Ladder Metamodeling and PLC Program Validation through Time Petri Nets

Abstract

Ladder Diagram (LD) is the most used programming language for Programmable Logical Controllers (PLCs). A PLC is a special purpose industrial computer used to automate industrial processes. Bugs in LD programs are very costly and sometimes are even a threat to human safety. We propose a model driven approach for formal verification of LD programs through model-checking. We provide a metamodel for a subset of the LD language. We define a time Petri net (TPN) semantics for LD programs through an ATL model transformation. Finally, we automatically generate behavioral properties over the LD models as LTL formulae which are then checked over the generated TPN using the modelcheckers available in the Tina toolkit. We focus on race condition detection.

An overview of model checking practices on verification of PLC software

“…They provide a seven-step modelling procedure for their methodology; however, their discussions do not include any aspect about automation of their process. Recently, in Mokadem et al's study [83] [8], a model-driven approach is applied by using ATL [62] transformations over LDs to produce timed Petri nets, which then can be automatically transformed to Tina models. A rule-based translation is used in LD translation, and race conditions are handled by checking whether the stabilizing inputs yield to stable outputs.…”

“…Latest achievements in cloud computing and Internet of Things area awaken a demand in industrial automation on gathering and integrating information from data sources to enterprise software systems via PLCs. Many mainstream PLC hardware/software manufacturers released Web-based versions of their PLC visualization and programming software like Siemens, 8 Beckhoff 9 and other software manufacturers. 10,11,12,13 PLC hardware manufacturers even make it possible to access the PLC software by Web servers that runs on the PLC itself.…”

“…For most of the studies, the process was completed in an order of minutes while automatic translation is performed in around half of the studies. In the study by Bender et al [8], performance was discussed over the size of the Petri net model, which is used as an intermediate format so it was considered likewise in the comparison table as well. In Table 8, we can see that most of the studies include explicit examples of the properties (especially safety properties) that were checked.…”

Method for translating ladder diagrams to ordinary Petri nets

“…There have been several attempts to construct PNs from LDs. Bender, Combemale, Crégut, Farines, Berthomieu and Vernadat [17] performed the translation using time PNs with read-arcs. Despite the clear description of the working process, the reachability analysis of the time PNs with read-arcs is of high computational complexity.…”

“…The dynamic behavior, which the PN model describes, embraces that of the PLC system, depicts each phase in the executing process of the LD program, indicates the logic relations among the variables, and shows how the outputs vary. Therefore, the PN model is applicable to the simulation of its corresponding LD program, and the verification of the properties of the LD program such as liveness, reversibility and being free of race [17]. It is inevitable I II III I II III I II III I II III scan cycle 1 scan cycle 2 scan cycle 3 scan cycle 4 scan cycle 5 scan cycle 6 …”