Design considerations for modeling modes in cyber-physical systems

A design pattern for the mode logic of GPCA (generic patient controlled analgesia) that allows for flexible, understandable, and maintainable models is proposed. During requirements analysis, the various modes of drug infusion of GPCA were visualized from the user's perspective. While the sequential structure better matches the user's conceptual view of how operational modes work, it was clearly not suitable from the model's flexibility and manageability perspective. Other approaches to capture the mode logic include tabular formats such as Parnas tables and SCR. It is found that the graphical notations are more suitable for early modeling efforts to easily communicate to domain experts and to get clarity on requirements. Also, the tables for describing the mode logic become unmanageable with increase in the number of features and parameters that influence the mode logic. While the proposed parallel structuring is flexible and maintainable, from a verification perspective, care has to be taken while specifying properties to verify the mode logic. When the modal behavior is expected to be mutually exclusive, a sequential pattern naturally captures it. But in the parallel pattern, the logic of deciding active features is handled by the arbiter.