Developing Functional Safety Requirements using Process Model Variables

In ISO 26262, the top-level safety goals are derived using the Hazard Analysis and Risk Assessment. Functional safety requirements (FSRs) are then derived from these safety goals in the concept phase (ISO 26262-3:2011). The standard does not call out a specific method to develop these FSRs from safety goals. However, ISO 26262-8:2011, Clause 6, does establish requirements to ensure consistent management and correct specification of safety requirements with respect to their attributes and characteristics throughout the safety lifecycle. Hence, there are expectations on the part of system engineers to bridge this gap. The method proposed in this paper utilizes concepts from process modeling to ensure the completeness of these requirements, eliminate any external inconsistencies between them and improve verifiability. The goals of process modeling are to understand the current state of the process in detail, define the desired state of the process and implement techniques to change the state. The process model variables provide the appropriate context needed to define these states. These principles are commonly adopted in the fields of software development and chemical engineering. They are also being used to improve safety in aviation and industrial operations with some success. In an ISO 26262-based product development, process modeling can help define all the safety-relevant attributes of a system and analyze them. With this information, the functional safety requirements can be written with a high level of rigor. Electronic Throttle Control is used in this paper as an example to illustrate the advantages of the proposed method.