Qualitative Fault Modeling in Safety Critical Cyber Physical Systems

ABSTRACTOne of the key requirements for designing safety critical cyber physical systems (CPS) is to ensure resiliency. Typically, the cyber sub-system in a CPS is empowered with protection devices that quickly detect and isolate faulty components to avoid failures. However, these protection devices can have internal faults that can cause cascading failures, leading to system collapse. Thus, to guarantee the resiliency of the system, it is necessary to identify the root cause(s) of a given system disturbance to take appropriate control actions. Correct failure diagnosis in such systems depends upon an integrated fault model of the system that captures the effect of faults in CPS as well as nominal and faulty operation of protection devices, sensors, and actuators. In this paper, we propose a novel graph based qualitative fault modeling formalism for CPS, called, Temporal Causal Diagrams (TCDs) that allow system designers to effectively represent faults and their effects in both physical and cyber sub-systems. The paper also discusses in detail the fault propagation and execution semantics of a TCD model by translating to timed automata and thus allowing an efficient means to quickly analyze, validate and verify the fault model. In the end, we show the efficacy of the modeling approach with the help of a case study from energy system.