Exploring Validity Frames in Practice

Model-Based Systems Engineering (MBSE) provides workflows, methods, techniques and tools for optimal simulation-based design and realization of complex Software-Intensive, Cyber-Physical Systems. One of the key benefits of this approach is that the behavior of the realized system can be reasoned about and predicted in-silico, before any prototype has been developed. Design models are increasingly used after the system has been realized as well. For example, a (design) digital twin can be used for runtime monitoring to detect and diagnose discrepancies between the simulated and realized system. Inconsistencies may arise, however, because models were used at design time that are not valid within the operating context of the realized system. It is often left to the domain expert to ensure that the models used are valid with respect to their realized counterpart. Due to system complexity and automated Design-Space Exploration (DSE), it is increasingly difficult for a human to reason about model validity. We propose validity frames as an explicit model of the contexts in which a model is a valid representation of a system to rule out invalid designs at design time. We explain the essential and conceptual, yet practical, structure of validity frames and a process for building them using an electrical resistor in the optimal design of a high-pass filter as a running example. We indicate how validity frames can be used in a DSE process, as well as for runtime monitoring.