Model Checking PLC Software Written in Function Block Diagram

Abstract

Abstract-The development of Programmable Logic Controllers (PLCs) in the last years has made it possible to apply them in ever more complex tasks. Many systems based on these controllers are safety-critical, the certification of which entails a great effort. Therefore, there is a big demand for tools for analyzing and verifying PLC applications. Among the PLCspecific languages proposed in the standard IEC 61131-3, FBD (Function Block Diagram) is a graphical one widely used in rail automation. In this paper, a process of verifying FBDs by the NuSMV model checker is described. It consists of three transformation steps: FBD→TextFBD→tFBD→NuSMV. the novel step introduced here is the second one: it reduces the state space dramatically so that realistic application components can be verified. The process has been developed and tested in the area of rail automation, in particular interlocking systems. As a part of the interlocking software, a typical point logic has been used as a test case.

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“…PLCs are often programmed in multiple ways. The most common of which include the use of ladder logic [61]; function blocks [62] and high level programming languages [63], [64].…”

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“…A possible solution to meet this objective is to apply formal verification techniques ( [2]) on the specification of the control logic ( [4], [13]) or the PLC code that implements this logic ( [1], [5], [6], [9], [12]). These techniques are based on an exhaustive analysis of a state space which represents the specification or the PLC code according to the verification objective.…”

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“…The application of formal methods for PLCs has been studied in previous work for many years (e.g., Frey and Litz (2000); Canet et al (2000); Bauer et al (2004); Sülflow and Drechsler (2008); Yoo et al (2008); Pavlović and Ehrich (2010)), but either they do not address the modelling of real systems, or they do not apply verification, thus they do not have to apply any specific technique to handle the state space explosion problem.…”