Understanding control function and failure from a process perspective

In control design, fault-identification and fault tolerant control, the controlled process is usually perceived as a dynamical process, captured in a mathematical model. The design of a control system for a complex process, however, begins typically long before these mathematical models become relevant and available. To consider the role of control functions in process design, a good qualitative understanding of the process as well as of control functions is required. As the purpose of a control function is closely tied to the process functions, its failure has a direct effects on the process behaviour and its function. This paper presents a formal methodology for the qualitative representation of control functions in relation to their process context. Different types of relevant process and control abstractions are introduced and their application to formal analysis of control failure modes from a process perspective is presented. Finally anticipated applications in context of offline analysis and online supervisory control are discussed.