Design of integral sliding mode observers for state, fault and unknown input reconstruction

This paper proposes a new technique for fault diagnosis and estimation of states and unknown inputs in a class of nonlinear systems. The novelty of this approach lies in the design of two nonlinear observers which incorporate a combination of sliding and integral control actions. The observers are networked together for online information interchange. The first observer is used for fault diagnosis, and the second is used for the unknown inputs. It is shown that under certain conditions, the proposed observer is able to reduce chattering without compromising on estimation accuracy. Another significant advantage of the proposed approach is that the network of two interconnected integral sliding mode observers permits the relaxation of the fault isolability from the unknown inputs (in an appropriate sense), which has been a major problem previously