Dynamic System on State-Space, and Sinusoidal Signal

Hello!

I have a set of 15 differential equations that describe a dynamic system, and 4 of the independent variables of said system are angles, which in turn makes so that other 4 dependant variables of said angles be sinusoidal functions. My intention was to describe this model in a state-space block structure, as to make things simpler, but it just so happens that a dynamic system of differential equations can't be easily translated into state-space form when there are angles as variables, or sinusoidal funcions. My control theory professor answered me that a state-space system must be of linear differential equations. But I wonder if there may be any way to come around this problem without needing to discretize the sine functions.

Thank you. Vicente.