A safety analysis method for control software in coordination with FMEA and FTA

Abstract

In this study, we proposed a method to improve the safety level of control software (CSW) by managing the CSW’s design information and safety analysis results, and combining failure mode and effects analysis (FMEA) and fault tree analysis (FTA). Here, the CSW is developed using structured analysis and design methodology. In the upper stage of the CSW’s development process, as the input of the preliminary design information (data flow diagrams (DFDs) and control flow diagrams (CFDs)), the causes of undesirable events of the CSW are clarified by FMEA, and the countermeasures are reflected in the preliminary design information. In the lower stage of the CSW’s development process, as the inputs of the detailed design information (DFDs and CFDs in the lower level) and programs, the causes of the specific undesirable event are clarified by FTA, and the countermeasures are reflected in the detailed design specifications and pro-grams. The processes are repeated until the impact of undesirable events become the acceptable safety level. By applying the proposed method to the CSW installed into a communication control equipment on the space system, we clarified several undesirable events and adopted adequate countermeasures. Consequently, a safer CSW is developed by applying the proposed method.