Automated formal analysis of temporal properties of Ladder programs

Abstract

Programmable Logic Controllers are industrial digital computers used as automation controllers in manufacturing processes. The Ladder language is a programming language used to develop software for such controllers. In this work, we consider the description of the expected behaviour of a Ladder program under the form of a timing chart, describing a scenario of execution. Our aim is to prove that the given Ladder program conforms to the expected temporal behaviour given by such a timing chart. Our approach amounts to translating the Ladder code, together with the timing chart, into a program for the Why3 environment for deductive program verification. The verification proceeds with the generation of verification conditions: mathematical formulas to be checked valid using automated theorem provers. The ultimate goal is twofold. On the one hand, by obtaining a complete proof, one verifies the conformity of the Ladder code with respect to the timing chart with a high degree of confidence. On the other hand, in the case the proof is not fully completed, one obtains a counterexample, illustrating a possible execution scenario of the Ladder code which does not conform to the timing chart.