Obtaining formal models from Ladder diagrams

Programmable Logic Controllers (PLC) have been used in industrial environment to control several different tasks. Such devices can be programmed using one of the languages specified in the IEC 61131–3 standard, such as Ladder or Functional Logic Diagrams. PLCs are widely used in a number of critical environment, such as in the extraction processes of oil and gas. For this reason, systems running on PLCs requires a high level of reliability. In general, it is necessary to perform formal verification to achieve such a level of reliability. Formal behavioral models of the system are built to simulate and verify desired properties. Among many available formalisms, Petri Nets (PN) have been used successfully although the task of modeling demands extra time in the development process. Aiming to amortize time invested in formal verification, this paper presents a method to automatically obtain formal models, described with a class of PN called Coloured Petri Nets, directly from Ladder diagrams. With these models in hand, it is not only possible to simulate the system but it is also possible to apply automatic verification techniques.