Verification and Optimization of a PLC Control Schedule

We report on the use of the SPIN model checker for both the verification of a process control program and the derivation of optimal control schedules. This work was carried out as part of a case study for the EC VHS project (Verification of Hybrid Systems), in which the program for a Programmable Logic Controller (PLC) of an experimental chemical plant had to be designed and verified. The intention of our approach was to see how much could be achieved here using the standard model checking environment of SPIN/Promela. As the symbolic calculations of real-time model checkers can be quite expensive it is interesting to try and exploit the efficiency of established non-real-time model checkers like SPIN in those cases where promising work-arounds seem to exist. In our case we handled the relevant real-time properties of the PLC controller using a time-abstraction technique; for the scheduling we implemented in Promela a so-called variable time advance procedure. For this case study these techniques proved sufficient to verify the design of the controller and derive (time-)optimal schedules with reasonable time and space requirements

Model checking embedded system designs

We survey the basic principles behind the application of model checking to controller verification and synthesis. A promising development is the area of guided model checking, in which the state space search strategy of the model checking algorithm can be influenced to visit more interesting sets of states first. In particular, we discuss how model checking can be combined with heuristic cost functions to guide search strategies. Finally, we list a number of current research developments, especially in the area of reachability analysis for optimal control and related issues

What is the method in applying formal methods to PLC applications?

The question we investigate is how to obtain PLC applications with confidence in their proper functioning. Especially, we are interested in the contribution that formal methods can provide for their development. Our maxim is that the place of a particular formal method in the total picture of system development should be made very clear. Developers and customers ought to understand very well what they can rely on or not, and we see our task in trying to make this explicit. Therefore, for us the answer to the question above leads to the following questions: Which parts of the system can be treated formally? What formal methods and tools can be applied? What does their successful application tell (or does not) about the proper functioning of the whole system

Obtaining Formal Models through Non-Monotonic Refinement

When designing a model for formal verification, we want to\ud be certain that what we proved about the model also holds for the system we modelled. This raises the question of whether our model represents the system, and what makes us confident about this. By performing so called, non-monotonic refinement in the modelling process, we make the steps and decisions explicit. This helps us to (1) increase the confidence that the model represents the system, (2) structure and organize the communication with domain experts and the problem owner, and (3) identify rational steps made while modelling. We focus on embedded control systems

The Construction of Verification Models for Embedded Systems

The usefulness of verification hinges on the quality of the verification model. Verification is useful if it increases our confidence that an artefact bahaves as expected. As modelling inherently contains non-formal elements, the qualityof models cannot be captured by purely formal means. Still, we argue that modelling is not an act of irrationalism and unpredictable geniality, but follows rational arguments, that often remain implicit. In this paper we try to identify the tacit rationalism in the model construction as performed by most people doing modelling for verification. By explicating the different phases, arguments, and design decisions in the model construction, we try to develop guidelines that help to improve the process of model construction and the quality of models

Compositional verification of industrial control systems : methods and case studies

The main obstacles in the formal verification of industrial control systems are the lack of precise semantics for its programming languages, and the complexity problems which arise during the verification process. This work addresses both issues by defining an operational semantics for Sequential Function Charts, a widely-used language for Programmable Logic Controllers (PLCs), and by presenting modular and compositional methods to reduce the complexity arising from parallel structures in the system. These methods are illustrated by the verification of two PLC-controlled chemical batch plants

Capturing Assumptions while Designing a Verification Model for Embedded Systems

A formal proof of a system correctness typically holds under a number of assumptions. Leaving them implicit raises the chance of using the system in a context that violates some assumptions, which in return may invalidate the correctness proof. The goal of this paper is to show how combining informal and formal techniques in the process of modelling and formal verification helps capturing these assumptions. As we focus on embedded systems, the assumptions are about the control software, the system on which the software is running and the system’s environment. We present them as a list written in natural language that supplements the formally verified embedded system model. These two together are a better argument for system correctness than each of these given separately

The programmable logic controller : its prehistory, emergence and application

Programmable Logic Controllers (PLCs) are widely used devices controlling industrial machines and processes and many other diverse applications, requiring primarily, combinatorial logic and sequential control. The PLC is a hidden technology, little known by the general public and overlooked in academic historical studies of technology. The research reported in this thesis aims to address this lack of awareness. The thesis explores the development of sequential and combinatorial logic control technologies, the emergence of the PLC, its subsequent development and its industrial applications. Patents and first- hand accounts and experiences from senior industrial engineers in a number of diverse manufacturing industries have been used as the primary research sources since, as a hidden technology, academic historical accounts are sparse. This approach illustrates, through using the PLC as an example, a potential method of studying other, unrelated hidden technologies. The research has revealed the influence of geography, industrial settings and earlier engineering practices on the design, selection and application of PLC control technologies, and comments on the how these influences define specific communities of practice