Safe operation of control systems - Principles and methods

Control systems play a major role in the management of modern technological systems. They are found in many industrial and transport systems, where they contribute to the accomplishment of complex missions. For this reason, it is important to anticipate and control malfunctions, which can have serious economic and human consequences. Operational safety (OS) methods can be used to analyze and find solutions to this problem, whether in the design or redesign phases of control systems.

This article presents the main dependability methods that can be used to design safe control systems, taking into account the behaviors of the systems with which they interact. The lay reader can consult the articles [S 8 250] , [AG 4 670] and [R 7 595] to acquire basic knowledge of dependability, such as the concepts of reliability, failure rate, MTTF (mean time to failure), maintainability, repair rate, MTTR (mean time to repair), availability, MUT (mean up time), etc.

The study of the operational safety of control systems cannot be based on "classical" methods, which have a number of limitations (see [S 8 250] ). The main ones are: the "physical" (non-interaction) and probabilistic (hazards induced by interactions) independence of components from each other and from the system environment, the binary behaviors of components (operation/panic), the exclusion of "non-coherent" behaviors of certain systems and the notion of "masked" failures (one failure can mask another, or even compensate for it, etc.) or dynamic behaviors and temporal aspects (systems whose physical quantities significantly influence their failure characteristics and vice versa, the order of appearance of sequences of events changes the final state of the system).) or dynamic behavior and temporal aspects (systems whose physical quantities significantly...