Differential Algebraic Modelling of Nonlinear Systems

This paper discusses how the concepts of differential algebra can be used in the modelling of physical systems. In particular it is shown that the concepts of ranking and characteristic set can be used to give a structure to the set of physical equations. The characteristic set makes it easy to find the number of inputs and the order of the system. The question of observability can also be investigated using the characteristic set. In particular one can see that the order of an input output differential equation will be the same as the system order if and only if all variables are observable from the output.