Force as a flow variable

Abstract

The paper stems from an assessment of the suitability of bond graphs for modelling mechanical systems. Although mathematically rigorous, difficulties arise with intuitive interpretation of bond graphs. The source of the difficulty is the semantics commonly adopted and the way that they relate to traditional interpretations of mechanical system dynamics. Bond graphs represent dynamic systems as energy manipulators with the flow of energy given by the product of two power variables, commonly described as the flow and effort variables. In the mechanical domain, velocity is commonly described as the flow variable, and force as the effort variable. The physical interpretation of mechanical systems and the analogies across systems domains are much improved if force is adopted as the flow variable and the second power variable (i. e. velocity) is called the potential variable. Force can be represented as the flow of a mechanical charge, where the total charge stored in a mass is equal to its momentum. It displays all the properties of a flow variable. Mass is then analogous to electrical capacitance, and mechanical compliance is analogous to electrical inductance.