Abstract
This paper investigates the problem of sensor fault estimation for systems with monotone nonlinearities and unknown inputs. To the best of our knowledge, such a particular problem is treated for the first time, as only actuator faults have been estimated for this class of nonlinear systems in the literature. The design method is based on an unknown input observer and is particularly effective for integer-order systems and fractional-order systems. This can be regarded as a second contribution in the paper. Indeed, to the best of our knowledge, no papers dealing with the observer design problem for fractional-order systems with monotone nonlinearities exist in the literature. In order to validate the theoretical results, two simulation examples are given, including fractional-order examples and the “integer-order” flexible joint electric drive system.