Robots, and more generally mechanical systems, are types of a physical system. This is why it is important to study and control these systems using information about their particular structure that describes their particular nature.
In discussing physical systems, concepts like energy, interconnection and interaction, become of substantial importance. Furthermore, during the modeling and control tasks, the results we obtain should be independent from artificial co-ordinates that people use to analyse the results of their work. This has lead to the concept of co-ordinate free description...