Hans-Michael Hanisch

Verification of distributed control systems in intelligent manufacturing

This paper presents an application of formal methods for validation of flexible manufacturing systems controlled by distributed controllers. A software tool verification environment for distributed applications (VEDA) is developed for modeling and verification of distributed control systems. The tool provides an integrated environment for formal, model-based verification of the execution control of function blocks following the new international standard IEC61499. The modeling is performed in a closed-loop way using manually developed models of plants and automatically generated models of controllers.

Deadlock Prevention Based on Structure Reuse of Petri Net Supervisors for Flexible Manufacturing Systems

Deadlocks are an undesirable situation in automated flexible manufacturing systems (FMS). Their occurrences often deteriorate the utilization of resources and may lead to catastrophic results. Finding an optimal supervisor is NP-hard. A computationally efficient method often ends up with a suboptimal one. This paper develops a deadlock prevention method that makes a good tradeoff between optimality and computational tractability for a class of Petri nets, which can model many FMS. The theory of regions guides our efforts toward the development of near-optimal solutions for deadlock prevention. Given a plant net, a minimal initial marking is first decided by structural analysis, and an optimal live controlled system is computed. Then, a set of inequality constraints is derived with respect to the markings of monitors and the places in the model such that no siphon can be insufficiently marked. A method is proposed to identify the redundancy condition for constraints. For a new initial marking of the plant net, a deadlock-free controlled system can be obtained by regulating the markings of the monitors such that the inequality constraints are satisfied, without changing the structure of the controlled system. The near-optimal performance of a controlled net system via the proposed method is shown through several examples.

Closed-Loop Modeling in Future Automation System Engineering and Validation

This paper presents a new framework for design and validation of industrial automation systems based on systematic application of formal methods. The engineering methodology proposed in this paper is based on the component design of automated manufacturing systems from intelligent mechatronic components. Foundations of such componentspsila information infrastructure are the new IEC 61499 architecture and the automation object concept. It is illustrated in this paper how these architectures, in conjunction with other advanced technologies, such as Unified Modeling Language, Simulink, and net condition/event systems, form a framework that enables pick-and-place design, simulation, formal verification, and deployment with the support of a suite of software tools. The key feature of the framework is the inherent support of formal validation techniques achieved on account of automated transformation among different system models. The paper appeals to developers of automation systems and automation software tools via showing the pathway to improve the system development practices by combining several design and validation methodologies and technologies.

A Signal Extension for Petri Nets and its Use in Controller Design

Petri nets have a longstanding history in controller design and implementation, and it is often told that they have the dominating position in this field due to the close relationship between Petri nets and Graphcet or other sequential function diagrams. We want to show that there are some severe problems when Petri nets are used to describe both: the controller and the plant under control. The nature of these problems is the lack of a concept of input and output signals and, moreover, the lack of a formal concept to interconnect systems modeled by Petri nets via signals. Although this is often neglected, it turns out to be a key issue if a model of the closed-loop behavior must be established as a precondition for formal controller verification. We present a signal extension for Petri nets and means for interconnecting an arbitrary number of those extended Petri nets. We come up with the conclusion that the resulting models are suited for controller verification. Although the graphical appearance shows similarities to Petri nets, the resulting models are no longer Petri nets.

A modeling approach for verification of IEC1499 function blocks using net condition/event systems

This paper presents a preliminary report on verification of discrete control applications defined by a new international standard draft IEC 1499. As a first step to verification, the structures presented in the IEC1499 are modeled with Net Condition/Event Systems, for which there exist formal methods and tools of proofing various qualitative and quantitative properties. The paper illustrates the methodology of modeling and outlines further steps towards the full-scale verification of execution control of IEC1499 applications.

Formal validation of intelligent-automated production systems: towards industrial applications

This paper introduces a framework for formal modelling and validation of automation systems destined to be used directly by control engineers. The framework is based on a modelling formalism of Net Condition/Event Systems (NCES), which is graphical, modular and typed. This allows for the modelling of realistic hierarchically organised automation systems in a closed plant-controller loop. The framework consists of methodologies and tools, which enable formal analysis of automation systems. The framework is to be used for the improvement of safety characteristics, reliability and robustness of such systems by means of prediction of potential faults and deadlocks.

Reconfiguration of Distributed Embedded-Control Systems

This paper deals with distributed multiagent reconfigurable embedded-control systems following the component-based International Industrial Standard IEC61499 in which a function block (FB) is an event-triggered software component owning data and a control application is a distributed network of FBs. We define an architecture of reconfigurable multiagent systems, where a reconfiguration agent modeled by nested state machines is affected to each device of the execution environment to apply local automatic reconfigurations, and a coordination agent is proposed for any coordination between devices in order to guarantee safe and adequate distributed reconfigurations. A communication protocol is proposed in our research to handle coordinations between agents by using well-defined coordination matrices. We define, in addition, Extensible Markup Language (XML) based implementations for both kinds of agents, where XML code blocks are exchanged between devices. The contributions of the paper are applied to two benchmark production systems available in our laboratory.

A Comparative Study of Synthesis Methods for Discrete Event Controllers

The purpose of this paper is to illustrate and compare some of the existing synthesis methods for discrete event controllers. We summarize four of the well-known methodologies in the context of plant model, specification model, and controller synthesis procedure. Then, by applying the models to two working examples, we explore and compare the descriptive power of the methods and their capabilities for handling various types of specifications.

Reconfiguration Protocol for Multi-Agent Control Software Architectures

This paper deals with distributed multi-agent reconfigurable embedded control systems following the International Industrial Standard IEC 61499 in which a Function Block (FB) is an event-triggered software component owning data and a control application is a network of distributed blocks that should satisfy functional and temporal properties according to user requirements. We define an architecture of reconfigurable multi-agent systems in which a Reconfiguration Agent is affected to each device of the execution environment to apply local reconfigurations, and a Coordination Agent is proposed for coordinations between devices in order to guarantee safe and adequate distributed reconfigurations. A communication protocol is proposed to handle coordinations between agents by using well-defined Coordination Matrices. We specify both reconfiguration agents to be modeled by nested state machines, and the Coordination Agent according to the formalism Net Condition-Event Systems (NCES) which is an extension of Petri nets. To validate the whole architecture, we check by applying the model checker SESA in each device functional and temporal properties to be described according to the temporal logic “Computation Tree Logic.” We have also to check all possible coordinations between devices by verifying that whenever a reconfiguration is applied in a device, the Coordination Agent and other concerned devices react as described in user requirements. The papers contributions are applied to two Benchmark Production Systems available in our research laboratory.

Object-oriented modular place/transition formalism for systematic modeling and validation of industrial automation systems

We introduce a framework for formal modeling and validation of automation systems destined to use by control engineers. The framework is based on a modeling formalism of net condition/event systems which is graphical, modular, and typed. This allows for modeling of realistic hierarchically organized automation systems in a closed-loop. The framework consists of methodologies and tools, which enable formal analysis of automation systems. The framework is to be used for improvement of safety characteristics, reliability and robustness of such systems by means of prediction of potential faults and deadlocks.