Janette Cardoso

Possibilistic Petri nets

This paper presents the possibilistic Petri net model which combines possibility logic with Petri nets with objects. The main feature of this model is to allow one to reason about the aspects of uncertainty and change in dynamic discrete event systems. The paper presents relevant concepts of Petri nets with objects and possibility logic and how imprecision and vagueness are introduced in the marking of a Petri net with objects. The marking of a net is imprecise, or in a more general way, fuzzy, in order to represent an ill-known knowledge about a system state. A new marking updating according to the fuzzy marking such defined is also discussed. An example of shop door monitoring is presented that illustrates our approach.

Distributed Simulation of Heterogeneous and Real-Time Systems

This work describes a framework for distributed simulation of cyber-physical systems (CPS). Modern CPS comprise large numbers of heterogeneous components, typically designed in very different tools and languages that are not or not easily compose able. Evaluating such large systems requires tools that integrate all components in a systematic, well-defined manner. This work leverages existing frameworks to facilitate the integration offers validation by simulation. A framework for distributed simulation is the IEEE High-Level Architecture (HLA) compliant tool CERTI, which provides the infrastructure for co-simulation of models in various simulation environments as well as hardware components. We use CERTI in combination with Ptolemy II, an environment for modeling and simulating heterogeneous systems. In particular, we focus on models of a CPS, including the physical dynamics of a plant, the software that controls the plant, and the network that enables the communication between controllers. We describe the Ptolemy extensions for the interaction with HLA and demonstrate the approach on a flight control system simulation.

Mathtutor: A Multi-Agent Intelligent Tutoring System

In this paper we propose a multi-agent intelligent tutoring system building tool that integrates different formalisms in order to facilitate the teacher task of developing the contents of a tutorial system and at the same time to provide adaptiveness and flexibility in the presentation. The adopted formalisms are ground logic terms for the student model, data-bases for the domain model and object Petri nets for the pedagogical model. The interaction between the student and each agent of the system is controlled by an object Petri net, automatically translated into a rule-based expert system. The object Petri net tokens are composed by data objects that contain pointers to the student model and to the domain knowledge, stored into a data-base of texts, examples and exercises. The object Petri net transitions are controlled by logical conditions that refer to the student model and the firing of these transitions produce actions that update this student model.

Uncertainty handling in quantitative BDD-based fault-tree analysis by interval computation

In fault-tree analysis, probabilities of failure of components are often assumed to be precise. However this assumption is seldom verified in practice. There is a large literature on the computation of the probability of the top (dreadful) event of the fault-tree, based on the representation of logical formulas in the form of binary decision diagrams (BDD). When probabilities of atomic propositions are ill-known and modelled by intervals, BDD-based algorithms no longer apply to the computation of the top probability interval. This paper investigates this question for general Boolean expressions, and proposes an approach based on interval methods, relying on the analysis of the structure of the Boolean formula. The considered application deals with the fault-tree-based analysis of the reliability of aircraft operations.

Network latency and packet delay variation in cyber-physical systems

The problem addressed in this paper is the limitation imposed by network elements, especially Ethernet elements, on the real-time performance of timecritical systems. Most current network elements are concerned only with data integrity, connection, and throughput with no mechanism for enforcing temporal semantics. Existing safety-critical applications and other applications in industry require varying degrees of control over system-wide temporal semantics. In addition, there are emerging commercial applications that require or will benefit from tighter enforcement of temporal semantics in network elements than is currently possible. This paper examines these applications and requirements and suggests possible approaches to imposing temporal semantics on networks. Modelbased design and simulation is used to evaluate the effects of network limitations on time-critical systems.

Interpretation of UML sequence diagrams as causality flows

UML Interaction diagrams (Sequence diagrams (SD), Collaboration diagrams or Communication diagrams) lack a formal semantics; they include some amount of ambiguity so that formal techniques cannot be applied e.g. for automatically deriving the behavior of the objects (as Statecharts or State-Transition diagrams) from the SD where they appear. To overcome this situation, this paper first analyses and simplifies the semantic relationships among the concepts that intervene in the definition of SDs: synchronous / asynchronous, activator andreturn messages. Then it proposes an interpretation of the precedence association among messages that yields to an operational semantics of SDs that orders the actions of emitting and receiving messages according to a causality relationship.

Communication protocols as first-class components of multiagent systems

Communication protocols are widely recognised as essential to an efficient coordination among agents. Accordingly, this paper proposes to consider protocols as first-class entities, so that an agent that wants to initiate a conversation creates an instance of the appropriate protocol, called a Moderator, in charge of managing this conversation. Then, agents intending to take part in the conversation send their communication actions to the Moderator, which guarantees the respect of the protocols rules. Cooperative Objects, a high-level Petri net language, may be used to specify, validate and implement Moderators. From a software engineering point of view, the resulting MAS architecture features many advantages.

Evaluating the Uncertainty of a Boolean Formula with Belief Functions

In fault-tree analysis, probabilities of failure of components are often assumed to be precise and the events are assumed to be independent, but this is not always verified in practice. By giving up some of these assumptions, results can still be computed, even though it may require more expensive algorithms, or provide more imprecise results. Once compared to those obtained with the simplified model, the impact of these assumptions can be evaluated. This paper investigates the case when probability intervals of atomic propositions come from independent sources of information. In this case, the problem is solved by means of belief functions. We provide the general framework, discuss computation methods, and compare this setting with other approaches to evaluating the uncertainty of formulas.

A method for modelling adaptive interactions in Intelligent Tutoring Systems

This paper proposes a method to couple the domain, student and pedagogical models in an Intelligent Tutoring System. The goal of the proposed method is to produce a personalised version of the domain model with respect to the student model, without burdening the teacher with the task of specifying how this personalisation is to be done. The pedagogical model is defined through Object Petri Net, whose transitions control the interaction decisions, according to conditions that refer to the student model and feedback. All student model management is automatically included in the pedagogical model by the authoring tool.

Toward a formalism to study the scheduling of cyber-physical systems simulations

This paper presents ongoing work on the formalism of Cyber-Physical Systems (CPS) simulations. These systems are distributed real-time systems, and their simulations might be distributed or not. In this paper, we propose a model to describe the modular components forming a simulation of a CPS. The main goal is to introduce a model of generic simulation distributed architecture, on which we are able to execute a logical architecture of simulation. This architecture of simulation allows the expression of structural and behavioural constraints on the simulation, abstracting its execution. We will propose two implementations of the execution architecture based on generic architectures of distributed simulation: The High Level Architecture (HLA), an IEEE standard for distributed simulation, and one of its open-source implementation of RunTime Infrastructure (RTI): CERTI. The Distributed Simulation Scheduler (DSS), an Airbus framework scheduling predefined models. Finally, we present the initial results obtained applying our formalism to the open-source case study from the ROSACE case study.