Designing Functional Safety Systems: A Pattern Language Approach

Human beings, at least most of us, want to feel and be safe. This is one of the fundamental needs of an organism. However, several of the processes and machines used in current societies introduce hazards that could and can harm us causing unnecessary pain and financial losses. Still, our modern societies need these processes and machines to operate so we cannot really be without them. Fortunately, there are ways to reduce risks introduced by systems around us to a tolerable level.This thesis considers the design and development of safety-related systems and safetyrelated parts of control systems referred to as functional safety systems. These systems implement safety functions that reduce risks introduced by machines, processes, and other systems. That is, the functions affect the system under control so that the likelihood of occurrence or severity of consequences are reduced.The design and development of safety systems is typically regulated by laws and standards. This increases the cost of safety system development and therefore eventually also the product in which it is incorporated. However, from a manufacturer viewpoint, safety in all its forms is also a potential asset for the companies developing, producing, and selling the systems. An increase in efficiency to develop and design safety systems offers the potential for a larger margin or increased sales due to the reduced price.One way to support design and development efficiency is to apply good design methods and solutions in form of design patterns. In this thesis, a design pattern language for the development and design of functional safety systems is introduced. The purpose of the language is to support the designers in their task to design and implement safety functions in machines and processes. The language considers various aspects of the development and design of safety systems starting from the initial phases of hazard and risk analysis, followed by the selection of the hazard and risk reduction methods, and concluding with the hardware and software structure, functionality, and design principles considerations. Finally, a functional safety system may, and often does, co-exist and co-operate with a control system. Therefore, a part of the pattern language takes this aspect into account.To compile the design pattern language and the included patterns a design science research approach complemented with grounded theory approach is applied The data to identify the patterns is collected from literature, personal experience, interviews, and discussions with industry representatives and people engaged with the design or use of systems including safety systems or functionality. Like the patterns have evolved during the research, so has the approach to identify, document, and process the patterns