Georg Frey

Formal methods in PLC programming

A detailed generic model of the control design process is introduced and discussed. It is used for surveying different formal approaches in the context of PLC programming. The survey focuses on formal methods for verification and validation (V&V). The varying works in this area are categorized using three criteria: the general approach (A) to the task (model based, constraint based or without a model), the formalism (F) (Petri net, automata, etc.,) used to state the formal description, and the method (M) (model-checking, reachability analysis, etc.,) used to analyze the properties. Based on these three criteria (A-F-M) a three letter code for V&V approaches is introduced. Some works from the multitude of V&V research are presented and categorized using this new system.

Towards a Model-Driven IEC 61131-Based Development Process in Industrial Automation*

The IEC 61131-3 standard defines a model and a set of programming languages for the development of industrial automation software. It is widely accepted by industry and most of the commercial tool vendors advertise compliance with it. On the other side, Model Driven Development (MDD) has been proved as a quite successful paradigm in general-purpose computing. This was the motivation for exploiting the benefits of MDD in the industrial automation domain. With the emerging IEC 61131 specification that defines an object-oriented (OO) extension to the function block model, there will be a push to the industry to better exploit the benefits of MDD in automation systems development. This work discusses possible alternatives to integrate the current but also the emerging specification of IEC 61131 in the model driven development process of automation systems. IEC 61499, UML and SysML are considered as possible alternatives to allow the developer to work in higher layers of abstraction than the one supported by IEC 61131 and to more effectively move from requirement specifications into the implementation model of the system.

Automatic implementation of Petri net based control algorithms on PLC

Signal interpreted Petri nets (SIPN) show good properties in modeling and analyzing control algorithms. With SIPN, logic controllers are modeled by places-setting output signals-and transitions between those places-depending on Boolean functions of input signals. This model has interesting dynamics: Firstly, several transitions can fire simultaneously. Secondly, there can be iterated firing of transitions before a new stable marking is reached. In standard PLC programming languages, there is no direct means to implement these dynamics. In this contribution a method for the generation of transparent PLC code from SIPN is presented (using instruction list or ladder diagram). The code is transparent because there is a one-to-one correspondence between SIPN elements and code segments. A new analytical method to guarantee the correct dynamics of the implemented code is presented and compared to existing approaches. The new method results in a special ordering of the code segments. The presented analysis of the SIPN shows if a transition t/sub a/ can fire in an iteration after transition t/sub b/. In this case, the code segment describing t/sub b/ has to be processed prior to the one of t/sub a/. A solution for the case, where the analysis results in an impossible ordering is also presented.

Reconfigurable Coordination of Distributed Discrete Event Control Systems

Dynamic reconfigurability is receiving more and more attention from both academy and industry, which means the ability to flexibly modify system functions by adding/removing hardware/software components, modifying logic relation between components, or updating particular system data at runtime without sacrificing the system performance. A distributed reconfigurable discrete event control system (DRDECS) is composed of several networked reconfigurable subsystems. In order to realize system functions, these reconfigurable subsystems communicate and coordinate with each other, since any casually reconfiguration applied to a subsystem may cause risks to others, or even to the safety of the whole system. This brief proposes a new coordination method for a DRDECS, where each subsystem is modeled by a reconfigurable timed net condition/event system. A virtual coordinator together with a communication protocol between it and subsystems is developed in order to achieve two aims: 1) to coordinate subsystems with an optimal coordination solution using judgement matrices while multiple subsystems require global reconfigurations and 2) to reduce exchanged messages between the coordinator and these subsystems. Furthermore, for the purpose of checking functional and temporal properties of a DRDECS with this virtual coordinator, a computation tree logic-based model checking method is applied. Finally, a hypothetic manufacturing plant is used as a running example to illustrate this brief.

PLC programming with signal interpreted Petri nets

In this paper a graphical editor to design Programmable Logic Controller (PLC) programs using Signal Interpreted Petri Nets (SIPN) is presented. SIPN are an extension of condition event Petri nets that allow the handling of input and output signals. The presented tool, SIPN Editor, has been developed using DiaGen which is an environment for rapidly developing diagram editors from a formal specification of the diagram language. The SIPN Editor supports the translation of SIPN into input code for the model checker SMV. Using SMV, the SIPN can be verified before it is automatically translated into Instruction List code according to the IEC 61131-3 standard. This code can be downloaded on nearly every PLC.

An MDD process for IEC 61131-based industrial automation systems

Model Driven Development (MDD) has been proved as a quite successful paradigm in general-purpose computing and is currently exploited in the embedded systems domain. On the other hand, the majority of industrial automation systems is developed based on the IEC 61131 standard. This standard defines a model and a set of programming languages for the development of industrial automation software and it is supported by the majority of the commercial tools in this domain. This work proposes an MDD process to increase the productivity and reliability of the development process of industrial automation systems. Piping and instrumentation diagrams are considered as source of requirements for process control engineering and SysML is used to reduce the gap between these diagrams and the 61131 based design specs of the control system. The foundation for a SysML profile to facilitate the exploitation of SysML in this domain is described. This profile will allow the developer to work in higher layers of abstraction than the one supported by IEC 61131 and effectively move from requirement specifications into the implementation model of the system.

Component-Oriented Modeling of Thermoelectric Devices for Energy System Design

Thermoelectric (TE) devices are used in the form of Peltier coolers and as TE generators, with the latter producing electrical energy from waste heat, based on the Seebeck effect. In both cases, modeling of the TE device is a prerequisite for the design and control verification of the resulting overall energy system. To this end, the model has to be integrated seamlessly in an overall system model containing other electrical, thermodynamic, or even mechanical components. Following this premise, this paper presents a component-based model for TE devices described in the Modelica language. The model incorporates the temperature dependences of decisive material properties (Seebeck coefficient, thermal conductivity, and electrical resistivity) in 1-D spatial resolution. With the help of few additional geometrical parameters, e.g., the thickness of TE legs, the model is capable of describing the dynamic behavior of the TE device in accordance with the experimental results.

Verification and validation of control algorithms by coupling of interpreted Petri nets

Interpreted Petri nets (IPN) are an extension of the basic Petri net (PN) framework. They are ordinary PN with additional components to model the information flow to and from the PN. This contribution gives a general overview of the control design process. Furthermore, it shows how the different tasks in this process, especially analysis, verification and validation, can be successfully solved within the formal IPN framework.

Evaluation of response time in ethernet-based automation systems

This paper presents a method to assess response time of automation system architectures including industrial switched Ethernet networks using client/server protocols. The method relies upon modeling the behavior of the components of these architectures in the form of hierarchical timed colored Petri nets and upon simulation of these models. A case study exemplifies the method and shows how it can facilitate design of automation systems including this kind of industrial Ethernet networks.

Migration of a PLC Controller to an IEC 61499 Compliant Distributed Control System: Hands-on Experiences

IEC 61499 ushers in a new trend of software development in the area of Industrial Process Measurement and Control System (IPMCS). This new standard simplifies the development of distributed IPMCS applications through inclusion of re-usability, encapsulation and modularity. IEC 61499, due to its close resemblance with Object-Oriented (OO) paradigm also paves the way to integrate modeling techniques like UML into the development process of the distributed IPMCS applications. Yet a remarkable challenge is there to integrate or re-design the systems, elements or components designed using a monolithic language like the ones on Programmable Logic Controllers (PLCs). In this work it is attempted to share the experiences faced while attempting to migrate a PLC controlled centralized laboratory application into an IEC 61499 compliant distributed control application. The targeted distributed control system consists of network-enabled controllers where the IEC 61499 compliant control sub-applications should run. It needs to be mentioned that the development process did not start from analysis of the specification of the instrument to be controlled but rather from a formal specification of it written in Signal Interpreted Petri Nets (SIPN).