Abstraction by Time-Scale in Qualitative Simulation* Abstract (Extended Abstract)

Qualitative simulation faces an intrinsic problem of scale: the number of limit hypotheses grows exponentially with the number ofparameters approaching limits. We present a method called Time-Scale Abstractionfor structuring a complex system as a hierarchy of smaller, interacting equilibrium mechanisms. Within this hierarchy, a given mechanism views a slower one as being constant, and a faster one as being instantaneous. A perturbation to a fast mechanism may be seen by a slower mechanism as a displacement ofa monotonic function constraint. (In the full paper, we demonstrate the time-scale abstraction hierarchy using the interaction between the water and sodium balance mechanisms in medical physiology, an example drawn from a larger, fully implemented, program.) Where the structure ofalarge system permits decomposition by time-scale, this abstraction method permits qualitative simulation of otherwise intractibly complex systems