Extension of STPA to Analyze Decisions and Behaviors of Human as Controlled Process in Human-Machine Coexistence Environment.

To realize an autonomous control system that operates in cooperation with humans, safety design that considers human decisions and behaviors is essential. Systems- Theoretic Accident Model and Processes (STAMP) and System- Theoretic Process Analysis (STPA) can analyze interactions between systems and humans that cause hazards, such as mistakes by human operators. However, it is hard to extract the behaviors of humans who are out in the same field as autonomous devices and work in cooperation with the systems as hazard causal factors with conventional STPA methods. In this study, we propose an extended STPA method that can systematically analyze hazard causal factors in the behaviors of humans who coexist and cooperate with autonomous control systems in the same field as autonomous devices. The proposed method (1) defines information that is given to humans by the systems and may cause hazardous human behavior as unsafe control actions and (2) identifies hazard causal factors with guidewords improved to be applicable to humans. Through a case study on a system that controls automated guided vehicles in an area where humans are present, it was shown that the proposed method can extract human's erroneous decisions and behaviors that may cause hazardous situations as hazard causal factors.