccessful and successful termination in order to stay closer to the analogies in automata theory. We prove that any basic parallel process is rooted branching bisimulation equivalent to a regular process communicating with a bag (also called a parallel pushdown automaton) and therefore we can regard the bag as the prototypical basic parallel process.This result is closely related to the fact that any contextfree process is either rooted branching bisimulation equivalent or contrasimulation equivalent to a regular process communicating with a stack, a result that is the analogy in process theory of the language theory result that any contextfree language is the language of a pushdown automaton. 

Article: Preface.

Conference Paper: A ProcessTheoretic Look at Automata.

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ABSTRACT: The three classical process algebras CCS, CSP and ACP present several dierences in their respective technical machinery. This is due, not only to the dierence in their operators, but also to the terminology and \way of thinking" of the community which has been (and still is) working with them. In this paper we will rst discuss such dierences and try to clarify the dierent usage of terminology and concepts. Then, as a result of this discussion, we dene a generic process algebra where each basic mechanism of the three process algebras (including minimal xpoint based unguarded recursion) is expressed by an operator and which can be used as an underlying common language. We show an example of the advantages of adopting such a language instead of one of the three more specialized algebras: producing a complete axiomatization for Milner's observational con gruence in the presence of (unguarded) recursion and static operators. More precisely, we provide a syntactical characterization (allowing as many terms as possible) for the equa tions involved in recursion operators, which guarantees that transition systems generated by the operational semantics are nitestate. Viceversa we show that every process ad mits a specication in terms of such a restricted form of recursion. We then present an axiomatization which is groundcomplete over such a restricted signature. Notably, we also show that the two standard axioms of Milner for weakly unguarded recursion can be expressed by using just a single axiom. 
Conference Paper: A ContextFree Process as a Pushdown Automaton
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ABSTRACT: A wellknown theorem in automata theory states that every contextfree language is accepted by a pushdown automaton. We investigate this theorem in the setting of processes, using the rooted branching bisimulation and contrasimulation equivalences instead of language equivalence. In process theory, different from automata theory, interaction is explicit, so we realize a pushdown automaton as a regular process communicating with a stack.  [Show abstract] [Hide abstract]
ABSTRACT: Process algebra is the study of distributed or parallel syst ems by algebraic means. Originating in computer science, process algebra has been extended in recent years to encompass not just discrete event, reactive systems, but also continuously evolving phenomena, resulting in socalled hybrid process algebras. A hybrid process algebra can be used for the specification, simulation, control and verification of embedded systems in combination with their environment, and for any dynamic system in general. As the vehicle of our exposition, we use the hybrid process algebra χ (Chi). The syntax and semantics of χ are discussed, and it is explained how equational reasoning can simplify, among others, tool implementations for simulation and verification. Finally, a bottle filling line example is introduced to illustrate system anal ysis by means of equational reasoning.
Publication Stats
3k  Citations  
39.60  Total Impact Points  
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Institutions

19912015

Technische Universiteit Eindhoven
 • Department of Mathematics and Computer Science
 • Department of Mechanical Engineering
Eindhoven, North Brabant, Netherlands


19702006

University of Amsterdam
 Department of Computer Science
Amsterdamo, North Holland, Netherlands


1992

Centrum Wiskunde & Informatica
Amsterdamo, North Holland, Netherlands
