The importance of being persistent [qualitative simulation]

Summary form only given. The QSIM algorithm of B. Kuipers (1985) is used to perform qualitative simulation of continuous dynamic physical systems described using `weak´ modelling primitives. The algorithm is nonconstructive in that at each new state generation, every possible (in a local sense) qualitative transition of each system variable is considered. The algorithm then refutes impossible (in a global sense) transitions of the system considered as a whole by applying a succession of filters. A phenomenon known as `chattering´ has been described and investigated in the algorithm. It is shown that chattering can be split into (at least) two quite distinct phenomena and that in one case it is the nonconstructive nature of QSIM that causes chattering. A constructive algorithm for performing qualitative simulation of continuous dynamic physical systems is presented. This algorithm is event-based and uses the notion of persistence, so that events are only caused by the behaviour of the system and not by the algorithm used to investigate that behaviour. This eliminates one aspect of chattering without recourse to additional information. The algorithm is discussed