Stefan Kowalewski

Verification of logic controllers for continuous plants using timed condition/event-system models

An approach to the formal verification of logic controllers for processes with switched continuous dynamics is presented. The method builds on modular, timed discrete event models of the plant and the controller. Subsystems with continuous dynamics are approximated algorithmically. The formal verification consists of determining the reachable discrete states of the resulting model and comparing it to a set of undesired states. For this purpose, the tool HyTech is applied. The approach is illustrated by the treatment of a process engineering example.

Comparing Timed and Hybrid Automata as Approximations of Continuous Systems

We describe two approaches to derive Timed and Linear Hybrid Automata from continuous models given as systems of ordinary differential equations. A semiquantitative modeling method is applied which yields a qualitative description of the system dynamics and quantitative bounds for the residence times in the discrete states or the state variable derivatives. We discuss the problem of spurious trajectories and illustrate the two aproaches by means of a simple process engineering example. Results of a reachability analysis obtained with the tool HyTech are presented.

On the Generation of Timed Discrete Approximations for Continuous Systems

In this contribution we present two procedures to systematically derive timed discrete approximations from continuous models. Both methods are based on a rectangular state space partition and aim at mapping continuous dynamic behaviours described by ODE-systems with switched inputs onto timed state transition systems: In the first approach the transitions between the discrete states are determined by analysing the flow between rectangular cells of the state space. The second one uses numerical integration of the ODE-system between partitions of the boundaries of the cells. The application of both approaches is illustrated by a chemical process example. The paper discusses completeness and consistency properties of the approximation mappings as well as issues of accuracy and computational effort.

An Algorithm for the Approximative Analysis of Rectangular Automata

Rectangular automata are well suited for approximate modeling of continuous-discrete systems. The exact analysis of these automata is feasible for small examples but can encounter severe numerical problems for even medium-sized systems. This paper presents an analysis algorithm that uses conservative overapproximation to avoid these numerical problems. The algorithm is demonstrated on a simple benchmark system consisting of two connected tanks.

Automatic safety analysis of computer-controlled plants

Abstract The paper describes an approach to apply the formal technique of model checking to the verification of logic controllers within the safety analysis of processing plants. In order to investigate plant safety in an early design phase in which only basic information is available, we set up plant and controller models in a qualitative and modular fashion. In a first step, the computer-controlled plant is partitioned into functional units, named modules , and the communication between different modules is represented graphically in a so-called process control event diagram (PCED). The PCED can be transformed into a formal model in which the behaviour of each module is described in terms of logical expressions for the modules’ input, state and output variables. Based on the formal model, the method of model checking can be applied to determine algorithmically whether the system fulfils a set of given safety requirements. Specifically, we use the tool symbolic model verifier (SMV) to determine whether the plant can reach states that are, in some sense, critical for the plant operation. The whole approach is illustrated by application to an industrial computer-controlled tube reactor.

An environment for model-checking of logic control systems with hybrid dynamics

This paper presents the modeling environment VERDICT which intends to make the method of formal verification applicable to logic controlled chemical plants. The tool offers textual and graphical editors to set up models of plant and controller. The plant model can be specified as hybrid dynamical or (timed) discrete systems, which are then translated automatically into models that are verifiable with existing model-checking tools. Furthermore, VERDICT allows one to include the controller model by specifying it in Instruction List (IL) code directly. The paper illustrates the different features of the framework, the underlying modeling paradigm, and some aspects of the analysis using a simple technical example.

Verification of Logic Controllers for Continuous Plants

This contribution deals with the formal verification of logic controllers for processes with switched continuous dynamics. We present a consistent method for treating this problem in a rather general formulation and describe the corresponding computer support. The approach is characterized by the integration of available analysis tools for real-time and hybrid systems from computer science and a signal-flow oriented modular modeling framework for mixed discrete-continuous systems. The formal verification is based on determining the reachable discrete states of the resulting model and comparing it to a set of undesired states. The approach is illustrated by the modeling and analysis of a process engineering example.

VERDICT-a tool for model-based verification of real-time logic process controllers

This paper provides an overview of a project in which a practically applicable tool for the algorithmic formal verification of logic controllers (including timers) for processes with at least piecewise continuous dynamics is developed. In contrast to existing verification techniques from computer science, the presented approach is based on a model of the uncontrolled plant. Condition/event systems are used as the underlying modeling framework. They provide the possibility to build discrete, real-time, and modular models. The paper presents the basic tool architecture and discusses the current project status.

Diskrete Modellierung verfahrenstechnischer Prozesse zur Steuerungsverifikation

Der Beitrag befaßt sich mit der Fragestellung, welche besonderen Anforderungen die Verifikation von Steuerungen für verfahrenstechnische Prozesse an die dazu notwendige Modellierung stellt und wie diese erfüllt werden können. Es wird ein Ansatz vorgestellt, der auf einer modularen Modellstruktur aufbaut und ein über mehrere Abstraktionsebenen abgestuftes Vorgehen ermöglicht. Dabei stehen zur Beschreibung von Teilsystemen drei Modelltypen zur Verfügung: diskrete, zeitbewertete und umschaltbare kontinuierliche Modelle. Als einheitliche Modellform kommen die Bedingung/Ereignis-Systeme nach Sreenivas und Krogh zum Einsatz. Der Beitrag beschreibt die Grundlagen der Modellierungsmethodik und stellt ein Verfahren zur algorithmischen Approximation von kontinuierlichen Teilsystemen durch zeitbewertete diskrete Modelle als einen wesentlichen Bestandteil des Ansatzes vor.

Verification of IL Programs with an Explicit Model of their PLC Execution

In this paper we present an approach for the automatic translation of control programs implemented in Instruction List (IL) into a suitable model for verification. This model includes a detailed, generic description of the execution mode of Programmable Logic Controllers (PLCs). The approach is integrated into a tool environment for formal verification.