Differential flatness and control of nonlinear systems

The purpose of this communication is to investigate the connection between output relative degrees and differential flatness property of non linear systems. Considering the relative degrees of the outputs of a non linear system, necessary and sufficient conditions for differential flatness are displayed. Then, it is shown that the application of the non linear inverse approach to an output observable and output differential flat system is identical with the differential flat control approach. The application of the non linear control approach to a four rotor aircraft is considered. A trajectory tracking system based on a two layers non linear inverse control structure is then proposed.