System-Theoretic Process Analysis (STPA) is a technique, based on System-Theoretic Accident Model and Process (STAMP), to identify hazardous control actions, loss scenarios, and safety requirements. STPA is considered a rather complex technique and lacks formalism, but there exists a growing interest in using STPA in certifications of safety-critical systems development. SysML is a modeling language for systems engineering. It enables representing models for analysis, design, verification, and validation of systems. In particular, the free software TTool and the model-checker UPPAAL enable formal verification of SysML models. This paper proposes a method that combines STPA and SysML modeling activities in order to allow simulation and formal verification of systems' models. An automatic door system serves as example to illustrate the effectiveness of the proposed approach.
STAMP (System-Theoretic Accident Model and Processes) techniques such as STPA (System- Theoretic Process Analysis) and STPA-Sec (STPA for Security) have been applied only in an adhoc manner, without the aid of tools. More recently, tools have been proposed to help the application of STPA and STPA-Sec. Most of the tools focus on user experience issues and do not cover all the aspects of STPA and STPA-Sec. Three aspects of tools are systematization, automation and analysis completeness. Systematization allows the analysis to be performed in a more disciplined way while automation allows a more time efficient analysis. Analysis’ completeness is the analysis coverage in a given domain. We identify the essential requirements supporting business and stakeholders' needs for a STAMP based tool. We propose a STAMPcompliant web application, named WebSTAMP, for STPA and STPA-Sec. WebSTAMP is intended to aid analysts throughout the analysis process in a more automated and comprehensive way, and it aims to be a collaborative tool. We illustrate how the requirements are implemented in the current version of WebSTAMP with an example of use. The results show that WebSTAMP assists analysts to conduct safety and security analyses in a more systematic, automated and comprehensive manner.
Systems of today are becoming more complex; they have many levels of the control hierarchy, are software-intensive, use different networks, have increasing processing power, use a diversity of devices, and require more integration. Systems-Theoretic Process Analysis (STPA) is a technique that is being used to analyze the safety of those systems at the concept stage. For the design phase, STPA can be combined with SysML modeling activities, including simulation and formal verification of systems models to produce the control software more efficiently. However, for the design phase, when starting from the STPA analysis there is no support to elaborate the control algorithm. Building the control algorithm is one of the most difficult tasks in the design phase. We propose a method to synthesize the control algorithm for safetycritical systems from the STPA analyses and the functional requirements. Our method maps the control structure (STPA) into a block diagram (SysML), and it uses the STPA results to generate an initial state machine diagram (SysML) for automated controllers, actuators, and sensors. We use our method to generate the control algorithms for an Adaptive Cruise Control system. We evaluate the synthesized algorithms by performing model simulation and formal verification. This illustrates that our method is a systematic way to synthesize control algorithms that satisfy both safety and functional requirements.INDEX TERMS Safety, Systems Modeling Language, Model checking, Control system synthesis, System analysis and design.
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