Formal Qualitative Physics-Based Reasoning for Functional Decomposition of Engineered Systems

Functional decomposition is an important task in early systems engineering and design, where the overall function of the system is resolved into the functions of its components or subassemblies. Conventionally, this task is performed manually, because of the possibility of multiple solution paths and the need for understanding the physics phenomena that could realize the desired effects. To this end, this paper presents a formal method for functional decomposition using physics-based qualitative reasoning. The formal representation includes three parts: (1) a natural language lexicon that can be used to detect the changes of physical states of material and energy flows, (2) a set of causation tables that abstracts the knowledge of qualitative physics by capturing the causal relations between the various quantities involved in a physical phenomenon or process, and (3) a process-to-subgraph mapping that translate the physical processes to function structure constructs. The algorithms use the above three representations and some additional topological reasoning to synthesize and assemble function structure graphs that are decompositions of a given black box model. The paper presents the formal representations and reasoning algorithms, and illustrates this method using an example function model of an air-heating device. It also presents the software implementation of the representations and the algorithms and uses it to validate the method’s ability to generate multiple decompositions from a black box function model.