Cyber-Physical Systems (CPS) are complex engineered systems which consist of physical components with an underlying cyber network. The three main components of a cyber-physical System are: physical system, networking and communications element and a distributed cyber system. The primary challenge for cyber-physical systems is to understand what happens when various sub-systems, which have been developed in an isolated environment, are integrated. CPS studies need to ensure sub-systems that had been designed in isolation to meet certain specifications, when combined, do not cause the overall system to fail. The crux of cyber-physical research is thus to find a common platform to bind all these different components, so as to monitor the overall system performance.
This dissertation discusses how to unify these different aspects and tackles the issue of synthesizing, verifying and monitoring highly diverse environments by introducing the concept of Unified Invariants. In this dissertation, a smart grid has been used to implement and validate this concept of Unified Invariants towards building a robust cyber-physical system. There are several ways to compromise the reliable operation of a smart grid. Examples of such contingent events are voltage collapse, line overloading and dynamic instability. Physical system invariants have been developed to identify and thwart such events which threaten the integrity of the physical system. These physical invariants have be integrated with cyber controllers to ensure a safe, stable and reliable operation of the smart grid. This is an unique concept and differs from previous methods in the fact that while earlier methods have tried to compose functionality of each domain of the cyber-physical world, the Unified Invariant method serves as a transformative approach to express and impose system properties that are common to all the domains (cyber, physical, networking). The net outcome of such an approach is that the resulting CPSs will be safe and stable at the system level, rather than just the sub-system level. --Abstract, page iii
