Supervision and Safety of Complex Systems

Chapter 8The APPRODYN Project:Dynamic Reliability Approaches toModeling Critical Systems ¹

 

 

 

8.1. Context and aims

8.1.1. Context

The safety of high criticity industrial systems relies heavily on relatively complex program systems (with regards to code size, architecture, number of variables examined, etc.). Due to the complexity of the interactions between physical processes and their control, the standard methods of safety assessment are limited. These methods involve constructing a structural function in the form of a temporally invariable Boolean equation (a combinatorial approach) of the variables representing the components' states. These approaches, however, are shown to be limited in the cases of “non-coherent systems”.

Some systems cannot be represented by a temporally invariable Boolean function. Their structure function has been proved to be correctly represented by a language of events. This involves using a formalism of state-transition type to represent the behavior of the system allowing us to show the sequence of events rather than their combinations in the form of cuts or bonds. This is the first dynamic aspect of the system's structure function. Beyond this first issue, the concept of ‘dynamic reliability' covers a range of properties highlighted in the literature over recent decades that can be summarized as follows:

– the system's states are defined by a finite state machine (FSM). These states correspond to combinations of component states. Their number ...