System Theoretic Approach for Determining Causal Factors of Quality Loss in Complex System Design

Abstract

Identifying the factors that could lead to the loss of quality is difficult for large complex product systems. Traditional design methods such as Failure Modes and Effects Analysis (FMEA), Fault Tree Analysis (FTA), and Robust Design have been proven effective at identifying component failures, but are less effective for causes of quality loss that involve interactions between components, software flaws, or external noises. This research applied System Theoretic Process Analysis (STPA) to a case study at Cummins, Inc. The case study was a technology change to a subsystem in a new product development project. The intent of this case was to determine if STPA, developed for safety engineering and hazard analysis, would be effective in identifying causes of quality losses. The results of the case study were compared to the traditional design methods. STPA allowed design teams to identify more causal factors for quality losses than FMEA or FTA, especially those involving component interactions, software flaws, and external noises. STPA was also found to be complementary to Robust Design methods.