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Generating sign conventions in reusable models

Establishing proper sign convention in physical system models is not a trivial task. If it is done carelessly, severe problems can result with respect to the interpretation and computation of the resulting model. Moreover, setting up a sign convention should not be confounded with other modelling decisions. We propose a systematic procedure for generating sign conventions in physical systems models. The conventions are expressed as orientations of the energetic interactions represented in bond graphs. An optimal sign convention for the overall system is established through propagation in the completed model. In a model library only the submodels of active components are stored with orientations. Our approach is based on the notion of through and across variables, being domain dependent labels of the effort and flow variables. The algorithm complies with the requirements for sign conventions given in the literature and has been implemented in software and tested. Two examples show how this algorithm works in practice.