On the existence of unknown input observers for state affine systems up to output injection

In this paper, the design of an unknown input observer is considered. The main contribution consists in the obtention of a sufficient condition to design an observer which estimates a part of the state independently of the knowledge of some inputs. Based on the geometric approach, a sufficient condition for the existence of an unknown input observer for state affine systems up to output injection is given. This approach is illustrated in a numerical example.